EP4732053A1

EP4732053A1 - Optical fibres and uses thereof

Info

Publication number: EP4732053A1
Application number: EP24739654.2A
Authority: EP
Inventors: Julian Fells; Zipei SONG; Tongyu Liu; Martin Booth
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2023-06-23
Filing date: 2024-06-24
Publication date: 2026-04-29
Also published as: CN121399512A; WO2024261497A1

Abstract

There is provided an optical fibre comprising a first portion and a second portion, wherein: the second portion is a void portion comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre within the void portion; the first portion and the second portion are substantially mode-matched; and the one or more voids are at least partially filled with a filler having a material property different to the material property of a material of the optical fibre.

Description

OPTICAL FIBRES AND USES THEREOF

The present disclosure relates to optical fibres having voids machined therein, methods for manufacturing optical fibres having voids therein, and applications of optical fibres having voids therein.

Optical fibres are used for a wide range of applications including communications and various types of environmental sensing. The propagation of light in optical fibres is heavily affected by the properties of the material of the optical fibre. Much research has been conducted into optimising the materials and combination of materials used in optical fibres intended for different applications. However, it can nonetheless be difficult to find suitable materials or material combinations that have the desired properties of particular applications. In particular, the properties of optical fibre materials may change with temperature and pressure, thereby causing unwanted effects on the propagation of light in the fibres.

It is known to provide optical fibres with holes along their length in order to alter the properties of the optical fibre. However, such fibres are typically manufactured by drawing a specially-prepared preform, which places restrictions on the structures of holes that can be provided. In addition, sections of fibres containing holes must be joined, usually by splicing, to sections of standard optical fibre in order to be able to interface with other parts of an optical system. This increases manufacturing complexity and can create weak points in the fibre where sections are joined together. In addition, there is typically a modemismatch caused by splicing dissimilar fibres, resulting in a high interface loss.

In view of these problems, it would be desirable to provide optical fibres that can be manufactured more flexibly with more easily tuneable properties.

According to a first aspect of the invention, there is provided an optical fibre comprising one or more voids, wherein: the one or more voids extend longitudinally along the length of the optical fibre within a void portion, the void portion being a continuous and integral portion of the optical fibre; a longitudinal extent of each of the one or more voids is less than a longitudinal extent of the void portion; and the one or more voids are sealed from an exterior of the optical fibre and at least partially filled with a filler having a material property different to the material property of a material of the optical fibre.

Providing voids containing filler allows for tuning the properties of the optical fibre by varying the properties of the filler and the arrangement of the voids. Providing the voids within integral portions of the fibre provides improved mechanical strength relative to spliced fibres, and greater flexibility in the design and arrangement of the voids.

Optionally, the one or more voids comprise two or more voids filled with different fillers. Optionally, the material property of the fillers differs between the different fillers. This provides additional flexibility in tuning properties of the fibre.

Optionally, the material property is an optical property, optionally refractive index. Optionally, the one or more voids are configured such that the optical property of the filler affects light transmitted through the optical fibre. This can allow for stronger, direct effects on light in the fibre, rather than relying on indirect effects such as changing expansion properties of the fibre material.

Optionally, a change with temperature of the material property of the filler is different to, optionally opposite to, a change with temperature of the material property of the material of the optical fibre. Optionally, the one or more voids are configured such that the change with temperature of the material property of the filler at least partially compensates for the change with temperature of the material property of the material of the optical fibre. This can allow for the voids to reduce an effect of temperature on the fibre, making its properties more stable over a wider range of conditions.

Optionally, wherein a cross-sectional area of at least one of the one or more voids varies along the length of the optical fibre. Optionally, the cross-sectional area reduces away from a centre of the void along the length of the optical fibre for at least a portion of the length of the void. Optionally, a central axis of the one or more voids extends along a direction inclined to the longitudinal axis of the optical fibre for at least a portion of the one or more voids. This can allow for variations in the properties of the fibre along its length and across its cross-section to achieve more localised effects.

Optionally, the cross-section of at least one of the one or more voids is non-circular. This can provide greater flexibility in how the filler affects light in the fibre and how the holes and filler respond to external conditions such as pressure compared to existing fibres drawn from preforms having substantially circular drilled holes.

Optionally, the material property is an optical property, optionally refractive index; the one or more voids are configured such that the optical property of the filler affects light transmitted through the optical fibre; and a variation of the cross-sectional area of the one or more voids and/or an orientation of the central axis of the one or more voids are such that light transmitted through the optical fibre experiences a substantially continuous variation in an effect of the optical property on the transmission of the light along the length of the optical fibre. Continuous changes in optical properties reduce reflections that are caused by discontinuous interfaces within the fibre, thereby improving transmission performance.

Optionally, the optical fibre comprises a core and a cladding surrounding the core, and the one or more voids comprise at least one cladding void within the cladding. Optionally, the cladding void is configured such that transmission of light within the optical fibre is affected by the filler within the cladding void, optionally wherein the at least one cladding void is adjacent to the core. Cladding voids can allow for weaker interaction of the filler with light in the fibre, which is primarily guided by the core, allowing for more controlled effects on light in the fibre.

Optionally, the cladding void extends such that the filler is in contact with the core. Optionally, the cladding void extends such that the filler is not in contact with the core. Contact of the filler with the core may or may not be desirable in different applications depending on the strength of interaction desired between the filler and light guided by the fibre core.

Optionally, the one or more voids comprise a plurality of cladding voids within the cladding, optionally at least two cladding voids, optionally at least four cladding voids, optionally at least six cladding voids. More cladding voids can allow for greater control over the effect of the filler on light propagating in the fibre.

Optionally, the plurality of cladding voids is arranged symmetrically around the core. Optionally, a distance between the cladding voids and the core varies along the length of the cladding void. This can also allow for greater control over the effect of the filler on light in the fibre, and variation of the effect along the length or around the circumference of the fibre.

Optionally, the optical fibre further comprises one or more access voids extending from the one or more voids towards an exterior surface of the optical fibre. Access voids allow for the voids within the fibre to be filled from an exterior surface of the fibre during manufacture.

Optionally, the one or more access voids are sealed at the exterior surface of the optical fibre. Optionally, the access voids are sealed by: a blocking member within the access void, optionally wherein the blocking member is entirely within the access void; or melting the material of the optical fibre at the exterior surface of the optical fibre. Sealing the access voids ensures that the voids remain filled following manufacture and that the effect of the voids on light in the fibre is consistent during use of the fibre.

Optionally, the one or more voids are formed by etching a material of the optical fibre via the one or more access voids prior to sealing of the one or more access voids. Optionally, the optical fibre is formed by drawing a preform, and the one or more voids are formed after the optical fibre is drawn. Forming the voids by etching, particularly after drawing of the fibre, provides greater control of the shape, position, and extent of the voids compared to existing techniques where channels are defined prior to drawing.

Optionally, the one or more voids extend entirely within the void portion and/or a boundary of the one or more voids is defined entirely within the void portion. Optionally, the void portion does not comprise any interface between longitudinally adjacent solid materials. As mentioned above, forming the voids within the continuous and integral void portion improves mechanical strength of the fibre. Optionally, the one or more voids are entirely filled with the filler. This provides uniform properties through the void.

Optionally, the filler comprises a gas, optionally nitrogen or air. Optionally, the filler comprises a non-gaseous material, optionally a liquid. Optionally, the filler comprises glycerol or a glycerol-water mixture. Different fillers may be appropriate for different applications, for example depending on the refractive index or other properties required.

Optionally, the filler comprises a liquid crystal. This may be useful in applications where changing the properties of the filler is advantageous.

Optionally, a refractive index of the filler is within 0.1 of a refractive index of the material of the optical fibre, optionally within 0.05, optionally within 0.01, optionally within 0.005, optionally within 0.001. Optionally, the optical fibre comprises a core and a cladding surrounding the core, and the refractive index of the filler is approximately equal to the refractive index of a material of the cladding at a reference temperature. Matching the refractive index of the filler closely to the material of the optical fibre reduces disruption to the waveguiding of the optical fibre and its mode properties.

Optionally, the optical fibre has an outer diameter of between 25 pm and 300 pm, optionally between 100 pm and 300 pm, optionally approximately 125 pm or 250 pm. These diameters are useful for typical optical fibre applications.

Optionally, the optical fibre is a single mode optical fibre. Optionally, the optical fibre is an antiresonant or negative curvature fibre. These types of fibres can be preferred for particular applications.

Optionally, the optical fibre comprises silica. This is a common material for optical fibres that has low manufacturing costs.

Optionally, the optical fibre comprises crystal material, optionally single-crystal material. Optionally, the optical fibre is a crystal-derived fibre, optionally a sapphire derived fibre. Including at least a portion of crystal material in the optical fibre allows for some of the advantageous properties of crystal materials to be used in the optical fibre.

Optionally, the optical fibre is a crystal optical fibre, optionally a single-crystal optical fibre. Optionally, the optical fibre comprises sapphire, diamond, or yttrium aluminium garnet (YAG) crystal. Crystalline optical fibres can be more resilient to extremes of temperature and pressure than other common optical fibre materials.

Optionally, the crystal is doped, optionally with a rare-earth element. This can provide further control over the optical properties of the optical fibre.

Optionally, the optical fibre comprises a Bragg grating. Bragg gratings cause reflections of light at a defined wavelength that can be affected by the conditions of the fibre. This allows the fibre to be used for a wide array of environmental sensing applications.

Optionally, the optical fibre comprises a core and a cladding surrounding the core, and the Bragg grating is at least partially located in the core. Optionally, the Bragg grating is provided by periodic modification of a material of the core. This allows the Bragg grating to interact strongly with light in the fibre, which is typically most localised mainly around the core.

Optionally, the Bragg grating is provided by periodic modification of a material of the cladding, optionally wherein the periodic modification is adjacent to the core. This may be preferred if it is difficult or undesirable to modify the core of the fibre.

Optionally, the material property is an optical property and the one or more voids are configured such that the optical property of the filler affects the Bragg wavelength of the Bragg grating. Optionally, the one or more voids at least partially surround the Bragg grating. This can allow the filler to modify the optical behaviour of the Bragg grating.

Optionally, the longitudinal extent of the one or more voids is at least as long as a longitudinal extent of the Bragg grating. Optionally, the one or more voids extend at most 1 mm, optionally at most 0.5mm past either end of the Bragg grating. This ensures a uniform effect of the filler along the entire length of the grating.

Optionally, the one or more voids are configured such that a change with temperature of the material property of the filler at least partially compensates for an effect on the Bragg wavelength of the Bragg grating of a change with temperature of the material property of the material of the optical fibre, optionally wherein the material property is an optical property. This can stabilise the behaviour of the Bragg grating with respect to temperature. Optionally, the one or more voids are configured such that a magnitude of the change with temperature of the Bragg wavelength of the Bragg grating is at most 5pm/°C, optionally at most 2pm/°C, optionally at most 1pm/°C over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C. This can allow more accurate sensing of quantities such as pressure or strain without the confounding effect of temperature shifts, or more stable filtering using the grating.

Optionally, the one or more voids are configured such that a change with temperature of the material property of the filler increases a magnitude of a change with temperature of the Bragg wavelength of the Bragg grating relative to an optical fibre without the one or more voids, optionally wherein the material property is an optical property. Optionally, the one or more voids are configured such that a magnitude of a change with temperature of the Bragg wavelength of the Bragg grating is at least 20pm/°C, optionally at least 30pm/°C, optionally at least 40pm/°C, optionally at least 50pm/°C over a temperature range of at least 20°C, optionally at least 40°C. This can make the effect of temperature on the grating more pronounced, allowing for more sensitive detection of temperature shifts using the grating.

Optionally, the Bragg wavelength of the Bragg grating reduces with increasing temperature.

Optionally, the optical fibre comprises two separate Bragg gratings. This allows for greater design flexibility in the properties of the optical fibre.

Optionally, the material property is an optical property and the one or more voids are configured such that the optical property of the filler affects the Bragg wavelengths of the two Bragg gratings differently. Optionally, the one or more voids are configured such that the optical property of the filler affects the Bragg wavelength of one of the two Bragg gratings and does not affect the Bragg wavelength of the other of the two Bragg gratings. This can allow for differential sensing or other applications by seeing how changes in conditions affect the two Bragg gratings differently, for example if one grating is stabilised against the effects of temperature and the other is not. Another example is if the one or more voids are configured such that a change with temperature of the material property of the filler increases the magnitude of a change with temperature of the Bragg wavelength for only one of the two separate Bragg gratings, and optionally the change with temperature of the material property of the filler at least partially compensates for an effect on the Bragg wavelength of the other of the two separate Bragg gratings of a change with temperature of the material property of the material of the optical fibre. This latter example could be achieved using different fillers in different voids.

Optionally, the optical fibre comprises a core and a cladding surrounding the core, and the two Bragg gratings are spaced apart longitudinally along the core. This can be a straightforward way to manufacture multiple Bragg gratings in the optical fibre.

Optionally, the optical fibre comprises a core and a cladding surrounding the core; one of the two Bragg gratings is located in the cladding; and the other of the two Bragg gratings is located in the core. This reduces a longitudinal extent of the two Bragg gratings as a whole, making the assembly more compact. This also ensures that the Bragg gratings are in the same longitudinal position within the fibre, ensuring both are experiencing environmental conditions at the same point along the fibre. This provides improved consistency in sensing applications.

Optionally, the one of the two Bragg gratings is located in a waveguide optically coupled to the core such that a proportion of light guided by the core is transferred into the waveguide. This allows both Bragg ratings to interact with light in the fibre at a similar level of intensity.

Optionally, the material property is an optical property and the one or more voids comprise at least one void configured such that transmission of light within the waveguide is affected by the optical property of the filler within the void, optionally wherein the at least one void is adjacent to the waveguide. This allows both gratings to be affected by the filler.

Optionally, the one or more voids comprise a cladding void configured to provide the waveguide within the cladding; the cladding void is optically coupled to the core such that a proportion of light guided by the core is transferred into the cladding void; and the one of the two Bragg gratings is provided by periodic modification of the material of the optical fibre adjacent to the cladding void. This can allow light to interact more directly with the filler in the waveguide.

Optionally, the one or more voids comprise an isolation void configured to at least partially isolate one of the Bragg gratings from strain within the optical fibre. Optionally, wherein the isolation void surrounds an end of the one of the Bragg gratings. Optionally, the isolation void surrounds at least 50%, optionally at least 75%, optionally at least 90%, optionally 100% of the length of the one of the Bragg gratings. This allows differential strain sensing between the two Bragg gratings by decoupling one grating from the effect of strain. Differential sensing can then be performed to account for the effects of other factors such as temperature.

Optionally the optical fibre comprises a core and a cladding surrounding the core; the one of the two Bragg gratings is located in the cladding; the other of the two Bragg gratings is located in the core; and the one of the two Bragg gratings is located in a waveguide optically coupled to the core such that a proportion of light guided by the core is transferred into the waveguide. This allows both gratings to be located in a more similar longitudinal position, making the assembly more longitudinally compact. This also ensures that the Bragg gratings are in the same longitudinal position within the fibre, ensuring both are experiencing environmental conditions at the same point along the fibre. This provides improved consistency in sensing applications.

Optionally, the optical fibre is configured to exhibit birefringence. Birefringence will mean that light with different polarisations is affected differently as it propagates in the fibre. This allows for differential sensing because external conditions may affect the two propagation modes differently, such that the condition can be determined from the difference between behaviour of the polarisations.

Optionally, the optical fibre comprises a core and a cladding surrounding the core, and the optical fibre comprises one or more stress-inducing regions arranged around the core to contribute to the birefringence. Optionally, the stress-inducing regions comprise laser-exposed regions. This allows for tuning the birefringent properties.

Optionally, the optical fibre comprises a core and a cladding surrounding the core, and the one or more voids comprise a plurality of voids arranged around the core with a symmetry such as to contribute to the birefringence. This allows for further ways of tuning the birefringent properties.

Optionally, the plurality of voids comprises two voids arranged along a first diameter of the optical fibre on opposite sides of the core. Optionally, no voids are provided along a second diameter of the optical fibre perpendicular to the first diameter. This is a convenient way to arrange the voids to create birefringent effects.

Optionally, the optical fibre comprises a Bragg grating, and the birefringence is such that the Bragg wavelength of the Bragg grating differs for light having different polarisations, optionally wherein the polarisations are orthogonal. Using a Bragg grating creates an easily-identified peak that can be used for sensing application.

Optionally, the birefringence is such that reflection peaks for the light having different polarisations around the corresponding Bragg wavelengths of the Bragg grating are resolvable when a pressure within the one or more voids is substantially equal to a pressure external to the optical fibre, optionally wherein the reflection peaks are separated by at least a full-width at half maximum of the reflection peaks. This ensures that the peaks are resolved at no external pressure difference, making it easier to calibrate and measure pressure using the sensor.

Optionally, the material property is an optical property and the one or more voids are configured such that an effect of the optical property of the filler on light guided by the optical fibre differs for light having different polarisations, optionally wherein the polarisations are orthogonal. This can allow for differential tuning of the properties of the two birefringent modes by the filler. Optionally, the optical fibre comprises a waveguide separate to a core of the optical fibre. This allows for placing additional features in the fibre at the same longitudinal position as a feature in the core.

Optionally, the waveguide is formed by one or more modified regions of the optical fibre in which an optical property of the optical fibre differs from the optical property of a material of the optical fibre surrounding the one or more modified regions, optionally wherein the optical property is refractive index. This creates a feature in the fibre that can guide light.

Optionally, the one or more modified regions comprises one or more laser-exposed regions. Optionally the one or more modified regions comprise one or more voids extending longitudinally along the length of the optical fibre. These are both convenient methods for creating the regions that can be integrated into the manufacturing process for the voids.

Optionally, the optical fibre is configured to exhibit birefringence and the one or more modified regions are configured to contribute to the birefringence. This allows for birefringent effects as discussed above.

Also provided is a strain sensor comprising the optical fibre of the first aspect of the invention, wherein the optical fibre comprises a Bragg grating. By measuring the wavelength of the Bragg grating, it is possible to determine a strain applied to the optical fibre. The filler in the voids allows for tuning of the properties of the fibre to enhance or enable strain sensing for particular environments and applications.

Optionally, the strain sensor further comprises a controller configured to determine a strain applied to the optical fibre based on the Bragg wavelength of the Bragg grating. This allows for readout of the sensor.

Optionally, the optical fibre is configured to exhibit birefringence; and the controller is configured to determine the strain based on a difference between the Bragg wavelength of the Bragg grating for light having different polarisations. This type of differential sensing can be more robust than sensing based on an absolute value of the Bragg wavelength.

Also provided is a system for sensing strain and/or temperature comprising the optical fibre of the first aspect of the invention, wherein the optical fibre comprises two separate Bragg gratings, and wherein a change with temperature of the Bragg wavelength of the two Bragg gratings is different. This can allow the effects of strain and temperature to be separated because of the different effects on the two gratings, so that temperature and strain can both be accurately determined.

Optionally, the system further comprises a controller configured to determine a strain applied to the optical fibre and a temperature of the optical fibre based on the Bragg wavelengths of the two Bragg gratings. This allows for readout of the strain and temperature.

Also provided is a pressure sensor comprising an optical fibre according to the first aspect of the invention, wherein the optical fibre comprises a Bragg grating. External pressure shifts the Bragg wavelength. Optical fibres can be particularly useful as pressure sensors in constrained or extreme environments.

Optionally, the pressure sensor is configured such that a pressure difference between a pressure of the filler and an external pressure applied to the optical fibre affects the Bragg wavelength of the Bragg grating. This allows for control of the pressure sensing by the properties of the filler, for example to increase sensitivity.

Optionally, the pressure sensor further comprises a controller configured to determine the external pressure applied to the optical fibre based on the Bragg wavelength of the Bragg grating. This allows for readout of the pressure.

Optionally, the optical fibre is configured to exhibit birefringence; the pressure difference affects the birefringence; and the controller is configured to determine the external pressure applied to the optical fibre based on a difference between the Bragg wavelength of the Bragg grating for different polarisations of light. Differential sensing can allow for more accurate and sensitive sensing than determining an absolute value of wavelength. Birefringence allows this using different polarisations of light.

Optionally, the optical fibre comprises a coating; a change with temperature of a material property of the coating affects light transmitted through the optical fibre; and the one or more voids are configured such that a change with temperature of the material property of the filler at least partially compensates for a combined effect on light transmitted through the optical fibre of a change with temperature of the material property of the material of the optical fibre and the change with temperature of the material property of the coating. Coatings are commonly used to protect optical fibres, but can affect how they respond to external conditions such as temperature, for example because the coating has a different coefficient of thermal expansion to the material of the optical fibre. Compensating for this allows coatings to be used while stabilising the fibre properties.

According to a second aspect of the invention, there is provided an optical fibre comprising: one or more voids at least partially filled with a filler, the filler having a material property different to the material property of a material of the optical fibre; and a coating, wherein: a change with temperature of a material property of the coating affects light transmitted through the optical fibre; and the one or more voids are configured such that a change with temperature of the material property of the filler at least partially compensates for a combined effect on light transmitted through the optical fibre of a change with temperature of the material property of the material of the optical fibre and the change with temperature of the material property of the coating. Coatings are commonly used to protect optical fibres, but can affect their optical properties. Compensating for this allows coatings to be used while stabilising the fibre properties.

Optionally, the one or more voids extend longitudinally along the length of the optical fibre within a void portion, the void portion being a continuous and integral portion of the optical fibre. Optionally, a longitudinal extent of each of the one or more voids is less than a longitudinal extent of the void portion. Forming the voids within continuous and integral material avoids weaknesses that can be created by joining and splicing fibres.

Optionally, the one or more voids are sealed from an exterior of the optical fibre. This allows the effect of the filler to be determined and controlled consistently.

Optionally, the coating comprises polyacrylate or polyimide. Optionally, the coating is metallic. These are common coating types that are used for many optical fibre applications.

Optionally, the material property is an optical property, optionally refractive index. This allows the propagation of light in the fibre to be tuned by the filler.

Optionally, the optical fibre comprises a Bragg grating. This creates a well-defined detectable peak in the fibre spectrum that can be used for various sensing applications.

Optionally, a change with temperature of a material property of the coating affects the Bragg wavelength of the Bragg grating; and the one or more voids are configured such that the change with temperature of the material property of the filler at least partially compensates for the combined effect on the Bragg wavelength of the Bragg grating of the change with temperature of the material property of the material of the optical fibre and the change with temperature of the material property of the coating. Optionally, the one or more voids are configured such that a magnitude of the change with temperature of the Bragg wavelength of the Bragg grating is at most 5pm/°C, optionally at most 2pm/°C, optionally at most 1pm/°C over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C. Compensating for the effect on the Bragg wavelength stabilises the peak for sensing applications.

Optionally, the optical fibre comprises a core and a cladding surrounding the core, and the one or more voids comprise a core void within the core. A core void allows for very strong interaction between the filler material and light in the fibre, which is usually most strongly localised in the core.

According to a third aspect of the invention, there is provided an optical fibre comprising: one or more voids; and a Bragg grating, wherein: the one or more voids extend longitudinally along the length of the optical fibre; the one or more voids are at least partially filled with a filler having an optical property different to that of a material of the optical fibre; the optical fibre comprises a core and a cladding surrounding the core; and the one or more voids comprise a core void within the core. A core void allows for very strong interaction between the filler material and light in the fibre, which is usually most strongly localised in the core.

Optionally, the core consists substantially of the core void. This means propagation of the light is primarily determined by the filler material.

Optionally, the Bragg grating is provided by periodic modification of the cladding. This allows for Bragg gratings with some types of filler such as liquid or gas fillers that may not be able to exhibit periodic modification.

Optionally, the core is aligned with a central axis of the optical fibre. This creates a symmetrical cladding around the core.

Optionally, the Bragg grating is provided by periodic modification of the filler in the core void, optionally wherein the filler comprises a liquid crystal. Optionally, the filler is polymerizable and the periodic modification comprises periodic polymerisation, optionally wherein the filler comprises monomers and a photo-initiator and the periodic polymerisation is performed using a laser. This allows for a Bragg grating in the core, where interaction with light is strongest, even with liquid fillers.

Optionally, the optical fibre further comprises one or more electrodes configured to apply an electric field to the filler to affect the material property of the filler. This can allow for tuning of the properties of the filler even after manufacture.

According to a fourth aspect of the invention, there is provided an optical fibre comprising one or more voids, wherein: the one or more voids extend longitudinally along the length of the optical fibre; the one or more voids are at least partially filled with a filler having a material property different to that of a material of the optical fibre; and the optical fibre further comprises one or more electrodes configured to apply an electric field to the filler to affect the material property of the filler. Electrodes allows for tuning of the properties of the filler even after manufacture.

Optionally, the filler comprises a liquid crystal. This is a well-understood class of material that can have its properties tuned by electric fields.

Optionally, the electric field is configured to affect one or more of the refractive index, the absorption, and the scattering loss of the filler. Optionally, the filler exhibits birefringence and the electric field is configured to affect one or more of the magnitude of the birefringence, and the angle of an optic axis of the birefringence. These properties will allow flexibility in affecting the propagation of light in the fibre.

Optionally, the optical fibre comprises a Bragg grating, and the electric field is configured to affect a Bragg wavelength of the Bragg grating. The Bragg grating creates an easily-detected peak that can be advantageous for sensing applications.

According to a fifth aspect of the invention, there is provided a crystal optical fibre comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre. Crystal fibres can be more resilient than other types of optical fibre such as silica, allowing them to be used in extreme conditions. Providing voids allows for tuning the properties of the optical fibre by varying the arrangement of the voids.

Optionally, the one or more voids are at least partially filled with a filler having a material property different to that of a material of the optical fibre, optionally wherein the material property is an optical property, optionally refractive index. Providing voids containing filler allows for tuning the properties of the optical fibre by varying the properties of the filler.

Optionally, a longitudinal extent of the one or more voids is less than a length of the optical fibre. Closed voids within the fibre allow for more straightforward joining to other components that interface with optical fibres. Closed voids are also advantageous for pressure sensing and to prevent fluids from entering the voids and affecting the properties of the filler.

Optionally, the one or more voids extend within a void portion, the void portion being a continuous and integral portion of the optical fibre, optionally wherein a longitudinal extent of the one or more voids is less than a longitudinal extent of the void portion. Providing the voids within an integral portion improves structural performance relative to spliced or joined fibres.

Optionally, a central axis of the one or more voids extends along a direction inclined to the longitudinal axis of the optical fibre for at least a portion of the one or more voids. This can be used to create a more gradual transition of properties along the fibre, reducing losses and reflections around the voids.

Optionally, the optical fibre is an antiresonant or negative curvature fibre. These types of fibres can be preferred for particular applications.

Optionally, the optical fibre comprises a single-crystal optical fibre, optionally sapphire, diamond, or yttrium aluminium garnet (YAG) crystal. These types of crystal are known for use as optical fibres, so have well-understood properties.

Optionally, the optical fibre is doped, optionally with a rare-earth element. This can provide further control over the optical properties of the optical fibre.

According to a sixth aspect of the invention, there is provided a method for forming one or more voids in an optical fibre comprising: selectively exposing the optical fibre to laser radiation to define one or more exposed regions within the optical fibre; contacting the optical fibre with an etchant, wherein the etchant etches the exposed regions at a higher rate than regions of the optical fibre not exposed to laser radiation, such that the one or more voids are formed by etching of the exposed regions; at least partially filling the one or more voids with a filler having a material property different to that of a material of the optical fibre; and sealing the one or more voids from an exterior of the optical fibre.

Existing methods for creating optical fibres with holes along their length typically rely on processing the fibre preform, and then drawing the preform. This creates significant limitations on the structure of holes that can be created in the final fibre. The present method allows for forming voids of near-arbitrary shape and size within the optical fibre with much greater precision than existing methods.

Optionally, wherein the one or more exposed regions comprise a high exposure region and a low exposure region, wherein the high exposure region is exposed to a higher dose of laser radiation than the low exposure region. Optionally, the higher dose of laser radiation is achieved by varying one of more of laser power, a speed of scanning of the laser, and laser beam profile. This can allow for finer control of the manufacturing process.

Optionally, the etchant etches the high exposure region at a higher rate than the low exposure region. This causes differential etching rates during the etching step to more precisely control the process of forming of the voids.

Optionally, selectively exposing the optical fibre to laser radiation further comprises defining one or more access regions extending to an external surface of the optical fibre, such that each of the one or more exposed regions is connected to the external surface by at least one of the one or more access regions. This allows the voids to be formed from any external surface, rather than only from the ends of the fibre as with common existing methods.

Optionally, during the step of contacting the optical fibre with an etchant, the access regions are initially etched to form one or more access voids, the access voids allowing the etchant to contact the exposed regions within the optical fibre. This allows access to etch the exposed regions that will form the voids.

Optionally, sealing the one or more voids comprises sealing the one or more access voids at the exterior surface of the optical fibre. This prevents later changes to the content of the voids. Optionally the access voids are sealed by: inserting a blocking member within the access void, optionally wherein the blocking member is entirely within the access void; or melting the material of the optical fibre at the exterior surface of the optical fibre, optionally wherein the melting is performed using a laser or an electric arc. These steps are readily integrated into the manufacturing process.

Optionally, the method further comprises a step of forming one or more stress-inducing regions in the optical fibre. Optionally, forming the one or more stress-inducing regions comprises selectively exposing the optical fibre to laser radiation. Stress-inducing regions can be used to affect the optical properties of the fibre, providing greater flexibility in the design of the fibre.

Optionally, the one or more stress-inducing regions are configured to contribute to birefringence of the optical fibre. Birefringence is advantageous for various sensing applications as discussed above.

Optionally, the step of forming the one or more stress-inducing regions is carried out prior to the step of contacting the optical fibre with an etchant, the stress-inducing regions being separated from the exposed regions. Optionally, the step of forming the one or more stress-inducing regions is carried out after the step of contacting the optical fibre with an etchant. If stress-inducing regions are formed before etching, they should be defined to prevent etchant affecting the stress-inducing regions. Otherwise, the stress-inducing regions can be formed after etching the voids.

Optionally, the method further comprises flushing the etchant from the one or more voids prior to filling the one or more voids. This prevents contamination of the filler with etchant or changes to the void shape over time.

Optionally, the etchant comprises potassium hydroxide, optionally having a concentration of at least 5 mol, optionally at least 8 mol. This etchant is effective at etching typical optical fibre materials.

Optionally, the laser radiation comprises infrared or visible light. Optionally, the laser radiation has a wavelength between 700 nm and 900 nm, optionally between 750 nm and 850 nm, optionally approximately 790 nm. Optionally, the laser radiation has a wavelength between 450 nm and 650 nm, optionally between 500 nm and 600 nm, optionally approximately 530 nm. These wavelengths are effective at creating the modifications to typical fibre materials required for defining the areas to be etched.

Optionally, the laser radiation is provided by a laser beam generated using a laser system. Optionally, the laser beam is a pulsed laser beam. Optionally, pulses of the pulsed laser beam have a duration of at most 1 ps, optionally at most 500 fs, optionally at most 200 fs, optionally at most 100 fs. This can provide finer control over the laser properties.

Optionally, wherein the one or more exposed regions comprise a high exposure region and a low exposure region, and the high exposure region is exposed to a higher dose of laser radiation than the low exposure region, the higher dose of laser radiation is achieved by varying one of both of laser pulse energy and pulse repetition rate. Controlling these properties allows for fine control of the laser dose applied to the optical fibre material.

Optionally, selectively exposing the optical fibre to laser radiation comprises applying a correction to an active optical element of the laser system to modify wavefront properties of the laser beam to counteract an effect of aberration on laser focus. This allows for fine control of the regions within the fibre that are exposed to the laser radiation by allowing the laser to be precisely directed within the fibre material.

Optionally, the method further comprising forming a Bragg grating in the optical fibre. As discussed above, Bragg gratings have a variety of applications in sensing using optical fibres.

Also provided is an optical fibre produced using the method of the sixth aspect of the invention.

According to a seventh aspect of the invention, there is provided an optical fibre comprising a first portion and a second portion, wherein: the second portion is a void portion comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre within the void portion; the first portion and the second portion are substantially mode-matched; and the one or more voids are at least partially filled with a filler having a material property different to the material property of a material of the optical fibre.

Advantageously, providing voids containing filler allows for tuning the properties of the optical fibre by varying the properties of the filler and the arrangement of the voids and substantially mode matching portions of the optical fibre enables losses to be reduced.

Optionally, the mode field diameter of the first portion for a given wavelength and temperature is substantially the same as the mode field diameter of the second portion for the given wavelength and temperature. This minimises loss in the light passing between the first portion and the second portion.

Optionally, wherein the first portion and the second portion each comprise: a core; and a cladding, wherein the respective cladding surrounds the respective core.

Optionally, wherein the refractive index of the core of the first portion is substantially the same as the refractive index of the core of the second portion, optionally wherein the refractive index of the core of the first portion is between 0.002 and 0.007 greater than the refractive index of the cladding surrounding the core of the first portion, optionally wherein the refractive index of the core of the first portion is approximately 0.005 greater than the refractive index of the cladding surrounding the core of the first portion. Matching the refractive indices of the core of the first portion and the second portion results in low loss transition between the first portion and the second portion.

Optionally, the maximum lateral dimension of the core of the first portion in a direction substantially perpendicular to the longitudinal axis of the optical fibre is substantially the same as the maximum lateral dimension of the core of the second portion in the direction substantially perpendicular to the longitudinal axis of the optical fibre at an interface between the first portion and the second portion. Matching the dimension of the core of the first portion and the second portion results in low loss transition between the first portion and the second portion.

Optionally, the dopant concentration of the core of the first portion is substantially the same as the dopant concentration of the core of the second portion. Beneficially, the use of substantially the same dopant concentrations means that the core size in the void portion can be better matched to the first portion.

Optionally, the first portion is a sealing portion and the one or more voids are at least partially sealed by the sealing portion. Optionally, the one or more voids are at least partially sealed from an exterior of the optical fibre at an interface between an end face of the sealing portion and an end face of the void portion. Optionally, the one or more voids are sealed from an exterior of the optical fibre at a further interface between the other end face of the void portion and an end face of a further sealing portion. Optionally, the sealing portion comprises a standard single mode fibre having a core surrounded by a cladding, wherein the cladding comprises a substantially solid material that at least partially seals the one or more voids of the void portion. Optionally, the further sealing portion comprises a standard single-mode fibre having a core surrounded by a cladding, wherein the cladding comprises a substantially solid material that at least partially seals the one or more voids of the void portion.

Beneficially, the sealing portion ensures that the voids remain filled following manufacture and that the effect of the voids on light in the fibre is consistent during use of the fibre.

Optionally, the optical fibre is a single mode optical fibre. Providing preferred communication for particular applications.

Optionally, the mode field diameter is between 9 and 12 micrometres, optionally wherein the mode field diameter is between 9.6 and 11.2 micrometres, optionally wherein the mode field diameter is approximately 10.5 micrometres, optionally wherein the given wavelength is within the C-band, L-band and/or S-band. Advantageously, such mode-matching enables single mode communication with reduced loss.

Optionally, the core of the first portion and the core of the second portion each have a maximum lateral dimension greater than 5.5 micrometers and less than 11 micrometres in a direction substantially perpendicular to the longitudinal axis of the optical fibre at the interface between the first portion and the second portion, optionally wherein the core of the first portion and the core of the second portion each have a maximum lateral dimension between 6 and 10.5 micrometres in a direction substantially perpendicular to the longitudinal axis of the optical fibre at the interface between the first portion and the second portion, optionally wherein the core of the first portion and the core of the second portion each have a maximum lateral dimension of between 7 and 10 micrometres in a direction substantially perpendicular to the longitudinal axis of the optical fibre at the interface between the first portion and the second portion, optionally wherein the core of the first portion and the core of the second portion each have a maximum lateral dimension of between 8 and 9 micrometres in a direction substantially perpendicular to the longitudinal axis of the optical fibre at the interface between the first portion and the second portion, optionally wherein the core of the first portion and the core of the second portion each have a maximum lateral dimension of approximately 8.2 micrometres in a direction substantially perpendicular to the longitudinal axis of the optical fibre at the interface between the first portion and the second portion. Advantageously, such optical fibres provide reduced losses in standard single mode optical fibre systems.

Optionally, the first portion comprises a bridging portion configured to progressively convert a mode field diameter along the longitudinal axis of the bridging portion from a first mode field diameter to a second mode field diameter, wherein the second mode field diameter is substantially the same as the mode field diameter of the void portion for a given wavelength and temperature. Optionally, the first portion comprises a bridging portion comprising a core having a cross-sectional area that varies along the longitudinal axis of the bridging portion from a first cross-sectional area to a second cross-sectional area, wherein the second cross-sectional area has a maximum lateral dimension that is substantially the same as the maximum lateral dimension of a core of the void portion. Beneficially, the bridging portion has the effect of sealing a filled fibre and also has the effect of acting as an adiabatic mode converter to convert from the one mode size of the filled fibre to another mode size of the further fibre. Hence it is possible to use a filled fibre which is very different in mode-field diameter to the further optical fibre.

Optionally, the first portion comprises a lens portion having a cross-sectional area that varies along the longitudinal axis of the lens portion, thereby progressively to convert the mode field diameter along the longitudinal axis of the lens portion from a first mode field diameter to a second mode field diameter, wherein the second mode field diameter is substantially the same as the mode field diameter of the void portion for a given wavelength and temperature. Beneficially, such conversion helps to minimise coupling loss in multi-component optical systems.

Optionally, the optical fibre comprises a core and a cladding surrounding the core, and the Bragg grating is at least partially located in the core. Optionally, the Bragg grating is provided by periodic modification of a material of the core. This allows the Bragg grating to interact strongly with light in the fibre, which is typically most localised mainly around the core. Optionally, the Bragg grating is provided by periodic modification of a material of the cladding, optionally wherein the periodic modification is adjacent to the core. This may be preferred if it is difficult or undesirable to modify the core of the fibre.

Optionally, the longitudinal extent of the one or more voids is at least as long as a longitudinal extent of the Bragg grating. Optionally, the one or more voids extend at most 1 mm, optionally at most 0.5mm past either end of the Bragg grating. This ensures a uniform effect of the filler along the entire length of the grating. Optionally, the one or more voids are configured such that a change with temperature of the material property of the filler at least partially compensates for an effect on the Bragg wavelength of the Bragg grating of a change with temperature of the material property of the material of the optical fibre, optionally wherein the material property is an optical property, optionally wherein the one or more voids are configured such that a magnitude of the change with temperature of the Bragg wavelength of the Bragg grating is at most 5pm/°C, optionally at most 2pm/°C, optionally at most 1pm/°C over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C. This can allow more accurate sensing of quantities such as pressure or strain without the confounding effect of temperature shifts, or more stable filtering using the grating.

Optionally, the one or more voids are configured such that a change with temperature of the material property of the filler increases a magnitude of a change with temperature of the Bragg wavelength of the Bragg grating relative to an optical fibre without the one or more voids, optionally wherein the material property is an optical property, optionally wherein the one or more voids are configured such that a magnitude of a change with temperature of the Bragg wavelength of the Bragg grating is at least 20pm/°C, optionally at least 30pm/°C, optionally at least 40pm/°C, optionally at least 50pm/°C over a temperature range of at least 20°C, optionally at least 40°C, further optionally wherein the Bragg wavelength of the Bragg grating reduces with increasing temperature. This can make the effect of temperature on the grating more pronounced, allowing for more sensitive detection of temperature shifts using the grating.

Optionally, in the optical fibre comprises two separate Bragg gratings. This allows for greater design flexibility in the properties of the optical fibre.

Optionally, the material property is an optical property and the one or more voids are configured such that the optical property of the filler affects the Bragg wavelengths of the two Bragg gratings differently, optionally wherein the one or more voids are configured such that the optical property of the filler affects the Bragg wavelength of one of the two Bragg gratings and does not affect the Bragg wavelength of the other of the two Bragg gratings. This can allow for differential sensing or other applications by seeing how changes in conditions affect the two Bragg gratings differently, for example if one grating is stabilised against the effects of temperature and the other is not. Another example is if the one or more voids are configured such that a change with temperature of the material property of the filler increases the magnitude of a change with temperature of the Bragg wavelength for only one of the two separate Bragg gratings, and optionally the change with temperature of the material property of the filler at least partially compensates for an effect on the Bragg wavelength of the other of the two separate Bragg gratings of a change with temperature of the material property of the material of the optical fibre. This latter example could be achieved using different fillers in different voids.

Optionally: the optical fibre comprises a core and a cladding surrounding the core; one of the two Bragg gratings is located in the cladding; and the other of the two Bragg gratings is located in the core. This reduces a longitudinal extent of the two Bragg gratings as a whole, making the assembly more compact. This also ensures that the Bragg gratings are in the same longitudinal position within the fibre, ensuring both are experiencing environmental conditions at the same point along the fibre. This provides improved consistency in sensing applications.

Optionally: the one or more voids comprise a cladding void configured to provide the waveguide within the cladding; the cladding void is optically coupled to the core such that a proportion of light guided by the core is transferred into the cladding void; and the one of the two Bragg gratings is provided by periodic modification of the material of the optical fibre adjacent to the cladding void. This can allow light to interact more directly with the filler in the waveguide.

Optionally, the one or more voids comprise an isolation void configured to at least partially isolate one of the Bragg gratings from strain within the optical fibre. Optionally, the isolation void surrounds an end of the one of the Bragg gratings, optionally wherein the isolation void surrounds at least 50%, optionally at least 75%, optionally at least 90%, optionally 100% of the length of the one of the Bragg gratings. This allows differential strain sensing between the two Bragg gratings by decoupling one grating from the effect of strain. Differential sensing can then be performed to account for the effects of other factors such as temperature.

Optionally: the optical fibre comprises a core and a cladding surrounding the core; the one of the two Bragg gratings is located in the cladding; the other of the two Bragg gratings is located in the core; and the one of the two Bragg gratings is located in a waveguide optically coupled to the core such that a proportion of light guided by the core is transferred into the waveguide. This allows both gratings to be located in a more similar longitudinal position, making the assembly more longitudinally compact. This also ensures that the Bragg gratings are in the same longitudinal position within the fibre, ensuring both are experiencing environmental conditions at the same point along the fibre. This provides improved consistency in sensing applications.

Optionally, the optical fibre comprises a core and a cladding surrounding the core, and the optical fibre comprises one or more stress-inducing regions arranged around the core to contribute to the birefringence, optionally the stress-inducing regions comprise laser-exposed regions. This allows for tuning the birefringent properties.

Optionally the plurality of voids comprises two voids arranged along a first diameter of the optical fibre on opposite sides of the core, further optionally wherein no voids are provided along a second diameter of the optical fibre perpendicular to the first diameter. This is a convenient way to arrange the voids to create birefringent effects.

Optionally, the material property is an optical property and the one or more voids are configured such that an effect of the optical property of the filler on light guided by the optical fibre differs for light having different polarisations, optionally wherein the polarisations are orthogonal. This can allow for differential tuning of the properties of the two birefringent modes by the filler.

Optionally, the optical fibre comprises a waveguide separate to a core of the optical fibre. This allows for placing additional features in the fibre at the same longitudinal position as a feature in the core.

Optionally, the waveguide is formed by one or more modified regions of the optical fibre in which an optical property of the optical fibre differs from the optical property of a material of the optical fibre surrounding the one or more modified regions, optionally wherein the optical property is refractive index, optionally wherein the one or more modified regions comprises one or more laser-exposed regions. This creates a feature in the fibre that can guide light.

Optionally, the one or more modified regions comprise one or more voids extending longitudinally along the length of the optical fibre. These are both convenient methods for creating the regions that can be integrated into the manufacturing process for the voids.

There is also provided a strain sensor comprising: the optical fibre described herein; and a controller configured to determine a strain applied to the optical fibre based on the Bragg wavelength of the Bragg grating. By measuring the wavelength of the Bragg grating, it is possible to determine a strain applied to the optical fibre. The filler in the voids allows for tuning of the properties of the fibre to enhance or enable strain sensing for particular environments and applications.

Optionally: the optical fibre is configured to exhibit birefringence; and the controller is configured to determine the strain based on a difference between the Bragg wavelength of the Bragg grating for light having different polarisations. This allows for readout of the sensor.

There is also provided a system for sensing strain and/or temperature comprising: the optical fibre comprising two separate Bragg gratings, wherein a change with temperature of the Bragg wavelength of the two Bragg gratings is different; and a controller configured to determine a strain applied to the optical fibre and a temperature of the optical fibre based on the Bragg wavelengths of the two Bragg gratings. This can allow the effects of strain and temperature to be separated because of the different effects on the two gratings, so that temperature and strain can both be accurately determined. This allows for readout of the strain and temperature.

There is also provided a pressure sensor comprising: the optical fibre comprising a Bragg grating; and a controller configured to determine the external pressure applied to the optical fibre based on the Bragg wavelength of the Bragg grating, wherein the pressure sensor is configured such that a pressure difference between a pressure of the filler and an external pressure applied to the optical fibre affects the Bragg wavelength of the Bragg grating. External pressure shifts the Bragg wavelength. Optical fibres can be particularly useful as pressure sensors in constrained or extreme environments.

Optionally: the optical fibre is configured to exhibit birefringence; the pressure difference affects the birefringence; and the controller is configured to determine the external pressure applied to the optical fibre based on a difference between the Bragg wavelength of the Bragg grating for different polarisations of light. Differential sensing can allow for more accurate and sensitive sensing than determining an absolute value of wavelength. Birefringence allows this using different polarisations of light Optionally, where there are two Bragg gratings, one of the two Bragg gratings has a Bragg wavelength that decreases with increasing temperature and the other of the two Bragg gratings has a Bragg wavelength that responds differently to increasing temperature. Advantageously, the two Bragg reflection wavelengths move in opposite directions with increasing temperature, with the separation being indicative of the temperature. This means far greater discrimination can be made between strain and temperature.

Optionally, the refractive index of the filler is greater than the refractive index of the material of the optical fibre. Beneficially, a fibre Bragg grating associated with such filler operates with a steeper gradient as part of the Bragg wavelength vs temperature characteristic, thereby affording higher temperature sensitivity.

Optionally, the one or more voids comprise two or more voids filled with different fillers, optionally wherein the material property of the fillers differs between the different fillers. This provides additional flexibility in tuning properties of the fibre.

Optionally, wherein the material property is an optical property, optionally refractive index. Optionally, the one or more voids are configured such that the optical property of the filler affects light transmitted through the optical fibre. This can allow for stronger, direct effects on light in the fibre, rather than relying on indirect effects such as changing expansion properties of the fibre material.

Optionally, a cross-sectional area of at least one of the one or more voids varies along the length of the void portion, optionally wherein the cross-sectional area reduces away from a centre of the void along the length of the optical fibre for at least a portion of the length of the void. Optionally, a central axis of the one or more voids extends along a direction inclined to the longitudinal axis of the optical fibre for at least a portion of the one or more voids. This can allow for variations in the properties of the fibre along its length and across its cross-section to achieve more localised effects.

Optionally: the material property is an optical property, optionally refractive index; the one or more voids are configured such that the optical property of the filler affects light transmitted through the optical fibre; and a variation of the cross-sectional area of the one or more voids and/or an orientation of the central axis of the one or more voids are such that light transmitted through the optical fibre experiences a substantially continuous variation in an effect of the optical property on the transmission of the light along the length of the optical fibre. Continuous changes in optical properties reduce reflections that are caused by discontinuous interfaces within the fibre, thereby improving transmission performance.

Optionally, the one or more voids comprise at least one cladding void within the cladding. Optionally, the cladding void is configured such that transmission of light within the optical fibre is affected by the filler within the cladding void, optionally wherein the at least one cladding void is adjacent to the core. Cladding voids can allow for weaker interaction of the filler with light in the fibre, which is primarily guided by the core, allowing for more controlled effects on light in the fibre.

Optionally, either: a) the cladding void extends such that the filler is in contact with the core; or b) the cladding void extends such that the filler is not in contact with the core. Contact of the filler with the core may or may not be desirable in different applications depending on the strength of interaction desired between the filler and light guided by the fibre core. Optionally, the one or more voids comprise a plurality of cladding voids within the cladding, optionally at least two cladding voids, optionally at least four cladding voids, optionally at least six cladding voids, more cladding voids can allow for greater control over the effect of the filler on light propagating in the fibre.

Optionally wherein the plurality of cladding voids is arranged symmetrically around the core. Optionally, a distance between the cladding voids and the core varies along the length of the cladding void. This can also allow for greater control over the effect of the filler on light in the fibre, and variation of the effect along the length or around the circumference of the fibre.

Optionally, the optical fibre is formed by drawing a preform, and the one or more voids are formed after the optical fibre is drawn.

Optionally, the one or more voids extend entirely within the void portion and/or a boundary of the one or more voids is defined entirely within the void portion.

Optionally, the one or more voids are entirely filled with the filler. This provides uniform properties through the void. Optionally, the filler comprises a gas, optionally nitrogen or air. Optionally, the filler comprises a non-gaseous material, optionally a liquid. Different fillers may be appropriate for different applications, for example depending on the refractive index or other properties required.

Optionally wherein the filler comprises a liquid crystal. This may be useful in applications where changing the properties of the filler is advantageous.

Optionally, a refractive index of the filler is within 0.1 of a refractive index of the material of the optical fibre, optionally within 0.05, optionally within 0.01, optionally within 0.005, optionally within 0.001.

Optionally, the optical fibre comprises a core and a cladding surrounding the core, and the refractive index of the filler is approximately equal to the refractive index of a material of the cladding at a reference temperature. Matching the refractive index of the filler closely to the material of the optical fibre reduces disruption to the waveguiding of the optical fibre and its mode properties

Optionally, the optical fibre is an antiresonant, or negative curvature fibre. These types of fibres can be preferred for particular applications.

Optionally, the optical fibre comprises crystal material, for example single-crystal material, optionally wherein the optical fibre is a crystal-derived fibre, for example a sapphire derived fibre. Including at least a portion of crystal material in the optical fibre allows for some of the advantageous properties of crystal materials to be used in the optical fibre.

Optionally, the optical fibre is a crystal optical fibre, optionally a single-crystal optical fibre. Optionally, the optical fibre comprises sapphire, diamond, or yttrium aluminium garnet (YAG) crystal. Crystalline optical fibres can be more resilient to extremes of temperature and pressure than other common optical fibre materials. Optionally, the crystal is doped, optionally with a rare-earth element. This can provide further control over the optical properties of the optical fibre.

Optionally, the optical fibre comprises: a coating, wherein: a change with temperature of a material property of the coating affects light transmitted through the optical fibre; and the one or more voids are configured such that a change with temperature of the material property of the filler at least partially compensates for a combined effect on light transmitted through the optical fibre of a change with temperature of the material property of the material of the optical fibre and the change with temperature of the material property of the coating. Coatings are commonly used to protect optical fibres, but can affect how they respond to external conditions such as temperature, for example because the coating has a different coefficient of thermal expansion to the material of the optical fibre. Compensating for this allows coatings to be used while stabilising the fibre properties.

Optionally, either: a) the coating comprises polyacrylate or polyimide; or b) the coating is metallic. These are common coating types that are used for many optical fibre applications.

Optionally, a change with temperature of a material property of the coating affects the Bragg wavelength of the Bragg grating; and the one or more voids are configured such that the change with temperature of the material property of the filler at least partially compensates for the combined effect on the Bragg wavelength of the Bragg grating of the change with temperature of the material property of the material of the optical fibre and the change with temperature of the material property of the coating, optionally wherein the one or more voids are configured such that a magnitude of the change with temperature of the Bragg wavelength of the Bragg grating is at most 5pm/°C, optionally at most 2pm/°C, optionally at most 1pm/°C over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C. Compensating for the effect on the Bragg wavelength stabilises the peak for sensing applications.

According to an eighth aspect of the invention, there is provided an optical fibre comprising a first portion and a second portion, wherein: the second portion is a void portion comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre within the void portion; and the one or more voids are at least partially filled with a filler having a material property different to the material property of a material of the optical fibre, wherein the first portion and the second portion each comprise: a core; and a cladding, wherein the respective cladding surrounds the respective core and wherein the maximum lateral dimension of the core of the first portion in a direction substantially perpendicular to the longitudinal axis of the optical fibre is substantially the same as the maximum lateral dimension of the core of the second portion in the direction substantially perpendicular to the longitudinal axis of the optical fibre at an interface between the first portion and the second portion.

Advantageously, matching the core dimension of a portion of optical fibre with a void to another portion of optical fibre reduces optical losses at the transition between the portions.

According to a ninth aspect of the invention, there is provided an optical fibre comprising a void portion comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre within the void portion; and the one or more voids are at least partially filled with a filler having a material property different to the material property of a material of the optical fibre, wherein the void portion comprises: a core; and a cladding, wherein the cladding surrounds the core and wherein the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fibre is greater than 5.5 micrometres and less than 11 micrometres, optionally wherein the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fibre is between 6 and 10.5 micrometres, optionally wherein the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fibre is between 7 and 10 micrometres, optionally wherein the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fibre is between 8 and 9 micrometres, optionally wherein the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fibre is approximately 8.2 micrometres.

Beneficially, an optical fibre with a void having such dimensions can be adjoined with a standard single mode optical fibre to provide the benefits associated with filled voids whilst ensuring single mode transmission with minimal losses.

According to a tenth aspect of the invention, there is provided a device comprising two Bragg gratings, wherein one of the two Bragg gratings has a Bragg wavelength that decreases with increasing temperature and the other of the two Bragg gratings has a Bragg wavelength that responds differently to increasing temperature. Advantageously, such a device provides enhanced discrimination between temperature and strain which can be used in conjunction with the further features described herein, or independently thereof.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols represent corresponding parts, and in which

Fig. 1 A and Fig. 1 B show an optical fibre having voids in a void portion;

Fig. 2 is a flowchart of a method of forming the optical fibre of Fig. 1 A and Fig. 1 B;

Fig. 3A and Fig. 3B show an unmodified optical fibre;

Fig. 4A and Fig. 4B show the optical fibre of Fig. 3A and Fig. 3B after exposure to laser radiation;

Fig. 5A and Fig. 5B show the optical fibre of Fig. 4A and Fig. 4B after contacting the fibre with an etchant;

Fig. 6A and Fig. 6B show an optical fibre which has been exposed to different intensities of laser radiation; Fig. 7A and Fig. 7B show the optical fibre of Fig. 6A and Fig. 6B after contacting the fibre with an etchant;

Fig. 8A and Fig. 8B show the optical fibre of Fig. 5A and Fig. 5B after the voids have been filled with an etchant;

Fig. 9A and Fig. 9B show the optical fibre of Fig. 8A and Fig. 8B after the voids have been sealed;

Fig. 10A and Fig. 10B show an optical fibre where the access voids do not extend along a radial direction; Fig. 11A shows an unmodified optical fibre having a coating;

Fig. 11 B shows the optical fibre of Fig. 11 A after a portion of the coating has been removed and the optical fibre exposed to laser radiation;

Figs. 12A to Fig. 12F show cross-sectional views of optical fibres with different voids containing a filler;

Figs. 13A to Fig. 13D show changes in refractive index of the optical fibre with temperature;

Fig. 14 shows the optical fibre of Fig. 11 B with a Bragg grating in the core;

Fig. 15 shows the optical fibre of Fig. 1A and Fig. 1 B with a Bragg grating in the core;

Fig. 16 shows the optical fibre of Fig. 10A and Fig. 10B with a Bragg grating in the core;

Fig. 17 shows the optical fibre of Fig. 7A and Fig. 7B with a Bragg grating in the core;

Fig. 18 shows the change in Bragg wavelength with temperature for three different optical fibres with Bragg gratings;

Fig. 19A to Fig. 19C show the reflection spectra for the three optical fibres in Fig. 18 at different temperatures;

Fig. 20 shows the change in Bragg wavelength for a coated optical fibre compensated using voids containing filler;

Fig. 21 shows an optical fibre with two Bragg gratings in the core;

Fig. 22 shows sampled Bragg gratings;

Fig. 23A to Fig. 23C show spectra of the sampled Bragg gratings of Fig. 22;

Fig. 24A and Fig. 24B show an optical fibre with a waveguide in the cladding containing a Bragg grating;

Fig. 25A and Fig. 25B show an optical fibre where a waveguide in the cladding is formed with a void filled with filler;

Fig. 25C and Fig. 25D show a multicore optical fibre where a central core is formed with a void filled with filler;

Fig. 26A and Fig. 26B show an optical fibre with a strain-isolated Bragg grating in the cladding;

Fig. 27A to Fig. 27D show birefringent optical fibres;

Fig. 28A and Fig. 28B show an optical fibre pressure sensor designed to have non-zero birefringence at zero external pressure difference;

Fig. 29A and Fig. 29B show an alternative design of optical fibre pressure sensor designed to have non-zero birefringence at zero external pressure difference;

Fig. 30A and Fig. 30B show a side-hole fibre 110 which has been made birefringent by laser exposure;

Fig. 31 A and Fig. 31 B show examples of pressure sensors created in polarisation maintaining optical fibres;

Fig. 32 shows a system for measuring environmental variables such as strain or temperature using an optical fibre;

Fig. 33A to Fig. 33C show example strain and temperature measurements using the system of Fig. 32

Fig. 34A to Fig. 34E show example pressure measurements at different temperatures using the system of Fig. 32;

Fig. 35A and Fig. 35B show example pressure measurements from pressure sensors which have non-zero birefringence at low pressure difference;

Fig. 36A shows a thermally tuned optical filter;

Fig. 36B shows spectra of the filter of Fig. 36A;

Fig. 37 shows an add/drop multiplexer;

Fig. 38A shows a temperature stable laser;

Fig. 38B shows spectra of the laser of Fig. 38A;

Fig. 39A shows a tuneable laser;

Fig. 39B shows spectra of the laser of Fig. 39A;

Fig. 40 shows a tuneable fibre laser;

Fig. 41 to Fig. 41 D show optical fibres having voids in the core; Fig. 42A and Fig. 42B show an optical fibre containing electrodes for electrical tuning;

Fig. 43A and Fig. 43B show an electrically tuneable optical fibre with a core void;

Fig. 44A to Fig. 44D show various different example arrangements of electrodes and voids within the cross-section of the optical fibre;

Fig. 45A shows an electrically tuneable optical filter;

Fig. 45B shows spectra of the filter of Fig. 45A;

Fig. 46A and Fig. 46B show an unmodified crystal optical fibre;

Fig. 47A and Fig. 47B show the crystal optical fibre of Fig. 46A and Fig. 46B after exposure to laser radiation;

Fig. 48A and Fig. 48B show the crystal optical fibre of Fig. 47A and Fig. 47B after contacting the fibre with an etchant;

Fig. 49A and Fig. 49B show a crystal optical fibre that also has a core void;

Fig. 50 shows a pressure sensor formed using a crystal optical fibre;

Fig. 51 shows an alternative pressure sensor design using a crystal optical fibre;

Fig. 52A and Fig. 52B show a substrate after exposure to laser radiation;

Fig. 53A and Fig. 53B shows the substrate of Fig. 52A and Fig. 52B after contacting with an etchant;

Fig. 54A and Fig. 54B show the substrate of Fig. 53A and Fig. 53B after the voids have been filled with a filler;

Fig. 55 shows an example device containing a Bragg grating;

Fig. 56A and Fig. 56B show simulated shifts in Bragg wavelength;

Fig. 57A shows a design of an optical fibre with a Bragg grating and high thermal sensitivity of its Bragg wavelength;

Fig. 57B shows measured reflection spectra of the fibre of Fig. 57A at different temperatures;

Fig. 58A shows a design of an optical fibre with a Bragg grating and compensated thermal sensitivity of its Bragg wavelength;

Fig. 58B shows measured reflection spectra of the fibre of Fig. 58A at different temperatures;

Fig. 59 shows the Bragg wavelength peak of the optical fibres of Fig. 57A and Fig. 58A;

Fig. 60 shows a flowchart of a method of forming the optical fibre of Fig. 61 A, Fig. 61 B, Fig. 62 and Fig. 63;

Fig. 61 A and Fig. 61 B show a mode-matched optical fibre;

Fig. 62 shows an optical fibre with bridging portions;

Fig. 63 shows an optical fibre with a lens portion; and

Fig. 64 shows the change in wavelength as a function of temperature change for two optical fibres with Bragg gratings.

Optical fibres can be used for a variety of applications, including sensing applications. Environmental conditions on the optical fibre, such as strain, pressure, or temperature, affect the material properties of the optical fibre. This in turn has effects on the propagation of light in the fibre that can be detected and used to derive the conditions around or in the fibre.

For example, an optical fibre pressure sensor can be made from side-hole optical fibre. This optical fibre has a core along its length surrounded by a cladding of lower refractive index material. Within the cladding there are two air holes which run longitudinally along the length of the fibre, such that they are either side of the core within the cross-section.

Within the core of the optical fibre there is a Bragg grating which consists of a periodic modulation of the core refractive index along its length. When placed within an optical fibre, the Bragg grating may be referred to as a fibre Bragg grating (FBG). FBGs reflect light at a Bragg wavelength, A>, determined by the pitch of the Bragg grating, A, and the effective refractive index, nett, of the optical waveguide according to m b=2n_eftA, where m is an integer.

When there is a pressure difference between the air within the fibre and the outside of the fibre, the fibre becomes distorted, resulting in a stress-induced birefringence. This means that the fibre has two orthogonal polarisation axes within the cross-section, termed the fast and slow axes. The orthogonal polarisation axes have different effective refractive indices. Since the two polarisations have different effective refractive indices, the Bragg grating will reflect one polarisation axis at one wavelength and the other polarisation axis at a different wavelength.

The difference between the effective refractive index of the two polarisation axes and hence the difference in Bragg wavelength is determined by the pressure difference between the inside and outside of the optical fibre. Hence the pressure external to the fibre can be determined by the wavelength separation between the two Bragg wavelength peaks.

There are a number of limitations with existing fibre Bragg grating pressure sensors. There is a limit to the dimensions and the shape of the holes that can be produced within the optical fibre. The air-hole fibre needs to spliced to conventional fibre in order to couple light into it, which can result in large losses due to the mode-mismatch between the fibres. This coupling loss can be significant if there a number of sensors concatenated, as each preceding sensor in the chain will incur 4 splice losses as the sensors operate in reflection and there are two splices per sensor. There is also no suitable fibre which will operate at extremely high temperatures above 1000°C. A further issue is that it is difficult to measure low pressures where there is very little separation in the wavelength peaks.

The present invention includes a method of manufacture that allows FBG pressure sensors to be created within standard optical fibre, thereby avoiding the need to splice fibres and allowing low coupling losses. The method allows arbitrary shaped voids to be created, allowing greater pressure sensitivities to be achieved. This also allows pressure sensors to be created in singlecrystal fibres in order to be able to withstand ultrahigh temperatures above 1000°C.

An optical fibre 1 containing a Bragg grating 31 can also act a strain sensor, since any strain imparted will result in a change in pitch and hence affect the Bragg wavelength. However, the Bragg grating 31 is also sensitive to temperature, since temperature will change the effective refractive index through the thermo-optic effect as well as the pitch by thermal expansion. The fact that Bragg gratings 31 are sensitive to both strain and temperature is a problem as it is not possible to determine if a change in the Bragg wavelength is due to a change in temperature, a change in strain, or a combination of both.

Existing solutions to this problem include placing the FBGs in a housing which strains the fibre and relieves the strain under thermal expansion. This improves temperature performance but strains the fibre such that the device cannot be used as a strain sensor. Alternatively, “strain-relief jackets” can be used to isolate FBGs so that they are not affected by strain. However, these add cost and the extra bulk limits scenarios they can be deployed in. Other techniques to discriminate between temperature and strain include dual wavelength or dual polarisation sensing and tapered fibres. However, none of these techniques give a significant change in response to allow good discrimination between the effects of temperature and strain with good signal to noise ratio.

In addition to sensing, FBGs are also used for other functions, such as optical filters and to stabilise lasers. These applications often require that the Bragg wavelength remains constant and it can be a problem if the wavelength changes with temperature. Sometimes it is required to be able to tune the wavelength by incorporating a heating element and in these circumstances a greater temperature sensitivity would be desirable.

The ability of the present invention to create arbitrary void shapes also opens up a variety of other possibilities that enable improved performance in applications such as strain and temperature sensing. Applications and advantages of embodiments of the present invention include strain measurement with low temperature cross-sensitivity, independent strain and temperature measurements, low-cost temperature insensitive lasers and optical filters, low-cost tuneable add-drop multiplexers, and tuneable fibre lasers. Other applications include fabricating ‘holey’ single-crystal single mode fibres, and refractive index sensors. The invention allows both temperature insensitivity and high temperature sensitivity without compromising signal to noise ratio and without adding cost and bulk. The invention concerns an optical fibre 1, such as shown in Fig. 1 A and Fig. 1 B. The optical fibre 1 may comprise silica. This optical fibre 1 may have an outer diameter of between 25 |um and 300 pm, optionally between 100 pm and 300 pm, optionally approximately 125 pm or 250 pm. The optical fibre 1 is typically made of silica and 125 pm in diameter. Other common diameters include 50 pm, 80 pm, 250 pm, and 425 pm. Larger diameters may be preferred for pressure sensing applications because larger diameter fibres are more sensitive to changes in pressure. The optical fibre 1 comprises a core 3 and a cladding 5 surrounding the core 3.

The optical fibre 1 may be a single mode optical fibre. However, there are a multitude of other fibre types possible. For example, the optical fibre 1 may be an anti-resonant or negative curvature fibre.

The optical fibre 1 may comprise crystal material, optionally single-crystal material. In such cases the optical fibre 1 may comprise some crystal (or single-crystal) material portions and may also comprise portions of other materials such as silica. For example, the optical fibre 1 may be a crystal-derived fibre, optionally a sapphire derived fibre. Examples of crystal-derived fibres are described in Dragic, P., Hawkins, T., Foy, P. et al. “Sapphire-derived all-glass optical fibres”, Nature Photon 6, 627-633 (2012).

Further, the optical fibre 1 may be a crystal optical fibre, optionally a single-crystal optical fibre. In this case, the material of the optical fibre 1 is entirely or substantially entirely composed of the crystal material. For example, the optical fibre 1 may comprise sapphire, diamond, or yttrium aluminium garnet (YAG) crystal. The crystal material in the optical fibre 1, whether this provides all or part or the optical fibre 1 , may be doped, optionally with a rare-earth element. Other fibre types include fibres having a pure silica core, photonic crystal, polymer, hydrogel etc.

A single-mode silica fibre has a core 3 which is typically approximately 9 pm in diameter and doped with germanium to have a higher refractive index (e.g. around 10³ higher). Around the core 3 is undoped cladding 5. In a single-mode fibre, only a single transverse mode is able to propagate. The mode is predominantly within the core 3, but it has an evanescent field which extends into the cladding 5. It can be characterised by an effective refractive index, n_eft, which has a value at a particular wavelength determined by the core diameter as well as the core and cladding refractive indices at that wavelength.

The optical fibre 1 comprises one or more voids 9. The one or more voids 9 extend longitudinally along the length of the optical fibre 1 within a void portion 10.

The void portion 10 is a continuous and integral portion of the optical fibre 1. The void portion 10 does not comprise any interface between longitudinally adjacent solid materials. Within the void portion 10, there are no longitudinal joins or splices in the material of the optical fibre 1. Apart from the voids 9 and any other structures defined within it, the void portion 10 is formed of a single material. The voids 9 are formed, as will be discussed further below, by starting with a single, continuous, integral material and removing part of the material of the void portion 10 internally.

The one or more voids 9 may extend entirely within the void portion 10. A boundary of the one or more voids 9 may be defined entirely within the void portion 10. A longitudinal extent of each of the one or more voids 9 may be less than a longitudinal extent of the void portion 10. A longitudinal extent of every void 9 within the optical fibre 1 may be less than a longitudinal extent of the void portion 10, i.e. such that there are no voids 9 within the optical fibre 1 that have a longitudinal extent greater than the longitudinal extent of the void portion 10.

Forming the voids 9 within the void portion 10 in this manner means that there is no need to splice two sections of fibre together to form the optical fibre 1. Splicing different types of fibres results in a mode-mismatch and additional loss. It can also result in structural weaknesses at the point of the splice. This problem is particularly acute in applications using FBGs. When a string of FBGs is concatenated using splicing, each FBG would have 2 splice losses at its ends. If light is reflected from the last FBG in the chain it would see 4x this splice loss per FBG before it, so loss builds up quickly.

The process where voids 9 are defined within the void portion 10 is also much more scalable in manufacturing. A large number of optical fibres 1 can be mounted on a tray and processed together. There is no need to splice the ends of each individual fibre separately, which requires time-consuming alignment of the fibre cores to micrometre precision.

The one or more voids 9 are sealed from an exterior of the optical fibre 1 and at least partially filled with a filler 15. The filler 15 has a material property different to the material property of a material of the optical fibre 1. The one or more voids 9 may be entirely filled with the filler 15. The filler 15 may be a fluid. The filler may comprise a gas, optionally nitrogen or air. The filler 15 may comprise a non-gaseous material, optionally a liquid. The filler 15 may comprise glycerol or a glycerol-water mixture. The filler 15 may comprise a liquid crystal. The filler 15 may comprise monomers and photo-initiator, such that a laser may be used to selectively polymerise the filler. The filler 15 may be a glass. The filler 15 or the material of the optical fibre 1 may be chalcogenide glass or borosilicate glass. The filler 15 may have a lower melting temperature than the material of the optical fibre 1. The one or more voids may be filled by heating the optical fibre 1 and the filler 15 to a temperature between the melting point of the filler and the melting point of the material of the optical fibre 1. The filler 15 may then be solidified following filling of the one or more voids by allowing the filler 15 to cool below its melting temperature. The filler 15 may comprise a polymer. The filler 15 may be supplied as a liquid polymer resin. The one or more voids 9 may comprise two or more voids 9 filled with different fillers 15. The material property of the fillers 15 may differ between the different fillers 15.

Voids 9 containing fluid filler 15 are extremely difficult to create with existing manufacturing techniques that use splicing. Fluid leaks out of the ends of the fibre during splicing and can be vaporised during the splicing process, leading to non-uniform filling of the voids 9. In addition, it is difficult to efficiently fill voids 9 from either end over long sections of fibre 1. This prevents scalable manufacturing processes, because it is necessary to splice very short lengths of fibre with liquid within them.

As mentioned above, the invention also concerns a method for forming one or more voids 9 in an optical fibre 1, in order to produce optical fibres 1 such as those shown in Fig. 1 A and Fig. 1 B. Fig. 2 is a flowchart of the method.

Fig. 3A shows an unmodified optical fibre 1 in profile, and Fig. 3B shows a cross-section of the unmodified optical fibre 1. Typically, the optical fibre 1 may be silica and have a diameter of approximately 125 pm, with a germanium doped core of approximately 9 pm diameter. However, other sizes and materials are possible as mentioned above. The optical fibre 1 may be formed by drawing a preform, and the one or more voids 9 are formed after the optical fibre 1 is drawn.

The optical fibre 1 may have initially had a coating 23, for example of a polymer material such as 250 pm diameter polyacrylate or 150 pm diameter polyimide. The coating 23 may be removed S5 prior to performing the remaining steps of the method. The coating may be removed S5 by any suitable method, such as chemical removal or ablation. The coating 23 may be removed along substantially the entire longitudinal extent of the void portion 10, or even from the entire length of the optical fibre 1.

The method comprises selectively exposing S10 the optical fibre 1 to laser radiation to define one or more exposed regions 7 within the optical fibre 1. Fig. 4A shows where the optical fibre 1 has been modified by exposing it to laser radiation. Fig. 4B shows a cross-section of the optical fibre 1 of Fig. 4A.

The exposed regions 7 are formed where the energy delivered to the material of the optical fibre 1 by the laser radiation is above a predetermined threshold, which causes a modification of the material of the optical fibre 1. The laser radiation is focussed within the material of the optical fibre 1 at a focal spot. The energy delivered to the material by the laser radiation is above the predetermined threshold at the focal spot, but below the predetermined threshold outside of the focal spot. In this way, only portions of the material of the optical fibre 1 that are exposed to the focal spot are modified to form exposed regions. The laser radiation may pass through other portions of the material of the optical fibre without forming exposed regions 7. This allows exposed regions 7 to be formed entirely within the material of the optical fibre 1.

The focal spot is moved through the material of the optical fibre 1 to define exposed regions larger than the focal spot. This can be achieved by moving the optical fibre 1 while keeping the position of the focal spot fixed, by moving the focal spot while keeping the position of the optical fibre 1 fixed, or by moving both the optical fibre 1 and the focal spot simultaneously.

The exposed regions 7 define regions of the optical fibre 1 to be subsequently selectively removed. When silica or other common optical fibre materials such as sapphire are exposed to high energy laser pulses, nanogratings are formed. When material which has been so exposed is put into an etchant, the etchant will remove the exposed regions preferentially compared with the pristine unmodified material.

The laser radiation may be provided by a laser beam generated using a laser system. Selectively exposing S10 the optical fibre 1 to laser radiation may comprise applying a correction to an active optical element of the laser system. The correction modifies wavefront properties of the laser beam to counteract an effect of aberration on laser focus. Aberration is caused in particular by refraction at the external surface of the optical fibre 1 and at the interface between the core 3 and cladding 5. This can distort the focal spot at which the laser beam is focussed, leading to inconsistent results when exposing the material of the optical fibre 1. Counteracting the effect of aberration allows much more accurate control of the position, size, and shape of the focal spot of the laser beam within the internal volume of the optical fibre 1. This allows arbitrary regions to be accurately exposed within the optical fibre 1 to form the exposed regions 7.

The laser radiation may comprise infrared or visible light. For example, the laser radiation may have a wavelength between 700 nm and 900 nm, optionally between 750 nm and 850 nm, optionally approximately 790 nm. The laser radiation may have a wavelength between 450 nm and 650 nm, optionally between 500 nm and 600 nm, optionally approximately 530 nm.

The laser beam may be a pulsed laser beam. For example, the laser system may be a femtosecond laser system. Pulses of the pulsed laser beam may have a duration of at most 1 ps, optionally at most 500 fs, optionally at most 200 fs, optionally at most 100 fs. Pulses of the pulse laser beam may have an energy of at least 0.1 pi J, optionally at least 0.15 pi J, optionally at least 0.2 pi J, optionally at least 0.25 pi J.

Selectively exposing S10 the optical fibre 1 to laser radiation may further comprise defining one or more access regions 17. Similar to the exposed regions 7, the access regions 17 are regions in which an energy delivered to the material of the optical fibre by the laser radiation is above the predetermined threshold. This means that the material of the optical fibre 1 is modified in the access regions 17 in the same manner as in the exposed regions 7. The access regions 17 can be defined in the same way as the exposed regions 7 by moving the focal spot through the material of the optical fibre 1.

Each access region 17 is contiguous with or adjoins at least one of the exposed regions 7. The access regions 17 extend to an external surface of the optical fibre 1 , such that each of the one or more exposed regions 7 is connected to the external surface by at least one of the one or more access regions 17. The access regions 17 create a continuous path of laser-modified material from the outside of the optical fibre 1 through into the exposed regions 7. Preferably, every point within the exposed regions 7 is connected to the external surface of the optical fibre 1 by a continuous path through laser-modified material.

The external surface to which the access regions 17 extend is preferably a radial ly-outermost surface of the optical fibre 1, rather than a flat surface at a longitudinal end of the optical fibre 1. The provides a shorter path from the outside of the optical fibre 1 to the exposed regions 7.

The method further comprises contacting S20 the optical fibre 1 with an etchant. The etchant etches the exposed regions 7 at a higher rate than regions of the optical fibre 1 not exposed to laser radiation, such that the one or more voids 9 are formed by etching of the exposed regions 7. That is, the etchant preferentially etches the exposed regions 7. The optical fibre 1 may be contacted S20 with the etchant for a period of time long enough such that substantially all of the exposed regions 7 and access regions 17 are removed. The period of time may be at least 2 hours, optionally at least 5 hours, optionally at least 10 hours, optionally at least 15 hours, optionally at least 24 hours. As discussed above, the exposed regions 7 are regions where the material of the optical fibre 1 has been exposed to laser radiation such that the energy delivered to the material is above a predetermined threshold. This causes modification of the material such that the etchant etches the exposed regions 7 at a higher rate than unexposed regions. Some portions of the material of the optical fibre 1 may be exposed to laser radiation during the step of selectively exposing S10 the optical fibre 1, but without the energy delivered being above the predetermined threshold. Such portions will be etched by the etchant at a rate substantially the same as the rate of etching for regions of the material of the optical fibre 1 that have not been exposed to laser radiation at all.

Where access regions 17 are formed, during the step of contacting the optical fibre 1 with an etchant, the access regions 17 are initially etched to form one or more access voids 19. Consequently, the optical fibre 1 comprises one or more access voids 19 extending from the one or more voids 9 towards an exterior surface of the optical fibre 1. Since the access regions 17 adjoin the exposed regions 7, the access voids 19 are connected to the voids 9. The access voids 19 therefore create a continuous flow path between the outside of the optical fibre 1 and the voids 9 through the access voids 19. The access voids 19 allow the etchant to contact the exposed regions 7 within the optical fibre 1. This means that the one or more voids 9 are formed by etching a material of the optical fibre 1 via the one or more access voids 19 prior to sealing S40 of the one or more access voids 19 (sealing is discussed further below).

Having at least two access voids 19 allows more efficient filling of the voids 9 with etchant during the etching process. Two access voids 19 also permits capillary filling of the voids 9 when using a liquid filler 15, as described later. However, having more than two access voids 19 along the length of the optical fibre 1 can be advantageous because it allows longer voids 9 along the length of the optical fibre 1 to be formed and filled with having excessively long etch and filling times. For example, the optical fibre 1 may have access voids 19 regularly spaced along its length, in particular along the length of the void portion 10. A largest longitudinal distance between consecutive access voids 19 along the length of the void portion 10 may be at most 10mm, optionally at most 5mm, optionally at most 2mm. A largest longitudinal distance between consecutive access voids 19 along the length of the void portion 10 may be at most 50%, optionally at most 25% of a longitudinal extent of the void 9 to which the access voids 19 are connected.

Fig. 5A and Fig. 5B show the optical fibre 1 after it has been put in a etch solution such that the etchant contacts the optical fibre 1. The etchant preferentially removes the exposed regions 7 where the material of the optical fibre 1 has been modified to form nanogratings. The etchant may comprise potassium hydroxide, optionally having a concentration of at least 5 mol, optionally at least 8 mol. The etchant may be heated to above room temperature, for example to at least 50°C, optionally at least 70°C. A suitable etchant is 8 mol/l KOH heated to 85°C using a hotplate. An ultrasonic bath or water bath filled with the etch solution can also be used. Any other suitable etchant can be used instead of potassium hydroxide, for example hydrogen fluoride (HF).

As shown in Fig. 6A and Fig. 6B, the one or more exposed regions 7 may comprise a high exposure region 11 and a low exposure region 13. The high exposure region 11 is exposed to a higher dose of laser radiation than the low exposure region 13 during the step of selectively exposing S10 the optical fibre. In other words, more energy is delivered to the material of the optical fibre 1 by the laser radiation in the high exposure region 11 than in the low exposure region 13. This causes more significant modification of the material of the optical fibre 1 in the high exposure region 11 than in the low exposure region 13. Due to the increased modification of the material in the high exposure region 11, the etchant etches the high exposure region 11 at a higher rate than the low exposure region 13 during the step of contacting S20 the optical fibre with the etchant.

The higher dose of laser radiation in the high exposure region 11 may be achieved by varying one of more of laser power, a speed of scanning of the laser, and laser beam profile. Where a pulsed laser beam is used, the higher dose of laser radiation in the high exposure region 11 may be achieved by varying one of both of laser pulse energy and pulse repetition rate. In Fig. 6A and Fig. 6 B, the centre of the exposed region 7 is exposed to a higher dose of laser radiation than the areas at the edges of the exposed region 7. This allows the etchant to flow along the centre of the exposed region 7 initially and then progress towards the edges. This avoids a problem whereby the portion of the exposed region 7 nearest the entry point at the surface of the optical fibre 1 becomes over-etched as it is in contact with the etchant for longer. Fig. 7A and Fig. 7B show the fibre from Fig. 6A and Fig. 6B after etching.

Fig. 6A to Fig. 7B also demonstrate an embodiment in which a cross-sectional area of at least one of the one or more voids 9 varies along the length of the optical fibre 1. Cross-sectional area here refers to a cross-section taken in a plane perpendicular to the length of the optical fibre 1. The shape, size, and orientation of the voids 9 is determined by the exposing S10 of the optical fibre 10 to the laser radiation. The cross-sectional area may reduce away from a centre of the void 9 along the length of the optical fibre 1 for at least a portion of the length of the void 9. The cross-section of at least one of the one or more voids 9 may be non-circular. For example, the cross-section of the void 9 may be rectangular, oval, or have the shape a segment of a circle. A central axis of the one or more voids 9 may extend along a direction inclined to the longitudinal axis of the optical fibre 1 for at least a portion of the one or more voids 9. The central axis of the void 9 here refers to an axis substantially along the longest dimension of the void 9. This is typically similar to the longitudinal axis of the optical fibre 1.

The method further comprises, once the one or more voids 9 have been formed, at least partially filling the one or more voids 9 with a filler 15 having a material property different to that of a material of the optical fibre 1. Fig. 8A and Fig. 8B show the optical fibre 1 of Fig. 5A and Fig. 5B after the microchannels have been filled with the filler 15.

The method may comprise flushing the etchant from the one or more voids 9 prior to filling the one or more voids 9 with the filler 15. This can be achieved with any suitable solvent in order to remove residual etchant. Removing residual etchant prevents further etching of the voids 9 that could distort their shape. It also prevents contamination of the filler 15 that could alter its material properties.

The voids 9 may be filled using any suitable technique depending on the nature of the filler 15. For example, where the filler 15 is a liquid, the voids 9 can be filled via capillary action by dropping the filler onto one of the access voids 19, whilst leaving another of the access voids 19 clear. Alternatively, the voids 9 could be pressure filled. Filling the one or more voids 9 also includes the possibility of simply allowing an ambient fluid, such as air, to enter the voids 9. For example, the optical fibre 1 may be removed from the etchant solution and then, after a possible flushing step, left in, for example, an ambient environment or a chamber filled with an inert gas such as nitrogen before the voids 9 are sealed. This would allow the surrounding fluid to fill the voids 9.

As mentioned above, the filler 15 may be a glass. The filler 15 may have a lower melting temperature than the material of the optical fibre 1. The one or more voids may be filled by heating the optical fibre 1 and the filler 15 to a temperature between the melting point of the filler and the melting point of the material of the optical fibre 1. The filler 15 may then be solidified following filling of the one or more voids by allowing the filler 15 to cool below its melting temperature.

Filling the voids using the access voids 19 from the side (i.e. via a radial ly-outermost surface of the optical fibre 1 rather than a longitudinal end of the optical fibre 1) avoids the need to attempt to splice fibres with liquid coming out of the end, as is necessary in prior art methods. The arc temperature of a fibre splicer is high enough to melt the material of the optical fibre 1 , and therefore can evaporate many common liquid fillers 15. The present method avoids the need to splice fibres to define voids 9, and therefore avoids this issue.

The method further comprises sealing S40 the one or more voids 9 from an exterior of the optical fibre 1 following filling S30 of the voids 9. Fig. 9A and Fig. 9B show the optical fibre 1 of Fig. 8A and Fig. 8B after the voids 9 have been sealed.

Sealing S40 can be achieved with an adhesive, for example a UV curable adhesive. Alternatively, the material of the optical fibre 1 could be melted. Melting could be achieved with an arc as used in a typical splicing machine, or using a laser to selectively melt the material at the external surface of the optical fibre 1.

Where the optical fibre 1 comprises access voids 19, sealing S40 the one or more voids 9 comprises sealing the one or more access voids 19 at the exterior surface of the optical fibre 1. Consequently, in the finished optical fibre 1, the one or more access voids 19 are sealed at the exterior surface of the optical fibre 1.

The access voids 19 may be sealed by inserting a blocking member 21 within the access void 19. Consequently, the finished optical fibre 1 comprises a blocking member 21 within the access void 19. The blocking member 21 may be entirely within the access void 19. The blocking member may comprise an adhesive as mentioned above. Alternatively, the access voids 19 may be sealed by melting the material of the optical fibre 1 at the exterior surface of the optical fibre 1. The melting may be performed using a laser or an electric arc.

In the examples shown so far, the access voids 19 have generally extended along a radius of the optical fibre 1. However, this is not essential in general. Fig. 10A and Fig. 10B show an example in which the access voids 19 extend along a non-radial direction.

As mentioned above, the optical fibre 1 may initially have had a coating that is removed S5 prior to the processing carried out to form and fill the voids 9. Once the voids 9 have been filled and sealed, the method may further comprise recoating S45 the optical fibre 1. This results in the final optical fibre 1 as shown in Fig. 1 A and Fig. 1 B, which comprises a coating 23. The coating 23 may comprise a plastic or polymer material. Polyacrylate is a common optical fibre coating, but high-temperature applications a polyimide coating may be used instead. The recoating S45 may be performed using a standard recoating machine and process.

Instead of removing S5 the optical fibre coating 23 prior to processing along substantially the entire longitudinal extent of the void portion 10 or from the entire length of the optical fibre 1, it is also possible to leave the coating 23 on if it is resistant to the etchant. In this case, removing S5 the coating 23 comprises selectively removing the coating 23 to form gaps 25 in the regions where the access voids 19 will meet the external surface of the optical fibre 1. The coating 23 may be removed S5 by any suitable process, for example laser ablation. Laser ablation may be carried out using the same laser system as used for selectively exposing. The exposing S10 of the optical fibre 1 to laser radiation then comprises focussing the laser radiation through the coating 23.

Fig. 11 A shows a coated optical fibre 1 prior to carrying out the method. Fig. 11 B shows the optical fibre 1 after selectively removing S5 the coating 23 and exposing S10 the optical fibre 1 to form the exposed regions 7. The optical fibre 1 can then be contacted S20 with the etchant, such that etchant enters though the gaps 25 in the coating 23. The voids 9 can then be filled S30 and sealed S40 as discussed above. Following this the coating 23 can be reinstated in the gaps 25 to form a uniform coating 23 again in the finished optical fibre 1.

The present method provides much greater flexibility to produce voids 9 in optical fibres customised to individual requirements than existing methods. The precise dimensions and positions of the voids 9 can be defined for each individual optical fibre 1 and even varied along its length without having to have a new fibre preform produced each time and without having to splice fibres together. This greatly reduces manufacturing time, complexity and cost.

Having described the method for forming optical fibres 1 according to the present invention, embodiments of optical fibres 1 produced using the method will now be described that can be used in various different applications.

The one or more voids 9 are configured such that the optical property of the filler 15 affects light transmitted through the optical fibre 1. As mentioned above, the filler 15 has a material property different to the material property of a material of the optical fibre 1. This allows the filler 15 to be used to tune the optical properties of the optical fibre 1 in the void portion 10. Importantly, the filler 15 may also have a change in the material property with an environmental variable (such as temperature, pressure, or strain) that differs from a change in the material property with the environmental variable of the material of the optical fibre 1. This allows the change in the optical properties of the optical fibre 1 with the environmental variable to also be controlled by appropriate choice of filler 15.

The material property of the optical fibre 1 and the filler 15 may be an optical property, optionally refractive index. The filler 15 may have a refractive index that differs from the refractive index of the material of the optical fibre 1. Nonetheless, the refractive index of the filler 15 will preferably not differ too much from the refractive index of the material of the optical fibre 1. The refractive index of the filler may be within 0.1 of a refractive index of the material of the optical fibre, optionally within 0.05, optionally within 0.01, optionally within 0.005, optionally within 0.001. Where the optical fibre 1 comprises a core 3 and a cladding 5 surrounding the core 3, the refractive index of the filler 15 may be approximately equal to the refractive index of a material of the cladding 5 at a reference temperature. Matching the refractive index of the filler 15 approximately can reduce reflections and losses in the optical fibre 1, but is also important in order not to disrupt the waveguiding of the optical fibre 1 and the mode properties of the optical fibre 1. Ideally, the optical fibre 1 forms a "weakly guiding" waveguide so that the optical mode extends into the cladding 5. This requires a relatively small refractive index difference between the core 3 and cladding 5. If the refractive index of the cladding 5 (which is affected by the refractive index of the filler 15) is larger than that of the core 3, then there is no waveguiding. If the refractive index of the cladding 5 is too large, then the optical fibre 1 will become multimoded, where more than one transverse spatial modes exists.

The filler 15 may have a change in refractive index with temperature, dn/dT, which is different to that of the material of the optical fibre 1. The change in the refractive index of the filler 15 with temperature may be negative.

In general it is preferable to avoid sudden transitions in the effective optical properties experienced by light transmitted in the optical fibre 1. Sudden transitions can lead to reflections and losses that have adverse effects on the transmission of light. To reduce this effect, a variation of the cross-sectional area of the one or more voids 9 and/or an orientation of the central axis of the one or more voids 9 may be such that light transmitted through the optical fibre experiences a substantially continuous variation in an effect of the optical property on the transmission of the light along the length of the optical fibre 1. For example, as shown in Fig. 7A and Fig. 7B or Fig. 10A and Fig. 10B, the cross-sectional area of the voids 9 may smoothly increase from zero at both longitudinal ends of the voids 9 to its maximum value. Since light transmitted in the optical fibre 1 is localised most strongly in the centre of the optical fibre 1 around the core 3, the effect of the filler 15 on light propagating in either direction along the optical fibre 1 is gradually increased towards the centre of the void 9 as the cross-sectional area of the void 9 increases. Alternatively or additionally, the cross-sectional area of the void 9 may be substantially constant, but the distance between the void 9 and the core 3 may increase towards either end of the void 9. This also means that the effect of the filler 15 on light transmitted in the fibre reduces towards either end of the void 9.

As discussed above, the optical fibre 1 may comprise a core 3 and a cladding 5 surrounding the core 3. In this case, the one or more voids 9 may comprise at least one cladding void within the cladding 5.

Fig. 12A to Fig. 12F show cross-sectional views of different optical fibres 1 with voids 9 containing a filler 15. Light transmitted in the optical fibre 1 is predominantly confined to and guided by the core 3. However, the evanescent field of the light extends into the surrounding cladding 5 and consequently into the filler 15 within the voids 9. The material property of the cladding 5, such as refractive index, is modified by the presence of the filler 15. This means that the cladding void is configured such that transmission of light within the optical fibre 1 is affected by the filler 15 within the cladding void.

If the filler 15 has a material property which changes depending on environmental variables such as temperature or electric field then the effective material property of the cladding 5 experienced by light transmitted in the optical fibre 1 will also change. By adjusting the size, shape, and placement of the voids 9, the area over which the filler 15 is present can be adjusted. Thereby the amount of overlap of the optical fibre mode with the filler 15 and the strength of the interaction with light in the fibre can be controlled.

The at least one cladding void may be adjacent to the core 3. As shown in Fig. 12C, the cladding void may extend such that the filler 15 is in contact with the core 3. Alternatively, as shown in Fig. 12A, Fig. 12B, and Fig. 12D to Fig. 12F, the cladding void may extend such that the filler 15 is not in contact with the core 3.

The one or more voids may comprise exactly one void 9. Fig. 12B shows such an example with a relatively small gap between the core 3 and the filler 15 in the void 9. In this example, the core 3 is completely surrounded and suspended within the void 9 in the void portion 10. This achieves a very high overlap of the optical mode with the filler 15.

The one or more voids 9 may comprise a plurality of cladding voids within the cladding 5. As shown in Fig. 12A, the plurality of cladding voids may comprise at least two cladding voids. Fig. 12A shows two voids 9, each covering approximately half of the circumference around the core 3 and with a gap between the voids 9.

As shown in Fig. 12E and Fig. 12F, the plurality of cladding voids may comprise at least four cladding voids. Fig 12E shows an arrangement with 4 elliptical voids 9 in very close proximity to the core 3 to give a high overlap of the optical mode with the filler 15. Fig. 12F shows an arrangement with 4 circular voids 4 which are further from the core 3 to reduce the overlap as a design parameter.

As shown in Fig. 12C and Fig. 12D, the plurality of cladding voids may comprise at least six cladding voids. Fig. 12C shows a spoke arrangement where the overlap of the optical mode localised at the core 3 with the material is reduced compared to the example of Fig. 12B. Fig. 12D shows a spoke arrangement with a larger gap between the core 3 and the filler 15, reducing further the overlap of the optical mode with the filler 15.

The plurality of cladding voids may be arranged symmetrically around the core 3. Radial symmetry around the core 3 reduces polarisation-dependent effects, leading to a more uniform effect of the filler 15 on different polarisation modes within the optical fibre 1. A distance between the cladding voids and the core 3 may vary along the length of the cladding void. This can contribute to the gradual transition of material properties experienced by light in the optical fibre 1, as discussed above.

As mentioned above, the filler 15 may have a change in the material property with environmental variables that differs from that of the material of the optical fibre 1. This can be used to compensate for the change of the material property of the optical fibre 1, thereby stabilising its behaviour with respect to the environmental variable. Alternatively, the change of the material property of the optical fibre 1 can be enhanced. The latter may increase sensitivity for sensing applications.

In particular, a change with temperature of the material property of the filler 15 may be different to, optionally opposite to, a change with temperature of the material property of the material of the optical fibre 1.

The one or more voids 9 may be configured such that the change with temperature of the material property of the filler 15 at least partially compensates for the change with temperature of the material property of the material of the optical fibre 1.

Fig. 13A to Fig. 13D show example refractive index profiles across the optical fibre cross-section illustrating this effect.

Fig. 13A shows the refractive index profile of an unmodified step index single mode silica optical fibre 1. The core refractive index is higher than the cladding refractive index. The optical mode is predominantly localised within the core 3, but also extends into the cladding 5.

Fig. 13B shows the refractive index profile of the same unmodified optical fibre, but at a higher temperature. The refractive index of both the core 3 and the cladding 5 has increased and hence the effective refractive index, n_eft, experienced by the optical mode has also increased. Standard optical fibre is made of silica which has a refractive index of 1.4440 at 1550nm and a refractive index variation with temperature dn/dT = 7.97* 10^-6 °C^-1.

Fig. 13C shows the refractive index profile of an optical fibre 1 according to the present invention which has voids 9 filled with a filler 15. The refractive index of the filler has been chosen in this example to be lower than the refractive index of the core 3 and higher than the refractive index of the material of the cladding 5. However, equally the refractive index of the filler could have been chosen to be lower than the original silica cladding refractive index.

Fig. 13D shows the refractive index profile of the same optical fibre as Fig. 13C at a higher temperature. The refractive indices of the core 3 and the cladding 5 have increased, but the refractive index of the filler 15 has decreased. The effective refractive index, n_ett, experienced by the optical mode will be somewhere between the core and cladding refractive indices. The effect of the filler 15 on the effective refractive index is therefore to oppose the change due to the core and cladding refractive indices.

One example of a suitable filler 15 with a large difference in dn/dT is glycerol. Glycerol has a refractive index of 1.4473 at 1550 nm and a dn/dT = -225x10^-6 °C"¹. The glycerol can be appropriately diluted with distilled water to form an aqueous glycerol solution to reduce its refractive index such that it is well matched to the cladding 5 over the required operating temperature range. As an alternative to glycerol, it is possible to use a proprietary refractive index oil, such as one from Cargille Laboratories (www.cargille.com). For example, Cargille Series AA liquid 1.44000 refractive index (specified at 589.3 nm, 25°C) has a refractive index of 1.43 at 1550 nm and a temperature coefficient of -395x10^-6 °C^-1. As the chosen filler 15 introduced has a dn/dT which is of a much larger magnitude than silica, even a relatively low interaction with the optical mode can have a significant effect on the effective refractive index.

The net change in effective refractive index, dnett/dT, with temperature may be positive or negative depending on the application. The net change may also be designed to be approximately zero for some range of temperatures. The value of dn_eft/dT is determined by (i) the refractive index of the filler 15, nnu (ii) the refractive index change with temperature of the filler 15, dna/dT, (Hi) the amount of filler 15 in the cladding 5; and (iv) the proximity of the filler 15 to the core 3. Diluting a liquid filler 15 with a solvent has the effect of reducing (i) and (ii). By appropriately adjusting these parameters it is possible to manipulate the dneft/dT to achieve a desired value and change of the effective refractive index.

As mentioned above, in many situations the optical fibre 1 may comprise a coating 23. This is desirable for many reasons, including protecting the optical fibre 1 from damage because uncoated optical fibres can be fragile and easily broken. However, the coating 23 may also affect the behaviour of light transmitted in the optical fibre 1. In particular, a change with temperature of a material property of the coating 23 may affect light transmitted through the optical fibre 1.

Although the coating 23 may be too far from the core 3 to interact significantly with the light localised in the core, it can affect the light in other ways. For example, a coefficient of expansion of the coating 23 with temperature may differ from that of the material of the optical fibre 1. This can cause strain in the optical fibre 1 as temperature changes, which can in turn affect the refractive index of the material of the optical fibre 1. This can elongate the optical fibre 1 through thermal expansion of the coating 23. For optical fibres 1 containing Bragg gratings (discussed further below) this can increase the pitch of the Bragg grating, thereby changing the Bragg wavelength.

For this reason, in some embodiments, the one or more voids 9 may be configured such that a change with temperature of the material property of the filler 15 at least partially compensates for a combined effect on light transmitted through the optical fibre 1 of a change with temperature of the material property of the material of the optical fibre 1 and the change with temperature of the material property of the coating 23. This enables coated optical fibres to be thermally compensated, meaning that they are better protected against damage. It also means they can be embedded in structures without temperature influencing their characteristics.

For some applications, it may be advantageous to split the light transmitted in the optical fibre 1 along multiple paths within the optical fibre 1. Examples of such applications will be discussed further below. In such cases, the optical fibre 1 may comprise a waveguide 41 separate to a core 3 of the optical fibre 1.

Fig. 24A and Fig. 24B show an example of such an arrangement. The waveguide 41 may be located in the cladding 5 of the optical fibre 1. The waveguide 41 may be substantially parallel to the core 3 of the optical fibre 1 for at least part of the length of the waveguide 41. For example, at least 50% of the length of the waveguide, optionally at least 75% of the length of the waveguide 41.

The waveguide 41 may be arranged such that light is coupled from the core 3 of the optical fibre 1 to the waveguide 41. This can be achieved using a waveguide coupler 45. The waveguide coupler 45 may be a portion of the length of the waveguide 41 where the waveguide 41 is within a predetermined coupling distance of the core 3 such that light is evanescently coupled from the core 3 to the waveguide 41.

The waveguide 41 may be formed by one or more modified regions of the optical fibre 1 in which an optical property of the optical fibre 1 differs from the optical property of a material of the optical fibre 1 surrounding the one or more modified regions. The optical property may be refractive index. The one or more modified regions may comprise one or more laser-exposed regions. This allows the waveguide 41 to be formed by exposure to laser radiation using the same laser setup as is used for forming the voids 9, thereby reducing manufacturing complexity.

The waveguide 41 may be configured such that transmission of light in the waveguide 41 is affected by the filler 15 in at least one of the one or more voids 9. The one or more voids 9 may comprise at least one void adjacent to the waveguide 41. Alternatively, the one or more modified regions that form the waveguide 41 may comprise one or more voids 9 extending longitudinally along the length of the optical fibre 1. In this case, the filler 15 in the void 9 essentially forms the waveguide 41, allowing for strong interaction between the filler 15 and light in the waveguide 41.

A change with temperature in a refractive index of the waveguide 41 may be is different to a change with temperature in a refractive index of the core 3. This can be achieved by forming the optical fibre 1 such that a material of the waveguide 41 is different to a material of the core 3. In particular, the material of the waveguide 41 and the material of the core 3 may comprise glass. Where the optical fibre is a multicore fibre, the waveguide may be provided by a second core, for example a core that is not the central core running along the longitudinal axis of the fibre. The second core may comprise a material having a different refractive index to the core 3 of the optical waveguide. The material of both the core 3 and the second core may comprise a glass. As discussed further below, both the waveguide 41 and the core 3 may comprise a Bragg grating.

In further embodiments, the optical fibre may comprise at least two waveguides at different positions within a crosssection of the optical fibre. A change with temperature in a refractive index of the first waveguide of the at least two waveguides may be different to a change with temperature in a refractive index of a second waveguide of the at least two waveguides. One or both of the waveguides may be provided by cores of the optical fibre, for example where the optical fibre has one or two cores. Where the optical fibre is a multicore fibre, the two waveguides may be provided by different cores of the optical fibre. However, in some embodiments, neither waveguide may be provided by a core, for example where the waveguides are formed by other changes in the material of the optical fibre such as stress waveguides. One or both of the waveguides may be formed by laser modification of the material of the optical fibre. A material of the first waveguide may be different to a material of the second waveguide, optionally the material of the first waveguide and the material of the second waveguide comprise glass.

In some applications discussed further below, the optical fibre 1 is configured to exhibit birefringence. The one or more modified regions may be configured to contribute to the birefringence.

As mentioned above, various sensors can be constructed from optical fibres containing Bragg gratings. The present invention is also advantageously applied in embodiments where the optical fibre 1 comprises a Bragg grating 31.

Fig. 14 to Fig. 17 illustrate various embodiments analogous to those already described, but containing a Bragg grating 31. Fig. 14 corresponds to Fig. 11 B, Fig. 15 corresponds to Fig. 1A, Fig. 16 corresponds to Fig. 10A, and Fig. 17 correspond to Fig. 7A. Voids 9 may be formed as described above in an optical fibre 1 that already contains a Bragg grating 31. Alternatively, the Bragg grating may be formed during the method, such that the method further comprises forming a Bragg grating 31 in the optical fibre 1. This can be achieved by modifying the refractive index of the material of the optical fibre 1.

One method used to form a Bragg grating is to make the material of the optical fibre 1 photosensitive (for example by subjecting it to hydrogen or doping the core with a dopant such as boron) and exposing it to an ultraviolet (UV) laser. The periodicity can be imparted by splitting the output of a high power UV laser into two beams and creating an interference pattern on the optical fibre 1. Alternatively, a phase mask can be used to create an interference pattern on the optical fibre 1.

Another method is to use a laser (e.g. at a wavelength of 790 nm or 532 nm) to directly modify the refractive index at points along the length of the optical fibre 1. Laser radiation is focussed at a focal spot in the optical fibre 1 to create modified regions in which the refractive index is raised periodically along a length of the optical fibre 1. This can be achieved, for example, by used pulsed laser radiation and moving the optical fibre 1 and/or the laser focal spot at a constant speed to get equally spaced modified regions. Suitable parameters for a typical optical fibre are -190 fs pulse duration with a pulse energy around 0.15 pi J. Alternative types of FBG include: a) a chirped FBG, in which the pitch of the modified regions varies along the length of the optical fibre 1; b) a long period FBG, in which the grating has a much larger period resulting in a coupling from the core to lossy cladding modes); c) a sampled FBG (discussed further below in relation to Fig. 22); or d) a n-phase-shifted FBG, which has a narrow optical passband within the Bragg reflection band.

The Bragg grating 31 may be formed after the voids 9, but is preferably formed before the voids 9 where possible. This is because the additional structure of the voids 9 within the optical fibre 1 can make it more difficult to accurately focus a laser within the optical fibre 1 to form the Bragg grating 31.

Where the optical fibre 1 comprises a core 3 and a cladding 5 surrounding the core 3, the Bragg grating 31 may be at least partially located in the core 3, optionally entirely located within the core 3. The Bragg grating 31 may be provided by periodic modification of a material of the core 3. Alternatively or additionally, the Bragg grating 31 may be provided by periodic modification of a material of the cladding 5. The periodic modification may be adjacent to the core 3, which creates a stronger interaction between the periodic modification and light in the core 3. A Bragg grating 31 in the cladding 5 may also be provided using a void 9 filled with a filler 15 where it is possible to periodically modify the filler 15. For example, the filler 15 may be a liquid crystal. The filler 15 may comprise monomers and photo-initiator such that a laser may be used to selectively polymerise the filler periodically along the length of the void 9 to form a Bragg grating.

As discussed above, the one or more voids 9 are configured such that the material property of the filler 15 affects light transmitted in the optical fibre 1. Where the material property of the optical fibre 1 is an optical property, the one or more voids 9 may be configured such that the optical property of the filler 15 affects the Bragg wavelength of the Bragg grating 31. This allows advantageous modification of the Bragg wavelength and its change with respect to environmental variables such as temperature according to the needs of particular applications.

The voids 9 and filler 15 may be localised to the just the length of the optical fibre 1 where the Bragg grating 31 is located. This reduces the optical losses along the optical fibre 1 as a whole. As shown in Fig. 15 and Fig. 17, the one or more voids 9 may at least partially surround the Bragg grating 31. The longitudinal extent of the one or more voids 9 may be at least as long as a longitudinal extent of the Bragg grating 31. The one or more voids 9 may extend at most 1mm, optionally at most 0.5mm past either end of the Bragg grating 31. This ensures a uniform effect of the filler 15 on the Bragg grating 31 along its entire length.

The one or more voids 9 may be configured such that a change with temperature of the optical property of the filler 15 at least partially compensates for an effect on the Bragg wavelength of the Bragg grating 31 of a change with temperature of the optical property of the material of the optical fibre 1. This can be achieved by appropriate choice of the composition of the filler 15 and the size, shape, and arrangement of the voids 9 within the optical fibre 1. The change in the Bragg wavelength with temperature may be substantially cancelled at a predetermined wavelength over a predetermined range of temperature.

The net change in effective refractive index, dneft/dT, with temperature may be positive or negative depending on the application. The net change may also be designed to be approximately zero for some range of temperatures. The value of dn_eft/dT is determined by factors such as those discussed above. By appropriately adjusting these factors it is possible to manipulate the dneft/dT. In turn this can be used to modify the change in Bragg wavelength with temperature AAtJAT.

The net change in Bragg wavelength shift is also determined by the thermal expansion of the optical fibre. The change in Bragg wavelength, is given by where £, is the thermo-optic coefficient, P_e is the photoelastic coefficient, a is the thermal expansion coefficient of the material of the optical fibre 1, s is the strain, and AT \s the temperature change. In practice for a silica fibre, a is small in comparison to the thermo-optic effect by around an order of magnitude, so the Bragg wavelength shift is dominated by the thermo-optic effect. Taking a into account, n_ett and dneft/dT can be adjusted as described above to flatten the change in Bragg wavelength over a required temperature range. For example, the one or more voids 9 may be configured such that a magnitude of the change with temperature of the Bragg wavelength of the Bragg grating 31 is at most 5pm/°C, optionally at most 2pm/°C, optionally at most 1pm/°C over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C. A plurality of voids 9, optionally containing different fillers 15, can be used in order to flatten the temperature response over a wider temperature range.

As well as flattening the temperature response, the one or more voids 9 may be configured to enhance the temperature response. The one or more voids 9 may be configured such that a change with temperature of the optical property of the filler 15 increases a magnitude of a change with temperature of the Bragg wavelength of the Bragg grating 31 relative to an optical fibre 1 without the one or more voids 9. This could be advantageous to provide improved sensitivity in temperature-sensing applications. The change in Bragg wavelength with temperature may be negative, such that the Bragg wavelength of the Bragg grating 31 reduces with increasing temperature. The negative change in the Bragg wavelength may be larger than the positive change in Bragg wavelength for an optical fibre 1 without the one or more voids 9. For example, the one or more voids 9 may be configured such that a magnitude of a change with temperature of the Bragg wavelength of the Bragg grating 31 is at least 20pm/°C, optionally at least 30pm/°C, optionally at least 40pm/°C, optionally at least 50pm/°C over a temperature range of at least 20°C, optionally at least 40°C.

Fig. 18 shows graphs of the change in Bragg wavelength versus change in temperature for illustrative examples of an optical fibre. The graph for an unmodified silica optical fibre containing an FBG 31 but without voids 9 (FBG#1) shows a linear positive (+10 pmC ¹) change in Bragg wavelength with temperature. An optical fibre 1 with an FBG 31 and comprising voids 9 configured for high temperature sensitivity (FBG#2) shows a large negative gradient. An optical fibre 1 with an FBG 31 and comprising voids 9 configured to be thermally compensated (FBG#3) shows a flat curve with a minimum point and negative and positive gradients either side of the minimum point.

Fig. 19A to Fig. 19C show spectra for the same three Bragg gratings as Fig. 18 at two temperatures, To and Ti (Ti>To). Fig. 19A shows the spectra for the unmodified silica FBG (FBG#1), where there is a shift to longer wavelengths with increasing temperature. Fig. 19B shows the spectra for a high sensitivity FBG (FBG#2) where there is a much larger shift at higher temperatures, but this time to shorter wavelengths. Fig. 19C shows the spectra for a temperature compensated FBG (FBG#3), where the wavelength shift is much smaller than that in Fig. 19A.

Where the optical fibre 1 comprises a coating 23, a change with temperature of a material property of the coating 23 may affect the Bragg wavelength of the Bragg grating 31, similarly as described above. This may also be the case if the optical fibre 1 is embedded within or bonded to a substrate. In this situation, the thermal expansion of this material can also be taken into account when determining the Bragg wavelength. In this case, the Bragg wavelength shift is given by where a_c is the thermal expansion coefficient of the coating 23. In this case (1-P₉)cfc may dominate over The temperature dependence can be compensated if the fibre can be modified such that 5, is made substantially equal to - (1-P_e)cfc.

In this case, the one or more voids 9 may be configured such that the change with temperature of the material property of the filler 15 at least partially compensates for the combined effect on the Bragg wavelength of the Bragg grating 31 of the change with temperature of the material property of the material of the optical fibre 1 and the change with temperature of the material property of the coating 23. For example, the one or more voids 9 may be configured such that a magnitude of the change with temperature of the Bragg wavelength of the Bragg grating 31 is at most 5pm/°C, optionally at most 2pm/°C, optionally at most 1pm/°C over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C. The combined optical fibre 1 and coating 23 may have a Bragg wavelength shift with temperature which has a stationary point. The change in Bragg wavelength with temperature may be negative at temperatures below the stationary point and positive at temperatures above the stationary point.

Fig. 20 shows compensation of a coated optical fibre or an optical fibre 1 embedded within or bonded to a substrate. Fig. 20 shows (i) the Bragg wavelength shift associated with the thermal expansion, (1-P₉)cfc, with a large positive gradient, (ii) the Bragg wavelength shift associated with the thermo-optic effect, with a corresponding negative gradient, and (iii) the resultant Bragg grating shift, which is flattened and close to zero. This response has a stationary point where there is a minimum and a negative gradient and positive gradient either side.

The optical fibre 1 may comprise two separate Bragg gratings 31. This may enable differential sensing applications. The one or more voids 9 may be are configured such that the optical property of the filler affects the Bragg wavelengths of the two Bragg gratings 31 differently. For example, the one or more voids 9 may be configured such that the optical property of the filler 15 affects the Bragg wavelength of one of the two Bragg gratings 31 and does not affect the Bragg wavelength of the other of the two Bragg gratings 31.

The two Bragg gratings 31 can be provided in a variety of different configurations. Where the optical fibre 1 comprises a core 3 and a cladding 5 surrounding the core 3, the two Bragg gratings 31 may both be provided at least partially in the core 3 and spaced apart longitudinally along the core 3.

Fig. 21 shows an optical fibre 1 with two FBGs 31 in close proximity. The optical fibre 1 of Fig. 21 can be used to measure both strain and temperature. The first Bragg grating 31-1 is an unmodified silica FBG. The second Bragg grating 31-2 is a high-sensitivity FBG where the voids 9 are configured to increase a magnitude of a change with temperature of the Bragg wavelength of the Bragg grating 31-2. Since FBG 31-1 has a low positive temperature coefficient (change in Bragg wavelength with temperature) and FBG 31-2 has a high negative change with temperature coefficient, it is possible to measure the change in the Bragg wavelength of both Bragg gratings and solve simultaneous equations to obtain independent measurements of strain and temperature. For example, matrix linear algebra may be used.

Fig. 22 shows a sampled Bragg grating in which parts of the grating are missing periodically. These can be used order to effect a wider tuning range. FBG 31 -A has gaps with a different period to FBG 31 -B. FBG 31 -A can be used in place of FBG 31-1 and FBG 31-B can be used in place of FBG 31-2 in Fig. 21.

Fig. 23A to Fig. 23C show spectra for the sampled FBGs. Fig. 23A shows FBG 31 -A and Fig. 23B shows FBG 31-B. Each FBG has a comb of spectral peaks, but FBG 31 -A has a wider spacing than FBG 31-B. Only one peak from each of FBG 31 -A and FBG 31-B overlap (the central one in this example) so this will be used as the wavelength of light emitted from a laser into the optical fibre during sensing applications. Fig. 23C shows the spectrum of FBG 31-B after it has been tuned relative to FBG 31 -A by a change in temperature or strain. Now a different peak from FBG 31-B aligns with a peak of FBG 31 -A and hence the wavelength of the emitted light has changed. However, the shift in wavelength of the emitted light is greater than the amount FBG 31-B has been tuned. Further relative shifts in FBG 31-B result in further jumps in wavelength. In order to access wavelengths in between, both FBGs can be tuned together.

Alternatively to both Bragg gratings 31 being located in the core 3, one of the two Bragg gratings may be located in the cladding 5, and the other of the two Bragg gratings located in the core 3.

The one of the two Bragg gratings located in the cladding 5 may be located in a waveguide 41 such as discussed above. The waveguide 41 is optically coupled to the core 3 via a waveguide coupler 45 such that a proportion of light guided by the core 3 is transferred into the waveguide 41. The one or more voids 9 may then comprise at least one void 9 configured such that transmission of light within the waveguide 41 is affected by the optical property of the filler 15 within the void 9. The at least one void 9 may be adjacent to the waveguide 41.

Fig. 24A and Fig. 24B show an optical fibre 1 with two FBGs in close proximity at the same point along the optical fibre length. FBG 31-1 is in the core 3 of the optical fibre 1. Within the cladding 5 of the optical fibre 1 iFs a waveguide 41. The waveguide 41 may be formed by exposure of the material of the cladding 5 to laser radiation to modify the material of the cladding 5. The first part of the waveguide 41 is in close proximity to the core 3 to form an evanescent waveguide coupler 45 such that light is coupled out from the core 3 into the waveguide 41. A Bragg grating 31-3 is then formed within this additional waveguide by creating an additional periodic modulation of the refractive index along the waveguide 41 length, for example by exposure to laser radiation as described above. Voids 9 are formed around the waveguide 41 and filled with the filler 15. This allows strain and temperature to be independently determined as illustrated in Fig. 23, but with the advantage that the two FBGs are at the same point along the length of the optical fibre 1. This can reduce confounding effects such as a difference in temperature of the optical fibre 1 along its length.

The second Bragg grating can also be provided in an alternative manner within the cladding. The one or more voids 9 may comprise a cladding void configured to provide the waveguide 41 within the cladding 5. The cladding void acting as the waveguide 41 is optically coupled to the core 3 via a waveguide coupler 45 such that a proportion of light guided by the core 3 is transferred into the cladding void. The one of the two Bragg gratings within the cladding 5 is then provided by periodic modification of the material of the optical fibre 1 adjacent to the cladding void acting as the waveguide 41.

Fig. 25A and Fig. 25B show an example of this arrangement. A high sensitivity FBG 31-4 is provided in close proximity to a silica FBG 31-1 at the same point along the length of the optical fibre 1. FBG 31-4 is formed within a waveguide 41 formed by a void 9 in the cladding 5 of the optical fibre 1 filled with filler 15. Light is coupled into the waveguide 41 by defining it such that it is in close proximity to the core 3 at one end to form an evanescent waveguide coupler 45 to couple light from the core 3 into the waveguide 41. The Bragg grating 31-4 is formed by inscribing rings of higher refractive index material in the silica around the void 9 forming the waveguide 41. Having the filler 15 in the waveguide 41 rather than surrounding the waveguide 41 in the cladding 5 enables a much stronger interaction between the filer and light, resulting in a higher change in the effective index with temperature.

An alternative way to compensate for cross-sensitivity of strain and temperature can be to isolate one of the Bragg gratings from strain. To achieve this, the one or more voids 9 may comprise an isolation void 47 configured to at least partially isolate one of the Bragg gratings 31 from strain within the optical fibre 1.

An example is shown in Fig. 26A and Fig. 26B, which show a strain-insensitive FBG 31-5 in an optical fibre 1. In this example, the isolation void 47 removes part of the cladding 5 of the optical fibre 1 to mechanically isolate the Bragg grating 31-5 from strain in the optical fibre 1. This means that any strain of the optical fibre 1 does not result in strain being imparted onto FBG 31-5. This makes the Bragg grating 31-5 to be insensitive to strain, allowing FBG 31-5 to act as a temperature sensor, independent of strain. The isolation void 47 may surround an end of the one of the Bragg gratings 31-5. The isolation void 47 may surround at least 50%, optionally at least 75%, optionally at least 90%, optionally 100% of the length of the one of the Bragg gratings 31-5. The isolation void 47 may cylindrically surround the waveguide 41, leaving some cladding material between the waveguide 41 and the isolation void 47.

The optical fibre 1 in Fig. 26A and Fig. 26B comprises a core 3 and a cladding 5 surrounding the core 3. The one of the two Bragg gratings 31-5 that is to be strain-isolated is located in the cladding 5. The other of the two Bragg gratings 31-1 is located in the core 3. The one of the two Bragg gratings 31-5 in the cladding 5 is located in a waveguide 41. The waveguide 41 is optically coupled to the core 3 via a waveguide coupler 45 such that a proportion of light guided by the core 3 is transferred into the waveguide 41.

The waveguide 41 may be formed as described above. In particular, FBG 31-5 could be made more temperature sensitive by forming the waveguide 41 using a void 9 filled with filler 15 as discussed in relation to Fig. 25A and Fig. 25B. Using a high dn/dT filler would then increase temperature sensitivity. Alternatively or additionally, voids 9 with a high dn/dT filler may be incorporated into the cladding 5 where they would affect the waveguide 41. In Fig. 26A and Fig. 26B, the conventional FBG 31-1 in the core 3 of the optical fibre can additionally be used for strain measurement by taking into account the temperature measured by 31-5. However, it is not essential to also include the second Bragg grating 31-1 as well as the strain-isolated Bragg grating 31- 5. Plural waveguides 41 with Bragg gratings 31 may be provided along the length of the optical fibre 1, each individually coupled to the core 3.

Multicore fibres are used for 3D shape sensing. They consist of a central core and a series of cores around the central core, typically equally spaced within the cross-section. Four core fibres and seven core fibres are common. For example, the optical fibre may comprise at least four cores. A refractive index of a central core of the at least four cores may be different and/or have a different temperature dependence to a refractive index of the other cores of the at least four cores. Bragg gratings may be written into each core at intervals along the fibre. By determining the strain of each Bragg grating, it is possible to determine the shape in which the fibre is bent.

Fig. 25C shows a custom fibre which is drawn with a void 9 along its central axis. Around the void 9 within the cross section are doped fibre cores 3a, for example germanium-doped cores. This fibre can be drawn by a conventional “stack and draw” technique. The optical fibre 1 may be drawn from a preform comprising a doped core and a void 9.

Following the drawing of the fibre, the void 9 may be filled with a glass having a lower melting temperature than a material of the optical fibre, as shown in Fig. 25D. For example, a length of the fibre is taken and a low melting point glass is injected into the void 9 on the central axis at an elevated temperature in a furnace. The fibre is allowed to cool such that the glass solidifies to form a filled void 9a.

A low melting point glass in this context refers to a glass having a melting point significantly lower than a melting temperature of the material of the optical fibre 1 , for example at least 200 degrees Centigrade lower, optionally at least 500 degrees Centigrade lower, optionally at least 700 degrees Centigrade lower, optionally at least 1000 degrees Centigrade lower. For example, the material of the optical fibre 1 maybe silica glass, which typically has a melting point around 1700 degrees Centigrade. The low melting point glass may be a glass having a melting point below 1000 degrees Centigrade, optionally below 800 degrees Centigrade. The low melting point glass may be a glass having a melting point in a range of 400-600 degrees Centigrade.

The low melting point glass is selected to have a refractive index which is slightly above the fibre refractive index (for example 0.005 above) but with a change in refractive index with temperature dn/dT which is different to standard silica. There is a wide range of optical glasses with low melting points and varying refractive index parameters available from Schott. For example, FK5HTi, F2, FK, SKF etc.

The central core now creates a waveguide with the silica cladding, except that the change in the effective index of the waveguide with temperature is now different to the other doped cores. In other words, the doped core provides one of the waveguide and the core, and the filled void provides the other of the waveguide and the core. This allows the fibre to be used as a shape sensor, but with the added benefit of temperature compensation. By comparing the mean wavelength shift of the outside cores relative to the straight position and the wavelength shift of the central core, the temperature can be determined. This can then be used as a correction factor for the shape sensor. It would also be possible just to use one of the outside cores in combination with the central core for simpler applications.

The optical fibre 1 may be configured to exhibit birefringence. This can be achieved in a variety of ways. For example, where the optical fibre 1 comprises a core 3 and a cladding 5 surrounding the core 3, the optical fibre 1 may comprise one or more stress-inducing regions 49 arranged around the core 3 to contribute to the birefringence.

The fibre may exhibit birefringence prior to formation of the one or more voids 9. Alternatively or additionally, stressinducing regions can be formed during manufacture of the optical fibre 1 , in which case the method further comprises a step of forming the one or more stress-inducing regions 49 in the optical fibre 1 . The one or more stress-inducing regions 49 are configured to contribute to birefringence of the optical fibre 1. The stress-inducing regions 49 may comprise laser-exposed regions, and forming the one or more stress-inducing regions 49 may comprise selectively exposing the optical fibre 1 to laser radiation.

The step of forming the one or more stress-inducing regions 49 may be carried out prior to the step of contacting S20 the optical fibre 1 with an etchant. In this case, the stress-inducing regions 49 should be separated from the exposed regions 7 such that the stress-inducing regions 49 are not etched by the etchant. Alternatively, the step of forming the one or more stressinducing regions 49 may be carried out after the step of contacting S20 the optical fibre 1 with an etchant.

Alternatively or additionally to including stress-inducing regions 49, the one or more voids 9 may comprise a plurality of voids 9 arranged around the core 3 with a symmetry such as to contribute to the birefringence. For example, the plurality of voids 9 may comprises two voids 9 arranged along a first diameter of the optical fibre 1 on opposite sides of the core 3 (i.e. exhibiting two-fold rotational symmetry around the longitudinal axis). No voids 9 may be provided along a second diameter of the optical fibre 1 perpendicular to the first diameter. Alternatively, voids 9 along different diameters of the optical fibre 1 may be filled with fillers 15 having different material properties.

Fig. 27A to Fig. 27D show examples of arrangements where the optical fibre has birefringence, such that orthogonal polarisations of light have different effective refractive indices. Fig. 27A shows an elliptical core birefringent fibre, which has liquid filled channels in one radial direction. Fig. 27B shows a similar arrangement, but with a rectangular core 3.

Fig. 27C shows a PANDA birefringent fibre, which has stress-inducing regions 49 to give a stress-induced birefringence, with voids 9 containing filler 15 in one radial direction. Here the filler 15 is introduced into the voids 9 along an access void 19 in an axis orthogonal to the stress rods. Fig. 27D shows a standard optical fibre which has been made birefringent by creating stressinducing regions 49 by laser exposure. This creates high stress in these regions, such that a stress-induced birefringence results. There are also voids 9 filled with filler 15 in the orthogonal axis. The voids 9 are arranged only along one diameter of the optical fibre 1 , and no voids 9 are present along the perpendicular diameter. This means that only one of the two orthogonal polarisations of light interacts significantly with the filler 15. This allows for strain-temperature discrimination, because the change with temperature of the refractive index is different for the two orthogonal polarisations of light.

The optical fibre 1 may comprise a Bragg grating 31. The birefringence of the optical fibre 1 may be such that the Bragg wavelength of the Bragg grating 31 differs for light having different polarisations, optionally wherein the polarisations are orthogonal.

A Bragg grating in birefringent fibre will have different Bragg wavelengths for two orthogonal polarisations. If one polarisation has a different sensitivity to an environmental variable to the other, then that variable may be determined by the wavelength separation of the Bragg reflection peaks. For example, temperature could be measured. The wavelength separation may also be an indication of the external pressure. Also, if both polarisations have the same strain sensitivity, then strain can be measured by the absolute wavelength of the peaks.

For example, in the arrangements of Fig. 27A to Fig. 27D, the stress-inducing regions 49 induce birefringence so there are two Bragg reflection peaks. One of the peaks is affected by the filler 15, for example to have a higher change in refractive index with temperature, but the other peak does not. This means temperature measurements can be derived by the difference between the two peaks, while strain measurements can be determined by the positions of the peaks, which should be similarly affected by strain.

The birefringence may be such that reflection peaks for the light having different polarisations around the corresponding Bragg wavelengths of the Bragg grating 31 are resolvable when an environmental variable, such as pressure, within the one or more voids 9 is substantially equal to the environmental variable external to the optical fibre 1, i.e. at zero pressure difference. This ensures that the two peaks are fully separated. Preferably the reflection peaks are separated by at least a full-width at half maximum of the reflection peaks at zero pressure difference. The separation of the two peaks may also be temperature dependent.

Fig. 28A and Fig. 28B show a pressure sensor which has been designed so that it is birefringent even at zero pressure difference. Fig. 28A shows a cross-section of the optical fibre 1 with two sets of laser inscribed regions. One set is the exposed regions 7 used to define the regions to be removed through contacting S20 the optical fibre 1 with an etchant. This set has access regions 17 connecting the exposed regions 7 to the external surface of the optical fibre 1. The other set of laser inscribed regions is the stress-inducing regions 49 used to impart stress on the optical fibre 1. These are not connected to access regions 17. Fig. 28B shows the optical fibre 1 after etching such that there are voids 9 and the additional stress-inducing regions 49, which have not been exposed to the etchant. For pressure-sensing applications, the filler 15 is preferably air or another inert gas such as nitrogen.

The stress-inducing regions 49 result in a stress-induced birefringence in the optical fibre 1, even for no pressure difference between the internal pressure in the voids 9 and external pressure on the optical fibre 1. The additional birefringence caused by the stress-inducing regions 49 results in the wavelength peaks of the Bragg grating 31 for two orthogonal polarisations being separated even at zero pressure difference. This makes it easier to measure low pressures, because the peaks in the spectra from the two polarisations will be distinguishable from each other. This allows side-hole optical fibre pressure sensors to operate at low pressures without requiring polarisation management. It means that a sensor can be interrogated by an incoherent or a polarisation scrambled light source and the wavelength peaks of the orthogonal polarisations will be distinguishable.

The one or more voids 9 may also be configured such that an effect of the optical property of the filler 15 on light guided by the optical fibre 1 differs for light having different polarisations, optionally wherein the polarisations are orthogonal. Fig. 29A and Fig. 29B show a modified version of the design in Fig. 28A and Fig. 28B where in addition to a bow-tie shape of the voids 9, the voids 9 comprise additional portions to increase the pressure-induced birefringence in the optical fibre 1. Fig. 29A shows the device after laser exposure and Fig. 29B shows the device after etching.

Using birefringence, a single FBG is able to give information about both strain and temperature at the same time. An advantage over having two separate FBGs is that the FBGs are physically co-located so give information from precisely the same physical location. Another advantage is that the two FBG peaks are close to each other in wavelength and track each other, so the number of sensors for a given source bandwidth is not significantly reduced. With two separate FBGs you may only be able to sense half the number of locations.

Fig. 30A and Fig. 30B show a side-hole fibre 110 which has been made birefringent by laser exposure. Fig. 30A shows a fibre which was drawn with side-holes in the fibre. Fig. 30B shows the fibre after it has been exposed to laser radiation to create stress-inducing regions 49. This allows a conventional air-hole fibre to be made birefringent, so that FBGs written within it are separated in wavelength even at zero pressure difference.

Fig. 31 A and Fig. 31 B show examples of pressure sensors created in polarisation maintaining optical fibres. Instead of fabricating voids 9 within a standard optical fibre, the voids 9 are fabricated within a polarisation-maintaining fibre. As the polarisation maintaining fibre is already birefringent, this gives the birefringence desired at zero pressure difference. Fig. 31A shows voids 9 created in a “bow-tie” polarisation maintaining fibre which has bow-tie shaped stress-inducing regions 49. Fig. 31 B shows voids created in a PANDA polarisation maintaining optical fibre which has circular stress-inducing regions 49. The voids 9 can be created by exposure to laser radiation and subsequent etching as described above.

The optical fibres 1 discussed above may be incorporated into system for use in various applications such as environmental sensing or optical filtering and control.

Fig. 32 shows a system for measuring environmental variables such as strain or temperature using an optical fibre 1 according to the present invention including a Bragg grating 31. Light from a tuneable laser 61 passes through an optical circulator 65 and into an optical fibre 1. In this system, a polarisation scrambler 63 is also present to scramble the polarisation of the laser light. This is useful when using a birefringent fibre where information is to be obtained from multiple orthogonal polarisations of light. However, it is not necessary in general and may be omitted, in particular where the system will not be using information from multiple different polarisations. Alternatively, the scrambler 63 could be replaced by polarisation control to allow the response of the optical fibre 1 to different polarisations to be measured. A polarisation controller could be used to tune the polarisation of the laser 61 to each polarisation axis for each FBG individually to measure the response of different polarisation modes.

In this example, the optical fibre 1 contains plural Bragg gratings 31a, 31b, 31c, but this is in general not essential for all applications and in some situations one or two Bragg gratings may be sufficient. Each FBG 31a, 31b, 31c has a different Bragg wavelength, with the wavelength separation large enough to accommodate changes in the Bragg wavelength associated with the variables being measured. Light reflected from each FBG passes back through the optical circulator 65 to the detector 69. A controller 67 sweeps the wavelength of the laser 61 by applying electrical signals and records the detector 69 response to measure the spectrum of the FBG array within the optical fibre 1.

An example application is a strain sensor comprising an optical fibre having a Bragg grating. The voids 9 may be configured to provide reduced temperature sensitivity of the strain sensor, as described above. The strain sensor may be configured to measure the Bragg wavelength of the Bragg grating. The controller 67 may be configured to determine a strain applied to the optical fibre 1 based on the Bragg wavelength of the Bragg grating. Where the optical fibre 1 is configured to exhibit birefringence, the controller 67 may be configured to determine the strain based on a difference between the Bragg wavelength of the Bragg grating 31 for light having different polarisations.

Another application is a system for sensing strain and/or temperature comprising an optical fibre 1 having two Bragg gratings 31. The change with temperature of the Bragg wavelength of the two Bragg gratings 31 is different. For example, one FBG may be a standard FBG in standard silica fibre, while the voids 9 are configured to affect the Bragg wavelength of the other FBG. Alternatively, both FBGs may be affected by the voids 9, but the voids 9 affecting each FBG may have different fillers 15 and/or different shapes and sizes and/or different positions relative to the Bragg grating. The system may comprise a controller configured to determine a strain applied to the optical fibre 1 and a temperature of the optical fibre 1 based on the Bragg wavelengths of the two Bragg gratings 31. As mentioned above, this could be achieved using linear algebra techniques to separate the effects of temperature and strain on the two Bragg gratings 31.

Rather than having two separate Bragg gratings, strain and temperature could also be sensed simultaneously using a birefringent fibre where the two orthogonal polarisations have different responses to temperature and/or strain. Fig. 33A to Fig. 33C show an example of operation of a system for sensing strain and temperature comprising a birefringent optical fibre 1.

Fig. 33A shows the sensor at a temperature TO and a strain SO. SO is a non-zero reference strain such that the optical fibre 1 is still under some tension and not slack. There are two reflection peaks, one for each polarisation, since each polarisation has a different effective refractive index. Fig. 33B shows the sensor at the same temperature TO when the fibre is subjected to a strain S1 >S0. Under the applied strain both reflection peaks move together to longer wavelengths. Fig. 33C shows the sensor at temperature T1 >T0, with the same strain SO applied. Here the two reflection peaks separate in wavelength, such that one shifts to longer wavelengths and the other shifts to shorter wavelengths. Hence the sensor can be used to determine strain via the absolute shift in the peaks and temperature via the separation in the reflection peaks.

A pressure sensor may also be created comprising an optical fibre 1 including a Bragg grating 31. The pressure sensor may be configured such that a pressure difference between a pressure of the filler 15 in the one or more voids 9 and an external pressure applied to the optical fibre 1 affects the Bragg wavelength of the Bragg grating. The pressure sensor may comprise a controller 67 such as shown in Fig. 32 configured to determine the external pressure applied to the optical fibre 1 based on the Bragg wavelength of the Bragg grating 31. Where the optical fibre is configured to exhibit birefringence, the pressure difference may affect the birefringence. In this case, the controller may be configured to determine the external pressure applied to the optical fibre based on a difference between the Bragg wavelength of the Bragg grating for different polarisations of light.

A system such as shown in Fig. 32 can also be used to provide the pressure sensor. Light reflected from the Bragg gratings 31a, 31b, and 31c is detected by the detector 69. The controller 67 unit sweeps the wavelength of the tuneable laser 61 and reads the signals from the detector 69, for example by sampling it with an analogue-to-digital converter. The controller 67 then records the reflection spectra of the optical fibre 1 , determines the wavelength of each peak and converts the information into pressure and temperature measurements.

Instead of using a swept tuneable laser 61 and a polarisation scrambler 63 or polarisation control, it is possible to use an incoherent source such as a light emitting diode (LED), superluminescent diode or a length of rare earth doped optical fibre which is optically pumped with a semiconductor laser or any optical amplifier to generate amplified spontaneous emission.

Fig. 34A to Fig. 34E shows the reflected spectra from an optical fibre pressure sensor, such as that shown in Fig. 7A and Fig. 7B. Fig. 34A shows the spectrum at Temperature T=T0 and with substantially no pressure difference between the voids 9 and the exterior of the fibre, AP=0. Here there is no induced birefringence and the wavelength spectra for the two orthogonal polarisations overlap. Only a single reflection peak is observed. Fig. 34B shows the spectrum at the same temperature T=T0 but with a slight increase in the pressure difference to AP=P1>0. There are now two overlapping, partially separated spectra for the two orthogonal polarisations. Fig. 34C shows the spectrum at the same temperature T=T0, but at a higher-pressure difference AP=P2>P1. Now the spectra of the two polarisations are fully separated. Fig. 34D shows the spectrum at temperature T=T0 and pressure difference AP=P3>P2. Here the spectra for the two orthogonal polarisations have been separated further still. Fig. 34E shows the spectra at a higher temperature, T=T1 >T0, but such that the pressure difference is the same as Fig. 34C with AP=P2. Here the wavelength separation is substantially the same as Fig. 34C, but the overall spectrum has shifted to longer wavelengths. Using this system, it is therefore possible to determine the pressure from the wavelength separation and the temperature from the absolute wavelength. This permits independent measurement of pressure and temperature.

Fig. 35A and Fig. 35B show spectra from pressure sensors which have additional birefringence at low pressure difference, for example through the use of stress-inducing regions 49 as described above. Fig. 35A shows a pressure sensor at zero pressure difference at T=T0 and AP=0. Here the spectrum shows two peaks for the two orthogonal polarisations which are separated and distinguishable. Fig. 35B shows the spectrum at a higher pressure AP=P1 where the wavelength separation of the two polarisations has been increased.

Other applications of the present invention include Bragg grating optical filters that are temperature stable. Bragg gratings can be used as optical filters, and compensating for the temperature dependence of their Bragg wavelength allows the filter to remain stable over a range of temperatures. This could also be applied to create a Bragg grating-stabilised laser in which at least one of the reflectors is a Bragg grating with voids 9 that at least partially compensate for the temperature dependence of the Bragg wavelength. A semiconductor stabilised laser or a fibre laser with temperature-insensitive Bragg gratings may also be created.

In addition to highly stable optical filters and lasers, the optical fibres 1 of the present invention can also be used to create highly tuneable optical filters and lasers. If the voids 9 and filler 15 are configured to increase a magnitude of a change with temperature of the Bragg wavelength, this can be used to create optical fibres 1 containing highly temperature-sensitive Bragg gratings for tuning. Using such an optical fibre can create a Bragg grating optical filter which is temperature tuned. A tuneable laser could be provided in which at least one of the reflectors is provided by an optical fibre comprising a highly temperaturesensitive Bragg grating. A tuneable fibre laser or sampled-grating tuneable fibre laser could be created using two highly temperature-sensitive Bragg gratings.

Fig. 36A shows a thermally-tuneable FBG optical filter. An optical fibre with a highly temperature sensitive Bragg grating 31 is coupled to a Peltier element 71 that provides a thermoelectric heat pump device. The Peltier element 71 is electrically controlled to provide a set temperature by a controller 67. By changing the set temperature, the optical filter wavelength is tuned. As the temperature sensitivity of the highly temperature sensitive Bragg grating 31 is significantly higher than a standard FBG, a far greater tuning range is achievable. Fig. 36B shows the spectra of the filter of Fig. 36A at temperature TO and temperature T1>T0.

Another application is an add-drop multiplexer, such as shown in Fig. 37. This is used to ‘drop’ wavelength channels on a particular optical fibre and replace them with wavelength channels from another source. The dropped channel can be routed to a different destination. A thermally-tuneable FBG 31 is used to select which wavelength to drop. In Fig. 37, the dropped wavelength is set to the third wavelength channel, but in general any channel can be dropped. The input channels enter the first optical circulator 65 and are passed onto the tuneable FBG 31. The third wavelength channel is reflected back from the FBG 31 via the first optical circulator 65 and is dropped out. The remaining channels pass through the FBG 31 onto the second circulator 66. Here they are combined with an ‘added’ channel before being sent to the required destination. The wavelength of the FBG 31 can be tuned to other wavelengths to add/drop different wavelength channels. The wavelength of the FBG 31 can be tuned to be outside the wavelength range of all the channels so that all of the channels pass through.

Fig. 38A shows an application to provide a temperature stable laser. A laser cavity is formed with a semiconductor gain medium 75 which has a high reflectivity rear facet 79 and an antireflection coated front facet 77. Light is coupled out of the front facet 77 and onto a lensed fibre 73 (provided by a tapered fibre). Within the optical fibre 1 there is a temperature insensitive FBG 31 which serves as a narrowband front reflector for the laser. The wavelength of the FBG 31 determines the wavelength of the emitted light from the laser. By having a thermally-i nsensi ti ve FBG 31 , the laser wavelength will undergo less wavelength change with changes in ambient temperature. Fig. 38B demonstrates that the emitted spectrum remains substantially unchanged with a change in temperature.

Fig. 39A shows an application to provide a tuneable laser. A laser cavity is formed with a semiconductor gain medium 75 with a high reflectivity rear facet 79 and a tuneable FBG 31 as the front reflector. The tuneable FBG 31 can be thermally tuned as discussed above. The wavelength of the emitted light is determined by the wavelength of the FBG 31. Hence tuning the FBG wavelength via the controller 67 has the effect of tuning the laser wavelength. Fig. 39B shows example spectra as the laser wavelength is tuned.

Fig. 40 shows an application to provide a tuneable fibre laser. The gain medium is a doped optical fibre 120 (for example doped with erbium or any other suitable dopant including rare earth dopants such as erbium-ytterbium). The doped optical fibre 120 is spliced at splices 85 to optical fibres 1 containing tuneable Bragg gratings 31-1 and 31-2. The doped fibre 120 is optically pumped via couplers 81 using pump lasers 83 (e.g. at 980 nm or 1480nm). There may be co- and/or counter-propagating pump light. The tuneable FBGs, 31-1 and 31-2 provide the reflectors and are tuned via the controller 67.

As discussed above, the voids 9 may include one or more cladding voids that are located in the cladding 5 of an optical fibre 1 that has a core 3 and a cladding 5 surrounding the core 3. Additionally or alternatively, the one or more voids 9 may comprise a core void within the core 3. A core void allows a much stronger interaction between light transmitted in the fibre and the filler 15, since the optical mode is predominantly localised within the core 3. This can allow much higher temperature sensitivity sensors and tuneable optical devices with a much larger tuning range. The core 3 may be aligned with a central axis of the optical fibre 1.

The core void may be completely or partially filled with the filler 15. The core void is located within the core 3, but may not completely replace the core 3. In other examples, the core 3 may consist substantially of the core void. Where both core voids and cladding voids are present, the cladding voids may be filled with a filler 15 having a material property different to the material property of the filler 15 in the core void. There may be plural core voids within the core 3. The plural core voids may be filled with the same filler 15, or with different filler 15 having different material properties.

Where the fibre has a core void, it may also comprise a Bragg grating 31. The Bragg grating 31 may be provided by periodic modification of the cladding 5, for example adjacent to and/or surrounding the core void. The Bragg grating may be formed by rings of modified material around the core 3. Alternatively or additionally, where the core void does not entirely replace the core 3, the Bragg grating 31 may be created by a periodic modification of the remaining solid parts of the core 3.

Fig. 41A to Fig. 41 D shows the fabrication of optical fibres 1 with core voids containing filler 15 in the core 3, as well cladding voids containing filler 15 in the cladding 5. Fig. 41A shows the optical fibre 1 before filling the voids 9. The voids 9 may be etched from appropriately defined exposed regions 7 as described above. Alternatively, the voids 9 may be formed by conventional techniques, where a suitably defined preform is drawn to produce an optical fibre 1 containing voids 9.

As can be seen, the voids 9 comprise a core void in the core 3 of the optical fibre 1. Fig. 41 B shows the optical fibre 1 where a filler 15-A has been filled into the cladding voids and a filler 15-B with a higher refractive index has been filled into the core void. Fig. 41 C shows a Bragg grating 31 has been inscribed in the remaining solid regions of the optical fibre 1 around the core void by modifying the material of the optical fibre to create an exposed region 7 surrounding the core void. Fig. 41 C shows a crosssection where the exposed region 7 is present, but to create a Bragg grating, the exposed region 7 would only be present periodically along the length of the optical fibre 1. Fig 141 D shows an optical fibre 1 with two different fillers 15-A and 15-C in the cladding voids in addition to the filler 15-B in the core void. The three different fillers 15-A, 15-B, and 15-Ccould be different materials with different refractive indices and/or a different change in refractive index with temperature dn/dT. The fillers 15-A and 15-B in the cladding voids could also be in different proximity to the core void or be filled by different amounts. Using more than one filler allows the thermal response of the Bragg grating 31 to be flattened over a wider range of temperatures. Rather than providing the Bragg grating 31 by periodic modification of the material around the core void, the Bragg grating 31 may be provided by periodic modification of the filler 15 in the core void, similarly as discussed above for the cladding voids. For example, the filler 15 may comprise a liquid crystal. The filler 15 may be polymerizable and the periodic modification may comprise periodic polymerisation. For example, the filler 15 may comprise monomers and a photo-initiator and the periodic polymerisation may be performed using a laser to selectively polymerise the filler 15 periodically along the length of the core void.

As well as thermal tuning of the properties of the optical fibre 1 as discussed above, it is possible to electrically tune the optical fibre containing voids 9 filled with filler 15. To achieve this, the optical fibre may comprise one or more electrodes 93 configured to apply an electric field to the filler 15 to affect the material property of the filler 15.

Fig. 42A and Fig. 42B show an electrically tuneable optical fibre 1. The filler 15 used in this case is a liquid crystal or another material with material properties which are altered by the presence of an electric field. In addition to the voids 9 in the cladding 5 containing the liquid crystal filler, there are also electrodes 93 in the cladding 5. The electrodes 93 may be formed using voids 9 created similarly as for the voids 9 that are filled with filler 15, but then filling the voids 9 with an electrically conductive material. Filling the voids 9 with the electrically conductive material could be done, for example, by flowing an electrically conductive material such as indium-tin solder into the void 9 while above its melting point (around 125°C for indium-tin solder).

An electric field generator 91 is used to apply an electric field across the liquid crystal filler 15 in order to change its material properties, for example its optical properties. The electric field generator 91 may be any suitable device such as a bipolar square-wave generator coupled to a high voltage amplifier. By changing the magnitude of the applied electric field, material properties such as the refractive index of the filler 15 is changed. This affects the effective refractive index experienced by light transmitted in the optical fibre 1. This can also be used to change the Bragg wavelength of an FBG 31 in the optical fibre 1 , such as shown in Fig. 42A and Fig. 42B. Other material properties that can be changed may include the magnitude of the birefringence, the angle of the optic axis of a birefringent material, the optical absorption, or the scattering loss. The electric field may change the effective refractive index of at least one polarisation axis of the optical fibre. The electric field may change the effective refractive index of both polarisations unequally or substantially equally.

Fig. 43A and Fig. 43B show an optical fibre 1 containing an electrically tuneable FBG 31 in which the one or more voids 9 comprise a core void filled with liquid crystal. Instead of liquid crystal, any other liquid whose material properties change in response to an applied electric field may also be used. Access voids 19 allow the core void to be filled with the liquid crystal or other suitable filler 15. Voids 9 in the cladding are filled with an electrically conductive material to form the electrodes 93 as discussed above.

Fig. 44A to Fig. 44D show various different example arrangements of the materials within the cross-section of the optical fibre 1. Fig. 44A shows rectangular voids containing liquid crystal 95 either side of an elliptical core 3, with electrodes 93 either side of the liquid crystal 95. The internal surfaces of the voids 9 containing liquid crystal 95 have alignment surfaces 97 with structuring that aids liquid crystal alignment. These alignment surfaces 97 may be created using exposure to laser radiation. Fig. 44B shows an elliptical core void filled with liquid crystal 95 with rectangular electrodes 93 either side. Fig. 44C shows a rectangular core 3 with rectangular voids 9 containing liquid crystal 95 and rectangular electrodes 93. Fig. 44D shows a rectangular core void containing liquid crystal 95 with rectangular electrodes 93. In all of these examples, it is not necessary for the voids to contain liquid crystal 95, and any other suitable filler 15 may be used instead that changes its material property in response to exposure to an electric field.

These electrically-tuneable embodiments allow low-cost tuneable optical components such as optical filters. One of the largest costs with active optical devices is not the device itself but the packaging cost associated with aligning optical fibres to the active device. By fabricating an electrically active component within the optical fibre 1 itself, the need for a fibre alignment process is removed, thereby removing cost and manufacturing complexity.

Using these embodiments, a Bragg grating optical filter can be created similar to the temperature-tuneable optical filter described above. Fig. 45A shows an electrically tuneable FBG filter. An electrically tuneable FBG 31 is coupled to a controller 67 to set a particular Bragg wavelength of the Bragg grating 31 by tuning the material property of the filler 15 within the one or more voids 9 that affect the Bragg grating 31. Fig. 45B shows the corresponding spectra at two different applied voltages.

An electrically tuned FBG can also be used instead of a thermally tuned FBG to provide an add-drop multiplexer such as shown in Fig. 40. A tuneable laser can be provided in which at least one of the reflectors is an electrically tuneable Bragg grating. A tuneable fibre laser or a sampled-grating tuneable fibre laser can be created using two electrically tuneable Bragg gratings. An electrically tuned FBG can be used instead of a thermally tuned FBG to provide a tuneable laser such as shown in Fig. 39A.

Crystal optical fibres can be useful in particular applications where normal silica optical fibres are unsuitable. This may include extreme temperature environments where the higher melting point of a crystal optical fibre is relevant. The present invention allows for crystal optical fibres to be formed comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre. This has not historically been possible using conventional methods because crystal optical fibres cannot be drawn from a preform and retain their crystalline structure. However, the present method using exposure to laser radiation and subsequent etching of the exposed region 7 allows for voids 9 to be formed even in crystal optical fibres. This enables optical fibre sensors such as those described above to be created within a crystal optical fibre, such that the sensor can be used in extreme conditions such as high temperatures above 1000°C or extreme pressures.

Any suitable crystal optical fibre may be used. The optical fibre may be a microstructured fibre. The optical fibre may be an anti-resonant or negative curvature fibre. The optical fibre may comprise a single-crystal optical fibre, optionally sapphire, diamond, or yttrium aluminium garnet (YAG) crystal. The optical fibre may be doped, optionally with a rare-earth element. The voids may be formed in the optical fibre using the method described above.

Fig. 46A to Fig. 49B demonstrate the fabrication process in a crystal fibre. The crystal fibre may be a single crystal fibre. Fig. 46A and Fig. 46B show a piece of unmodified single-crystal (hard crystal) optical fibre 100, for example sapphire fibre. Fig. 47A and Fig. 47B show the crystal optical fibre 100 of Fig. 46A and Fig. 46B which has been selectively exposed S10 to laser radiation to define exposed regions 7 to be removed by selective etching and access regions 17 to allow for etchant to reach the exposed regions 7. A Bragg grating 31 has also been inscribed in the crystal fibre 100 by periodic modification of the material of the crystal optical fibre 100 using laser radiation. However, it is not essential that the crystal optical fibre 100 comprises a Bragg grating 31.

Fig. 48A and Fig. 48B show the crystal optical fibre 100 of Fig. 47A and Fig. 47B which has been contacted S20 with etchant to etch the access regions 17 and exposed regions 7 to form access voids 19 and voids 9. In this example, the crystal optical fibre 100 was etched in 8 mol/l KOH at 85°C, but any suitable etching process may be used. In this example, access voids 19 are used, which allow long fibres to be etched without excessively long etch times. However, it is also possible to avoid using access voids 19 by etching from the ends of the crystal optical fibre 100.

As shown in Fig. 48A and Fig. 48B, the voids 9 have been defined such that the crystal optical fibre 100 forms a microstructured optical fibre. The periodic array of voids 9 forms a guiding structure, with light guided within the central region. The microstructured optical fibre may be used to form a photonic crystal waveguide or a photonic bandgap waveguide.

The one or more voids 9 may be at least partially filled with a filler 15 having a material property different to that of a material of the crystal optical fibre 100. The material property may be an optical property such as refractive index. In the example of Fig. 48A and Fig. 48B, the filler is simply air, but other fillers 15 may be used depending on the application. For example, another inert gas such as nitrogen may be used. Fig. 49A and Fig. 49B show a hollow-core microstructured crystal optical fibre 100. Here the core 3 itself is also a void 9 and light is guided within the core void.

The voids 9 may be sealed from an exterior of the optical fibre 100 as described above. This can be achieved by sealing the access void 19, after filling the voids 9 with filler 15 if used. However, in the case of single crystal optical fibres 100, it is not always necessary to seal the voids 9 from the exterior. The single crystal material is very hard and robust, and if no filler 15 is used, it is not necessary to seal the voids to contain the filler 15. The voids 9 may be sealed if there are fluids expected to be present around the optical fibre 1 that would change the material properties within the voids 9 such that the behaviour of the optical fibre 1 would be detrimentally affected. For example, gas concentration within the voids 9 may change with temperature if the voids 9 are not sealed, which may negatively affect the performance of a temperature sensor using the single crystal optical fibre 100. In addition, the voids 9 should be sealed if the crystal optical fibre 100 is to be used for a pressure sensor, because this application relies on a differential in pressure between the voids 9 and the external environment, which cannot exist if the voids 9 are not sealed from the exterior of the optical fibre.

As for the optical fibres discussed above, a longitudinal extent of the one or more voids 9 may be less than a length of the crystal optical fibre 100. The one or more voids 9 may extend within a void portion. The void portion is a continuous and integral portion of the crystal optical fibre 100. A longitudinal extent of the one or more voids 9 may be less than a longitudinal extent of the void portion.

A central axis of the one or more voids 9 may extend parallel to the longitudinal axis of the crystal optical fibre. Alternatively, the central axis of the one or more voids 9 may extend along a direction inclined to the longitudinal axis of the crystal optical fibre 100 for at least a portion of the one or more voids 9. Any of the features of the optical fibre 1 and voids 9 discussed for other embodiments above may also be applied in the voids 9 of the crystal optical fibre 100. For example, the crystal optical fibre 100 may comprise a Bragg grating 31. This may be useful particularly in sensing applications.

The crystal optical fibre 100 may be configured to exhibit birefringence. This can be achieved using the voids 9, which can result in a stress induced birefringence in the crystal optical fibre 100 when the crystal optical fibre 100 is subjected to pressure. Stress-inducing regions 49 may also be provided to modify the birefringence. The crystal optical fibre 100 may be configured such that two orthogonal polarisation modes of light are reflected at different wavelengths by the Bragg grating 31. The wavelength separation of the Bragg wavelengths for the different polarisations may be used as an indication of the external pressure as described above.

The single crystal optical fibre 100 may have a waveguide 41 within it. The waveguide 41 may be a single-mode waveguide. The single-mode waveguide may have two orthogonal polarisation modes. The waveguide 41 may be any suitable waveguide such as a depressed cladding waveguide, a microstructured waveguide, a photonic crystal waveguide or an antiresonant waveguide. The waveguide 41 may be formed by modifying the refractive index of the crystal optical fibre 100. The waveguide may be formed by modifying the material of the crystal optical fibre 100 by exposing the crystal optical fibre 100 to laser radiation. The waveguide 41 may be formed using voids 9 that may be filled with a filler 15. For example, the crystal optical fibre 100 may be selectively etched to form the waveguide 41 by selectively removing material from the crystal optical fibre 100. This potentially allows single-mode, single-crystal optical fibres with lower loss and which can withstand higher temperatures than conventional silica optical fibres.

Single-crystal fibres such as sapphire are often used for ultrahigh temperature applications. Waveguides can be formed in crystal optical fibres by exposure to laser radiation. However, at high temperatures the laser-induced modification of the material of the crystal optical fibre may be diminished or removed entirely by effectively being annealed out. By using voids 9 to form the waveguide within the crystal optical fibre, the structural change to the material of the optical fibre becomes irreversible, because the voids 9 are changed to be filled with the filler 15, and so the structure will not be changed by subjecting it to extreme temperatures. Single-crystal optical fibres can be more optically lossy than conventional silica optical fibres, but by using voids 9 to change a substantial proportion of the waveguide material to be a lower-loss filler such as air, the loss in the crystal optical fibre can be reduced.

Fig. 50 shows an application of the invention to form a crystal optical fibre pressure sensor. A waveguide is created by exposing a region to laser radiation to form an exposed region 7 with reduced refractive index. In the middle of the exposed region 7, coaxial with the longitudinal axis of the crystal optical fibre 100, an unexposed part of the crystal is unmodified and therefore has a higher refractive index than the exposed region 7. This structure effectively creates a “cladding” of the exposed region 7 around the “core” of unexposed material. Light injected into the core is guided by the waveguide formed by the core/cladding interface.

In addition to the cladding/core structure, two voids 9 are formed by the method described above. The voids are sealed in any suitable manner as described above. For example, a high-temperature resistant ceramic adhesive may be used. The voids 9 result in the crystal optical fibre 100 being distorted under external pressure, creating a stress-induced birefringence. A Bragg grating is also written into the core 3. The crystal optical fibre 100 therefore operates as a pressure sensor in which the wavelength separation of the Bragg reflection peaks of the two polarisations is indicative of the pressure. Using a crystal optical fibre allows pressures to be measured at much higher temperatures, for example over 2000°C for a sapphire fibre.

Fig. 51 shows a microstructured crystal optical fibre pressure sensor. This crystal optical fibre 100 also has a waveguide written into it. The waveguide in this case is a microstructured waveguide. The waveguide consists of an unmodified core, surrounded by a periodic array of exposed regions 7. Some of the exposed regions 7 have been etched to form voids 9. The voids 9 are sealed from the exterior of the optical fibre. A Bragg grating is provided within the core. Under pressure, there will be a stress-induced birefringence in the optical fibre created by the array of voids 9. Pressure can therefore be measured by the separation in wavelength between the Bragg reflection peaks of the two orthogonal polarisation modes.

Fig. 52A to Fig. 54B show a device fabricated within a substrate 130 such as silica, quartz, borosilicate glass or other transparent material. This can be considered as a fibre that has no core, and in general any of the embodiments discussed above in relation to optical fibres 1 can equally be applied in a substrate, e.g. a planar substrate, as illustrated in Fig. 52A to Fig. 54B. The process is similar to fabrication in conventional optical fibre, except that the waveguide has no core. Instead, a waveguide 41 serving a similar purpose to an optical fibre core is created within the substrate 130 by exposing it to laser radiation to modify its material properties, for example to raise its refractive index. The waveguide 41 that provides an effective core can also be provided in any other suitable manner.

Fig. 52A and Fig. 52B show the substrate 130 after exposing S10 the substrate to laser radiation to create a waveguide 41, inscribe a Bragg grating 31 within the waveguide 41 and define exposed regions 7. Fig. 53A and Fig. 53B show the substrate 130 after it has been contacted S20 with etchant such that the exposed regions 7 are removed to form voids 9. Fig. 54A and Fig. 54B show the substrate 130 after the voids 9 have been filled with filler 15.

Some specific examples of optical fibres manufactured according to the methods and embodiments described above will now be discussed.

As discussed above, an FBG is a periodic modulation of the refractive index in an optical fibre. A narrow band centred at the Bragg wavelength is reflected when light is guided. The Bragg wavelength is dependent on the effective refractive index of the fibre and the pitch of the periodic grating, given by: m _B = 2n_e/.y /l where is the Bragg wavelength, n_eff is the effective refractive index of the fibre, m is an integer, and A is the pitch of the periodic structure. When there is a temperature or strain variation, both n_eff and A will vary. As a result, the shift in Bragg wavelength due to a change in temperature and strain is given by: where AA_B is the shift in Bragg wavelength due to the strain e or the variation in temperature AT, Pij are the Pockel’s coefficients of the stress-optic tensor, v is Poisson’s ratio, a is the thermal expansion coefficient and is the thermal response of the fibre.

An FBG sensor device with a much different thermal response is expected in order to discriminate temperature response from strain response. Although a mainly depends on the fibre material, is dependent on the effective refractive index of the FBG section given by:

In a conventional FBG fibre device is a constant equal to the thermo-optic coefficient of the fibre material (normally 7.97 x 10"⁶ °C^-1 in fused silica). The Bragg wavelength shift due to an increase in temperature is always positive across all temperature ranges. In order to improve the thermal response, a more significant is required. Thus a material with a different thermal-optic coefficient is needed in either the fibre’s core or cladding (since the evanescent field of a fibre’s waveguide extends into the cladding region). This can be achieved using microchannels formed by voids such as those described above. These may be filled with a filler such as glycerol solution.

In fact, there are several advantages to putting micro-fluidic glycerol solution in claddings rather than in cores. At first, micro-channels in cladding enable the FBG inscription in the fibre core. Furthermore, the guiding wave in liquid solution may appear to be lossy and potentially vulnerable to bending. In addition, since the cladding occupies more region than the core there is much more flexibility in designing devices with different thermal responses. Glycerol is an optimal choice as it is non-toxic and nonflammable with a much larger thermo-optic coefficient of -225 x 10^-6 °C^-1 than that of fused silica. Furthermore, pure glycerol has a refractive index of 1.4473 at 1550 nm at room temperature and can be easily tuned to a lower refractive index by dilution with water to match the refractive index of fused silica.

The cross-section of the FBG device was designed to be a ’double bow-tie’ pattern made of four identical trapezoidal sectors with sector angles of p as shown in Fig. 55. An FBG is written within the core region and micro-channels filled with glycerol are exposed from the side of the fibre without touching the fibre core, so that the proportion of glycerol solution in the cladding can be parameterized by the sector angles. In such a device as temperature increases, the refractive index of core n_co increases at a constant rate equal to the thermo-optic coefficient of fused silica. The total variation of the cladding index n_ci is affected by the two oppositely varying components of cladding indices which are the silica cladding n_{cl sl} and the glycerol cladding n_{cl gl}. The net thermal response of lies in between the thermal-optic coefficients of fused silica and that of pure glycerol, while the average cladding refractive index n_cl lies in between n_{cl gl} and n_{cl sl} . Both of them can be tuned by varying glycerol solution concentration and/or the glycerol proportion. A different glycerol solution concentration results in a much different n_cl but impacts less on ^7 because thermo-optic coefficients for both glycerol and water are much larger in magnitude than that of fused silica. The glycerol proportion essentially affects both of the parameters. For example, increasing the glycerol proportion effectively brings to the negative side as well as making the n_cl closer to n_{cl gl}. Therefore, there are have two variables of micro-fluid (the glycerol proportion and the mass concentration of glycerol in the glycerol-water solution C_mgl to control the two parameters of the evanescent field in the cladding region (the resultant cladding refractive index n_cl and the resultant thermal response of the fibres’ cladding ^).

The cladding parameters can be used to control the desired figures of merit for an FBGsensor device, among which the thermal response is the most important. The Bragg wavelength is proportional to the FBG’s refractive index n_eff. Therefore, the link between the cladding refractive index n_ci and the thermal response should be revealed by calculating n_eff in the waveguide. Solving for n_eff in such a complex geometry analytically is challenging. However, it is at least possible to demonstrate the factors affecting the effective index and carry out numerical simulation accordingly. Since the glycerol channels are placed in between the core and the cladding, the mode width depends on the glycerol refractive index. When n_{ci ai} ~ n_co the mode width expands, therefore variation in glycerol index impacts significantly on the effective index. When n_{cl gl} « n_co, the mode is confined within the core region. The effective index does not depend much on the glycerol index in this case. This indicates that by tuning n_cl itself it is possible to modify the of the fibre and hence the thermal response. Meanwhile, is derived from n_cl, the effects on —H due to become straig htforward. dT dT ^a

Another important figure of merit of the FBG sensor device is the operating temperature. Compared with a conventional FBG sensor where the thermal response is constant across all temperature ranges, the thermal response of the present FBG device varies depending on temperature. Therefore, the operating temperature is characterized as the temperature at which the device presents a desired thermal response. Generally, the operating temperature is shifted by n_cl and the range of operating temperature is enlarged or reduced by varying A fundamental limitation of the operating temperature is that, because a glycerol-water solution is used, the temperature should not drop below the freezing point or above the boiling point of either of the liquids.

The process to design a device is important. Provided is an exemplary general method to follow. A glycerol proportion is selected first to meet the requirement of the thermal response of the devices. Generally, a lower glycerol proportion results in a more positive thermal response and a larger glycerol proportion results in a more negative thermal response. Then the concentration of glycerol solution is tuned to set the operating temperature since the concentration of glycerol solution impacts slightly on but mainly on the absolute value of n_cl.

In order to achieve optimized parameters, FIMMWAVE (Photon Design Limited) mode solver software was used. The fibre used in the simulation was SMF28e+ with a core diameter of 9.2/zm and cladding diameter of 125/zm. A convergence test was carried out to choose an optimal effective cladding diameter of 50/zm to speed up the simulation. The core and cladding index of the fibre are 1.451 and 1.445 respectively. In the simulation, it was assumed that the thermo-optic coefficient of glycerol-water solution varies as concentration according to a simple addition: where — is the thermal-optic response of the material and C_m is the mass concentration of the liquid, labelled with subscript gl for glycerol and wa for water.

The height of each trapezoidal section micro-channels is 10/zm, and the axial length of the FBG and the micro-channels was 6 mm. The pitches were slightly tuned so that all Bragg wavelengths are not far from 1550 nm. The simulated shifts in Bragg wavelength are plotted against temperature with different glycerol proportions and glycerol concentration in Fig. 56A and Fig. 56B. Fig. 56A shows shifts for glycerol proportion from 0 (purely silica cladding) to 1 (purely glycerol cladding) with glycerol solution concentration of 85%. Fig. 56B shows shifts for glycerol concentration in weight from 80% to 90% with a glycerol proportion of 1. The raw data labelled with blue stars are fitted with second-order polynomials plotted with the solid red lines.

Notice that a general trend is that, as temperature increases, the thermal response of the Bragg wavelength also increases. This is because when n_clgl is further away from n_co the impact of glycerol cladding is less significant than that of silica cladding. And eventually, at a higher temperature, the devices have a positive thermal response since the silica cladding dominates in the cladding region. In contrast at a lower temperature where the glycerol refractive index is higher, the glycerol cladding dominates and the thermal response is therefore negative. A similar trend has been noticed from the concentration of glycerol solution as shown in Fig. 56B where the thermal response is more negative with a higher concentration of glycerol and more positive with a lower concentration of glycerol.

When C_mgl reaches 1 the device has the most negative thermal response, and when C_mgl is zero the micro-fluid is just water and a positive but reduced thermal response is expected. A different glycerol proportion not only shifts the thermal response but also changes the range of the operating temperature. One extreme case is when the glycerol proportion equals 1, where the device has a cladding made of glycerol solution. In this case, the variation in thermal response due to temperature is high and can approximately be fit into a quadratic function. When the glycerol proportion is zero the device is a conventional FBG with a constant thermal response. Therefore by varying the glycerol proportion between the two extreme cases, it can be decided whether or not to let the thermal response vary significantly, and in turn, manipulate the range of operating temperatures.

Optical fibres were fabricated according to an embodiment of the method described above. A regenerative femtosecond laser system (Light Conversion Pharos SP-06-1000-PP) was used to fabricate the necessary pattern inside the fibre. For fabrication, a second harmonic generation wavelength of 515nm was used with a pulse duration of 170fs. A half-waveplate was used to control the pulse energy from 10 to 300 nJ. An SLM (Hamamatsu X10468) was used to compensate for the aberration generated in the optical path of the whole system. The sample fibre is taped onto a microscope slide and placed on a motion stage (x, y: Aerotech ABL10100L and z: ANT95-3-V) to provide different focus positions for the laser. The objective used has a magnification ratio of 20 and NA of 0.5.

Second-order FBGs have been written in a single-mode fused silica fibre (Corning SMF28e+) by scanning the laser focus along the centre of the fibre. The microchannels are written and exposed to the fibre surface with a slightly higher pulse energy. The device was fabricated without immersion in oil thus aberration correction for the fibre surface in air was required.

The device was placed in a KOH solution for etching so that the laser-modified region will be removed. Potassium hydroxide (KOH) solution was selected as an etchant because of its high selectivity of up to 300 and the relatively fast etching rate of up to 300/zm/hour. The etched micro-channels were filled with glycerol solution by capillary effects.

A high thermal sensitivity FBG device can be used for temperature and strain discrimination. Ideally, the highest sensitivity is obtained with the entire evanescent field occupied by the glycerol cladding. However, a core surrounded entirely by fluid is mechanically unstable. Therefore some connections are left in between the glycerol channels as shown in Fig. 57 A.

Each of the four channels is about 7 /vm wide and 10 /vm long displayed symmetrically leaving approximately 0.5 /vm gap between each other and close contact with the core. The FBG is 2 mm long and the channels are 3 mm long with surface exposures every 1.5 mm to achieve roughly uniform etching along the fibre length. The FBG was written inside the fibre core with a pulse energy of 0.15uJ and a rep rate of 100 Hz controlled by a pulse picker. The writing speed was 1.071 /zm so after the strain induced by the tapes was relieved the Bragg wavelength was approximately 1550 nm. The micro-channels are written with pulse energy of 0.26/zJ, repetition of 250 kHZ at the speed of 0.1 mm/s. The microchannels were exposed to the surface without going through the fibre core. The device was then placed in 8 mol/L KOH 210 solution and etched for approximately 15 hours at 75 °C heated on a hotplate in the water bath.

Although it is desirable for the glycerol concentration to be as high as possible to improve the thermal sensitivity, it is also important that the glycerol concentration is controlled to perform a high sensitivity in the desired temperature range. A relatively high glycerol concentration of 85% was chosen.

A tuneable laser (ID Photonics, CoBrite) was used to produce a broadband spectrum near the Bragg wavelengths. The measured reflection spectrum of this device at temperatures from 22 °C to 70 °C is plotted in Fig. 57B and the peaks of which were plotted in Fig. 59. The thermal response is determined by the difference in Bragg wavelengths across each temperature point. At the temperature range of 20 °C to 40 °C the average thermal response was measured to be -35 pm/°C. Between 40°C and 70°C the average thermal response was measured to be -11 °C. The Bragg wavelengths of a normal FBG was also plotted to show a thermal sensitivity of 9.98 pm/°C The intensity of the peaks depended on the coupling coefficient given by: 7?_max = tanh² KL₃ where 7?_max indicates the maximum reflection at the Bragg wavelength, K is the coupling coefficient and L is the length of Bragg grating.

The peak reflection at 24 °C has dropped to 1/4 that of that at 70 °C At a lower temperature, the glycerol refractive index is closer to the core index. The model has expanded to the glycerol region thus the coupling coefficient is lower. And as a result, we expect a high thermal response but a low peak reflection. As temperature increases, the coupling coefficient gradually decreases, thus the thermal response decreases while the peak reflection increases. The bandwidth for the measured spectrum at different temperatures is approximately uniform at 5 nm.

An FBG sensor device with a compensated thermal response was fabricated that could be potentially used for laser stabilization. The non-linear thermal response depending on temperature enables the existence of a turning point where the thermal response crossed zero. In order to keep a low thermal response near the turning point, the glycerol proportion in the evanescent field was reduced by moving the micro-channels further away from the core, as shown in Fig. 58A. The proportion of glycerol in the evanescent field is approximately 0.35 at room temperature.

The four channels are circular in shape with a diameter of approximately 7 /vm leaving a larger gap of 1 /vm away from the core to allow some over-etch. The same parameters were used for FBG writing as for the high sensitivity device. The FBG is 2 mm long and the channels are 3 mm long with surface exposures every 1.5 mm. The microchannels are fabricated with a higher pulse energy of 0.290 to overcome the aberration caused by the asymmetric fibre top surface when the focal position is further away from the centre.

The glycerol concentration was chosen to be 85% so that the device is working at room temperature. The measured reflection spectrum at temperatures from 10 °C to 70 °C is plotted in Fig. 58B and showed an approximate bandwidth of 0.5 nm depending lightly on temperature conditions. The relative intensity at 24°C is approximately 1/3 of that at 70°C. This has been slightly improved compared with the high-sensitivity device since the modes are more confined inside the core. The Bragg wavelengths are plotted in Fig. 59 which shows a compensated thermal response of 0.97 pm/°C on average between a room temperature range of 14 °C to 50 °C.

Fig. 59 is a plot of measured reflected Bragg wavelength against temperature for the high negative thermal response FBG device shown in Fig. 57 A, the compensated thermal response FBG device shown in Fig. 58A, and a normal FBG.

Fig. 60 shows a flowchart for a method of forming an optical fibre with voids, such as the optical fibre 1001 described with reference to Fig. 61A to Fig. 63. The method provides an alternative way to form optical fibres with voids (such as microchannels) compared with that described with reference to Fig. 2, and enables customised portions of optical fibre containing voids to be spliced to conventional optical fibres in a way that advantageously reduces losses in light guided through the optical fibre, whilst allowing benefits associated with voids described herein to be applied to the optical fibre.

In order to form an optical fibre with voids to provide the effects described herein, the method shown at Fig. 60 can be used. The method involves splicing a customised portion of optical fibre to other optical fibre portions, which optionally are conventional optical fibres, such as standard single mode optical fibres with properties such as those set out in Table 1 below. Standard single mode optical fibres typically comprise a core extending along a longitudinal axis of the standard single mode optical fibre, surrounded by a cladding material. Typically, the core of a standard single mode optical fibre has circular crosssection along the longitudinal length of the standard single mode optical fibre with a core diameter of 8.2 micrometres. In further examples, the core diameter of a standard single mode optical fibre is greater than 5.5 micrometres and less than 11 micrometres. In further examples, the core diameter of a standard single mode optical fibre is between 6 and 10.5 micrometres. In further examples, the core diameter of a standard single mode optical fibre is between 8 and 9 micrometres. Typically, cladding of a standard single mode optical fibre comprises a substantially solid material, in contrast to the customised void portions described herein. In an example, the core of a standard single mode optical fibre comprises a material with a refractive index that is between 0.002 and 0.007 greater than the refractive index of the cladding material of the standard single mode optical fibre. In a further example, the core of a standard single mode optical fibre comprises material with a refractive index that is approximately 0.005 greater than the refractive index of the cladding material of the standard single mode optical fibre. In an example, the refractive index of the cladding of a standard single mode optical fibre under typical conditions is approximately 1.441. Optionally, the standard single mode optical fibres are configured to operate with wavelengths of electromagnetic radiation within the C-band (1530 nm to 1565 nm), L-band (1565 nm to 1625 nm) and/or S-band (1460 nm to 1530 nm). Advantageously, such standard single mode optical fibres can be used to at least partially seals the one or more voids of the void portion, as described herein.

An example of an optical fibre formed in accordance with the process set out at Fig. 60 is shown at Fig. 61A and Fig. 61 B. Fig. 61A and Fig. 61 B show a mode-matched optical fibre 1001 in profile and cross-section, respectively. There is shown a customised, void, portion of optical fibre 1004 in between two conventional portions 1002, 1006 of optical fibre. Optionally, the two conventional portions 1002, 1006 of optical fibre are standard single mode optical fibres.

The portions 1002, 1006 of conventional optical fibre that are spliced to the void portion 1004 of the optical fibre 1001 may have initially had a coating (not shown), for example of a polymer material such as 250 pm diameter polyacrylate or 150 pm diameter polyimide. The coating may be removed at step S105 of Fig. 60 prior to splicing the portions 1002, 1004, 1006 of the optical fibre 1001. The coating may be removed by any suitable method, such as chemical removal or ablation. The coating may be removed along substantially the entire longitudinal extent of the portions 1002, 1006, of conventional optical fibre, or even from the entire length of the conventional optical fibre(s).

At step S110, a custom portion of optical fibre containing one or more voids is formed. Optical fibres can be provided with voids, such as the voids 1009 in the void portion 1004 of the optical fibre 1001 shown at Fig. 61 A and Fig. 61 B, by drawing a custom fibre that has holes longitudinally along its length. The custom fibre may be provided with a core 1003. The core 1003 may have a circular cross-section along the length of the custom fibre. In an example, the core 1003 has the same shape and maximum lateral dimension as the core of the fibre(s) to which it is adjoined at the interface between the fibres. For example, where the core has a circular cross section extending along the longitudinal axis of the fibre, it may be provided with the same diameter and same core refractive index as the fibres to which it is subsequently adjoined at the interface, in order to provide mode-matching between the custom fibre and the conventional fibres. In further examples, mode-matching is achieved by selection of a combination of core diameter and core refractive index in order to provide mode-matching such that the mode field diameter of the custom portion of optical fibre matches the mode field diameter of the conventional optical fibre at the interface between the custom portion of optical fibre and the conventional optical fibre.

Around the core 1003 is undoped cladding 1005. The custom portion of the optical fibre is a void portion 1004 comprising one or more voids 1009. The one or more voids 1009 extend longitudinally along the length of the optical fibre 1001 within the void portion 1004. As discussed above, the optical fibre 1001 may comprise a core 1003 and a cladding 1005 surrounding the core 1003. In this case, the one or more voids 1009 may comprise at least one cladding void within the cladding 1005.

In an analogous fashion to the optical fibre 1 described with reference to Fig. 12A to Fig. 12F, the optical fibre 1001 can be provided with voids 1009 in different arrangements, in order to provide different effects. For example, Fig. 12A to Fig. 12F show cross-sectional views of different optical fibres 1 with voids 9 containing a filler 15 and equivalent cross-sectional voids 1009 can be formed in the void portion 1004 of the optical fibre 1001 shown at Fig. 61A and Fig. 61 B.

Following the formation of a custom fibre with voids 1009, the voids 1009 are at least partially filled with a filler 1015, which may be a polymerizable liquid, at step S115. The polymerizable liquid is subsequently optionally cured within the custom optical fibre. The filler 1015 has a material property different to that of a material of the optical fibre 1001 (for example, different to the material of the core 1003 and cladding 1005).

The voids 1009 may be filled using any suitable technique depending on the nature of the filler 1015. For example, where the filler 1015 is a liquid, the voids 1009 can be filled via capillary action by dropping the filler 1015 onto one end of the void 1009 whilst leaving the other end clear. Alternatively, the voids 1009 could be pressure filled. Filling the one or more voids 1009 also includes the possibility of simply allowing an ambient fluid, such as air, to enter the voids 1009. For example, the void portion 1004 may be, after a possible flushing step, left in, for example, an ambient environment or a chamber filled with an inert gas such as nitrogen before the voids 1009 are sealed. This would allow the surrounding fluid to fill the voids 1009.

The one or more voids 1009 are configured such that the optical property of the filler 1015 affects light transmitted through the optical fibre 1001. As mentioned above, the filler 1015 has a material property different to the material property of a material of the optical fibre 1001. This allows the filler 1015 to be used to tune the optical properties of the optical fibre 1001 in the void portion 1004. Importantly, the filler 1015 may also have a change in the material property with an environmental variable (such as temperature, pressure, or strain) that differs from a change in the material property with the environmental variable of the material of the optical fibre 1001. This allows the change in the optical properties of the optical fibre 1001 with the environmental variable to also be controlled by appropriate choice of filler 1015.

The material property of the optical fibre 1001 and the filler 1015 may be an optical property, optionally refractive index. The filler 1015 may have a refractive index that differs from the refractive index of the material of the optical fibre 1001. Nonetheless, the refractive index of the filler 1015 will preferably not differ too much from the refractive index of the material of the optical fibre 1001. The refractive index of the filler may be within 0.1 of a refractive index of the material of the optical fibre, optionally within 0.05, optionally within 0.01, optionally within 0.005, optionally within 0.001. Where the optical fibre 1001 comprises a core 1003 and a cladding 1005 surrounding the core 1003, the refractive index of the filler 1015 may be approximately equal to the refractive index of a material of the cladding 1005 at a reference temperature. Matching the refractive index of the filler 1015 approximately can reduce reflections and losses in the optical fibre 1001, but is also important in order not to disrupt the waveguiding of the optical fibre 1001 and the mode properties of the optical fibre 1001. Ideally, the optical fibre 1001 forms a "weakly guiding" waveguide so that the optical mode extends into the cladding 1005. This requires a relatively small refractive index difference between the core 1003 and cladding 1005. If the refractive index of the cladding 1005 (which is affected by the refractive index of the filler 1015) is larger than that of the core 1003, then there is no waveguiding. If the refractive index of the cladding 1005 is too large, then the optical fibre 1001 will become multimoded, where more than one transverse spatial modes exists.

Optionally, the refractive index of the filler 1015 is equal to, or greater than, the refractive index of the cladding 1005. Beneficially, such a relationship between refractive indices can be used to control the Bragg wavelength dependence on temperature for a Bragg grating associated with a void portion 1004, as described herein. The filler 1015 may have a change in refractive index with temperature, dn/dT, which is different to that of the material of the optical fibre 1001. The change in the refractive index of the filler 1015 with temperature may be negative.

Once the voids 1009 have been filled with filler 1015, the custom filled fibre and conventional optical fibre can be cleaved to provide suitable end faces for splicing the custom and conventional fibres together at step S120. The fibres can be cleaved using a diamond cutter, laser, or any suitable method to provide end faces that are cross-sectional faces substantially perpendicular to the longitudinal axis of the fibre. The void portion 1004 is then subsequently spliced to a conventional optical fibre at step S125, such that a first portion 1002 of conventional optical fibre adjoins the void portion 1004 at a splice at an interface 1008 between an end face of the first portion 1002 and an end face of the void portion 1004. The first portion 1002 of conventional optical fibre is spliced with the void portion 1004 of the custom optical fibre using any appropriate method, such as by electric arc or laser, to fuse the portions 1002, 1004 together.

Once the first portion 1002 of conventional optical fibre has been adjoined to the void portion 1004 of the custom optical fibre, the custom optical fibre is cleaved again at step S120 in order to provide an appropriate length of void portion 1004. In an example, the void portion 1004 is cleaved such that the void portion 1004 is less than 20 mm in length, optionally less than 10 mm in length and optionally between 5 and 10 mm in length, along its longitudinal axis. In further examples, the void portion 1004 is provided with any length that is appropriate for its application. Typically, the longitudinal length of the void portion 1004 is much shorter than the longitudinal length of the conventional optical fibre portions to which the void portion 1004 is spliced. The void portion 1004 of the custom optical fibre can be cleaved using a diamond cutter, laser, or any suitable method.

Once the adjoined void portion 1004 has been cleaved at step S120, the other end face of the void portion 1004 to that adjoined with the first portion 1002 of conventional optical fibre is adjoined to a further portion 1006 of conventional optical fibre at a splice interface 1010. The further portion 1006 of conventional optical fibre is spliced with the void portion 1004 of the custom optical fibre using any appropriate method, such as by electric arc or laser, to fuse the portions 1004, 1006 together.

Following the introduction of a custom void portion 1004 into a conventional optical fibre, a Bragg grating 1031 may be written into the void portion 1004 at step S130, optionally at least partially within the core 1003. Optionally, the Bragg grating 1031 is formed using a femtosecond laser system, as described with reference to the Bragg grating 31 of Fig. 14 to Fig. 17, for example. Beneficially, the use of a femtosecond laser system enables to the provision of a Bragg grating within an optical fibre having the same core diameter and dopant levels as used in conventional standard single-mode fibres.

The optical fibre 1001 is then optionally coated at step S135 in any appropriate manner, including as described herein with reference to the coating 23 of the optical fibre 1 shown at Fig. 1A.

Whilst the process described with reference to Fig. 60 shows a number of steps in a particular order, in further examples there are additional, fewer, and/or alternative steps that are implemented in order to provide an optical fibre with voids 1009 having the functionality described herein. For example, the voids 1009 of the void portion 1004 are optionally at least partially filled with filler material 1015 following splicing of the void portion 1004 to a first portion 1002 of a conventional optical fibre and prior to splicing the other end of the void portion 1004 to a further portion 1006 of conventional optical fibre. Beneficially, the filler material 1005 is sealed in the voids 1009 of the void portion by the portions 1002, 1006 of conventional fibre. Further, the method may not comprise the drawing of the fibre, but a pre-drawn fibre may be provided on which the other steps of the method are performed.

Similar to the method discussed in connection with Fig. 60, there may be provided a method of manufacturing a device comprising an optical fibre. The method is particularly suited for mass-producing devices.

The method comprises providing an optical fibre comprising one or more voids extending longitudinally along the length of the optical fibre. The optical fibre may be a custom-drawn optical fibre. The optical fibre comprises the one or more voids along its length. The method comprises filling the one or more voids with a liquid filler. The liquid filler is then solidified. Where the liquid filler comprises a polymer which can be solidified by exposure to ultraviolet (UV) light or by heat curing a polymer, the solidifying may be performed by curing the polymer by UV irradiation or heating. Alternatively, the filler may have a melting point lower than a melting point of a material of the optical fibre, for example the filler may comprise a low melting point glass as described above. The step of filling the one or more voids with the filler may then be performed at a temperature above the melting point of the filler and below the melting point of the material of the optical fibre.

The method further comprises splicing the optical fibre to a transmission fibre. A length of transmission fibre (which may be a solid step index fibre) is cleaved. The optical fibre having the filled voids is cleaved and spliced to the transmission fibre.

The splicing may be performed by laser splicing, for example using a focussed laser to selectively melt the material (e.g. glass) of the optical fibre and/or transmission fibre. Thereby, the laser splicing selectively targets portions of the optical fibre outside of the one or more voids, avoiding heating the filler to temperatures where it may be damaged or carbonise.

The filled optical fibre is then cleaved such that the length attached to the transmission fibre is a few millimetres long (e.g. 3, 5, 10 mm). Another length of transmission fibre is then spliced to the cleaved end.

A Bragg grating can then by written within the core of the filled fibre, for example using a femtosecond laser. The stripped section of the filled fibre and surrounding transmission fibre can then be recoated (for example with polyimide or polyacrylate). This process may be repeated multiple times to form an array of fibre Bragg grating devices along a length of fibre.

The method may further comprise providing a bridging fibre between the optical fibre and the transmission fibre. The bridging fibre may comprise one or more voids filled with a liquid having a melting point below a melting point of a material of the transmission fibre.

An alternative method of manufacturing a device comprising an optical fibre is to provide an optical fibre comprising one or more voids extending longitudinally along the length of the optical fibre, for example by drawing a custom optical fibre with one or more voids along its length in close proximity to the core.

The optical fibre is spliced to a solid transmission fibre at one or both ends. The optical fibre is then cleaved such that it is a few millimetres long and then spliced to another length of transmission fibre.

The method then comprises forming a hole in a side of the optical fibre. The holes, or access channels, are made through the side of the fibre to allow access to the voids. The hole provides access from outside the optical fibre to at least a first void of the one or more voids. The access channels may go through to one void and then further channels may be formed to connect up all of the voids. Alternatively, there may be an access channel formed for each void.

Forming the hole may comprise using a laser. For example, the access channels may be made by laser ablation or alternatively by defining an exposed region with a femtosecond laser and contacting this area with an etchant, similarly as described for other manufacturing methods above. In the latter case, forming the hole comprises: selectively exposing the optical fibre to laser radiation to define an exposed region within the optical fibre; and contacting the optical fibre with an etchant, wherein the etchant etches the exposed region at a higher rate than regions of the optical fibre not exposed to laser radiation, such that the hole is formed by etching of the exposed region.

The voids may then be filled through the access channels, for example with a polymerisable liquid. Preferably the whole void including the access channel is filled. Excess liquid is removed from the fibre surface.

The method further comprises sealing the hole. Sealing the hole may comprise solidifying the filler. For example, the fibre may be placed in a UV chamber to cure the polymer. This method has the effect of sealing the access channels at the same time as solidifying the polymer. It is otherwise difficult to seal the channel with a liquid inside because if an adhesive is used it tends to mix with the liquid filler and melting the fibre. The stripped region of the fibre can then be recoated with polyimide or polyacrylate. As a result of the process described with reference to Fig. 60, there may be provided an optical fibre 1001 as shown at Fig. 61A and Fig. 61 B. The void portion 1004 is positioned longitudinally along the length of the optical fibre 1001 between two further portions 1002, 1006 of conventional optical fibre 1001. In an example, the two further portions 1002, 1006 of conventional optical fibre are sealing portions 1002, 1006 which have the effect of sealing open ends of the voids 1009 of the void portion 1004.

The conventional fibre used to provide the first portion 1002 of the optical fibre 1001 has a core 1003’ surrounded by a cladding 1005’ and is spliced to the void portion 1004, which is a second portion of the optical fibre 1001, at an interface 1008 between an end face of the first portion 1002 of the optical fibre 1001 and an end face of the second, void, portion 1004, of the optical fibre 1001. A third portion 1006 comprises a core 1003’ surrounded by a cladding 1005. The third portion 1006 of the optical fibre 1001 is spliced to the second, void, portion 1004 of the optical fibre at an interface 1010 between a second end face of the second, void, portion 1004 of the optical fibre and an end face of the third portion 1006 of the optical fibre 1001. Open ends of the voids 1009 at either end of the void portion 1004 are sealed at the interfaces 1008, 1010 between the end faces of the void portion 1004 and end faces of the two further portions 1002, 1006 of the optical fibre 1001.

The optical fibre 1001 may comprise silica. This optical fibre 1001 may have an outer diameter of between 25 pm and 300 pm, optionally between 100 pm and 300 pm, optionally approximately 125 pm or 250 pm. The optical fibre 1001 is typically made of silica and 125 pm in diameter. Other common diameters include 50 pm, 80 pm, 250 pm, and 425 pm. Larger diameters may be preferred for pressure sensing applications because larger diameter fibres are more sensitive to changes in pressure. The optical fibre 1001 comprises a core 1003 and a cladding 1005 surrounding the core 1003.

The optical fibre 1001 may be a single mode optical fibre. However, there are a multitude of other fibre types possible. For example, the optical fibre 1003 may be an anti-resonant or negative curvature fibre.

The optical fibre 1003 may comprise crystal material, optionally single-crystal material. In such cases the optical fibre 1003 may comprise some crystal (or single-crystal) material portions and may also comprise portions of other materials such as silica. For example, the optical fibre 1001 may be a crystal-derived fibre, optionally a sapphire derived fibre. Examples of crystal- derived fibres are described in Dragic, P., Hawkins, T., Foy, P. et al. “Sapphire-derived all-glass optical fibres”, Nature Photon 6, 627-633 (2012).

Further, the optical fibre 1001 may be a crystal optical fibre, optionally a single-crystal optical fibre. In this case, the material of the optical fibre 1001 is entirely or substantially entirely composed of the crystal material. For example, the optical fibre 1001 may comprise sapphire, diamond, or yttrium aluminium garnet (YAG) crystal. The crystal material in the optical fibre 1001 , whether this provides all or part or the optical fibre 1001, may be doped, optionally with a rare-earth element. Other fibre types include fibres having a pure silica core, photonic crystal, polymer, hydrogel etc.

The optical fibre 1001 is arranged such that the void portion 1004 is substantially mode-matched to the other portions 1002, 1006 to which it is adjoined. In order to mode- match the void portion 1004 with one or both of the other portions 1002, 1006, the void portion 1004 is configured to match one or more properties of the other portions 1002, 1006 to which it is adjoined. For example, the void portion 1004 and the other portions 1002, 1006 may have a similar core size, similar core refractive index and/or similar mode field diameter.

For example, the cross-sectional shape and area of the core 1003 of the void portion 1004 is matched with the cross- sectional shape and area of the core 1003’ of the first portion 1002 of conventional optical fibre at the interface 1008 between the end faces of the respective portions of the optical fibre 1001. Optionally, matching the cross-section shape and area of the core 1003’ of the first portion 1002 of conventional optical fibre at the interface 1008 between the end faces of the respective portions of the optical fibre 1001 comprises matching a maximum lateral dimension of the core 1003 of the void portion 1004 such that it is substantially the same as a maximum lateral dimension of the core 1003’ of the first portion 1002 at the interface 1008, substantially perpendicular to the longitudinal axis of the optical fibre 1001. Where the cores 1003, 1003’ of the fibres are circular in cross section, the diameter of the cores may be matched. In further examples, the cross-sectional shapes of the cores are matched such that they are the same at the interface between the first portion 1002 and the void portion 1004.

Similarly, the cross-sectional shape and area of the core 1003 of the void portion 1004 is matched with the cross-sectional shape and area of the core 1003’ of the further portion 1006 of conventional optical fibre at the interface 1010 between the end faces of the respective portions of the optical fibre 1001. Optionally, matching the cross-section shape and area of the core 1003’ of the further portion 1006 of conventional optical fibre at the interface 1010 between the end faces of the respective portions of the optical fibre 1001 comprises matching a maximum lateral dimension of the core 1003 of the void portion 1004 such that it is substantially the same as a maximum lateral dimension of the core 1003’ of the further portion 1006 at the interface 1010, substantially perpendicular to the longitudinal axis of the optical fibre 1001. Where the cores 1003, 1003’ of the fibres are circular in cross section, the diameter of the cores may be matched. In further examples, the cross-sectional shapes of the cores are matched such that they are the same at the interface between the further portion 1006 and the void portion 1004.

In an example, the core 1003 of the void portion 1004 has a substantially circular cross-sectional profile perpendicular to the longitudinal axis of the optical fibre 1001 with a diameter of approximately 8.2 micrometres to match that of the conventional optical fibre at the interfaces 1008, 1100. In further examples, the core 1003 of the void portion 1004 has a substantially circular cross-sectional profile perpendicular to the longitudinal axis of the optical fibre 1001 with a diameter of greater than 5.5 micrometres and less than 11 micrometres to match that of the conventional optical fibre at the interfaces 1008, 1100. In further examples, the core 1003 of the void portion 1004 has a substantially circular cross-sectional profile perpendicular to the longitudinal axis of the optical fibre 1001 with a diameter of between 6 and 10.5 micrometres to match that of the conventional optical fibre at the interfaces 1008, 1100. In further examples, the core 1003 of the void portion 1004 has a substantially circular cross-sectional profile perpendicular to the longitudinal axis of the optical fibre 1001 with a diameter of between 7 and 10 micrometres to match that of the conventional optical fibre at the interfaces 1008, 1100. In further examples, the core 1003 of the void portion 1004 has a substantially circular cross-sectional profile perpendicular to the longitudinal axis of the optical fibre 1001 with a diameter of between 8 and 9 micrometres to match that of the conventional optical fibre at the interfaces 1008, 1100. Beneficially, matching the core size of the void portion 1004 with conventional optical fibres having core sizes with such dimensions provides improved single mode transmission with reduced losses, whilst benefitting from enhanced control through the use of voids.

In a further example, in order to provide a mode-matched optical fibre, the core refractive index of the core 1003 of the void portion 1004 of the optical fibre 1001 is matched to the core 1003’ refractive index of the first portion 1002 of conventional optical fibre of the optical fibre 1001 and optionally to the core refractive index of the core 1003’ of the further portion 1006 of conventional optical fibre of the optical fibre 1001. In an example, the core 1003’ of the first portion 1002 and/or the further portion 1006 comprises a material with a refractive index that is between 0.002 and 0.007 greater than the refractive index of the cladding material surrounding the respective cores 1003’ of the first portion 1002 and/or further portion 1006. In a further example, the core 1003’ of the first portion 1002 and/or further portion 1006 comprises material with a refractive index that is approximately 0.005 greater than the refractive index of the cladding material surrounding the respective cores 1003’ of the first portion 1002 and/or further portion 1006.

Mode-matching of the first portion 1002, void portion 1004 and/or further portion may be achieved by altering the core dimensions and/or refractive indices of the portion. Advantageously, the physical properties of the portions 1002, 1004, 1006 of the optical fibre 1001 are controlled in order to minimise losses associated with light passing through the interfaces 1008, 10101 between portions 1002, 1004, 1006 of the optical fibre 1001.

In a further example, the void portion 1004 is mode-matched to a portion 1002, 1006 of conventional optical fibre such that the mode field diameter of the void portion 1004 is substantially the same as the mode field diameter of the portion 1002, 1006 of conventional optical to which it is adjoined at the interface 1008, 1100 with the respective portion 1002, 1004 of conventional optical fibre.

The mode field diameter is defined at the position where intensity falls to 1/e² of the peak intensity. An approximation for the mode field diameter (MFD) is given by: where a is the core radius and V is the normalised frequency, given by: where a is the core radius, the wavelength, and m and n? are the core and cladding refractive indices respectively. Therefore, the mode field diameter of the void portion 1004 can be altered for a given wavelength of light by changing the refractive indices of the core 1003 and cladding 1005 of the void portion 1004 and/or the physical diameter of the substantially cylindrical core 1003.

Examples of conventional optical fibres to which the void portion 1004 may be mode-matched are shown in Table 1 below. The table provides a comparison of physical core diameter for commercial step-index single-mode fibres and the corresponding mode field diameter at a wavelength of 1550 nm and a wavelength of 1300 nm.

Table 1 Range of commercial step-index single-mode fibres. SMF-28 is often referred to as “standard single mode fibre”

In an example, the mode field diameter of the first portion 1002 of conventional optical fibre is substantially the same as the mode field diameter of the void portion 1004 of the optical fibre 1001. Optionally, the mode field diameter is between 9 and 12 micrometres, optionally between 9.6 and 11.2 micrometres, optionally approximately 10.5 micrometres. Optionally, the mode field diameter of the first portion 1002 of conventional optical fibre is substantially the same as the mode field diameter of the void portion 1004 of the optical fibre 1001 at a given temperature and wavelength. For example, under standard operating temperatures, the mode field diameter of the first portion 1002 of conventional optical fibre is substantially the same as the mode field diameter of the void portion 1004 of the optical fibre for wavelengths within the C-band, L-band and/or S-band (where C-band communication has a range of wavelengths of 1530 nm to 1565 nm, L-band communication has a range of wavelengths of 1565 nm to 1625 nm and S-band communication has a range of wavelengths of 1460 to 1530 nm).

The optical fibre 1001 optionally comprises a Bragg grating 1031 at least partially within the core 1003. Optionally, the optical fibre 1001 is a single mode optical fibre. Optionally, the Bragg grating 1031 is formed using a femtosecond laser system, as described with reference to the Bragg grating 31 of Fig. 14 to Fig. 17, for example. Beneficially, the use of a femtosecond laser system enables to the provision of a Bragg grating within an optical fibre having the same core diameter and dopant levels as used in conventional single-mode fibres.

In an example, the optical fibre 1001 is a single-mode silica fibre having a core 1003 which is typically approximately 9 pm in diameter and doped with germanium to have a higher refractive index (e.g. around 10³ higher). In a single-mode fibre, only a single transverse mode is able to propagate. The mode is predominantly within the core 1003, but it has an evanescent field which extends into the cladding 1005. It can be characterised by an effective refractive index, n_eft, which has a value at a particular wavelength determined by the core diameter as well as the core and cladding refractive indices at that wavelength.

Optionally, the refractive index of the filler 1015 has a change with temperature, dn/dT that is different to the change of refractive index with temperature of the surrounding material.

Whilst the optical fibre 1001 of Fig. 61A and Fig. 61 B is shown splice a customised void portion 1004 with conventional optical fibre portions 1002, 1006, in further examples, the void portion 1004 is spliced to other portions 1002, 1006 of optical fibres using one or more intermediate components. Fig. 62 shows an optical fibre 1001 ’ with bridging portions used to connect a void portion 1004 with lead out/in fibres with a significantly different core diameter, in contrast to the optical fibre 1001 described with reference to Fig. 61 A and Fig. 61 B.

There is shown a first portion 1002’ of the optical fibre 1001’ that is a lead out portion having a conventional optical fibre 1012 portion and a bridging portion 1022. There is also shown a further portion 1006’ of the optical fibre 1001 ’ that is a lead in portion having a conventional optical fibre 1016 portion and a further bridging portion 1026. The bridging portions 1022, 1026 are configured such that the cross-sectional area of the core 1003’ within the bridging portions 1022, 1026 varies across the longitudinal length of the bridging portions 1022, 1026. The cross-sectional area of the core 1003’ varies in order to bridge the difference in core diameter of the core 1003 of the void portion 1004 and the core diameter of the core 1003’ of the conventional optical fibre portions 1012, 1016 such that the maximum lateral dimension of the core 1003 of the void portion 1004 is substantially the same as the maximum lateral dimension of the core 1003’ of the bridging portion 1022, 1026 at the respective interface 1008, 1010 between the void portion 1004 and the respective bridging portion 1022, 1026. The bridging portions 1022, 1026 are configured to progressively change the mode field diameter for a given wavelength and temperature along the longitudinal axis of the optical fibre 1001 ’ between the void portion 1004 and the respective conventional optical fibre portions 1012, 1016.

Advantageously, the bridging portions 1022, 1026 not only seal the voids 1009 of the void portion 1004 a manner analogous to that described with reference to Fig. 61 A and Fig. 61 B, but they effectively act as adiabatic mode converters to convert from a first mode field diameter of the void portion 1004 of the optical fibre 1001’ to the second mode field diameter and/or third mode field diameter of the conventional optical fibre portions 1012, 1016, which may be significantly different. Advantageously, the benefits of a customised void portion 1004 are implemented in combination with an optical fibre having a very different mode field diameter. Whilst the configuration of Fig. 62 shows distinct portions, in further examples, the bridging portions 1022, 1026 are continuous and integral portions forming part of the respective conventional optical fibre portions 1012, 1016.

In a further example, Fig. 63 shows an optical fibre with a lens portion. The optical fibre 1001” has a void portion 1004 as described with respect to Fig. 61A, Fig. 61 B and Fig. 62. However, in contrast to the first portion 1002 of a conventional optical fibre, as shown with respect to the optical fibre 1001 of Fig. 61A and Fig. 61 B, there is shown a first portion 1002” of optical fibre having a lens portion 1032 and a bridging portion 1042. In further examples, the lens portion 1032 and bridging portion 1042 are formed as an integral, continuous entity. The lens portion 1032 is tapered thereby to convert the mode field diameter of the optical fibre 1001” to be compatible with a semiconductor laser. Accordingly, the lens portion 1032 has a cross-sectional area that varies along the longitudinal axis of the lens portion and is configured to progressively change mode field diameter for a given wavelength and temperature along the length of its longitudinal axis shared with the optical fibre 1001”. The optical fibre 1001” of Fig. 63 can be used in a grating stabilised laser, analogous to that described with reference to Fig. 38A, where the laser has high reflectivity rear facet and the Bragg grating 1031 acts as the other reflector. Beneficially, the lens portion 1032 converts the mode profile of the laser output to match that of the optical fibre 1001” to minimise the coupling loss.

Fig. 64 shows the change in wavelength as a function of temperature change for two optical fibres with Bragg gratings. As described with reference to Fig. 21, for example, an optical fibre 1 may comprise two separate Bragg gratings 31, wherein the one or more voids 9 are configured such that the optical property of the filler affects the Bragg wavelengths of the two Bragg gratings 31 differently. Similarly, the optical fibre 1001 described with reference to Fig. 61A and Fig. 60B may comprise two analogous, separate, Bragg gratings 1031, which can be provided in a variety of different configurations. Advantageously, the provision of two Bragg gratings 31, 1031 with particular properties enables enhanced determination of both strain and temperatures. The combination of a fibre Bragg grating 31, 1031 which has a Bragg reflection wavelength which has a negative gradient with temperature (such that the Bragg wavelength decreases with increasing temperature) and a Bragg grating which has a different temperature gradient (optionally a positive temperature gradient, or substantially zero temperature gradient) means that the two Bragg reflection wavelengths move in opposite directions with increasing temperature, with the separation being indicative of the temperature. This, therefore, enables enhanced discrimination between strain and temperature.

Whilst the properties of two Bragg gratings are described with reference to the optical fibres 1, 1001 of Fig. 1 to Fig 63, the advantageous properties provided by the combination of two Bragg gratings each with opposite signs of temperature coefficient are more widely applicable. For example, a device comprising a Bragg grating which has a Bragg reflection wavelength which has a negative gradient with temperature (such that the Bragg wavelength decreases with increasing temperature) and a Bragg grating which has a Bragg reflection wavelength which has a different temperature gradient (optionally a positive temperature gradient, or substantially zero temperature gradient) can be used simultaneously to provide enhanced strain and temperature discrimination. For example, where the two Bragg gratings are in thermal equilibrium, their respective responses to the same changes in temperature will be different. For example, the device comprising two Bragg gratings in thermal equilibrium may be a strain, pressure and/or temperature sensor. Optionally, the Bragg gratings in such a device form part of one or more waveguides, such as one or more optical fibres. In further examples, the Bragg gratings are implemented using any appropriate medium to provide the advantages described herein.

Example characteristics are shown at Fig. 64. In particular, there are two plots showing the change of Bragg wavelength as a function of temperature change. One plot is for a standard fibre Bragg grating response, with a temperature coefficient of +10 pm/°C. The other plot is for a modified fibre Bragg grating, with a temperature coefficient of around -50 pm/°C in the steep part of the characteristic. The modified fibre Bragg grating may be a modified Bragg grating 31, 1031 as described with reference to Fig. 1 to Fig. 63.

Advantageously, by measuring the Bragg wavelength of each fibre Bragg grating, the strain and temperature may be independently determined by linear algebra, for example using a matrix method. Each measurement gives two comparable Bragg wavelengths Ai and 2.2 from each of the two devices. Therefore, temperature and strain discrimination can be achieved by solving the matrix:

/A2.. _ K_el K_T1\ / e \ " e2 K_T2) T) where Ktj are the sensitivities for strain e and temperature change AT for the two fibre Bragg grating devices.

As described above for optical fibres 1, 1001 with voids 9, 1009 along their length, the filler 15, 1015 used to at least partially fill the voids 9, 1009 may comprise a material which has a change in refractive index with temperature, dn/DT, which is different to that of the material of the optical fibre 1, 1001. At certain temperatures within the operating range of the optical fibre 1, 1001, the refractive index of the filler 15, 1015 is greater than the refractive index of the cladding 5, 1005. Advantageously, where the refractive index of the filler 15, 1015 exceeds the refractive index of the cladding 5, 1005, a fibre Bragg grating 31, 1031 affected by the use of such filler 15, 1015 operates with a greater change in Bragg wavelength for a given temperature change - for example on the steeper part of the modified FBG curve of Fig. 64. Beneficially, this provides higher temperature sensitivity compared to the use of material filler 15, 1015 with a refractive index that is lower than that of the cladding 5, 1005.

It will be understood that features and properties detailed with respect to Fig. 1A to Fig. 59 may be implemented in combination with the devices and features described with respect to Fig. 60 to Fig. 64 and vice versa.

Aspects of the invention may also be described by the following numbered clauses. These are not the claims of this application, which follow under the heading CLAIMS below.

A1. An optical fibre comprising one or more voids, wherein: the one or more voids extend longitudinally along the length of the optical fibre within a void portion, the void portion being a continuous and integral portion of the optical fibre; a longitudinal extent of each of the one or more voids is less than a longitudinal extent of the void portion; and the one or more voids are sealed from an exterior of the optical fibre and at least partially filled with a filler having a material property different to the material property of a material of the optical fibre.

A2. The optical fibre of clause A1 , wherein the one or more voids comprise two or more voids filled with different fillers.

A3. The optical fibre of clause A2, wherein the material property of the fillers differs between the different fillers.

A4. The optical fibre of any of clauses A1 to A3, wherein the material property is an optical property, optionally refractive index.

A5. The optical fibre of clause A4, wherein the one or more voids are configured such that the optical property of the filler affects light transmitted through the optical fibre.

A6. The optical fibre of any of clauses A1 to A5, wherein a change with temperature of the material property of the filler is different to, optionally opposite to, a change with temperature of the material property of the material of the optical fibre.

A7. The optical fibre of clause A6, wherein the one or more voids are configured such that the change with temperature of the material property of the filler at least partially compensates for the change with temperature of the material property of the material of the optical fibre.

A8. The optical fibre of any of clauses A1 to A7, wherein a cross-sectional area of at least one of the one or more voids varies along the length of the optical fibre.

A9. The optical fibre of clause A8, wherein the cross-sectional area reduces away from a centre of the void along the length of the optical fibre for at least a portion of the length of the void.

A10. The optical fibre of any of clauses A1 to A9, wherein the cross-section of at least one of the one or more voids is noncircular.

A11. The optical fibre of any of clauses A1 to A10, wherein a central axis of the one or more voids extends along a direction inclined to the longitudinal axis of the optical fibre for at least a portion of the one or more voids.

A11a. The optical fibre of any of clauses A8 to A11, wherein: the material property is an optical property, optionally refractive index; the one or more voids are configured such that the optical property of the filler affects light transmitted through the optical fibre; and a variation of the cross-sectional area of the one or more voids and/or an orientation of the central axis of the one or more voids are such that light transmitted through the optical fibre experiences a substantially continuous variation in an effect of the optical property on the transmission of the light along the length of the optical fibre.

A12. The optical fibre of any of clauses A1 to A11, wherein the optical fibre comprises a core and a cladding surrounding the core, and the one or more voids comprise at least one cladding void within the cladding.

A13. The optical fibre of clause A12, wherein the cladding void is configured such that transmission of light within the optical fibre is affected by the filler within the cladding void, optionally wherein the at least one cladding void is adjacent to the core.

A14. The optical fibre of clause A12 or A13, wherein the cladding void extends such that the filler is in contact with the core.

A15. The optical fibre of clause A12 or A13, wherein the cladding void extends such that the filler is not in contact with the core.

A16. The optical fibre of any of clauses A12 to A15, wherein the one or more voids comprise a plurality of cladding voids within the cladding, optionally at least two cladding voids, optionally at least four cladding voids, optionally at least six cladding voids. A17. The optical fibre of clause A16, wherein the plurality of cladding voids is arranged symmetrically around the core.

A18. The optical fibre of any of clauses A12 to A17, wherein a distance between the cladding voids and the core varies along the length of the cladding void.

A19. The optical fibre of any of clauses A1 to A18, wherein the optical fibre further comprises one or more access voids extending from the one or more voids towards an exterior surface of the optical fibre.

A20. The optical fibre of clause A19, wherein the one or more access voids are sealed at the exterior surface of the optical fibre.

A20a. The optical fibre of clause A20, wherein the access voids are sealed by: a blocking member within the access void, optionally wherein the blocking member is entirely within the access void; or melting the material of the optical fibre at the exterior surface of the optical fibre.

A21. The optical fibre of clause A20, wherein the one or more voids are formed by etching a material of the optical fibre via the one or more access voids prior to sealing of the one or more access voids.

A22. The optical fibre of any of clauses A1 to A21, wherein the optical fibre is formed by drawing a preform, and the one or more voids are formed after the optical fibre is drawn.

A23. The optical fibre of any of clauses A1 to A22, wherein the one or more voids extend entirely within the void portion and/or a boundary of the one or more voids is defined entirely within the void portion.

A24. The optical fibre of any of clauses A1 to A23, wherein the void portion does not comprise any interface between longitudinally adjacent solid materials.

A25. The optical fibre of any of clauses A1 to A24, wherein the one or more voids are entirely filled with the filler.

A26. The optical fibre of any of clauses A1 to A25, wherein the filler comprises a gas, optionally nitrogen or air.

A27. The optical fibre of any of clauses A1 to A26, wherein the filler comprises a non-gaseous material, optionally a liquid.

A28. The optical fibre of clause A27, wherein the filler comprises glycerol or a glycerol-water mixture.

A29. The optical fibre of any of clauses A27 to A28, wherein the filler comprises a liquid crystal.

A30. The optical fibre of any of clauses A1 to A29, wherein a refractive index of the filler is within 0.1 of a refractive index of the material of the optical fibre, optionally within 0.05, optionally within 0.01, optionally within 0.005, optionally within 0.001.

A31. The optical fibre of clause A30, wherein the optical fibre comprises a core and a cladding surrounding the core, and the refractive index of the filler is approximately equal to the refractive index of a material of the cladding at a reference temperature. A32. The optical fibre of any of clauses A1 to A31 , wherein the optical fibre has an outer diameter of between 25 pm and 300 pm, optionally between 100 pm and 300 pm, optionally approximately 125 pm or 250 pm.

A33. The optical fibre of any of clauses A1 to A32, wherein the optical fibre is a single mode optical fibre.

A34. The optical fibre of any of clauses A1 to A33, wherein the optical fibre is an antiresonant or negative curvature fibre.

A35. The optical fibre of any of clauses A1 to A34, wherein the optical fibre comprises silica.

A36. The optical fibre of any of clauses A1 to A35, wherein the optical fibre comprises crystal material, optionally single-crystal material.

A37. The optical fibre of clause A36, wherein the optical fibre is a crystal-derived fibre, optionally a sapphire derived fibre.

A38. The optical fibre of any of clauses A1 to A34, wherein the optical fibre is a crystal optical fibre, optionally a single-crystal optical fibre.

A39. The optical fibre of any of clauses A36 to A38, wherein the crystal is doped, optionally with a rare-earth element.

A40. The optical fibre of any of clauses A36 to A39, wherein the optical fibre comprises sapphire, diamond, or yttrium aluminium garnet (YAG) crystal. A41. The optical fibre of any of clauses A1 to A40, wherein the optical fibre comprises a Bragg grating.

A42. The optical fibre of clause A41 , wherein the optical fibre comprises a core and a cladding surrounding the core, and the

Bragg grating is at least partially located in the core.

A43. The optical fibre of any of clauses A41 or A42, wherein the Bragg grating is provided by periodic modification of a material of the core.

A44. The optical fibre of any of clauses A41 or A42, wherein the Bragg grating is provided by periodic modification of a material of the cladding, optionally wherein the periodic modification is adjacent to the core.

A45. The optical fibre of any of clauses A41 to A44, wherein the material property is an optical property and the one or more voids are configured such that the optical property of the filler affects the Bragg wavelength of the Bragg grating.

A46. The optical fibre of any of clauses A41 to A45, wherein the one or more voids at least partially surround the Bragg grating.

A47. The optical fibre of any of clauses A41 to A46, wherein the longitudinal extent of the one or more voids is at least as long as a longitudinal extent of the Bragg grating.

A48. The optical fibre of any of clauses A41 to A47, wherein the one or more voids extend at most 1mm, optionally at most 0.5mm past either end of the Bragg grating.

A49. The optical fibre of any of clauses A41 to A48, wherein the one or more voids are configured such that a change with temperature of the material property of the filler at least partially compensates for an effect on the Bragg wavelength of the Bragg grating of a change with temperature of the material property of the material of the optical fibre, optionally wherein the material property is an optical property.

A50. The optical fibre of clause A49, wherein the one or more voids are configured such that a magnitude of the change with temperature of the Bragg wavelength of the Bragg grating is at most 5pm/°C, optionally at most 2pm/°C, optionally at most 1 pm/°C over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C.

A51. The optical fibre of any of clauses A41 to A48, wherein the one or more voids are configured such that a change with temperature of the material property of the filler increases a magnitude of a change with temperature of the Bragg wavelength of the Bragg grating relative to an optical fibre without the one or more voids, optionally wherein the material property is an optical property.

A52. The optical fibre of clause A51, wherein the one or more voids are configured such that a magnitude of a change with temperature of the Bragg wavelength of the Bragg grating is at least 20pm/°C, optionally at least 30pm/°C, optionally at least 40pm/°C, optionally at least 50pm/°C over a temperature range of at least 20°C, optionally at least 40°C.

A53. The optical fibre of clause A52, wherein the Bragg wavelength of the Bragg grating reduces with increasing temperature.

A54. The optical fibre of any of clauses A41 to A53, wherein the optical fibre comprises two separate Bragg gratings.

A55. The optical fibre of clause A54, wherein the material property is an optical property and the one or more voids are configured such that the optical property of the filler affects the Bragg wavelengths of the two Bragg gratings differently.

A56. The optical fibre of clause A55, wherein the one or more voids are configured such that the optical property of the filler affects the Bragg wavelength of one of the two Bragg gratings and does not affect the Bragg wavelength of the other of the two Bragg gratings.

A57. The optical fibre of any of clauses A54 to A56, wherein the optical fibre comprises a core and a cladding surrounding the core, and the two Bragg gratings are spaced apart longitudinally along the core.

A58. The optical fibre of any of clauses A54 to A56, wherein: the optical fibre comprises a core and a cladding surrounding the core; one of the two Bragg gratings is located in the cladding; and the other of the two Bragg gratings is located in the core.

A59. The optical fibre of clause A58, wherein the one of the two Bragg gratings is located in a waveguide optically coupled to the core such that a proportion of light guided by the core is transferred into the waveguide.

A60. The optical fibre of clause A59, wherein the material property is an optical property and the one or more voids comprise at least one void configured such that transmission of light within the waveguide is affected by the optical property of the filler within the void, optionally wherein the at least one void is adjacent to the waveguide.

A61. The optical fibre of clause A59 or A60, wherein: the one or more voids comprise a cladding void configured to provide the waveguide within the cladding; the cladding void is optically coupled to the core such that a proportion of light guided by the core is transferred into the cladding void; and the one of the two Bragg gratings is provided by periodic modification of the material of the optical fibre adjacent to the cladding void.

A62. The optical fibre of any of clauses A54 to A61, wherein the one or more voids comprise an isolation void configured to at least partially isolate one of the Bragg gratings from strain within the optical fibre.

A63. The optical fibre of clause A62, wherein the isolation void surrounds an end of the one of the Bragg gratings.

A64. The optical fibre of clause A63, wherein the isolation void surrounds at least 50%, optionally at least 75%, optionally at least 90%, optionally 100% of the length of the one of the Bragg gratings.

A65. The optical fibre of any of clauses A62 to A64, wherein: the optical fibre comprises a core and a cladding surrounding the core; the one of the two Bragg gratings is located in the cladding; the other of the two Bragg gratings is located in the core; and the one of the two Bragg gratings is located in a waveguide optically coupled to the core such that a proportion of light guided by the core is transferred into the waveguide.

A66. The optical fibre of any of clauses A1 to A65, wherein the optical fibre is configured to exhibit birefringence.

A67. The optical fibre of clause A66, wherein the optical fibre comprises a core and a cladding surrounding the core, and the optical fibre comprises one or more stress-inducing regions arranged around the core to contribute to the birefringence.

A68. The optical fibre of clause A67, wherein the stress-inducing regions comprise laser-exposed regions.

A69. The optical fibre of any of clauses A66 to A68, wherein the optical fibre comprises a core and a cladding surrounding the core, and the one or more voids comprise a plurality of voids arranged around the core with a symmetry such as to contribute to the birefringence.

A70. The optical fibre of clause A69, wherein the plurality of voids comprises two voids arranged along a first diameter of the optical fibre on opposite sides of the core.

A71. The optical fibre of clause A70, wherein no voids are provided along a second diameter of the optical fibre perpendicular to the first diameter.

A72 The optical fibre of any of clauses A66 to A71 , wherein the optical fibre comprises a Bragg grating, and the birefringence is such that the Bragg wavelength of the Bragg grating differs for light having different polarisations, optionally wherein the polarisations are orthogonal.

A73. The optical fibre of clause A72, wherein the birefringence is such that reflection peaks for the light having different polarisations around the corresponding Bragg wavelengths of the Bragg grating are resolvable when a pressure within the one or more voids is substantially equal to a pressure external to the optical fibre, optionally wherein the reflection peaks are separated by at least a full-width at half maximum of the reflection peaks.

A74. The optical fibre of any of clauses A66 to A73, wherein the material property is an optical property and the one or more voids are configured such that an effect of the optical property of the filler on light guided by the optical fibre differs for light having different polarisations, optionally wherein the polarisations are orthogonal.

A75. The optical fibre of any of clauses A1 to A74, wherein the optical fibre comprises a waveguide separate to a core of the optical fibre. A76. The optical fibre of clause A75, wherein the waveguide is formed by one or more modified regions of the optical fibre in which an optical property of the optical fibre differs from the optical property of a material of the optical fibre surrounding the one or more modified regions, optionally wherein the optical property is refractive index.

A77. The optical fibre of clause A76, wherein the one or more modified regions comprises one or more laser-exposed regions.

A78. The optical fibre of clause A76 or A77, wherein the one or more modified regions comprise one or more voids extending longitudinally along the length of the optical fibre.

A79. The optical fibre of any of clauses A76 to A78, wherein the optical fibre is configured to exhibit birefringence and the one or more modified regions are configured to contribute to the birefringence.

A80. A strain sensor comprising the optical fibre of clause A41 or any preceding clause dependent thereon.

A81. The strain sensor of clause A80, further comprising a controller configured to determine a strain applied to the optical fibre based on the Bragg wavelength of the Bragg grating.

A82. The strain sensor of clause A81, wherein: the optical fibre is configured to exhibit birefringence; and the controller is configured to determine the strain based on a difference between the Bragg wavelength of the Bragg grating for light having different polarisations.

A83. A system for sensing strain and/or temperature comprising the optical fibre of clause A54 or any preceding clause dependent thereon, wherein a change with temperature of the Bragg wavelength of the two Bragg gratings is different.

A84. The system of clause A83, further comprising a controller configured to determine a strain applied to the optical fibre and a temperature of the optical fibre based on the Bragg wavelengths of the two Bragg gratings.

A85. A pressure sensor comprising an optical fibre according to clause A41 or any preceding clause dependent thereon.

A86. The pressure sensor of clause A85, wherein the pressure sensor is configured such that a pressure difference between a pressure of the filler and an external pressure applied to the optical fibre affects the Bragg wavelength of the Bragg grating.

A87. The pressure sensor of clause A86, further comprising a controller configured to determine the external pressure applied to the optical fibre based on the Bragg wavelength of the Bragg grating.

A88. The pressure sensor of clause A86 or A87, wherein: the optical fibre is configured to exhibit birefringence; the pressure difference affects the birefringence; and the controller is configured to determine the external pressure applied to the optical fibre based on a difference between the Bragg wavelength of the Bragg grating for different polarisations of light.

B1. The optical fibre of any preceding clause, wherein: the optical fibre comprises a coating; a change with temperature of a material property of the coating affects light transmitted through the optical fibre; and the one or more voids are configured such that a change with temperature of the material property of the filler at least partially compensates for a combined effect on light transmitted through the optical fibre of a change with temperature of the material property of the material of the optical fibre and the change with temperature of the material property of the coating.

B2. An optical fibre comprising: one or more voids at least partially filled with a filler, the filler having a material property different to the material property of a material of the optical fibre; and a coating, wherein: a change with temperature of a material property of the coating affects light transmitted through the optical fibre; and the one or more voids are configured such that a change with temperature of the material property of the filler at least partially compensates for a combined effect on light transmitted through the optical fibre of a change with temperature of the material property of the material of the optical fibre and the change with temperature of the material property of the coating.

B3. The optical fibre of clause B2, wherein the one or more voids extend longitudinally along the length of the optical fibre within a void portion, the void portion being a continuous and integral portion of the optical fibre.

B4. The optical fibre of clause B2 or B3, wherein a longitudinal extent of each of the one or more voids is less than a longitudinal extent of the void portion.

B5. The optical fibre of any of clauses B2 to B4, wherein the one or more voids are sealed from an exterior of the optical fibre.

B6. The optical fibre of any of clauses B1 to B5, wherein the coating comprises polyacrylate or polyimide.

B7. The optical fibre of any of clauses B1 to B5, wherein the coating is metallic.

B8. The optical fibre of any of clauses B1 to B7, wherein the material property is an optical property, optionally refractive index.

B9. The optical fibre of any of clauses B1 to B8, wherein the optical fibre comprises a Bragg grating.

B10. The optical fibre of clause B9, wherein: a change with temperature of a material property of the coating affects the Bragg wavelength of the Bragg grating; and the one or more voids are configured such that the change with temperature of the material property of the filler at least partially compensates for the combined effect on the Bragg wavelength of the Bragg grating of the change with temperature of the material property of the material of the optical fibre and the change with temperature of the material property of the coating.

B11. The optical fibre of clause B10, wherein the one or more voids are configured such that a magnitude of the change with temperature of the Bragg wavelength of the Bragg grating is at most 5pm/°C, optionally at most 2pm/°C, optionally at most 1 pm/°C over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C.

C1. The optical fibre of any preceding clause, wherein the optical fibre comprises a core and a cladding surrounding the core, and the one or more voids comprise a core void within the core.

C2. An optical fibre comprising: one or more voids; and a Bragg grating, wherein: the one or more voids extend longitudinally along the length of the optical fibre; the one or more voids are at least partially filled with a filler having an optical property different to that of a material of the optical fibre; the optical fibre comprises a core and a cladding surrounding the core; and the one or more voids comprise a core void within the core.

C3. The optical fibre of clause C1 or C2, wherein the core consists substantially of the core void.

C4. The optical fibre of clause C3, wherein the Bragg grating is provided by periodic modification of the cladding.

C5. The optical fibre of any of clauses C1 to C4, wherein the core is aligned with a central axis of the optical fibre.

C6. The optical fibre of any of clauses C1 to C5, wherein the Bragg grating is provided by periodic modification of the filler in the core void, optionally wherein the filler comprises a liquid crystal.

C7. The optical fibre of clause C6, wherein the filler is polymerizable and the periodic modification comprises periodic polymerisation, optionally wherein the filler comprises monomers and a photo-initiator and the periodic polymerisation is performed using a laser.

D1. The optical fibre of any preceding clause, wherein the optical fibre further comprises one or more electrodes configured to apply an electric field to the filler to affect the material property of the filler.

D2. An optical fibre comprising one or more voids, wherein: the one or more voids extend longitudinally along the length of the optical fibre; the one or more voids are at least partially filled with a filler having a material property different to that of a material of the optical fibre; and the optical fibre further comprises one or more electrodes configured to apply an electric field to the filler to affect the material property of the filler.

D3. The optical fibre of clause D1 or D2, wherein the filler comprises a liquid crystal.

D4. The optical fibre of any of clauses D1 to D3, wherein the electric field is configured to affect one or more of the refractive index, the absorption, and the scattering loss of the filler.

D5. The optical fibre of any of clauses D1 to D4, wherein the filler exhibits birefringence and the electric field is configured to affect one or more of the magnitude of the birefringence, and the angle of an optic axis of the birefringence.

D6. The optical fibre of any of clauses D1 to D5, wherein the optical fibre comprises a Bragg grating, and the electric field is configured to affect a Bragg wavelength of the Bragg grating.

E1. A crystal optical fibre comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre.

E2. The optical fibre of clause E1, wherein the one or more voids are at least partially filled with a filler having a material property different to that of a material of the optical fibre, optionally wherein the material property is an optical property, optionally refractive index.

E3. The optical fibre of clause E1 or E2, wherein a longitudinal extent of the one or more voids is less than a length of the optical fibre.

E4. The optical fibre of any of clauses E1 to E3, wherein the one or more voids extend within a void portion, the void portion being a continuous and integral portion of the optical fibre, optionally wherein a longitudinal extent of the one or more voids is less than a longitudinal extent of the void portion.

E5. The optical fibre of any of clauses E1 to E4, wherein a central axis of the one or more voids extends along a direction inclined to the longitudinal axis of the optical fibre for at least a portion of the one or more voids.

E6. The optical fibre of any of clauses E1 to E4, wherein the optical fibre comprises a Bragg grating.

E7. The optical fibre of any of clauses E1 to E6, wherein the optical fibre is configured to exhibit birefringence.

E8. The optical fibre of any of clauses E1 to E7, wherein the optical fibre is an antiresonant or negative curvature fibre.

E9. The optical fibre of any of clauses E1 to E8, wherein the optical fibre comprises a single-crystal optical fibre, optionally sapphire, diamond, or yttrium aluminium garnet (YAG) crystal.

E10. The optical fibre of any of clauses E1 to E9, wherein the optical fibre is doped, optionally with a rare-earth element.

F1. A method for forming one or more voids in an optical fibre comprising: selectively exposing the optical fibre to laser radiation to define one or more exposed regions within the optical fibre; contacting the optical fibre with an etchant, wherein the etchant etches the exposed regions at a higher rate than regions of the optical fibre not exposed to laser radiation, such that the one or more voids are formed by etching of the exposed regions; at least partially filling the one or more voids with a filler having a material property different to that of a material of the optical fibre; and sealing the one or more voids from an exterior of the optical fibre.

F2. The method of clause F1 , wherein the one or more exposed regions comprise a high exposure region and a low exposure region, wherein the high exposure region is exposed to a higher dose of laser radiation than the low exposure region.

F3. The method of clause F2, wherein the higher dose of laser radiation is achieved by varying one of more of laser power, a speed of scanning of the laser, and laser beam profile.

F4. The method of clause F2 or F3, wherein the etchant etches the high exposure region at a higher rate than the low exposure region.

F5. The method of any of clauses F1 to F4, wherein selectively exposing the optical fibre to laser radiation further comprises defining one or more access regions extending to an external surface of the optical fibre, such that each of the one or more exposed regions is connected to the external surface by at least one of the one or more access regions.

F6. The method of clause F5, wherein during the step of contacting the optical fibre with an etchant, the access regions are initially etched to form one or more access voids, the access voids allowing the etchant to contact the exposed regions within the optical fibre.

F7. The method of clause F6, wherein sealing the one or more voids comprises sealing the one or more access voids at the exterior surface of the optical fibre.

F8. The method of clause F7, wherein the access voids are sealed by: inserting a blocking member within the access void, optionally wherein the blocking member is entirely within the access void; or melting the material of the optical fibre at the exterior surface of the optical fibre, optionally wherein the melting is performed using a laser or an electric arc.

F9. The method of any of clauses F1 to F7, wherein the method further comprises a step of forming one or more stressinducing regions in the optical fibre.

F10. The method of clause F9, wherein forming the one or more stress-inducing regions comprises selectively exposing the optical fibre to laser radiation.

F11. The method of clause F9 or F10, wherein the one or more stress-inducing regions are configured to contribute to birefringence of the optical fibre.

F12. The method of any of clauses F9 to F11, wherein the step of forming the one or more stress-inducing regions is carried out prior to the step of contacting the optical fibre with an etchant, the stress-inducing regions being separated from the exposed regions.

F13. The method of any of clauses F9 to F11, wherein the step of forming the one or more stress-inducing regions is carried out after the step of contacting the optical fibre with an etchant.

F14. The method of any of clauses F1 to F13, wherein the method further comprises flushing the etchant from the one or more voids prior to filling the one or more voids.

F15. The method of any of clauses F1 to F14, wherein the etchant comprises potassium hydroxide, optionally having a concentration of at least 5 mol, optionally at least 8 mol.

F16. The method of any of clauses F1 to F15, wherein the laser radiation comprises infrared or visible light.

F17. The method of clause F16, wherein the laser radiation has a wavelength between 700 nm and 900 nm, optionally between 750 nm and 850 nm, optionally approximately 790 nm.

F18. The method of clause F16, wherein the laser radiation has a wavelength between 450 nm and 650 nm, optionally between 500 nm and 600 nm, optionally approximately 530 nm.

F19. The method of any of clauses F1 to F18 wherein the laser radiation is provided by a laser beam generated using a laser system.

F20. The method of clause F19, wherein the laser beam is a pulsed laser beam.

F21. The method of clause F20 when dependent on clause F2, wherein the higher dose of laser radiation is achieved by varying one of both of laser pulse energy and pulse repetition rate.

F22. The method of clause F20 or F21, wherein pulses of the pulsed laser beam have a duration of at most 1 ps, optionally at most 500 fs, optionally at most 200 fs, optionally at most 100 fs.

F23. The method of any of clauses F19 to F22, wherein selectively exposing the optical fibre to laser radiation comprises applying a correction to an active optical element of the laser system to modify wavefront properties of the laser beam to counteract an effect of aberration on laser focus.

F24. The method of any of clauses F1 to F23, wherein the method further comprising forming a Bragg grating in the optical fibre.

F25. An optical fibre produced using the method of any of clauses F1 to F24.

G1. An optical fibre comprising a first portion and a second portion, wherein: the second portion is a void portion comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre within the void portion; the first portion and the second portion are substantially mode-matched; and the one or more voids are at least partially filled with a filler having a material property different to the material property of a material of the optical fibre.

G2. The optical fibre of clause G1 , wherein the mode field diameter of the first portion for a given wavelength and temperature is substantially the same as the mode field diameter of the second portion for the given wavelength and temperature.

G3. The optical fibre of clause G1 or clause G2, wherein the first portion and the second portion each comprise: a core; and a cladding, wherein the respective cladding surrounds the respective core.

G4. The optical fibre of clause G3, wherein the refractive index of the core of the first portion is substantially the same as the refractive index of the core of the second portion, optionally wherein the refractive index of the core of the first portion is between 0.002 and 0.007 greater than the refractive index of the cladding surrounding the core of the first portion, optionally wherein the refractive index of the core of the first portion is approximately 0.005 greater than the refractive index of the cladding surrounding the core of the first portion.

G5. The optical fibre of clause G3 or clause G4, wherein the maximum lateral dimension of the core of the first portion in a direction substantially perpendicular to the longitudinal axis of the optical fibre is substantially the same as the maximum lateral dimension of the core of the second portion in the direction substantially perpendicular to the longitudinal axis of the optical fibre at an interface between the first portion and the second portion.

G6. The optical fibre of any of clauses G3, G4, or G5 wherein the dopant concentration of the core of the first portion is substantially the same as the dopant concentration of the core of the second portion.

G7. The optical fibre of any of clauses G1 to G6, wherein the first portion is a sealing portion and the one or more voids are at least partially sealed by the sealing portion.

G8. The optical fibre of clause G7, wherein the one or more voids are at least partially sealed from an exterior of the optical fibre at an interface between an end face of the sealing portion and an end face of the void portion.

G9. The optical fibre of clause G8, wherein the one or more voids are sealed from an exterior of the optical fibre at a further interface between the other end face of the void portion and an end face of a further sealing portion.

G10. The optical fibre of any of clauses G1 to G11, wherein the optical fibre is a single mode optical fibre.

G11. The optical fibre of any of clauses G7 to G10, wherein the sealing portion comprises a standard single mode fibre having a core surrounded by a cladding, wherein the cladding comprises a substantially solid material that at least partially seals the one or more voids of the void portion.

G12. The optical fibre of any of any of clauses G2 to G11, wherein the mode field diameter is between 9 and 12 micrometres, optionally wherein the mode field diameter is between 9.6 and 11.2 micrometres, optionally wherein the mode field diameter is approximately 10.5 micrometres, optionally wherein the given wavelength is within the C-band, L-band and/or S-band.

G13. The optical fibre of any of clauses G5 to G12, wherein the core of the first portion and the core of the second portion each have a maximum lateral dimension greater than 5.5 micrometres and less than 11 micrometres in a direction substantially perpendicular to the longitudinal axis of the optical fibre at the interface between the first portion and the second portion, optionally wherein the core of the first portion and the core of the second portion each have a maximum lateral dimension between 6 and 10.5 micrometres in a direction substantially perpendicular to the longitudinal axis of the optical fibre at the interface between the first portion and the second portion, optionally wherein the core of the first portion and the core of the second portion each have a maximum lateral dimension of between 7 and 10 micrometres in a direction substantially perpendicular to the longitudinal axis of the optical fibre at the interface between the first portion and the second portion, optionally wherein the core of the first portion and the core of the second portion each have a maximum lateral dimension of between 8 and 9 micrometres in a direction substantially perpendicular to the longitudinal axis of the optical fibre at the interface between the first portion and the second portion, optionally wherein the core of the first portion and the core of the second portion each have a maximum lateral dimension of approximately 8.2 micrometres in a direction substantially perpendicular to the longitudinal axis of the optical fibre at the interface between the first portion and the second portion.

G14. The optical fibre of clause G9 or any preceding clause dependent thereon, wherein the further sealing portion comprises a standard single-mode fibre having a core surrounded by a cladding, wherein the cladding comprises a substantially solid material that at least partially seals the one or more voids of the void portion.

G15. The optical fibre of any of clauses G1 to G14, wherein the first portion comprises a bridging portion comprising a core having a cross-sectional area that varies along the longitudinal axis of the bridging portion from a first cross-sectional area to a second cross-sectional area, wherein the second cross-sectional area has a maximum lateral dimension that is substantially the same as the maximum lateral dimension of a core of the void portion..

G16. The optical fibre of any of clauses G1 to G14, wherein the first portion comprises a lens portion having a cross-sectional area that varies along the longitudinal axis of the lens portion, thereby progressively to convert the mode field diameter along the longitudinal axis of the lens portion from a first mode field diameter to a second mode field diameter, wherein the second mode field diameter is substantially the same as the mode field diameter of the void portion for a given wavelength and temperature. G17. The optical fibre of any of clause G1 to G16, wherein the optical fibre comprises a Bragg grating.

G18. The optical fibre of clause G17, wherein the optical fibre comprises a core and a cladding surrounding the core, and the

Bragg grating is at least partially located in the core.

G19. The optical fibre of any of clauses G17 or G18, wherein either: a) the Bragg grating is provided by periodic modification of a material of the core; or b) the Bragg grating is provided by periodic modification of a material of the cladding, optionally wherein the periodic modification is adjacent to the core.

G20. The optical fibre of any of clauses G17 to G19, wherein the material property is an optical property and the one or more voids are configured such that the optical property of the filler affects the Bragg wavelength of the Bragg grating.

G21. The optical fibre of any of clauses G17 to G20, wherein: a) the one or more voids at least partially surround the Bragg grating; and/or b) the longitudinal extent of the one or more voids is at least as long as a longitudinal extent of the Bragg grating; and/or c) the one or more voids extend at most 1mm, optionally at most 0.5mm past either end of the Bragg grating.

G22. The optical fibre of any of clauses G17 to G21, wherein the one or more voids are configured such that a change with temperature of the material property of the filler at least partially compensates for an effect on the Bragg wavelength of the Bragg grating of a change with temperature of the material property of the material of the optical fibre, optionally wherein the material property is an optical property, optionally wherein the one or more voids are configured such that a magnitude of the change with temperature of the Bragg wavelength of the Bragg grating is at most 5pm/°C, optionally at most 2pm/°C, optionally at most 1pm/°C over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C.

G23. The optical fibre of any of clauses G17 to G21, wherein the one or more voids are configured such that a change with temperature of the material property of the filler increases a magnitude of a change with temperature of the Bragg wavelength of the Bragg grating relative to an optical fibre without the one or more voids, optionally wherein the material property is an optical property, optionally wherein the one or more voids are configured such that a magnitude of a change with temperature of the Bragg wavelength of the Bragg grating is at least 20pm/°C, optionally at least 30pm/°C, optionally at least 40pm/°C, optionally at least 50pm/°C over a temperature range of at least 20°C, optionally at least 40°C, further optionally wherein the Bragg wavelength of the Bragg grating reduces with increasing temperature.

G23A. The optical fibre of any of clauses G1 to G23, wherein: the void portion is a continuous and integral portion of the optical fibre; a longitudinal extent of each of the one or more voids is less than a longitudinal extent of the void portion; and the one or more voids are sealed from an exterior of the optical fibre. G24. The optical fibre of any of clause G17 to G23A, wherein the optical fibre comprises two separate Bragg gratings.

G25. The optical fibre of clause G24, wherein the material property is an optical property and the one or more voids are configured such that the optical property of the filler affects the Bragg wavelengths of the two Bragg gratings differently, optionally wherein the one or more voids are configured such that the optical property of the filler affects the Bragg wavelength of one of the two Bragg gratings and does not affect the Bragg wavelength of the other of the two Bragg gratings.

G26. The optical fibre of clause G24 or clause G25, wherein the optical fibre comprises a core and a cladding surrounding the core, and the two Bragg gratings are spaced apart longitudinally along the core.

G27. The optical fibre of clause G24 or clause G25, wherein: the optical fibre comprises a core and a cladding surrounding the core; one of the two Bragg gratings is located in the cladding; and the other of the two Bragg gratings is located in the core.

G28. The optical fibre of clause G27, wherein the one of the two Bragg gratings is located in a waveguide optically coupled to the core such that a proportion of light guided by the core is transferred into the waveguide.

G29. The optical fibre of clause G28, wherein the material property is an optical property and the one or more voids comprise at least one void configured such that transmission of light within the waveguide is affected by the optical property of the filler within the void, optionally wherein the at least one void is adjacent to the waveguide.

G30. The optical fibre of clause G28 or clause G29, wherein: the one or more voids comprise a cladding void configured to provide the waveguide within the cladding; the cladding void is optically coupled to the core such that a proportion of light guided by the core is transferred into the cladding void; and the one of the two Bragg gratings is provided by periodic modification of the material of the optical fibre adjacent to the cladding void.

G31. The optical fibre of any of clauses G24 to G30, wherein the one or more voids comprise an isolation void configured to at least partially isolate one of the Bragg gratings from strain within the optical fibre.

G32. The optical fibre of clause G31 , wherein the isolation void surrounds an end of the one of the Bragg gratings, optionally wherein the isolation void surrounds at least 50%, optionally at least 75%, optionally at least 90%, optionally 100% of the length of the one of the Bragg gratings.

G33. The optical fibre of clause G31 or clause G32, wherein: the optical fibre comprises a core and a cladding surrounding the core; the one of the two Bragg gratings is located in the cladding; the other of the two Bragg gratings is located in the core; and the one of the two Bragg gratings is located in a waveguide optically coupled to the core such that a proportion of light guided by the core is transferred into the waveguide.

G34. The optical fibre of any of clauses G17 to G33, wherein the optical fibre is configured to exhibit birefringence.

G35. The optical fibre of clause G34, wherein the optical fibre comprises a core and a cladding surrounding the core, and the optical fibre comprises one or more stress-inducing regions arranged around the core to contribute to the birefringence, optionally the stress-inducing regions comprise laser-exposed regions.

G36. The optical fibre of clause G34 or clause G35, wherein the optical fibre comprises a core and a cladding surrounding the core, and the one or more voids comprise a plurality of voids arranged around the core with a symmetry such as to contribute to the birefringence, optionally the plurality of voids comprises two voids arranged along a first diameter of the optical fibre on opposite sides of the core, further optionally wherein no voids are provided along a second diameter of the optical fibre perpendicular to the first diameter.

G37. The optical fibre of any of clauses G34 to G36, wherein the optical fibre comprises a Bragg grating, and the birefringence is such that the Bragg wavelength of the Bragg grating differs for light having different polarisations, optionally wherein the polarisations are orthogonal. G38. The optical fibre of clause G37, wherein the birefringence is such that reflection peaks for the light having different polarisations around the corresponding Bragg wavelengths of the Bragg grating are resolvable when a pressure within the one or more voids is substantially equal to a pressure external to the optical fibre, optionally wherein the reflection peaks are separated by at least a full-width at half maximum of the reflection peaks.

G39. The optical fibre of any of clauses G34 to G38, wherein the material property is an optical property and the one or more voids are configured such that an effect of the optical property of the filler on light guided by the optical fibre differs for light having different polarisations, optionally wherein the polarisations are orthogonal.

G40. The optical fibre of any of clauses G17 to G39, wherein the optical fibre comprises a waveguide separate to a core of the optical fibre.

G41. The optical fibre of clause G40, wherein the waveguide is formed by one or more modified regions of the optical fibre in which an optical property of the optical fibre differs from the optical property of a material of the optical fibre surrounding the one or more modified regions, optionally wherein the optical property is refractive index, optionally wherein the one or more modified regions comprises one or more laser-exposed regions.

G42. The optical fibre of clause G41, wherein the one or more modified regions comprise one or more voids extending longitudinally along the length of the optical fibre.

G43. The optical fibre of clause G41 or clause G42, wherein the optical fibre is configured to exhibit birefringence and the one or more modified regions are configured to contribute to the birefringence.

G44. A strain sensor comprising: the optical fibre of any of clause G17 to G43; and a controller configured to determine a strain applied to the optical fibre based on the Bragg wavelength of the Bragg grating.

G45. The strain sensor of clause G44, wherein: the optical fibre is configured to exhibit birefringence; and the controller is configured to determine the strain based on a difference between the Bragg wavelength of the Bragg grating for light having different polarisations.

G46. A system for sensing strain and/or temperature comprising: the optical fibre of clause G24 or any preceding clause dependent thereon, wherein a change with temperature of the Bragg wavelength of the two Bragg gratings is different; and a controller configured to determine a strain applied to the optical fibre and/or a temperature of the optical fibre based on the Bragg wavelengths of the two Bragg gratings.

G47. A pressure sensor comprising: an optical fibre according to any of clauses G17 to G45; and a controller configured to determine the external pressure applied to the optical fibre based on the Bragg wavelength of the Bragg grating, wherein the pressure sensor is configured such that a pressure difference between a pressure of the filler and an external pressure applied to the optical fibre affects the Bragg wavelength of the Bragg grating.

G48. The pressure sensor of clause G47, wherein: the optical fibre is configured to exhibit birefringence; the pressure difference affects the birefringence; and the controller is configured to determine the external pressure applied to the optical fibre based on a difference between the Bragg wavelength of the Bragg grating for different polarisations of light.

G49. The optical fibre of any of clauses A54 to A66, or any of clauses A66 to A88, B1 to B11, C1 to C7 and D1 to D6, when dependent on any of clauses A54 to A66, or any of clauses G24 to G48, wherein one of the two Bragg gratings has a Bragg wavelength that decreases with increasing temperature and the other of the two Bragg gratings has a Bragg wavelength that responds differently to increasing temperature.

G50. The optical fibre of any of clauses A1 to G50, wherein the refractive index of the filler is greater than the refractive index of the material of the optical fibre.

G51. The optical fibre of any of clauses G1 to G50, wherein the one or more voids comprise two or more voids filled with different fillers, optionally wherein the material property of the fillers differs between the different fillers.

G52. The optical fibre of any of clauses G1 to G51, wherein the material property is an optical property, optionally refractive index.

G53. The optical fibre of clause G52, wherein the one or more voids are configured such that the optical property of the filler affects light transmitted through the optical fibre.

G54. The optical fibre of any of any of clauses G1 to G53, wherein a change with temperature of the material property of the filler is different to, optionally opposite to, a change with temperature of the material property of the material of the optical fibre.

G55. The optical fibre of clause G54, wherein the one or more voids are configured such that the change with temperature of the material property of the filler at least partially compensates for the change with temperature of the material property of the material of the optical fibre.

G56. The optical fibre of any of clauses G1 to G55, wherein a cross-sectional area of at least one of the one or more voids varies along the length of the void portion, optionally wherein the cross-sectional area reduces away from a centre of the void along the length of the optical fibre for at least a portion of the length of the void.

G57. The optical fibre of any of clauses G1 to G56, wherein the cross-section of at least one of the one or more voids is noncircular.

G58 The optical fibre of any of clauses G1 to G57, wherein a central axis of the one or more voids extends along a direction inclined to the longitudinal axis of the optical fibre for at least a portion of the one or more voids.

G59. The optical fibre of any of clauses G56 to G58, wherein: the material property is an optical property, optionally refractive index; the one or more voids are configured such that the optical property of the filler affects light transmitted through the optical fibre; and a variation of the cross-sectional area of the one or more voids and/or an orientation of the central axis of the one or more voids are such that light transmitted through the optical fibre experiences a substantially continuous variation in an effect of the optical property on the transmission of the light along the length of the optical fibre.

G60. The optical fibre of any of clauses G3 to G59, wherein the one or more voids comprise at least one cladding void within the cladding.

G61. The optical fibre of clause G60, wherein the cladding void is configured such that transmission of light within the optical fibre is affected by the filler within the cladding void, optionally wherein the at least one cladding void is adjacent to the core.

G62. The optical fibre of clause G60 or G61 , wherein either: a) the cladding void extends such that the filler is in contact with the core; or b) the cladding void extends such that the filler is not in contact with the core.

G63. The optical fibre of any of clause G60 to G62, wherein the one or more voids comprise a plurality of cladding voids within the cladding, optionally at least two cladding voids, optionally at least four cladding voids, optionally at least six cladding voids, optionally wherein the plurality of cladding voids is arranged symmetrically around the core.

G64. The optical fibre of any of clauses G60 to G63, wherein a distance between the cladding voids and the core varies along the length of the cladding void.

G65. The optical fibre of any of clauses G1 to G64, wherein the optical fibre is formed by drawing a preform, and the one or more voids are formed after the optical fibre is drawn.

G66. The optical fibre of any of clauses G1 to G66, wherein the one or more voids extend entirely within the void portion and/or a boundary of the one or more voids is defined entirely within the void portion.

G67. The optical fibre of any of any of clauses G1 to G66, wherein the one or more voids are entirely filled with the filler.

G68. The optical fibre of any of clauses G1 to G67, wherein the filler comprises a gas, optionally nitrogen or air.

G69. The optical fibre of any of clauses G1 to G68, wherein the filler comprises a non-gaseous material, optionally a liquid, further optionally wherein the filler comprises a liquid crystal.

G70. The optical fibre of any of any of clauses G1 to G69, wherein a refractive index of the filler is within 0.1 of a refractive index of the material of the optical fibre, optionally within 0.05, optionally within 0.01, optionally within 0.005, optionally within 0.001. G71. The optical fibre of clause G70, wherein the optical fibre comprises a core and a cladding surrounding the core, and the refractive index of the filler is approximately equal to the refractive index of a material of the cladding at a reference temperature. G72. The optical fibre of any of clauses G1 to G71 , wherein the optical fibre has an outer diameter of between 25 pm and 300 pm, optionally between 100 pm and 300 pm, optionally approximately 125 pm or 250 pm.

G73. The optical fibre of any of clauses G1 to G72, wherein the optical fibre is an antiresonant, or negative curvature fibre.

G74. The optical fibre of any of clauses G1 to G73, wherein the optical fibre comprises silica.

G75. The optical fibre of any of clauses G1 to G74, wherein the optical fibre comprises crystal material, for example singlecrystal material, optionally wherein the optical fibre is a crystal-derived fibre, for example a sapphire derived fibre.

G76. The optical fibre of any of clauses G1 to G75, wherein the optical fibre is a crystal optical fibre, optionally a single-crystal optical fibre.

G77. The optical fibre of clause G75 or clause G76, wherein: a) the optical fibre comprises sapphire, diamond, or yttrium aluminium garnet (YAG) crystal; and/or b) the crystal is doped, optionally with a rare-earth element.

G78. The optical fibre according to any of clauses G1 to G77 comprising: a coating, wherein: a change with temperature of a material property of the coating affects light transmitted through the optical fibre; and the one or more voids are configured such that a change with temperature of the material property of the filler at least partially compensates for a combined effect on light transmitted through the optical fibre of a change with temperature of the material property of the material of the optical fibre and the change with temperature of the material property of the coating.

G79. The optical fibre of clause G78, wherein either: a) the coating comprises polyacrylate or polyimide; or b) the coating is metallic.

G80. The optical fibre of any of clauses G78 to G79, wherein the optical fibre comprises a Bragg grating.

G81. The optical fibre of clause G80, wherein: a change with temperature of a material property of the coating affects the Bragg wavelength of the Bragg grating; and the one or more voids are configured such that the change with temperature of the material property of the filler at least partially compensates for the combined effect on the Bragg wavelength of the Bragg grating of the change with temperature of the material property of the material of the optical fibre and the change with temperature of the material property of the coating, optionally wherein the one or more voids are configured such that a magnitude of the change with temperature of the Bragg wavelength of the Bragg grating is at most 5pm/°C, optionally at most 2pm/°C, optionally at most 1 pm/°C over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C.

G82. The optical fibre of any of clauses G1 to G81 , wherein the optical fibre comprises a core and a cladding surrounding the core, and the one or more voids comprise a core void within the core.

G83. An optical fibre according to any of clauses G17 to G82, wherein the optical fibre comprises a core and a cladding surrounding the core; and the one or more voids comprise a core void within the core.

G84. The optical fibre of clause G82 or G83, wherein the core consists substantially of the core void.

G85. The optical fibre of clause G84, wherein the Bragg grating is provided by periodic modification of the cladding.

G86. The optical fibre of any of clauses G82 to G85, wherein the core is aligned with a central axis of the optical fibre.

G87. The optical fibre of any of clauses G82 to G86, wherein the Bragg grating is provided by periodic modification of the filler in the core void, optionally wherein the filler comprises a liquid crystal.

G88. The optical fibre of clause G87, wherein the filler is polymerizable and the periodic modification comprises periodic polymerisation, optionally wherein the filler comprises monomers and a photo-initiator and the periodic polymerisation is performed using a laser.

G89. The optical fibre of any of clauses G1 to G88, wherein the optical fibre further comprises one or more electrodes configured to apply an electric field to the filler to affect the material property of the filler.

G90. An optical fibre according to any of clauses G1 to G89, wherein the optical fibre further comprises one or more electrodes configured to apply an electric field to the filler to affect the material property of the filler.

G91. The optical fibre of clause G89 or G90, wherein the filler comprises a liquid crystal.

G92. The optical fibre of any of clauses G89 to G91, wherein the electric field is configured to affect one or more of the refractive index, the absorption, and the scattering loss of the filler.

G93. The optical fibre of any of clauses G89 to G92, wherein the filler exhibits birefringence and the electric field is configured to affect one or more of the magnitude of the birefringence, and the angle of an optic axis of the birefringence.

G94. The optical fibre of any of clauses G89 to G93, wherein the optical fibre comprises a Bragg grating, and the electric field is configured to affect a Bragg wavelength of the Bragg grating.

H1. An optical fibre comprising a first portion and a second portion, wherein: the second portion is a void portion comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre within the void portion; and the one or more voids are at least partially filled with a filler having a material property different to the material property of a material of the optical fibre, wherein the first portion and the second portion each comprise: a core; and a cladding, wherein the respective cladding surrounds the respective core and wherein the maximum lateral dimension of the core of the first portion in a direction substantially perpendicular to the longitudinal axis of the optical fibre is substantially the same as the maximum lateral dimension of the core of the second portion in the direction substantially perpendicular to the longitudinal axis of the optical fibre at an interface between the first portion and the second portion.

11. An optical fibre comprising a void portion comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre within the void portion; and the one or more voids are at least partially filled with a filler having a material property different to the material property of a material of the optical fibre, wherein the void portion comprises: a core; and a cladding, wherein the cladding surrounds the core and wherein the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fibre is greater than 5.5 micrometres and less than 11 micrometres, optionally wherein the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fibre is between 6 and 10.5 micrometres, optionally wherein the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fibre is between 7 and 10 micrometres, optionally wherein the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fibre is between 8 and 9 micrometres, optionally wherein the maximum lateral dimension of the core in a direction substantially perpendicular to the longitudinal axis of the optical fibre is approximately 8.2 micrometres.

J1. A device comprising two Bragg gratings, wherein one of the two Bragg gratings has a Bragg wavelength that decreases with increasing temperature and the other of the two Bragg gratings has a Bragg wavelength that responds differently to increasing temperature.

J2. The device of clause J1, wherein the device comprises an optical fibre, and the two Bragg gratings are provided within the optical fibre.

K1. A method of manufacturing a device comprising an optical fibre, the method comprising: providing an optical fibre comprising one or more voids extending longitudinally along the length of the optical fibre; filling the one or more voids with a filler; solidifying the filler; and splicing the optical fibre with the solidified filler to a transmission fibre. K1a. The method of claim K1, wherein the splicing is performed by laser splicing, optionally wherein the laser splicing selectively targets portions of the optical fibre outside of the one or more voids.

K2. The method of claim K1 or K1 a, wherein the filler is a polymer which can be solidified by exposure to ultraviolet light or by heat curing.

K3. The method of any of claims K1 to K2, wherein the filler has a lower melting point than a melting point of a material of the optical fibre, and the step of filling the one or more voids with the filler is performed at a temperature above the melting point of the filler and below the melting point of the material of the optical fibre, optionally wherein the filler is a low melting point glass.

K4. The method of any of claims K1 to K3, wherein the method further comprises providing a bridging fibre between the optical fibre and the transmission fibre, the bridging fibre comprising one or more voids filled with a liquid having a melting point below a melting point of a material of the transmission fibre.

L1. A method of manufacturing a device comprising an optical fibre, the method comprising: providing an optical fibre comprising one or more voids extending longitudinally along the length of the optical fibre; splicing the optical fibre to a transmission fibre at one or both ends; forming a hole in a side of the optical fibre, the hole providing access from outside the optical fibre to a first void of the one or more voids; filling the first void with a filler; and sealing the hole.

L2. The method of claim L1 , wherein sealing the hole comprises solidifying the filler.

L3. The method of claim L1 or L2, wherein forming the hole comprises using a laser

L3A. The method of claim L3, wherein forming the hole comprises using laser ablation.

L4. The method of claim L3, wherein forming the hole comprises: selectively exposing the optical fibre to laser radiation to define an exposed region within the optical fibre; and contacting the optical fibre with an etchant, wherein the etchant etches the exposed region at a higher rate than regions of the optical fibre not exposed to laser radiation, such that the hole is formed by etching of the exposed region.

M1. An optical fibre comprising at least two waveguides at different positions within a cross-section of the optical fibre, wherein a change with temperature in a refractive index of a first waveguide of the at least two waveguides is different to a change with temperature in a refractive index of a second waveguide of the at least two waveguides.

M2. The optical fibre of clause M 1 , wherein a material of the first waveguide is different to a material of the second waveguide.

M3. The optical fibre of clause M2, wherein the material of the first waveguide and the material of the second waveguide comprise glass.

M4. The optical fibre of clause M2 or M3, wherein: the optical fibre is drawn from a preform comprising a doped core and a void; following the drawing of the fibre, the void is filled with a glass having a lower melting temperature than a material of the optical fibre; and the doped core provides the first waveguide, and the filled void provides the second waveguide.

M5. The optical fibre of any of clauses M1 to M4, wherein both the first waveguide and the second waveguide comprise a Bragg grating.

M6. The optical fibre of any of clauses M1 to M5, wherein the optical fibre is a multicore fibre; the first waveguide and the second waveguide are provided by first and second cores of the optical fibre respectively; and a refractive index of the second core is different to a refractive index of the first core.

M7. The optical fibre of clause M6, wherein: the optical fibre comprises at least four cores; one of the at least four cores is a central core; the central core provides one of the first and second cores; and a refractive index of the central core has a different temperature dependence to a temperature dependence of a refractive index of the other cores of the at least four cores.

M8. The optical fibre of any of clauses M1 to M7, wherein the refractive index is an effective refractive index.

Claims

1. An optical fibre comprising a first portion and a second portion, wherein: the second portion is a void portion comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre within the void portion; the first portion and the second portion are substantially mode-matched; and the one or more voids are at least partially filled with a filler having a material property different to the material property of a material of the optical fibre.

2. The optical fibre of claim 1 , wherein the mode field diameter of the first portion for a given wavelength and temperature is substantially the same as the mode field diameter of the second portion for the given wavelength and temperature.

3. The optical fibre of claim 1 or 2, wherein the first portion and the second portion each comprise: a core; and a cladding, wherein the respective cladding surrounds the respective core.

4. The optical fibre of claim 3, wherein the refractive index of the core of the first portion is substantially the same as the refractive index of the core of the second portion, optionally wherein the refractive index of the core of the first portion is between 0.002 and 0.007 greater than the refractive index of the cladding surrounding the core of the first portion, optionally wherein the refractive index of the core of the first portion is approximately 0.005 greater than the refractive index of the cladding surrounding the core of the first portion.

5. The optical fibre of claim 3 or 4, wherein the maximum lateral dimension of the core of the first portion in a direction substantially perpendicular to the longitudinal axis of the optical fibre is substantially the same as the maximum lateral dimension of the core of the second portion in the direction substantially perpendicular to the longitudinal axis of the optical fibre at an interface between the first portion and the second portion.

6. The optical fibre of any preceding claim, wherein the first portion is a sealing portion and the one or more voids are at least partially sealed by the sealing portion.

7. The optical fibre of claim 6, wherein the one or more voids are at least partially sealed from an exterior of the optical fibre at an interface between an end face of the sealing portion and an end face of the void portion.

8. The optical fibre of claim 7, wherein the one or more voids are sealed from an exterior of the optical fibre at a further interface between the other end face of the void portion and an end face of a further sealing portion.

9. The optical fibre of any preceding claim, wherein the optical fibre is a single mode optical fibre.

10. The optical fibre of any preceding claim, wherein the first portion comprises a bridging portion comprising a core having a cross-sectional area that varies along the longitudinal axis of the bridging portion from a first cross-sectional area to a second cross-sectional area, wherein the second cross-sectional area has a maximum lateral dimension that is substantially the same as the maximum lateral dimension of a core of the void portion.

11. The optical fibre of any preceding claim, wherein the optical fibre comprises a Bragg grating.

12. The optical fibre of claim 11 , wherein the material property is an optical property and the one or more voids are configured such that the optical property of the filler affects the Bragg wavelength of the Bragg grating.

13. The optical fibre of claim 11 or 12, wherein the one or more voids are configured such that a change with temperature of the material property of the filler at least partially compensates for an effect on the Bragg wavelength of the Bragg grating of a change with temperature of the material property of the material of the optical fibre, optionally wherein the material property is an optical property, optionally wherein the one or more voids are configured such that a magnitude of the change with temperature of the Bragg wavelength of the Bragg grating is at most 5pm/°C, optionally at most 2pm/°C, optionally at most 1 pm/°C over a temperature range of at least 10°C, optionally at least 20°C, optionally at least 40°C.

14. The optical fibre of any preceding claim, wherein: the void portion is a continuous and integral portion of the optical fibre; a longitudinal extent of each of the one or more voids is less than a longitudinal extent of the void portion; and the one or more voids are sealed from an exterior of the optical fibre.

15. The optical fibre of any of claims 11 to 14, wherein the optical fibre comprises two separate Bragg gratings.

16. A system for sensing strain and/or temperature comprising: the optical fibre of claim 15, wherein a change with temperature of the Bragg wavelength of the two Bragg gratings is different; and a controller configured to determine a strain applied to the optical fibre and/or a temperature of the optical fibre based on the Bragg wavelengths of the two Bragg gratings.

17. A method of manufacturing a device comprising an optical fibre, the method comprising: providing an optical fibre comprising one or more voids extending longitudinally along the length of the optical fibre; filling the one or more voids with a filler; solidifying the filler; and splicing the optical fibre with the solidified filler to a transmission fibre.

18. The method of claim 17, wherein the splicing is performed by laser splicing, optionally wherein the laser splicing selectively targets portions of the optical fibre outside of the one or more voids.

19. The method of claim 17 or 18, wherein the filler is a polymer which can be solidified by exposure to ultraviolet light or by heat curing.

20. The method of any of claims 17 to 19, wherein the filler has a lower melting point than a melting point of a material of the optical fibre, and the step of filling the one or more voids with the filler is performed at a temperature above the melting point of the filler and below the melting point of the material of the optical fibre, optionally wherein the filler is a low melting point glass.

21. The method of any of claims 17 to 20, wherein the method further comprises providing a bridging fibre between the optical fibre and the transmission fibre, the bridging fibre comprising one or more voids filled with a liquid having a melting point below a melting point of a material of the transmission fibre.

22. A method of manufacturing a device comprising an optical fibre, the method comprising: providing an optical fibre comprising one or more voids extending longitudinally along the length of the optical fibre; splicing the optical fibre to a transmission fibre at one or both ends; forming a hole in a side of the optical fibre, the hole providing access from outside the optical fibre to a first void of the one or more voids; filling the first void with a filler; and sealing the hole.

23. The method of claim 22, wherein sealing the hole comprises solidifying the filler.

24. The method of claim 22 or 23, wherein forming the hole comprises using a laser

25. The method of claim 24, wherein forming the hole comprises using laser ablation.

26. The method of claim 24, wherein forming the hole comprises: selectively exposing the optical fibre to laser radiation to define an exposed region within the optical fibre; and contacting the optical fibre with an etchant, wherein the etchant etches the exposed region at a higher rate than regions of the optical fibre not exposed to laser radiation, such that the hole is formed by etching of the exposed region.

27. A method for forming one or more voids in an optical fibre comprising: selectively exposing the optical fibre to laser radiation to define one or more exposed regions within the optical fibre; contacting the optical fibre with an etchant, wherein the etchant etches the exposed regions at a higher rate than regions of the optical fibre not exposed to laser radiation, such that the one or more voids are formed by etching of the exposed regions; at least partially filling the one or more voids with a filler having a material property different to that of a material of the optical fibre; and sealing the one or more voids from an exterior of the optical fibre.

28. An optical fibre comprising at least two waveguides at different positions within a cross-section of the optical fibre, wherein a change with temperature in a refractive index of a first waveguide of the at least two waveguides is different to a change with temperature in a refractive index of a second waveguide of the at least two waveguides.

29. A crystal optical fibre comprising one or more voids, wherein the one or more voids extend longitudinally along the length of the optical fibre.

30. A device comprising two Bragg gratings, wherein one of the two Bragg gratings has a Bragg wavelength that decreases with increasing temperature and the other of the two Bragg gratings has a Bragg wavelength that responds differently to increasing temperature.