WO2009053717A2

WO2009053717A2 - Sensor element

Info

Publication number: WO2009053717A2
Application number: PCT/GB2008/003625
Authority: WO
Inventors: Gerard Franklyn Fernando; Ramani Salmalee Mehendran; Venkata Raj Machavaram; Stephen N. Kukureka
Original assignee: The University Of Birmingham
Priority date: 2007-10-26
Filing date: 2008-10-24
Publication date: 2009-04-30
Also published as: WO2009053717A3; GB0721251D0

Abstract

A sensor element comprises first and second channel devices and at least one reflector. Each of the first and second channel devices is directed towards the reflector(s) and is able to transmit radiation along its length towards the reflector, and to transmit reflected radiation away from the reflector. At least one of the channel devices is separated from the reflector. The first and second channel devices are arranged so that radiation may not be transmitted between the channel devices without reflection from the reflector.

Description

SENSOR ELEMENT

The present invention is concerned with sensor elements, and sensors containing such elements, capable of remote measurement of certain environmental parameters, and particularly to sensor elements and sensors capable of measuring several such parameters simultaneously .

GB 2 283 567 describes a prior art design of fibre optic sensor element. In this design, an optical fibre is cleaved and mounted in a housing such that the cleaved ends of the resulting fibres nearly abut one another (having only a small gap between them). Movement of the cleaved ends of the fibres relative to each other (e.g. lateral movement, caused by vibration, or longitudinal movement caused by thermal expansion of the housing or fibre, or by strain) alters the amount of light which is transmitted from one fibre to the other. The sensor element may therefore be attached (e.g. surface mounted) to a mechanical part, or even embedded in a material or structure, and used to measure remotely changes to the environment in which the sensor element is placed.

In one embodiment described in GB 2 283 567, the housing of the sensor element includes an aperture to allow ingress of material from the surrounding environment. For example, where the sensor is embedded in a fibre-reinforced composite (typically formed by saturating a mesh of fibres with liquid resin, which is then allowed to cure), the aperture will allow liquid resin to enter the gap between the optical fibres in the sensor element. Curing of the resin can then be monitored by following the changes in the refractive index of the gap between the fibres (which affects the amount of light transmitted between the fibres). However, the prior art sensors are restricted to measuring only a single parameter, such that sensors used to measure resin curing cannot concurrently or subsequently be used to measure strain or vibration. Furthermore, the operation of the sensor element described in GB 2 283 567 requires light to be provided to one of the optical fibres, with measurement taken of the light received by the other optical fibre; as a result, both ends of the sensor element must be connected to suitable apparatus in order to use the sensor element. This can limit the utility of the corresponding sensor in certain remote applications.

Thus, it would be advantageous to provide a sensor element having the advantages of light weight, small size, ruggedness and sensitivity, but which is also able to measure several parameters concurrently. It would also be advantageous if the sensor element required connection only at one end thereof. The present invention has been conceived with the above problems in mind.

According to the first aspect of the invention, there is provided a sensor element comprising first and second channel devices and at least one reflector, each of the first and second channel devices being directed towards the at least one reflector, at least one of the first and second channel devices being separated from the at least one reflector, and each of the first and second channel devices being capable of transmitting radiation along its length towards the at least one reflector and similarly transmitting reflected radiation received from the reflector, wherein the first and second channel devices are arranged such that radiation may not be transmitted between the first and second channel devices without reflection from the reflector. It will be understood that, where the sensor element comprises multiple reflectors, each channel device may be directed to a separate reflector. Alternatively, multiple or all channel devices may be directed to the same reflector, or there may be any combination of channel devices with reflectors. In all cases reference to the 'corresponding reflector' for a channel device is to the reflector that, in use, reflects radiation received from that channel device.

The sensor element may additionally comprise a third or any additional number of channel devices. In such an arrangement, the channel devices may be arranged such that radiation may be transmitted from one channel device and received by another channel device without reflection from a reflector, provided that there exists within the sensor element two (i.e. the first and second) channel devices for which such transmission is not possible.

In one embodiment, the first and second channel devices, and optionally any additional channel devices, are secured to a single common support. This allows the channel devices to be correctly positioned relative to one another, and improves the robustness of the sensor element. In one further embodiment, the support is located at an axially central region of the sensor element, and the first and second channel devices are secured to the periphery of the support. Additionally or alternatively, the reflector (or optionally, where the sensor element comprises more than one reflector, two or more of the reflectors) may be secured to the support. This provides a convenient means for ensuring that the channel devices are each correctly directed to a reflector. In such embodiments, the multiple channel devices or multiple reflectors may each be located at different longitudinal positions along the support, or multiple channel devices or reflectors may be located at the same longitudinal position.

Each channel device and/or reflector may be secured to the support by means of a direct connection (such as a fusion joint, adhesive (such as UV-activated adhesive, ceramic adhesive, or the like), etc.), by physical engagement with a corresponding part of the support (such as location of the channel device within a groove in the support) or by entrapping the channel device and support within an enclosing element (such as capillary tube or ring surrounding both the channel device and support) or any combination of these. In some embodiments, it may be advantageous for the channel device to move freely relative to the support in a longitudinal direction, whilst being retained adjacent to the support in a lateral direction; this may enable movement of the channel device in responses to thermal expansion or contraction, or in response to strain.

In one embodiment in which both a given channel device and the corresponding reflector are attached to a support, the support may be non- rigid (i.e. extensible under applied strain). Thus, the gap length between the channel device and the reflector will vary according to the strain applied between the reflector and the channel device, and the sensor element may be used to measure that strain.

In an alternative embodiment, the gap length between a channel device and the corresponding reflector may be isolated from any applied strain, such as for example by enclosing the reflector and the end of the channel device within a substantially rigid housing. In such cases, the gap length will still vary according to temperature, due to thermal expansion and contraction of elements such as the channel device, such that it is possible to use the sensor element to measure temperature independently of strain.

In a further embodiment, the support itself comprises a channel device. Thus, the support channel device may be used to transmit and/or receive radiation, in addition to those channel devices arranged around it. The support channel device may be arranged to interact with the or an additional reflector in a manner analogous to the other channel devices. Alternatively, the sensor element may further comprise a fibre-optic acoustic emission sensor component, or other suitable sensor components, and the support channel device may be arranged to transmit and/or receive radiation to or from the fibre-optic acoustic emission sensor component or any other sensor component(s).

In an alternative embodiment, even where the support does not comprise a channel device, the sensor element may comprise a suitable electrical or other sensor component, and the support may be of a type and arrangement to receive data therefrom (e.g. where the sensor component is an electrical or electronic sensor component, the support may comprise electrically conductive material and may be arranged to receive an electrical signal from the electrical or electronic sensor component) .

In one embodiment, the support may comprise a thermally stable material such as sapphire or a metal, and may consist for example of a rod of sapphire or metal. In a further embodiment, the channel devices may also comprise a thermally stable material such as sapphire. This would allow the sensor element to be used at elevated temperatures when compared to constructions having elements consisting of glass or plastics material. It will be understood that, in one embodiment of the first aspect of the invention, the sensor element may additionally comprise one or more electrical or other sensor components which do not transmit data via the support. Each such sensor component may be secured to the support (where present) in a manner analogous to that of any channel device or reflector.

According to a second aspect of the present invention, there is provided a sensor element comprising a primary channel device and a secondary channel device; the primary channel device comprising first and second discontinuous parts arranged co-axially and closely adjacent to one another, the closely adjacent ends of the first and second parts being enclosed in a housing from which at least the first part extends; and the housing having at least one reflective external surface, the secondary channel device being arranged to transmit radiation to, and/or receive reflected radiation from, the at least one reflective external surface of the housing.

In one embodiment, the secondary channel device is arranged to transmit radiation to the at least one reflective external surface of the housing such that at least a portion of the radiation is received by the same secondary channel device following reflection.

In one embodiment, the end surface of the second part of the primary channel device closely adjacent to the first part of the primary channel device is capable of acting as a reflector for radiation transmitted along the first part of the primary channel device. In one embodiment, the reflective external surface of the housing comprises a reflective coating applied to the surface of the housing. The reflective coating may comprise a metallic coating, a stacked coating of alternate dielectric layers, or other suitable coating.

According to a third aspect of the invention, there is provided a sensor comprising a sensor element of the first or second aspects of the invention, a source of radiation for transmitting radiation to at least one channel device of the sensor element, and a measuring device for measuring the reflected radiation transmitted along at least one channel device of the sensor element.

In one embodiment, the sensor comprises a single measuring device which measures the reflected radiation transmitted along all channel devices of the sensor element. Suitable such devices may include, for example, a Fourier transform infrared spectrometer (FTIRS). According to this embodiment, the channel devices may be coupled together to permit the signals from multiple channel devices to be received at a single input of the spectrometer. The radiation source may also be coupled to the channel devices to permit simultaneous transmission and measurement.

According to a fourth aspect of the present invention, there is provided a method of measuring a parameter comprising the steps of:

(a) providing a sensor element comprising a channel device and a reflector, the channel device being directed towards the reflector, and capable of transmitting radiation along its length towards the corresponding reflector and similarly transmitting reflected radiation away from the reflector;

(b) exposing the sensor element to the parameter to be measured; (c) transmitting radiation along the channel device of the sensor element so exposed, such that radiation reflected from the corresponding reflector is received by the channel device;

(d) measuring the reflected radiation transmitted by the channel device away from the corresponding reflector; and

(e) determining the value of the parameter according to the measured reflected radiation.

It should be understood that the term 'exposing the sensor element to the parameter', as used herein, refers to causing the optical properties of the sensor element to be influenced by the parameter. Where the channel device is separated from the corresponding reflector by a gap, this may comprise causing the optical properties of the gap between the end of the channel device and the reflector to be influenced by the parameter. Where the channel device is adapted for use in evanescent spectroscopy or comprises a Bragg grating, this may comprise causing the optical properties of the relevant section of the channel device to be influenced by the parameter.

For example, where the parameter to be measured is the tensile strain experienced by the environment in which the sensor element is mounted, exposing the gap between the channel device and corresponding reflector to the parameter may comprise connecting the channel device and the reflector to the environment such that the optical path length of the gap between the channel device and the reflector varies according to the tensile strain. Where the parameter to be measured is the curing (i.e cross-linking or polymerisation) of a resin in which the sensor element is to be embedded, exposing the gap between the channel device and corresponding reflector to the parameter may comprise allowing uncured resin to enter the gap between the channel device and the reflector, such that the optical path between the channel device and the reflector passes through the resin. Thus, the properties of the reflected radiation will be affected by the degree of curing of the resin, such as for example by spectroscopic absorption or as a result of changes in refractive index.

In one embodiment, the method provides simultaneous measurement of at least two parameters. For example, step (a) may comprise providing a sensor element comprising at least one additional channel device directed towards but separated from a reflector; step (b) may additionally comprise exposing the additional channel device and corresponding reflector to an additional parameter; step (c) may additionally comprise transmitting radiation along the additional channel device; step (d) may additionally comprise measuring reflected radiation transmitted by the additional channel device away from the corresponding reflector; and step (e) may additionally comprise determining the value of the additional parameter according to the measured radiation. Alternatively or additionally, step (a) may comprise providing a sensor element additionally comprising a suitable electrical or other sensor component; step (b) may additionally comprise exposing the electrical or other sensor component to an additional parameter; step (d) may additionally comprise receiving data from the electrical or other sensor component; and step (e) may additionally comprise determining the value of the additional parameter according to the data received.

The following comments and optional features apply to all aspects of the invention. In all aspects of the present invention, at least one channel device is directed towards, but separated from, a reflector. Without wishing to be bound by theory, it is believed that part of the radiation transmitted along such a channel device will undergo Fresnel reflection (a first reflection, forming a first reflected light beam) at the end of the channel device, due to differences in refractive index between the channel device and the material present in the gap between the channel device and the reflector. The remainder of the radiation will be transmitted to the reflector. Following reflection from the reflector (a second reflection), some radiation will re-enter the channel device to form a second reflected light beam; this will therefore result in interference due to superposition of the two first and second reflected light beams, commonly resulting in an observable fringe pattern. The interference fringe pattern will depend on the physical separation of the channel device and the reflector, and the refractive index of any material between the channel device and the reflector. Thus, any change to this separation distance (as a result of strain applied to the sensor element) or of the refractive index of material in the gap (as a result of e.g. the curing of resin) will produce a measurable change in the fringe pattern.

Any channel device may have one or more gaps located along its length, alternatively or in addition to any functionality located at the end of the channel device. Each such gap may create detectable fringes, which may be used to measure a parameter in the manner described above. Where there is more than one such gap along the length of a single channel devices, it is preferable that each gap should have different gap separations to allow the fringes from each gap to be identified using Fourier transformation. Other types of interaction with the light beam are also possible. For example, one or more Bragg gratings may be written in to a channel device. This will produce reflection of particular wavelengths of radiation (when the channel device is used with a broad-spectrum radiation source), depending on the grating separation. Thus, changes in the grating separation due to e.g. application of strain to the channel device, or to thermal expansion or contraction, will produce a measurable change in the wavelengths of radiation reflected.

A channel device may also be adapted for evanescent spectroscopy, in the manner familiar to the skilled person, for example by the reduction of cladding on an optical fibre. It is preferable for such channel devices to be located adjacent to the corresponding reflector, without any separation, in order to minimise any attenuation of the spectroscopic signal.

Each channel device may be used to provide an independent measurement of a parameter associated with that channel device, or with a gap between the channel device and a corresponding reflector. For example, if the gap for a particular channel device is open to the environment and the sensor element is embedded in an uncured fibre-reinforced composite, as described above, then that channel device can be used to measure the progress of curing of the resin in the composite, via changes in refractive index, or by spectroscopic measurement of chemical groups within the resin. The gap for a second channel device within the same sensor element might be closed to the environment, with that channel device being used to measure a further parameter, such as strain or temperature. Other parameters may be measured by performing spectroscopic analysis on the radiation returned along the channel device. This could indicate, for example, the presence of a chemical species which absorbs radiation of a particular frequency, and may therefore be used to track the progress of a chemical reaction.

In one embodiment, the sensor element further comprises one or more environmental indicators located in the gap between a channel device and the corresponding reflector. The optical or spectroscopic properties of such indicators should vary in response to environmental conditions such as temperature, pH, etc. In a further embodiment, the sensor element may comprise a matrix located in the gap between a particular channel device and the corresponding reflector, one or more environmental indicators being immobilised in the matrix. In another embodiment, the sensor element may comprise a barrier element enclosing the gap between a channel device and the corresponding reflector, one or more environmental indicators being located in the region enclosed by the barrier element to prevent escape from the sensor element.

In all such cases, the indicator should obviously be exposed to the environmental condition to be measured. For example, where the indicator is sensitive to pH, the barrier or matrix should be sufficiently porous to allow the exchange of hydrogen ions with the surrounding environment. Similarly, where the indicator is sensitive to temperature, the barrier or matrix should allow the exchange of heat with the surrounding environment.

In one embodiment, the channel device or any or all of the channel devices may each comprise an optical fibre. The sensor element may therefore be suitable for use with electromagnetic radiation from the visible or near-visible regions of the spectrum. In one embodiment of the method of the fourth aspect of the invention, therefore, step (a) comprises providing a sensor element comprising an optical fibre, and step (c) comprises transmitting electromagnetic radiation along that optical fibre. The narrow diameter of typical optical fibres allows the sensor element to be small, whilst the low signal attenuation allows the sensor element to be located at a position remote from remaining components of a sensor, such as the radiation source and detector. Furthermore, the flexibility of optical fibres increases the robust nature of the sensor element, allowing the sensor element to be used in a wide variety of operating environments.

Where a reflector of the present invention comprises a metallic coating, this may be formed by any suitable coating method. For example, the coating may be formed by sputter coating, by diffusion of metal atoms into a substrate, or by microlithography.

The invention will now de described by way of example, with reference to the accompanying Figures, in which:

Figure 1 shows a schematic section view of a prior art sensor element as described in GB 2 283 567;

Figure 2 shows an enlarged schematic section view of the sensor element of Figure 1 , depicting the optical pathways involved when the device is used according to the fourth aspect of the present invention; Figure 3 shows a schematic view of a sensor element according to the first aspect of the present invention;

Figure 4 shows a schematic view of a sensor element according to the second aspect of the present invention.

As shown in Figure 1 , the sensor element comprises an optical fibre which has been cleaved into first and second sections, 1 and 2. Each optical fibre consists of a central core Ia, 2a surrounded by cladding Ib, 2b. The fibre sections are aligned coaxially, and separated by a small gap 3.

The junction between the two optical fibre sections 1 and 2 is encased in a precision bore capillary tube 6. The optical fibre sections 1 and 2 are held in the precision bore capillary tube 6 by means of two sets of fusion joints 7 and 8, located towards either end of the capillary tube 6. The use of the precision bore capillary tube 6 ensures that the first and second optical fibre sections 1 and 2 are held in the necessary alignment.

As described in GB 2 283 567, a light source (not shown) may be connected to one of the optical fibre sections 1 , with the other optical fibre section 2 being connected to a detector (not shown). Light transmitted along the first optical fibre section 1 is refracted into a cone on exit from this optical fibre section. The amount of light received by the second optical fibre section 2 is therefore dependent on the ratio of the cross-sectional area of the second optical fibre section 2 (or more specifically, the core 2a thereof) to the area of the cone section at the same distance from the first optical fibre 1. Application of strain between the two ends of the sensor element causes the gap 3 between the two sections of optical fibre 1 and 2 to increase, thereby reducing the amount of light received by the second optical fibre section 2. Thus, changes in the strain can be measured by detecting changes in the quantity of light received at the detector.

The area highlighted in the circle II of Figure 1 is shown in greater detail in Figure 2.

As shown in Figure 2, it is possible to use the prior art sensor element of Figure 1 in the method according to the fourth aspect of the present invention. An incident light beam 11 is transmitted along the first optical fibre section 1 (or more strictly along the core Ia thereof). At the junction of the first optical fibre section 1 with the gap 3 , the change in refractive index (between fibre and air) causes some of the light to be reflected back along the first optical fibre section 1 (Fresnel reflection) as a first reflected light beam 12. The remaining light continues across the gap 3, where again a proportion of the light is reflected at the junction with the second optical fibre section 2. Some of this light will re-enter the first optical fibre section 1 as a second reflected light beam 13. Thus, as shown, the cleaved surface of the second optical fibre section 2 is acting as the reflector required by the fourth aspect of the present invention.

The second reflected light beam 13 differs in optical path from the first reflected light beam by 2n_aird , where n_air and d are the refractive index of air, and the separation distance between cleaved faces of the first and second optical fibre sections 1 and 2 inside the capillary respectively. The phase difference between spectral components of the two reflected light beams 12 and 13 produces an interference pattern modulated by the transfer function of the broad band light source. The interference pattern comprises of alternate dark and bright fringes whose minima and maxima correspond to path differences of (n +

and nλ respectively, where « = 0,l,2,Λ and λ is the corresponding wavelength. The interference pattern or the resultant intensity of the two superposed Fresnel reflections can be expressed from the following equations:

The complex electric field amplitude of the first and second Fresnel reflections can be written as t\_aE and t_]ar_a2t_a]e'^iφ+π)E respectively, where E is the complex amplitude of the light in the first optical fibre section 1 ;

_rΛ are the Fresnel coefficients of reflection and

transmission respectively at the interface between the first optical fibre

section 1 and air; are the Fresnel coefficients

of reflection and transmission at the interfaces between air and the second optical fibre section 2 and between air and the first optical fibre section 1

respectively; φ\ = ^m" — - and π are the phase changes due to path length v λ J difference between the Fresnel reflections and upon reflection at the interface between air and the second optical fibre section 2 respectively; n, , «₂(- "i) and n_a are the refractive indices of the first and second optical fibre sections

1 and 2 and air respectively. The amplitude E_R of the superposed Fresnel reflections is the sum of the amplitudes of the two Fresnel reflections which can be expressed as:

The intensity of the superposed Fresnel reflections can be expressed as: / - F - F - ¹ [R + /?(l - R.)² + 2R{\ ~ R)cos φ] and R + T = 1

where R\= r_]a ² = r_a2 ²) and τ(= t_]at_aϊ ) are the reflectivity and transmissivity of the air gap in between the first and second optical fibre sections 1 and 2 and

E (= /_o) is the intensity of the incident light beam 11 coupled into the first optical fibre section 1. Therefore the intensity of interference fringes produced as a result of superposition of the two Fresnel reflections varies in a sinusoidal fashion with phase difference between spectral components. The separation between the first and second optical fibre sections 1 and 2 or the absolute cavity length can be calculated from the corresponding Fabry-Perot interference as:

Cavity length = — -^

2Δ1 where A₁ and X₂ are the wavelengths corresponding to maxima (or minima) of bright fringes (or dark fringes) located at either end of the modulated interference pattern that correspond to a phase change of 2Nπ (or {IN + \)π ) where N = 0,l,2,3,K ; Aλ is the difference of these wavelengths; and n is the number of fringes in between X₁ and X₂ .

Measurement of the interference fringes can be undertaken using a Fourier transform infrared spectrometer, as described in Singh, M, Tuck, C & Fernando, G. F. , "Multiplexed fibre Fabry-Perot etalons for strain metrology" , Journal of Smart Materials and Structures, 8, 549-553, (1999).

Referring to Figure 3 , an optical fibre support 20 has one end located in a precision bore capillary tube 26 and secured thereto by two sets of fusion joints 27 and 28. A number of optical fibres (two optical fibres 21 and 22 are shown) are arranged parallel and adjacent to the optical fibre support 20, such that the ends of these fibres are separated from the precision bore capillary tube 26 by a small gap 23. It will be appreciated that although the optical fibres 21 and 22 are shown having identical spacing 23 from the precision bore capillary tube 26, in some cases it may be desirable for one or more of the optical fibres to have a spacing which is different from that of the other optical fibres.

The end surface of the precision bore capillary tube 26 adjacent to the optical fibres 21 and 22 is coated with a reflective coating 29. As a result, this end surface of the precision bore capillary tube 26 is able to act as a reflector.

Referring to Figure 4, the sensor element of Figure 1 additionally has a number of optical fibres (two optical fibres 31 and 32 are shown) arranged parallel and adjacent to the first optical fibre section 1. These additional optical fibres 31 and 32 are arranged so that the ends of these fibres are separated from the precision bore capillary tube by a small gap 33. As above, it will be appreciated that although the optical fibres 31 and 32 are shown having identical spacing 33 from the precision bore capillary tube 6, in some cases it may be desirable for one or more of the optical fibres to have a spacing which is different from that of the other optical fibres.

The end surface of the precision bore capillary tube 6 adjacent to the optical fibres 31 and 32 is coated with a reflective coating 39. As a result, this end surface of the precision bore capillary tube 6 is able to act as a reflector.

Claims

1. A sensor element comprising first and second channel devices and at least one reflector, each of the first and second channel devices being directed towards the at least one reflector, at least one of the first and second channel devices being separated from the at least one reflector, and each of the first and second channel devices being capable of transmitting radiation along its length towards the at least one reflector and similarly transmitting reflected radiation received from the reflector, wherein the first and second channel devices are arranged such that radiation may not be transmitted between the first and second channel devices without reflection from the reflector.

2. A sensor element as claimed in claim 1 , wherein the first and second channel devices are secured to a common support.

3. A sensor element as claimed in claim 2, wherein at least one reflector is additionally secured to the common support.

4. A sensor element as claimed in claim 3, wherein the common support is non-rigid.

5. A sensor element as claimed in claim 3, wherein the common support comprises a rigid housing enclosing the reflector and the corresponding end of each of the first and second channel devices.

6. A sensor element comprising a primary channel device and a secondary channel device; the primary channel device comprising first and second discontinuous parts arranged co-axially and closely adjacent to one another, the closely adjacent ends of the first and second parts being enclosed in a housing from which at least the first part extends; and the housing having at least one reflective external surface, the secondary channel device being arranged to transmit radiation to, and/or receive reflected radiation from, the at least one reflective external surface of the housing.

7. A sensor element as claimed in claim 6, wherein the secondary channel device is arranged to transmit radiation to the at least one reflective external surface of the housing such that at least a portion of the radiation is received by the same secondary channel device following reflection.

8. A sensor element as claimed in claim 6 or claim 7, wherein the end surface of the second part of the primary channel device closely adjacent to the first part of the primary channel device is capable of acting as a reflector for radiation transmitted along the first part of the primary channel device.

9. A sensor element as claimed in any one of claims 6 to 8, wherein the reflective external surface of the housing comprises a reflective coating applied to the surface of the housing.

10. A sensor comprising a sensor element as claimed in any preceding claim, a source of radiation for transmitting radiation to at least one channel device of the sensor element, and a measuring device for measuring the reflected radiation transmitted along at least one channel device of the sensor element.

11. A method of measuring a parameter comprising: providing a sensor element comprising a channel device and a reflector, the channel device being directed towards the reflector, and capable of transmitting radiation along its length towards the corresponding reflector and similarly transmitting reflected radiation away from the reflector; exposing the sensor element to the parameter to be measured; transmitting radiation along the channel device of the sensor element so exposed, such that radiation reflected from the corresponding reflector is received by the channel device; measuring the reflected radiation transmitted by the channel device away from the corresponding reflector; and determining the value of the parameter according to the measured reflected radiation.

12. A method as claimed in claim 11, wherein the method provides simultaneous measurement of at least two parameters.