WO2007077419A1

WO2007077419A1 - Optical fibre with angled core

Info

Publication number: WO2007077419A1
Application number: PCT/GB2006/004851
Authority: WO
Inventors: William Andrew Clarkson; Pu Wang
Original assignee: University Of Southampton
Priority date: 2006-01-05
Filing date: 2006-12-21
Publication date: 2007-07-12

Abstract

An optical fibre is heated and twisted so that the resulting optical fibre has a core that inclines at an angle with the longitudinal axis. The angled core (12) may be located at an end-facet (18) of the optical fibre and used to control the feedback of light reflected from that end-facet. In this way, the angled core may be used to suppress feedback for applications such as superluminescent sources or fibre amplifiers, or may be used to control the feedback for applications such as optical fibre lasers. Alternatively, the angled core may be used to attenuate certain modes propagating within the core.

Description

TITLE OF THE INVENTION

OPTICAL FIBRE WITH ANGLED CORE

BACKGROUND TO THE INVENTION

Field of invention

This invention relates to fibre-based optical sources and devices, and more particularly to novel fibre geometries for reducing unwanted optical feedback from a fibre end-facet or from the interface between a fibre core and another fibre core or a transparent optical material to improve the performance of the fibre source or the fibre device.

Description of related art

Optical fibre sources based on core-pumped and cladding-pumped rare-earth-doped optical fibres have seen rapid development over the last two decades driven by the needs of a range of applications. The main attractions of fibre-based sources are derived from their geometry, which offers relative immunity from thermal effects and flexibility in mode of operation. The tight confinement of the active ions within the small core region_allows relatively low thresholds and high gains to be achieved in oscillator and amplifier configurations respectively, as well as high efficiency. Another attraction of fibre-based sources is the array of commercially-available fibre- pigtailed devices that can be employed to help in delivering the light from one active element to the next, and to the final application, as well as for providing extra control of the light characteristics (e.g. power, pulse duration, wavelength spectrum, polarisation). There are many situations where unwanted optical feedback from a fibre end-facet, or from an interface between the fibre core and another fibre or transparent medium can be very detrimental to the performance of an optical fibre source or an optical fibre device. A striking example of this problem is in superfluorescent fibre sources [I]. Superfluorescent fibre sources (sometimes referred to as amplified spontaneous emission (ASE) fibre sources) exploit the high gain that can be achieved in an rare-earth-doped fibre core to amplify spontaneous emission to produce a broadband, spatially-coherent output. Such sources have a range of important applications, but output powers have been limited to levels far below those routinely achieved for more conventional fibre laser oscillator or master-oscillator power-amplifier (MOPA) configurations due to feedback from the fibre end facets which results in parasitic lasing [2]. Similarly, the maximum small-signal gain for a single fibre gain element is very often limited by the onset of parasitic lasing due to unwanted feedback from the fibre end facets. In some fibre oscillator configurations, optical feedback from one or both fibre end facets may also be detrimental to performance. For example, if an external cavity is employed to extend the functionality of the laser (e.g. for Q-switching, mode-locking or wavelength tuning), then feedback from the fibre end-facet adjacent to the external cavity will compete with the feedback provided by the external cavity and hence may impact adversely on the laser performance.

In addition, there are also many situations where unwanted optical feedback from interfaces in passive optical fibre devices or light delivery fibres (e.g. fibre pigtails for fibre- coupled semiconductor diode lasers) can also be detrimental to the performance of the device and/or the fibre system in which it is employed. For example, many fibre optic devices, including fiber couplers, combiners, splitters and grating dispersion compensators, have redundant ports or redundant optical fibre pigtails. The redundant pigtails need to be properly terminated to eliminate or suppress back- reflections from the unconnected fibre ends. Several methods for reducing optical feedback from fibre terminations have been developed to remedy this problem [3, 4]. In these approaches, back-reflections from fibre end terminations are reduced by employing a high absorption region at the end of the fibre, or by forming a plurality of loops in the optical fibre by winding it tightly around a spool. These methods suffer from the disadvantage that extra components are needed adding further cost to the overall system as well as extra complexity. The main physical origin of the optical feedback is Fresnel reflection [5], which results from a difference in the refractive indices of the core material and the adjacent fibre or transparent material (e.g. air) One standard method for reducing unwanted feedback is to 'angle-polish' the fibre end-facet so that most of the light reflected from the facet (due to Fresnel reflection) no longer couples to a guided mode of the core [6]. As a rough guide, this can result in a significant reduction in the effective feedback reflectivity provided that the facet is polished so that the angle between the normal to its surface and the longitudinal axis of the fibre is greater than θ_core/2 where θς_ore = arcsin (NA) and NA is the numerical aperture of the fibre core. However, as a result of imperfections in the polished surface, some of the incident light on the facet is scattered and a fraction of this light is then fed back along the core. Thus, this approach is not well-suited for fibre configurations (e.g. superfluorescent fibre sources) that require very low levels of optical feedback. A further drawback of this approach is that polishing of the fibre end is quite time consuming, adding to the overall cost. However, it is difficult to splice the angle-polished fibre end to other fibres (e.g. for extending functionality or for power delivery).

Another approach is to directly splice the fibre to another fibre or fuse it to a piece of transparent material of similar refractive index [7]. This approach exploits the reduction in the Fresnel reflection coefficient R=(ni-n₂)²/(ni+n₂)², when the refractive index (n,) for the fibre core and the refractive index (n₂) for spliced fibre or transparent material are well-matched. In practice, it very difficult to obtain a perfect match, so there will always be a residual Fresnel reflection at the interface which may be detrimental to the performance of the fibre source or device if very low levels of feedback are required. Similarly, a single layer or multi-layer dielectric anti- reflection coating may be used to reduce the reflection coefficient. However, it is difficult to reduce the reflectivity of the end facet to < 0.1-0.2% in this manner. Moreover, applying dielectric coatings to fibre end facets is both costly and inconvenient. Thus, the techniques for reducing unwanted optical feedback from a fibre end facet or from interfaces between different fibres in fibre sources and devices, described in the prior art, are of limited value for some applications. SUMMARY OF THE INVENTION

The approach for reducing optical feedback from fibre end-facets and from interfaces between fibres according to this invention uses a new fibre geometry to overcome the limitations of the prior art allowing a reduction in the residual feedback from one or more fibre ends or interfaces as required for power scaling of fibre-based superfluorescent sources, the achievement of very high small signal gains from single element fibre amplifiers, and for extending the functionality and enhancing the performance of various fibre laser configurations. The novel fibre geometry according to this invention can, if so desired, also be configured to provide a spatial mode filtering function for the transmitted and reflected light to improve the beam quality for fibre sources and devices that employ multi-mode cores. A further important feature of this invention is that it can also be applied to passive fibre devices to reduce or virtually eliminate optical feedback from a fibre end facet when required.

The approach for reducing optical feedback according to this invention exploits the fact that a fibre end-facet prepared by certain techniques (for example, cleaving or melting) can provide a very low scattering loss. However, the fibre end-preparation techniques that result in a low scattering loss are generally much more effective and easier to implement when the fibre's end-facet is perpendicular or substantially perpendicular to the longitudinal axis of the fibre. Under these circumstances optical feedback from the end-facet is dominated by Fresnel reflection at the interface, and hence is typically quite large (e.g. ~3.6% for a silica-air interface). However, in the present invention a novel fibre geometry is employed in which the core trajectory in the section of the fibre adjacent to one or both fibre end facets is inclined at an angle with respect to the fibre's longitudinal axis so that light propagating in the core is reflected from one or both end facets at an angle with respect to the core's axis at the facet and hence the effective feedback efficiency from one or both fibre end facets can be reduced dramatically compared to methods for reducing optical feedback described in the prior art. The approach for reducing optical feedback according to this invention has the attraction that very simple and inexpensive fibre end facet preparation techniques (e.g. cleaving and/or melting) that yield very low scattering losses can be employed. Moreover, the use of substantially perpendicular fibre end facets at both end of the fibre simplifies the launching of pump light in cladding- pumped fibre sources and simplifies the integration of the fibre source or fibre device within a more complex fibre-based system.

An alternative approach for reducing the unwanted feedback from fibre end facets is to use a fibre with a helical core trajectory within the inner-cladding. Such fibres have been studied primarily as a means for scaling fibre laser and fibre amplifier power levels whilst retaining good output beam quality [8, 9]. However, helical-core fibres suffer from many short comings compared to the fibre geometry according to this invention. Firstly, helical-core fibres have a constant helical pitch along their entire length which results in increased propagation loss for light in the core, due to the core's helical trajectory [10], and increased propagation loss for pump light in cladding-pumped configurations, due to reflection from the core-cladding interface. This greatly limits flexibility in design and limits the maximum pitch angle (i.e. the angle between the core trajectory and the longitudinal axis of the fibre) that can be used. This in turn limits the level of suppression of optical feedback from the end- facet that can be achieved before the core propagation loss becomes too high for most practical applications. By contrast, the fibre geometry for reducing unwanted optical feedback from the fibre end facets according to this invention allows much more flexibility in the overall fibre design and in the mode of operation of the fibre, since the angle of the core's trajectory with respect to the longitudinal axis of the fibre is large only in the section of fibre near to one or both end facets. In this way, the propagation losses in the core and in the cladding (i.e. for double-clad fibre geometries) can be made very small, and comparable with the core and cladding propagation losses for conventional straight-core fibres. Moreover, a larger angle between the core trajectory and the longitudinal axis of the fibre can be employed to further suppress unwanted feedback from the fibre end facet, since the angled-core trajectory need only be maintained for a very short distance (i.e. less than ~10mm) at the fibre ends. Another attraction of the fibre geometry according to this invention is that different core trajectories (i.e. core pitch angles) can be used at the two fibre ends to give different levels of optical feedback from the end facets, and hence added flexibility in mode of operation. The latter feature can be used in superfluorescent fibre configurations and in some laser oscillator configurations to yield a predominantly single direction output beam without resort to additional optical elements (e.g. mirrors or in-fibre Bragg gratings).

A further attraction of the fibre geometry according to this invention is that the required geometry can be fabricated in a relatively simple manner. In contrast, helical cores fibres are difficult and expensive to fabricate. One way to fabricate a fibre with a geometry according to the present invention, by way of example only, is to first of all fabricate a fibre with straight-core that is transversely offset from the longitudinal axis of the fibre and then heat the fibre and twist it to produce a permanent region where the core has an angled-trajectory with respect to the longitudinal axis of the fibre. The fibre can then be cleaved perpendicular to the longitudinal axis in the 'angled-trajectory' region to produce the required fibre end geometry necessary for suppressing feedback from the end-facet. The fibre core has a refractive index, nj, which is larger than the refractive index, n₂, of the surrounding cladding material, as required for guiding of light within the core region. The cladding is surrounded by an outer-cladding (or coating) of refractive index, n₃, which may be higher than the refractive index (n₂) of the inner-cladding, or may be lower than n₂ to allow pump light to be guided in the inner-cladding as required for cladding-pumped fibre configurations. The cross-section of the inner-cladding may be circular or non- circular (e.g. D-shaped) provided that the core is transversely off-set from the longitudinal axis about which the fibre can be permanently twisted when heated in at least one section of the fibre. Additionally, the core may be doped with an active laser ion (e.g a rare earth ion such as ytterbium, neodymium, erbium, thulium, holmium) or a combination of active laser ions to provide gain at various wavelengths and to facilitate efficient absorption of pump light from one or more pump lasers. Thus, this fibre geometry can be applied to active (light emitting) fibre devices and to passive devices. In the case of active fibre devices, this invention also comprises a pump laser for exciting the active ions in the fibre core. The pump laser may, for example, be a diode laser, a fibre laser source, a solid-state laser, or a combination of more than one pump laser of the same or different types. The invention also comprises a means for coupling pump light from said pump laser(s) into the active fibre through one or both end facets and/or through the side of the fibre. In all cases, the invention also comprises a fibre in which the core trajectory in the section of the fibre adjacent to one or both fibre end facets is inclined at an angle with respect to the fibre's longitudinal axis so that light propagating in the core is reflected from one or both end facets at an angle with respect to the core's axis at the facet, so that the effective feedback efficiency from one or both fibre end facets is reduced dramatically compared to methods for reducing optical feedback described in the prior art.

The novel fibre geometry for reducing unwanted feedback from a fibre end facet has many advantages over the prior art techniques described above, owing to the ease of its implementation, its improved performance and the flexibility in design it offers. In addition, to suppressing unwanted optical feedback, the fibre geometry can also provide a mode filtering function, by virtue of the core's curvature leading to a higher core propagation loss for higher order modes than for low order modes. The fibre geometry can be tailored, if required, to produce a very high loss for high order modes in order to improve the output beam quality (M² parameter). The reduction in unwanted optical feedback from one or both fibre end facets allows the construction of very high power fibre superfluorescent sources, very high gain fibre amplifiers and simpler fibre laser configurations with improved performance in some modes of operation.

According to a first aspect of the invention there is provided an optical fibre having a longitudinal axis, the optical fibre comprising: a core surrounded by a cladding, wherein the refractive index of the core is greater than the refractive index of the cladding; a first section in which the core follows a first path with respect to the longitudinal axis; and a second section in which the core follows a second path different to the first path, wherein the second path inclines at an angle with respect to a plane normal to the longitudinal axis. The optical fibre has a core that forms a non-perpendicular angle with respect to a plane normal to the longitudinal axis of the optical fibre as described above. The angle that the core forms with respect to a plane normal to the longitudinal axis may be constant or it may vary across the length of the second section. The core within the first section may follow a path that is substantially parallel to the longitudinal axis or it may follow a helical path. The core within the second section of the optical fibre may also follow a generally helical path with the axis of the helix substantially aligned with the longitudinal axis of the optical fibre. The term "helical" describes the general manner in which the core twists around the longitudinal axis, and is not meant to be construed by the strictest geometrical definition. In fact, the path may follow a uniform helix or, alternatively, the angle of the helix and/or the offset of the helix may vary along the second section of the optical fibre. The core in the second section may follow a helical path for a plurality of turns, or only a part of a turn.

In one embodiment the second section terminates at an end-facet.

The optical fibre may comprise a third section in which the core follows a third path that inclines at an angle with respect to a plane normal to the longitudinal axis. The angle of the third path may be the same, or may be different, to the angle of the second path and may also terminate at an end-facet. In fact, the optical fibre may comprise a plurality of further sections in which the core follows a path that inclines at an angle with respect to a plane normal to the longitudinal axis.

In one embodiment, the angle at which the core inclines with respect to a plane normal to the longitudinal axis is within the range: 0.5 degrees to 1 degree, 0.5 degrees to 89.5 degrees to 89 degrees, 89.5 degrees to 88 degrees, 89.5 degrees to 85 degrees, 89.5 degrees to 80 degrees, 89.5 degrees 70 degrees, 89.5 degrees to 60 degrees. Alternatively, the angle may be within the range: 89.5 degrees to 85 degrees,

85 degrees to 80 degrees, 80 degrees to 75 degrees, 75 degrees to 70 degrees, 70 degrees to 65 degrees, 65 degrees to 60 degrees.. In a further embodiment, a section of material is placed in contact with an end-facet of the optical fibre and has a refractive index that is substantially equal to the refractive index of the core. In this way, any reflection from the end-facet can be suppressed. Further suppression of the reflection from the end-facet may be attained by coating the end-facet with an anti-reflection coating.

In a further embodiment, the optical fibre further comprises a second cladding layer, which has a refractive index greater or lesser than the refractive index of the cladding. Thus, the second cladding may aid in confining waveguide modes or may provide a geometry suitable for cladding pumping.

In one embodiment the second section comprises a spliced region between a first optical fibre and a second optical fibre. Both optical fibres have cores that incline at an angle with respect to a plane normal to the longitudinal axis. These cores are aligned and the splice is performed to permanently bond the two optical fibres.

In a further embodiment, the core is doped with an active ion that is operable to absorb light at a first wavelength and to emit light at a second wavelengths. The first and the second wavelengths describe the peak wavelength of the absorption or the emission. The absorption or emission will exhibit a bandwidth around this peak wavelength. In fact, the first wavelength and the second wavelength may specify the peak wavelength of a broadband absorption or emission. The active ion may comprise any, or any combination, of the following ions: ytterbium, neodymium, erbium, thulium, holmium. Alternatively, the active ion may be any rare-earth ion, such as praseodymium, dysprosium or samarium.

One embodiment is a source for producing amplified spontaneous emission comprising: the doped optical fibre, wherein the angle of the second path at an end of the second section is chosen to suppress optical feedback into the core; and a pump source operable to emit light at the first wavelength of the active ion and couple the light into the optical fibre. In this instance, the end of the second section defines the plane normal to the longitudinal axis. In this embodiment, a super luminescent broadband optical output is obtained by designing the inclined core at the end-facets of the optical fibre to suppress a desired fraction of light from being coupled back into the core. This prevents laser oscillation that would narrow the output spectrum. Sources of amplified spontaneous emission with sufficient power are finding uses in many applications, such as low coherence interferometry and as pump sources for optical parametric oscillators.

A further embodiment is an optical fibre amplifier comprising: the doped optical fibre, wherein the angle of the second path at an end of the second section is chosen to suppress optical feedback into the core; a pump source operable to emit light at the first wavelength of the active ion and couple the light into the optical fibre; and a signal source operable to emit light at the second wavelength of the active ion and couple the light into the core of the optical fibre. In this instance, the end of the second section defines the plane normal to the longitudinal axis. In this embodiment, the feedback of light into the core is controlled so that the doped optical fibre acts as an amplifier. Prior art fibre amplifiers prevent laser oscillation by a combination of suppression of the optical feedback and a relatively high-power input signal, which saturates the gain available during amplification. However, the angled core within the second section of the optical fibre allows near complete suppression of feedback into the doped optical fibre, which prevents laser oscillation at much higher gain levels than is currently possible with prior art fibre amplifiers. Thus, the optical fibre amplifier of this embodiment may be used to amplify low-power signals without laser oscillation occurring.

A further embodiment is an optical fibre laser comprising: the doped optical fibre, wherein the angle of the second path at an end of the second section is chosen to control optical feedback into the core; and a pump source operable to emit light at the first wavelength of the active ion and couple the light into the optical fibre. In this instance, the end of the second section defines the plane normal to the longitudinal axis. In this embodiment, the angle at which the core is inclined to the longitudinal axis is chosen so that sufficient feedback is provided to cause the doped optical fibre to operate as a laser. Alternatively, the optical fibre laser of this embodiment may be used in an extended cavity laser. This requires the suppression of feedback from an end-facet involved with the extended cavity, since feedback from the end-facet would provide a secondary oscillator that would compete for gain with the oscillator that includes the extended cavity.

The above embodiments describe pumping a doped optical fibre to cause the active ions to emit light. The power of the light emitted from each end-facet of an optical fibre laser can be approximated by the following formula:

Where Pi is the power output from the first end-facet, P₂ is the power output from the second end-facet, Ri is the effective reflectivity at one end-facet and R₂ is the effective reflectivity at the remaining end-facet. This formula is only an approximation for the emission from each end-facet for an optical fibre laser, but it also provides a physical insight into the emission that would occur from certain optical fibre super luminescent sources. In the present case, the effective reflectivity at both end-facets is dependent upon the numerical aperture of the optical fibre (in the present embodiment, typically within the range of 0.03 to 0.25) and the angle that the core forms with respect to the longitudinal axis. The value of the effective reflectivity is relatively low, since the addition of the angled core reduces the effective reflectivity below the level of the standard Fresnel reflection. Thus, the first term on the right hand side of the above equation approaches unity, which leads to the ratio of the powers output from each end-facet being dictated by the square root of the ratio of the effective reflectivities at each end-facet. This square root relationship causes a small change in the ratio of the effective reflectivities to effect a significantly greater change in the ratio of the powers output from each end-facet. Thus, the power output from one end-facet can be designed to be significantly greater than the power output from the remaining end-facet. This is a significant advantage over prior art active optical fibre devices that exhibit similar power output from each end-facet. In the prior art devices, the power output from one end facet is used and the output power from the remaining end-facet is typically lost. Suppression of higher order modes is performed in one embodiment using an optical fibre with a core in the second section that tapers from a first diameter to a second diameter. The number of waveguide modes supported by the core reduces as the core diameter reduces, with the highest order modes being the first modes to lose confinement. The second section of the optical fibre may also be designed to act as a mode filter by designing the path of the core in the second section so that, of a plurality of spatial modes confined in the first section of the optical fibre, certain spatial modes experience significant attenuation. In fact, the path in the second section may be designed so that all of the modes confined in the first section of the optical fibre are significantly attenuated in the second section of the optical fibre.

A typical optical fibre coupler or splitter has at least one optical fibre connection that is not used for inputting or outputting an optical signal. In one embodiment a second section, in which the core follows a second path that inclines at an angle with respect a plane normal to the longitudinal axis, is formed in the unused optical fibre connection. The angle of the second path at an end of the second section is chosen to suppress the Fresnel reflection from that end of the second section, and in this way allows the unused optical fibre connection to be terminated. The termination may also be achieved if the second path is helical, and the angle of the second path with respect to a plane normal to the longitudinal axis being chosen to cause significant bending losses to modes supported in the unused optical fibre connection.

According to a second aspect of the invention there is provided a method of fabricating an optical fibre with a section of core that inclines at an angle with respect to a plane normal to the longitudinal axis of the optical fibre, comprising: providing an optical fibre having a core surrounded by a cladding, wherein the refractive index of the core is greater than the refractive index of the cladding; softening a section of the optical fibre by heating the section of the optical fibre; and twisting the section of the optical fibre with respect to the remaining optical fibre in order to cause the core within that section to incline at an angle with respect to a plane normal to the longitudinal axis. The twisted section of optical fibre may be formed in a manufactured optical fibre, or alternatively, the twisted section may be formed during manufacture of the optical fibre.

In one embodiment, an end-facet termination is formed within the section of the optical fibre. The end-facet termination may be formed by any end-facet termination technique including: cleaving, cleaving then melting the formed end-facet or cleaving then polishing the formed end-facet. The end-facet may be oriented substantially perpendicular to the longitudinal axis.

In one embodiment, the core is twisted so that the core forms a spiral around the longitudinal axis. The core may also be tapered from a first diameter to a second diameter within the section of the optical fibre. One option for tapering the core is to pull the opposite ends of the softened section away from each other, either before, during or after the twisting step.

In a further embodiment the optical fibre may comprise a second cladding, surrounding the cladding, with a refractive index that may be greater or lesser than the refractive index of the cladding.

In one embodiment, the core is doped with an active ion that is operable to absorb light at a first wavelength and to emit light at a second wavelength. The active ion may comprise any, or any combination, of the following ions: ytterbium, neodymium, erbium, thulium, holmium. Alternatively, the active ion may be any rare-earth ion, such as praseodymium, dysprosium or samarium.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings.

Figures l(a) and l(b) are schematic plan and side views respectively of a fibre end section and fibre end facet according to a preferred embodiment of the present invention.

Figures 2(a) and l(b) are schematic plan and side views respectively of a fibre end section and fibre end facet according to a second preferred embodiment of the present invention.

Figures 3 (a) and 3(b) are schematic end face views of two examples of different fibre cross-sectional designs that may be used for the present invention.

Figures 4(a) and 4(b) are schematic plan and side views of a fibre end section and fibre end facet according to a third preferred embodiment of the present invention

Figures 5 (a) and 5(b) are schematic plan and side views of two fibres in optical contact or joined by an appropriate means according to another preferred embodiment of the present invention

Figures 6(a) and 6(b) are schematic plan and side views of a superfluorescent fibre source according to another embodiment of the present invention.

Figures 7(a) and 7(b) are schematic plan and side views of a fibre amplifier according to another embodiment of the present invention.

Figures 8(a) and 8(b) are schematic plan and side views of a fibre laser according to another embodiment of the present invention. Figures 9(a) and 9(b) are schematic plan and side views of a fibre laser with an external feedback cavity configuration according to another embodiment of the present invention.

Figures 10(a) and 10(b) are schematic views of a fibre coupler configuration according to another embodiment of the present invention.

DETAILED DESCRIPTION

A preferred embodiment of the present invention will now be described, by way of example only:

With reference to figures l(a) and l(b), an optical fibre 10 comprising a single-mode or multimode core 12 with average refractive index ni, an inner-cladding 14 with refractive index n₂, an outer-cladding 16 with refractive index n₃, an end-facet 18 and with a longitudinal axis 20 is configured such that the core 12 is transversely off-set in a first direction (x direction) by distance δ. The core 12 is substantially straight and parallel to the longitudinal axis 20 over one section of the fibre and then inclined at an angle, θ, with respect to the longitudinal axis 20 at the end section of the fibre. Light propagating along the core 12 is incident on the fibre end-facet 18 at angle of incidence θ and is then reflected so that the reflected light experiences a high core propagation loss (due to the angle of the reflected light). Hence optical feedback from the fibre end-facet is dramatically reduced. In a preferred embodiment of the present invention, the end facet 18 is substantially perpendicular to the fibre's longitudinal axis 20 and is prepared by a method (e.g. cleaving or melting) that yields a low scattering loss for light incident on its surface. Moreover, the surface of the facet may be substantially flat or slightly curved depending on the preparation method employed. The above fibre geometry may be fabricated by taking a section of fibre with a straight, but transversely offset core, as for the first section of fibre shown in figures l(a) and l(b), removing a section of the outer-coating, and then heating the end section of the fibre and twisting the fibre about the longitudinal axis 20. The fibre can then be cleaved at a point within the twisted region to produce the desired fibre geometry. The outer-coating may then be re-applied if necessary. The angle θ between the core's trajectory and the longitudinal axis 20 is determined by the offset distance δ and by the twist angle per unit length of fibre β. Angle θ can be increased by increasing δ and/or β depending on the required level of suppression of feedback from the fibre end-facet. As a very rough guide, in order to achieve a relatively high suppression of the optical feedback from the end-facet, angle θ must be greater then [arcsin (NA)]/2. In general, the offset distance δ and the twist angle per unit length of fibre β are selected according to the application to give the required level of suppression of optical feedback from the facet at one or both ends of the fibre. In applications where different levels of optical feedback from the end facets at opposite ends of the fibre are needed (e.g. for a superflourescent fibre source with a predominantly single direction output) then the angle of the core's trajectory with respect to the longitudinal axis of the fibre can be made different at the two ends of the fibre (e.g. by using a different value for the twist angle per unit length of fibre β).

The core of the fibre may be doped with an active ion (e.g. a rare earth ion such as ytterbium, erbium, neodymium, thulium or holmium) or a combination of active ions so that the fibre can act as the gain medium, or a section of the gain medium in a laser oscillator, amplifier or superfluorescent source. The refractive index of the core material (ni) should be larger than the refractive index (n₂) of the inner-cladding 14 to allow light to be guided within the core. The refractive index (n₃) of the outer- cladding 16 can be larger or smaller than that of the inner-cladding depending on the application. For cladding-pumped laser oscillators, amplifiers and superfluorescent sources, n₂ > n₃ to allow pump light to be guided within the inner-cladding 14. In addition, the curvature of the core's trajectory in the section adjacent to the fibre end facets may be selected to give increased loss for high order modes compared to lower order modes to facilitate selection of a single-mode output beam when a multi-mode core is employed. Alternatively, the end section of the fibre may be tapered as well as twisted to so that the cross-sectional area of the fibre is decreased adjacent to the fibre end facet in order to provide the required level of mode filtering, or for mode- matching to an external device or application.

In another preferred embodiment of the invention, shown in figures 2(a) and 2(b), the section of fibre 10 with the twisted core trajectory may be extended for one or more rotations about the fibre's longitudinal axis 20, with constant or variable pitch, to relax the positioning tolerance for the end facet 18 and, to provide improved discrimination against high-order spatial modes, if required, whilst at the same time providing a reduction in unwanted optical feedback from the fibre end facet 18 by virtue of the core's path being at an angle with respect to the longitudinal axis 20 of the fibre. This geometry is particularly effective in applications where a very high level of suppression of optical feedback from the end facet is needed and no output from the fibre end facet is needed. In this case, suppression of optical feedback from the end facet may be achieved by using a fibre end geometry in which the core path is very tightly curved so that light propagating in the core is attenuated by the core's curvature and then the remaining light that is reflected from the end facet 18 experiences further attenuation by virtue of the light in the core being reflected from the end facet at an angle to the core's axis followed by a second pass of the section of fibre with the twisted core trajectory.

Figures 3 (a) and 3(b) show two examples of fibre cross-sectional geometries with the core transversely off-set from the longitudinal axis of the fibre. For the purpose of this description, the longitudinal axis 20 is defined as the axis about which the fibre may be rotated when heated and an appropriate twisting torque is applied. Figure 3 (a) shows a circular fibre with an off-set core 12 and figure 3(b) shows one example of a non-circular fibre with an off-set core 12. Non-circular (e.g. D-shaped) fibre cross- sections can provide improved pump absorption in cladding-pumped fibres sources with a relatively small core off-set compared to the inner-cladding diameter. A fibre with a core that is aligned to the longitudinal axis of the fibre may also be used provided that the end sections are prepared (e.g. by polishing or etching) to produce a non-circular fibre with the core transversely off-set from the fibre's longitudinal axis adjacent to one or both of the fibre end facets.

Figures 4(a) and 4(b) show plan and side views of another preferred embodiment of the invention in which the fibre end facet 18 is in optical contact or joined to another transparent material 22 of similar refractive index. In this case, the Fresnel reflection coefficient at the interface between the fibre 10 and the transparent optical material 22 is significantly reduced. As a result, optical feedback from the end facet 18 is now reduced by virtue of the combined effects of the core trajectory being at an angle to the fibre end-facet surface and the lower Fresnel reflection coefficient from the surface. This yields a further decrease in the level of unwanted feedback from the end-facet, and offers the further attraction that the transmitted beam expands by diffraction during its passage through the transparent material 22, and hence has a larger beam size when exiting the material. In this way, the damage threshold of the fibre device may be increased significantly. This embodiment of the invention is especially useful in high peak power pulsed fibre source configurations.

The transparent material 22 may have similar transverse dimensions to the fibre or larger transverse dimensions as required by the particular application. The length of the material should be selected so that the beam emerging from fibre's core does not impinge on the side walls of the material.

Figures 5(a) and 5(b) show schematic plan and side views of another preferred embodiment of the invention in which two fibres 10 and 10' with 'twisted' core trajectories are in optical contact or joined together via an appropriate means so that light propagating in the core 12 of a first fibre 10 is coupled directly into the core 12' of a second fibre 10'. The angles of the core 12 and core 12' with respect to their longitudinal axes 20 and 20' respectively, and the transverse positions of the both cores are substantially the same so that of light can propagate from core 12 to into core 12', or vice versa with low loss. The facets 18 and 18' of both fibres (prior to optical contact or joining) are substantially perpendicular to their respective longitudinal axes 20 and 20', so that optical feedback from the interface between cores 10 and 10' is reduced compared to the situation in prior art configurations where the core paths are parallel to the longitudinal axes of both fibres. This embodiment of the invention allows the integration of one or more fibres sources with one or more pig-tailed fibre components and devices (e.g. fibre isolators, fibre couplers, fibre pig- tailed modulators) with reduced optical feedback or back-reflections from interfaces between different fibres, and hence improved performance. In this embodiment of the invention the fibres that are joined in this manner may have cores doped with active laser ions or other dopants as required by the particular application. The final fibre system may have many fibres joined in the manner illustrated in figures 5(a) and 5(b) with further fibre end terminations, as shown in figures 1, 2, or 4, as required, when the beam enters a 'free-space' region within the optical system. The novel fibre geometry according to this invention has many applications in both optical fibre sources, passive fibre devices and in systems comprising fibre sources and fibre devices.

Figure 6, 7, 8, 9 and 10 show, by way of example, further embodiments of this invention.

Figures 6(a) and 6(b) show plan and side views respectively of a superfluorescent fibre source based on the novel fibre geometry according to this invention. The fibre core 12 is doped with an active ion (e.g. ytterbium, erbium, neodymium, thulium, holmium or other rare earth ion dopants) or a combination of active ions to provide gain over the appropriate wavelength regime. The diameter and numerical aperture of the core 12 are selected so that the core will guide only a single-spatial mode or multiple spatial modes. The core host material may be selected to provide the desired spectroscopic, optical, thermal and mechanical properties. In a preferred embodiment of the invention the core material is based on rare earth ion doped silica glass with further dopants added to provide the desired refractive index profile across the core region. The inner-cladding 14 of the fibre surrounding the core is fabricated from a lower refractive index glass (preferably pure silica), and the outer-cladding 16 is fabricated from a lower or higher refractive index material depending on the optical pumping scheme to be employed. In core-pumped fibre devices, the refractive index of the outer-cladding may be higher than the refractive index of the inner-cladding. In cladding-pumped devices, the outer-coating material has a lower refractive index so that pump light can be guided in the inner-cladding. More complex fibre geometries may be employed with additional layers and/or an arrangement of holes to provide the same basic functions. In all cases the fibre is characterised by a core region doped with one or more active ions, an inner-cladding region and one or more outer layers. The fibre core 10 is transversely offset with respect to a longitudinal axis 20 of the fibre in at least the two sections of fibre adjacent to the end facets, and the core has an angled trajectory with respect to the longitudinal axis 20 of the fibre at both end-facets 18 (which are substantially perpendicular to the longitudinal axis 20 of the fibre), so that optical feedback (due to Fresnel reflection) from the end facets 18 is reduced to suppress unwanted laser oscillation. The fibre is pumped through one or both end facets 18 and/or through the fibre's side by a pump source (not shown), which may comprise one or more pump lasers. The pump laser may for example be a diode laser, a fibre source or a solid-state laser. The pump light is injected into the fibre using an appropriate pump light delivery and focussing or coupling scheme (not shown). The combination of high gain in the core region 12 with very low optical feedback from the fibre end facets 18, made possible by this invention, leads to amplification of spontaneous emission and high power broad-band optical output (in a wavelength regime determined by the choice of active ion(s)) in output beams 30 and 32 from both fibre end-facets. The superfluorescent source according to this invention provides a broad wavelength band output which can be scaled to very high power levels (>100W) and offers many advantages over the approaches described in the prior art. In a preferred embodiment of the invention, the angles θi and G₂ of the core's trajectory at opposite fibre end facets are different, so that optical feedback from the fibre end facets is suppressed by different amounts at the two ends. In this way, the output power in one beam (i.e. 30 or 32) can be made much larger than the output power in the opposite beam (i.e. 32 of 30). In this way, the superfluorescent source can provide a predominantly single direction output. Additionally, the curvature of the core's path can be tailored, if required, to provide a mode filtering function in devices with multimode cores. Suppression of the optical feedback from one end of the fibre via the approach according to this invention may be sufficient to suppress lasing and hence obtain a broadband optical output in certain configurations. In this case, an angled core-trajectory with the respect to the fibre end-facet may be required at only one end of the fibre. The angle between the core trajectory and the fibre's longitudinal axis 20 should be large enough so that the light reflected from the end facet (at an angle to the core's axis) experiences a high loss. As a very rough guide, the angle should be chosen to be larger than [arcsin (NA)]/2, where NA is the numerical aperture of the core. In superfluorescent fibre configurations where suppression of optical feedback from one end is sufficient, at the opposite end of the fibre, the core's trajectory may be parallel to the longitudinal axis of the fibre 20 and hence substantially perpendicular to the fibre facet. Figures 7(a) and 7(b) show plan and side views respectively of a fibre amplifier based on the novel fibre geometry according to this invention. The fibre core 12 is doped with an active ion (e.g. ytterbium, erbium, neodymium, thulium, holmium or other rare earth ion dopants) or a combination of active ions to provide gain over the appropriate wavelength regime. The fibre design options are similar to those described above for the superfiuorescent source. The essential features are that the fibre core 12 is transversely offset with respect to a longitudinal axis 20 of the fibre 10 in at least the two sections of fibre adjacent to the end facets 18, and the core 12 has an angled trajectory with respect to the longitudinal axis 20 of the fibre at both end facets 18 (which are substantially perpendicular to the longitudinal axis 20 of the fibre), so that optical feedback (due to Fresnel reflection) from the end facets 18 is reduced to suppress laser oscillation. The fibre may be pumped through one or both end facets and/or through the fibre's side by a pump source (not shown), which may comprise one or more pump lasers. The pump laser may for example be a diode laser, a fibre source or a solid-state laser. The pump light is injected into the fibre using an appropriate pump light delivery and focussing or coupling scheme (not shown). Signal light 40 at the appropriate wavelength from another source (which may for example be a fibre laser, a fibre master-oscillator power-amplifier source, a superfluorescent fibre source, a diode laser or a solid-state laser) is coupled into the core of the fibre amplifier via a suitable coupling means (not shown) with an appropriate means for isolation (e.g. a Faraday isolator), if necessary, and the amplified output beam 42 is emitted from the core at the opposite end of the fibre. As a result of the very low optical feedback from the fibre end-facets provided by this invention, the fibre amplifier can provide a very high small signal gain. The angles θi and θ₂ of the core's trajectory at opposite fibre end facets may be the same or different provided that the combined level of suppression of feedback from the fibre end-facets 18 is sufficient to suppress parasitic laser oscillation. One or more amplifiers of this type may be arranged in series to form a multi-stage amplifier, with appropriate isolating means between each stage, to provide a higher gain and a higher output power. Figures 8(a) and 8(b) show plan and side views respectively of one example of a fibre laser oscillator based on the novel fibre geometry according to this invention. The fibre design options are similar to those described above for the previous examples of optical fibre sources. In this case, optical feedback from the end facets 18 is suppressed, but at a lower level than for a superfluorescent source or fibre amplifier so that the fibre operates as a laser oscillator. In a preferred embodiment, the different optical feedback efficiencies from opposite end facets are selected via the use of an appropriate fiber design, so that the output beam 52 from one end has a much higher power than the output beam 50 from the opposite end. This may be achieved, for example, by employing a fibre with substantially perpendicular fibre end facets 18 and with a core trajectory that is parallel to the longitudinal axis 20 of the fibre at one end facet and angled with respect to the longitudinal axis 20 of the fibre at the opposite end of the fibre. The angle between the core trajectory and the fibre's axis must be carefully selected so that threshold for lasing is low compared to the available pump power, in contrast to the situation for the superfluorescent source described above. This approach offers a very simple way to achieve predominantly single direction output from a laser oscillator without the need of in-fibre Bragg gratings or other optical components (e.g. mirrors). This approach is suitable for both core- pumped and cladding-pumped laser configurations. In a preferred embodiment of the invention, the fibre laser also comprises a pump source and an appropriate means for coupling light from said pump source into the fibre.

Figures 9(a) and 9(b) show plan and side views respectively of another example of a fibre laser oscillator based on the novel fibre geometry according to this invention. In this embodiment of the invention, feedback for laser oscillation is provided by a substantially perpendicular end facet at the output end of the fibre (which acts as the output coupler) and by an external feedback cavity 70 at the opposite end. The latter comprises one or more lenses to collect and condition the beam emerging from fibre end adjacent to the external cavity, and a reflecting means for feeding light back into the fibre, as necessary for laser oscillation. The reflecting means may for example be a mirror or a diffraction grating. The external feedback cavity may also comprise a Q-switch (active or passive) for pulsed operation or a mode-locking device (active or passive) for short pulse operation, or other optical elements or a combination of components so as to provide the desired means for selecting and controlling the lasing characteristics. For fibre laser cavity configurations of this type, to maximise the effectiveness of the external feedback cavity and relax the constraints on the design, it is essential that unwanted optical feedback from the fibre end-facet adjacent to the feedback cavity is minimised. This can be achieved in the manner described earlier (illustrated in figures 1, 2 and 4) by using a fibre with a core trajectory that is at an angle to the fibre's longitudinal axis 20 at the fibre end facet adjacent to the external cavity to suppress unwanted optical feedback. The laser output beam 60 emerges from the opposite end of the fibre, where the core 12 is preferably parallel to the longitudinal axis of the fibre and hence substantially perpendicular to the fibre end- facet. This approach can be applied to both core-pumped and cladding-pumped laser configurations. In a preferred embodiment of the invention, the fibre laser also comprises a pump source and an appropriate means for coupling light from said pump source into the fibre.

Figure 10(a) shows a plan view of a fibre coupler according to another embodiment of the invention. The fibre coupler 80 has two input ports 82 and output ports 84 and 86. In this example, the function of the coupler is to combine the outputs from the two input ports into a single fibre output port 84, so that port 86 is redundant. Optical feedback from the fibre end-facet of port 86 can be problematic, and so this port should ideally be terminated in manner that suppresses or eliminates retro-reflected light. This can be achieved (as shown in the expanded view) using a fibre end section geometry (according to the present invention) with a substantially perpendicular end- facet 18 and with the core trajectory 12 at an angle to the longitudinal axis 20 of the fibre, so that light is reflected (by the facet) at an angle to the core's axis and hence is strongly attenuated.

Figure 10(b) shows the plan view of a fibre coupler of similar construction to the one described above, but with a slightly modified fibre design for the redundant port. In this case, the core 12 is twisted one or more times about the fibre's longitudinal axis

20, so that optical feedback from the fibre end-facet is further attenuated by virtue of the light propagating through an extended region where the core has a curved trajectory. This allows for very simple suppression or elimination of unwanted optical feedback from the fibre end facet without resort to the more complicated and, in some cases, less effective approaches described in the prior art. The same approach may be applied to a number of different types of passive fibre device where unwanted optical feedback from fibre end-facets must be strongly suppressed or eliminated.

In summary, the novel fibre geometry according to this invention has many advantages over prior art techniques for controlling or suppressing optical feedback from fibre end-facets and fibre interfaces, and hence will find a huge range of applications in optical fibre sources, fibre devices and fibre systems.

REFERENCES:

[I]: "Rare-Earth Doped Fiber Lasers and Amplifiers", M. J. F Digonnet ed., 2^nd Edition, Marcel Dekker, Inc., 2001

[2]: "Broad-band diode-pumped ytterbium-doped fiber amplifier with 34dBm output power", J. M. Sousa, et al., IEEE Photonics Technology Letters, 11, 39, 1999

[3]: "Optical fiber for reducing optical signal reflections", George A. Pavlath, US patent no. US 5,970,197, 1999

[4]: "Reflection suppression for an optical fiber", Ronald L. Hodge and John J. Kenny, US patent application no. US 2005/0053350 Al, 2005

[5]: "Optics", Eugene Hecht, 4^th edition, Addison Wesley, 2001

[6]: "Spectral characteristics of high power 1.5 μm broadband super-luminescent fiber sources", P.F. Wysocki, et. al., IEEE Photonics Technology Letters, 2, 178, 1990

[7]: "Non-reflection optical fiber termination and method of manufacture of the same", Noriko Iwata and Kazuhiro Okamoto, US patent application no. US

2001/0017971 Al, 2001

[8]: "Cladding-pumped ytterbium-doped helical-core fiber laser", P. Wang, et. al., Technical Digest of Advanced Solid-State Photonics conference (Optical Society of America, Washington D.C.) paper MC4, 2005 [9]: "Cladding-pumped fiber with helical large-earth-doped core for fiber lasers and amplifiers", Gerald T. Moore and John R. Marciante, US patent no. US 6650664 Bl, 2003

[10]: "Radiation loss of a helically deformed optical fiber", D. Marcuse, Journal of the Optical Society of America, 66, 1025, 1976

Claims

1. An optical fibre having a longitudinal axis, the optical fibre comprising: a core surrounded by a cladding, wherein the refractive index of the core is greater than the refractive index of the cladding; a first section in which the core follows a first path with respect to the longitudinal axis; and a second section in which the core follows a second path different to the first path, wherein the second path inclines at an angle with respect to a plane normal to the longitudinal axis.

2. The optical fibre of claim 1, wherein the angle at which the second path inclines with respect to a plane normal to the longitudinal axis varies across the second section.

3. The optical fibre of claim 1 or claim 2, wherein the second path is helical.

4. The optical fibre of any of claims 1 to 3, wherein the second section terminates at an end facet of the optical fibre.

5. The optical fibre of any preceding claim, further comprising a third section in which the core follows a third path that inclines at an angle with respect to a plane normal to the longitudinal axis.

6. The optical fibre of any preceding claim, wherein the third section terminates at an end facet of the optical fibre.

7. The optical fibre of any preceding claim, wherein the angle at which the core inclines with respect to a plane normal to the longitudinal axis is within any one of the ranges 89.5 degrees to 89 degrees, 89.5 degrees to 88 degrees, 89.5 degrees to 85 degrees, 89.5 degrees to 80 degrees, 89.5 degrees 70 degrees, 89.5 degrees to 60 degrees.

8. The optical fibre of any preceding claim, wherein the angle at which the core inclines with respect to a plane normal to the longitudinal axis is within any one of the ranges 89.5 degrees to 85 degrees, 85 degrees to 80 degrees, 80 degrees to 75 degrees, 75 degrees to 70 degrees, 70 degrees to 65 degrees, 65 degrees to 60 degrees.

9. The optical fibre of any preceding claim, further comprising a section of material in contact with an end-facet of the optical fibre, wherein the refractive index of the section of material is substantially equal to the refractive index of the core.

10. The optical fibre of any preceding claim, further comprising an anti-reflection coating applied to an end-facet of the optical fibre.

11. The optical fibre of any preceding claim, wherein the core in the second section tapers from a first diameter to a second diameter.

12. The optical fibre of any preceding claim, further comprising a second cladding surrounding the cladding, wherein the refractive index of the second cladding may be greater or lesser than the refractive index of the cladding.

13. The optical fibre of any preceding claim, wherein the second section is comprised of a first optical fibre spliced to a second optical fibre, wherein the core of the first optical fibre at the splice and the core of the second optical fibre at the splice incline at an angle with respect to a plane normal to the longitudinal axis, and further wherein the core of the first optical fibre is aligned with the core of the second optical fibre.

14. The optical fibre of any preceding claim, wherein the first section is operable to confine a plurality of spatial modes and the second section is operable to attenuate a desired number of the plurality of spatial modes.

15. The optical fibre of any preceding claim, wherein the core is doped with an active ion operable to absorb light at a first wavelength and to emit light at a second wavelength.

16. The optical fibre of claim 15, wherein the active ion comprises one or a combination of ytterbium, neodymium, erbium, thulium, holmium.

17. A source of amplified spontaneous emission comprising: the optical fibre of claim 15 or 16, wherein the angle of the second path at an end of the second section is chosen to suppress optical feedback into the core; and a pump source operable to emit light at the first wavelength of the active ion and couple the light into the optical fibre.

18. An optical fibre amplifier comprising: the optical fibre of claim 15 or 16, wherein the angle of the second path at an end of the second section is chosen to suppress optical feedback into the core; a pump source operable to emit light at the first wavelength of the active ion and couple the light into the optical fibre; and a signal source operable to emit light at the second wavelength of the active ion and couple the light into the core of the optical fibre.

19. An optical fibre laser comprising: the optical fibre of claim 15 or 16, wherein the angle of the second path at an end of the second section is chosen to control optical feedback into the core; and a pump source operable to emit light at the first wavelength of the active ion and couple the light into the optical fibre.

20. An optical fibre coupler or splitter comprising the optical fibre of claim 14, wherein the angle of the second path at an end of the second section is chosen to suppress a Fresnel reflection from that end of the second section.

21. The optical fibre coupler or splitter of claim 20, wherein the second path is helical and the angle of the second path with respect to a plane normal to the longitudinal axis is chosen to cause a bending loss in the second section.

22. A method of fabricating an optical fibre with a section of core that inclines at an angle with respect to a plane normal to the longitudinal axis of the optical fibre, comprising: providing an optical fibre having a core surrounded by a cladding, wherein the refractive index of the core is greater than the refractive index of the cladding; softening a section of the optical fibre by heating the section of the optical fibre; and twisting the section of the optical fibre with respect to the remaining optical fibre in order to cause the core within that section to incline at an angle with respect to a plane normal to the longitudinal axis.

23. The method of claim 22, further comprising forming an end-facet termination within the section of the optical fibre.

24. The method of claim 23, wherein the forming an end-facet termination is performed by cleaving, cleaving then melting or cleaving then polishing the optical fibre.

25. The method of any of claim 23 or claim 24, wherein the end-facet is oriented substantially perpendicular to the longitudinal axis.

26. The method of any of claims 22 to 25, wherein twisting the section of the optical fibre causes the core to spiral around the longitudinal axis.

27. The method of any one of claims 22 to 26, further comprising: tapering the core within the section of the optical fibre from a first diameter to a second diameter either before, during or after the twisting step.

28. The method of any one of claims 22 to 27, wherein the optical fibre has a second cladding surrounding the cladding, wherein the refractive index of the second cladding may be greater or lesser than the refractive index of the cladding.

29. The method of any one of claims 22 to 28, wherein the core is doped with an active ion that absorbs light at a first wavelength and emits light at a second wavelength.

30. The optical fibre of claim 29, wherein the active ion comprises one or a combination of ytterbium, neodymium, erbium, thulium, holmium.