WO1996000905A1

WO1996000905A1 - Optical sensors

Info

Publication number: WO1996000905A1
Application number: PCT/GB1995/001513
Authority: WO
Inventors: Wamadeva Balachandran; Franjo Cecelja; Michael Berwick; Srboljub Radoslav Cvetkovic
Original assignee: University Of Surrey
Priority date: 1994-06-30
Filing date: 1995-06-28
Publication date: 1996-01-11
Also published as: GB9413179D0; AU2799595A

Abstract

An optical sensor has a sensing probe (10), a laser-diode light source (20), a photodetection unit (30), a fibre-optic link unit (40) for directing light from the light source (20) to the sensing probe (10) and from the sensing probe (10) to the photodetection unit (30) and a signal processing unit (50) for processing an output from the photodetection unit (30). The sensing probe (10) contains either an electro-optic sensing element (11) in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure or a magneto-optic sensing element (12) in the form of a crystal of manganese-doped cadmium telluride (Cd1-xMnxTe) with a cubic crystalline structure.

Description

OPTICAL SENSORS

BACKGROUND OF THE INVENTION The invention relates to optical sensors, and particularly to optical sensors including electro-optic sensing elements (sensitive to electric field) and magneto-optic sensing elements (sensitive to magnetic field) .

The invention relates particularly, though not exclusively, to such optical sensors that are suitable for the measurement of the electric and/or magnetic field components of weak electromagnetic fields, especially in the near field.

SUMMARY OF THE INVENTION The present invention provides an optical sensor comprising either an electro-optic sensing element in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure or a magneto-optic sensing element in the form of a crystal of manganese-doped cadmium telluride (Cd, Mn Te) with a cubic crystalline structure.

According to one aspect of the invention there is provided an optical sensor comprising, a light source, a sensing probe comprising a sensing element, polarising means for converting light from the light source to polarised light having a predetermined polarisation and analyser means, wherein said polarised light passes through the sensing element, the sensing element comprises either an electro-optic sensing element in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure which is effective to modify said predetermined polarisation according to an applied electric field or a magneto-optic sensing element in the form of a crystal of manganese-doped cadmium telluride (Cd, ^■—XMnXTe) with a cubic crystalline structure which is effective to modify said predetermined polarisation according to an applied magnetic field and said analyser means produces an output in response to the polarisation of the polarised light after the polarised light has passed through the sensing element, the sensing probe having an elongated configuration and said sensing element being located at one end of the sensing probe in spaced relationship to the polarising means and the analyser means, and the optical sensor further comprising means responsive to said output from the analyser means of the sensing probe to produce an electrical output representative of the applied electric or magnetic field.

According to another aspect of the invention there is provided an optical sensor comprising an electro-optic sensing element in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure, means for directing a beam of polarised light through said crystal of cadmium telluride (CdTe) in a direction perpendicular to the (110) crystallographic plane, the polarised light having a predetermined polarisation and the sensing element being effective to modify the predetermined polarisation according to an applied electric field, and means responsive to the polarisation of the polarised light after the polarised light has passed through the sensing element to produce an output representative of the applied electric field.

According to a further aspect of the invention there is provided an optical sensor comprising, a magneto-optic sensing element in the form of a crystal of manganese-doped cadmium telluride (Cd,~*-&MnXTe) with a cubic crystalline structure, means for directing a beam of polarised light through said crystal, wherein the beam passes through the crystal more than once, the polarised light has a predetermined polarisation, and the sensing element modifies said predetermined polarisation according to an applied magnetic field, and means responsive to the polarisation of the polarised light after the beam has passed through the sensing element to produce an output representative of the applied magnetic field.

DESCRIPTION OF THE DRAWINGS Optical sensors according to the invention are now described, by way of example only, with reference to the accompanying drawings of which:-

Figure 1 shows a diagrammatic view of an optical sensor according to the invention,

Figure 2a shows a longitudinal sectional view through an electro-optic sensing probe in the optical sensor of Figure 1,

Figure 2b shows a longitudinal sectional view through a magneto-optic sensing probe in the optical sensor of Figure 1,

Figure 3 is a block diagram showing a signal processing circuit in the optical sensor of Figure 1, and

Figure 4 is a longitudinal sectional view through a collimator used in the electro-optic and magneto-optic sensing probes shown in Figures 2a and 2b.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Figure 1, the optical sensor comprises a sensing probe 10, a light source 20 in the form of a laser diode, a photodetection unit 30, a fibre-optic link 40 for directing light from the light source 20 to the sensing probe 10 and from the sensing probe 10 to the photodetection unit 30, and a signal processing unit 50 for processing an output from the photodetection unit 30.

The sensing probe 10 may contain either an electro-optic sensing element (sensitive to electric field) or a magneto-optic sensing element (sensitive to magnetic field) and, in accordance with the present invention, the electro-optic sensing element comprises a crystal of cadmium telluride (CdTe) with a cubic crystalline structure, whereas the magneto-optic sensing element comprises a crystal of manganese-doped cadmium telluride (Cd,"~XMnXTe) with a cubic crystalline structure.

Crystalline cadmium telluride (the electro-optic sensing element) exhibits a linear electro-optic effect known as the Pockel's effect in which linear birefringence is induced in the crystalline material , in proportion to the strength of an applied electric field. This effect gives rise to a change in the ellipticity of polarised light propagating through the crystal, which can be expressed as a phase retardation δ between two mutually orthogonal linearly polarised components of the light. More specifically, the phase retardation δ is related to the component of electric field E perpendicular to the (110) crystallographic plane and, at the same time, perpendicular to the direction of propagation of the polarised light, by the equation

δ = n³r₄₁lE (1) λ where n and r.. are refractive index and electro-optic coefficient for the crystalline material respectively, λ is the wavelength of the propagating light and 1 is the distance traversed by the polarised light through the crystal. In the case of an alternating electric field, the phase retardation, hence the ellipticity of the polarised light is caused to oscillate at the frequency of the field.

Crystalline manganese-doped cadmium telluride (the magneto-optic sensing element) exhibits a linear magneto-optic effect known as the Faraday effect in which the polarisation plane of linearly polarised light is caused to rotate through an angle θ in proportion to the strength of an applied magnetic field. Ideally the rotation angle θ is related to the component of magnetic field H in the propagation direction of the linearly polarised light by the expression, θ = V1H → (2) where V is the Verdet constant for the crystalline material and 1 is the distance traversed by the linearly polarised light through the crystal. Again, in the case of an alternating magnetic field, the rotation angle θ is caused to oscillate at the frequency of the field.

The inventors have discovered that crystalline cadmium telluride and crystalline manganese-doped cadmium telluride respectively exhibit high sensitivity to applied electric and magnetic fields, especially at high frequencies of up to 2GHz and yet, compared with known electro-optic and magneto-optic materials are relatively insensitive to environmental changes.

Accordingly, the present materials find particular, though not exclusive application in the measurement of weak electric and magnetic fields against a background of environmental noise, as may arise, for example, in the near field of some r.f. transmitting antennas, such as the antenna of a mobile telephone.

The optical sensors described with reference to the accompanying drawings are designed to exploit the high sensitivity of the electro-optic and magneto-optic materials used. To this end, the electrical and electronic components of the sensor, namely the light source 20, the photodetection unit 30 and the signal processing unit 50 are shielded to reduce electromagnetic interference (EMI), and the sensing probe 10 is remotely coupled to the light source 20 and photodetection unit 30 by means of the fibre-optic link 40. The sensing probe 10 contains only dielectric materials and is electrically passive. These measures are effective to reduce perturbation of the measured field within the electro-optic or magneto-optic sensing element of the sensing probe.

Figure 2a of the accompanying drawings shows a longitudinal, sectional view through an electro-optic sensing probe which is sensitive to electric field and has an electro-optic sensing element 11 in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure, whereas Figure 2b shows a longitudinal, sectional view through a magneto-optic sensing probe which is sensitive to magnetic field and has a magneto-optic sensing element 12 in the form of a crystal of manganese-doped cadmium telluride

(Cd 1--xMnxTe)' with a cubic cry ^Jstalline structure.

The sensing probes shown in Figures 2a and 2b have a generally elongate con iguration, with the respective sensing element 11,12 in each probe being mounted at one extreme end, in remote, spaced relationship to other optical components in the probe.

This configuration reduces any perturbation of the measured field to which the optical components might otherwise give rise were they to be positioned in close proximity to the associated sensing element.

Referring now to Figure 2a, light from the laser-diode of source 20 is conducted to the sensing probe by a single mode fibre 41 in the fibre optic link 40, and a collimator 13 directs a beam B- of the received light along a precisely defined path.

Beam B- passes through a polariser 14, is reflected through 180 by a complementary reflector 15 and then passes through the electro-optic sensing element 11 towards a polarisation state analyser 16.

Polariser 14 comprises a polarising element 141 in the form of a Glan Thompson prism polariser which converts light received from source 20 to linearly polarised light having a defined polarisation plane, and a retardation element 142 in the form of a quarter-wave plate which is set to convert the linearly polarised light from the polarising element 141 to circularly polarised light.

As already explained, an applied electric field causes a change in the polarisation state of light passing through the electro-optic sensing element 11. In the present embodiment, an applied electric field converts the circularly polarised light in beam B- to elliptically polarised light having an ellipticity related to the strength of the applied field.

Thus, the ellipticity of polarised light in beam B. , after the beam has passed through the electro-optic sensing element 11 gives a measure of an applied electric field to which the sensing element is exposed.

It is not possible directly to measure the polarisation state of polarised light; however, polarisation state is measured indirectly using the polarimetric analyser 16 which produces a measurable modulation of light intensity in beam B, according to the polarisation state of the light.

In this embodiment, the polarimetric analyser 16 comprises a beam splitter 161, a right-angle prism 162 and two polarisers 163,164 having mutually orthogonal polarisation planes. The beam splitter 161 divides beam B into two beams B ,B of equal intensity and, in conjunction with right-angle prism 162 directs each beam ^B2'^B3 through a respective polariser 163,164. With this arrangement the polarisation state analyser 16 resolves elliptically polarised light in beam B- into two, mutually orthogonal, linearly polarised components whose relative intensities are related, in a known manner, to the required ellipticity representative of an applied electric field that is to be measured.

The output from each polariser 163,164 is supplied to a respective photodetector 31,32 in unit 30 via a respective collimator 17,18 and a respective multi-mode fibre 42,43 in fibre-optic unit 40, and each photodetector 31,32 produces a detection signal representative of the detected light intensity.

As already explained, in the case of an applied alternating electric field, the resulting ellipticity will oscillate at the frequency of the field. As a consequence of this, the intensity of light from each polariser 163,164 will also oscillate which, in turn, modulates the amplitude of the detection signal produced by the associated photodetector 31,32. The modulated detection signals are processed in respective channels of the signal processing unit 50. As shown in Figure 3, each channel C. ,C_ in unit 50 comprises an amplification stage 51,51' and a bandpass filter 52,52' having a centre frequency tuned to select a desired frequency range of applied electric field. In this embodiment, the passband of filters 52,52' is centred on the applied field frequency (e.g. 900 or 1800MHz) .

The amplified and filtered signals in each channel are combined in a differential amplifier 53 to produce an output 0/P(l) representative of the amplitude and frequency of the applied electric field to which the electro-optic sensing element 11 is exposed. As will be appreciated by those skilled in the art, the signals in channels C,,C_ could be combined in a variety of different ways to derive the required output signal.

The sensitivity of the electro-optic sensing element 11 to an applied electric field depends, inter alia, on the orientation of the cadmium telluride crystal and, for optimum sensitivity, it is found that beam B- should be aligned in a direction perpendicular to the (110) crystallographic plane. This orientation is found to offer the optimum sensor characteristics i.e. the maximum electro-optic effect. Furthermore, in this preferred orientation, the crystal has fixed, induced eigenaxes and is predominently sensitive to electric field in a single direction only; namely, the direction orthogonal to the (110) plane.

It has been found that for optimum sensitivity, the optical components in the sensing probe should preferably be aligned to an accuracy of better than 7mrad.

In this regard, the collimators 13,17,18 are of particular importance and these components have been specially designed with a view to improved sensitivity.

Referring to Figure 4, each collimator comprises a ball lens 60 which is accurately centred on the longitudinal axis of an associated optical fibre 61.

The lens 60 and the optical fibre 61 are mounted in respective, closely-fitting cylindrical holders 62 and 63. The axial position of the fibre within the holder 63 can be readily adjusted to optimise the spacing between the lens 60 and tip of the fibre 61. In the case of collimator 13, lens 60 forms light from fibre 61 into an outgoing beam B. having substantially the same diameter as that of the lens (typically 2mm), whereas in the case of collimators 17,18, lens 60 focuses light in an incoming beam ^B_,B_ onto the end face of the optical fibre.

The size of the electro-optic effect produced in a crystal of cadmium telluride and also the extent of optical absorption in this material (which should be as small as possible) depends upon the wavelength of light used.

Analysis shows that for optimum sensitivity, the preferred wavelength for cadmium telluride is 1300nm. By treating the front and rear surfaces S-.-S-, of the crystal with a view to reducing unwanted reflection of light at these surfaces, the sensitivity degradation is minimised. To this end, surfaces ^si'^S _p ^are polished, preferably to a quality better than λ/8 (where λ is the wavelength of light) and are provided with anti-reflective coatings AR which, in this example, would have a centre wavelength of 1300nm (the wavelength of light in beam B-).

Referring now to Figure 2b (the magneto-optic sensor), light from source 20 is conducted to the sensing probe 10 by a single mode fibre 41' in the fibre optic link 40, and a collimator 13" of a form described with reference to Figure 4, directs a beam B ' of the received light along a precisely defined path.

Beam B ' passes in turn through a polariser 14' and the magneto-optic sensing element 12 and is then reflected by 180 complementary reflection 15* back through the sensing element 12 towards a polarimetric analyser 16 ' .

Polariser 14* comprises a Glan Thompson prism polariser 141' and a half-wave plate 142' which, in combination, convert light from source 20 into linearly polarised light having a preset polarisation azimuth.

The effect of magneto-optic sensing element 12 is to rotate this preset polarisation plane through an angle θ which is proportional to an applied magnetic field in the propagation direction of beam B ' . As equation 2 above shows, the rotation angle θ is also proportional to the distance traversed by the beam through the magneto-optic sensing element 12, and so the effect of reflecting the beam back through the sensing element is to double the rotation angle, giving improved sensitivity. Clearly, a multi-path arrangement in which beam B- ' transverses the sensing element 12 more than twice would give even greater sensitivity.

The polarimetric analyser 16' comprises a beam splitter 161', a right-angle prism 162', and two polarisers 163', 164' (e.g. Glan Thompson prism polarisers) having mutually orthogonal polarisation planes, respectively set at +45° and -45 relative to the preset polarisation plane of polariser 14'. With this arrangement, analyser 16' resolves the linearly polarised light in beam B, ' into two mutually orthogonal, plane polarised components whose relative intensities are related, in known manner, to the rotation angle θ which represent the applied magnetic field that is to be measured.

As in the case of the sensing probe described with reference to Figure 2a, the output from each polariser 163 ',164' is supplied to a respective photodetector 31,32 in unit 30 via a respective collimator 17', 18' and a respective multi-mode fibre 42',43" in unit 40, and each photodetector 31,32 produces a detection signal representative of the detected light intensity.

The detection signals from photodetectors 31,32 are processed, as before, in respective channels ^ci'^C2 of the signal processing unit 50 and are combined in differential amplifier 53 to produce an output 0/P(2) representative of the measured rotation angle θ, in turn proportional to the applied magnetic field.

It has been found that the sensitivity of the magneto-optic sensing element 12 depends upon the proportion of manganese in the manganese-doped cadmium telluride crystal and, as in the case of the electro-optic sensing element 12, upon the wavelength of light used.

The proportion of manganese in the crystal affects the frequency response characteristics of the crystal (the higher the proportion of manganese the greater the frequency range) and also the absorption and temperature response characteristics of the material. Wavelength, on the other hand, affects the size of the magneto-optic effect and the degree of optical absorption. It has been found that the optimum wavelength is 650nm and that the optimum proportion of manganese in the crystalline material (Cd, X^—XMnXTe) is given by, x = 0.45, giving a frequency response in a range up to 2GHz and in this embodiment an optical absorption of about 35%.

It is envisaged that proportions of manganese in the range x = 0.43 to 0.48 are also particularly beneficial.

The sensitivity of the magneto-optic sensing element 12 does not depend critically upon orientation; however, in order to reduce such unwanted effects as variation of polarisation state due to intrinsic linear birefringence and scattering and or internal reflection of light, it is preferred to align the incident light beam B- * with the optic axis of the crystal, this being particularly beneficial in the case of a multipath configuration such as that described with reference to Figure 2b. In this embodiment the sensing element is cylindrical such that circular symmetry minimises the effect of induced linear birefringence.

As in the case of the electro-optic sensing arrangement described with reference to Figure 2a, by polishing the front and rear surfaces ^s-_t'S--, of the magneto-optic sensing element 12 and providing anti-reflective coatings (AR) at the polished surfaces, the sensitivity degradation is minimised. In this embodiment the anti-reflective coating would have a centre wavelenth of 650 nm.

For optimum sensitivity, the polarisers used in the electro-optic and magneto-optic sensing arrangements should have an extinction ratio of better than 1:10⁴, and the quarter-wave plate 142 (used in the electro-optic sensing arrangement) and the half-wave plate 142' (used in the magneto-optic sensing arrangement) should have a phase retardation accuracy better than lmrad.

It is also desirable that the single mode fibre 41,41' used to conduct light from the laser-diode to the sensing probe should be highly, linearly birefringent - a so-called Hi-Bi optical fibre. When correctly orientated, a Hi-Bi optical fibre will preserve the linear polarisation state of light from the laser-diode, and is thereby able to maintain a constant optical power to the sensing probe, even when the fibre is subjected to environmental changes (e.g. bending and temperature changes) which tend to have an adverse effect on the transmission characteristics of multi-mode or low-birefringence single mode fibres.

The level of output noise from the laser-diode should, of course, be as low as possible and, for optimum sensitivity it is desirable that the level of output noise should be less than 20pW(Hz) ^"•- . Since a high proportion of the output noise is derived from the electrical power supply to the laser, the supply noise should preferably not exceed lμA for a maximum laser-diode output power of 20mW. The laser diode has a stabilised power output and, in this embodiment, a stabilising control signal C_ is generated in a feedback circuit shown in Figure 3. Control signal C_ is derived from the signal processing unit 50. To this end, a respective low-pass filter 54,55 extracts a low frequency component (<lMHz) from each channel C-,C_ in unit 50 and an averaging circuit 56 averages the extracted, low frequency components to produce the stabilising control signal C_, which is then supplied to the laser-diode drive circuit 70.

Electro-optic and magneto-optic sensors described with reference to the drawings have high sensitivity to applied electric and magnetic fields over a frequency range up to about 2GHz. It has been determined that when suitably optimised, as described herein, the electro-optic sensor will have a resolution (i.e. minimum measurable electric field) of about 2Vm^~ , and the magneto-optic sensor will have a resolution (i.e. minimum measurable magnetic flux density) of about 0.25μT, and both sensors will have a dynamic range greater than 60dB, a linearity (in the respective ranges 2Vm to lOOOVm and 0.25μT to lmT) of better than 1% and a spatial resolution of approximately 1cm.

Furthermore, unlike many known field sensors, sensors according to the present invention are passive devices and are therefore inherently safe and have low power consumption.

INDUSTRIAL APPLICABILITY Sensors according to the invention find a multiplicity of different applications of which the following are merely examples:-

(i) Validation of a computer simulation of electromagentic field, especially in the near field.

(ii) Compliance testing against national and international standards and/or guidelines,

(iii) EMC hazard assessment, including dosimetry

(iv) Environmental and machine health monitoring

(v) RF and power line fault detection, and

(vi) Monitoring electric and/or magnetic field in hazardous environments e.g. in explosive or chemically aggressive environments.

Claims

1. An optical sensor comprising, a light source, a sensing probe comprising a sensing element, polarising means for converting light from the light source to polarised light having a predetermined polarisation and analyser means, 'wherein said polarised light passes through the sensing element, the sensing element comprises either an electro-optic sensing element in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure which is effective to modify said predetermined polarisation according to an applied electric field or a magneto-optic sensing element in the form of a crystal of manganese-doped cadmium telluride (Cd, MnXTe) with a cubic crystalline structure which is effective to modify said predetermined polarisation according to an applied magnetic field and said analyser means produces an output in response to the polarisation of the polarised light after the polarised light has passed through the sensing element, the sensing probe having an elongated configuration and said sensing element being located at one end of the sensing probe in spaced relationship to the polarising means and the analyser means, and the optical sensor further comprising means responsive to said output from the analyser means of the sensing probe to produce an electrical output representative of the applied electric or magnetic field.

2. An optical sensor as claimed in claim 1 comprising fibre-optic link means for guiding light from the light source to said polariser means and from the analyser means to the processing means.

3. An optical sensor as claimed in claim 2 wherein the fibre-optic link means includes a highly, linearly birefringent fibre (a Hi-Bi fibre) for guiding light from the light source to the polariser means.

4. An optical sensor as claimed in any one of claims 1 to 3 wherein said polarised light is incident at a first surface of the sensing element and is emergent from a second surface of the sensing element, and the sensing element is treated to reduce reflection of light at said first and second surfaces.

5. An optical sensor as claimed in claim 4 wherein said first and second surfaces of the sensing element are polished and bear respective anti-reflective coatings .

6. An optical sensor as claimed in any one of claims 1 to 5 wherein the sensing element is an electro-optic sensing element comprising said crystal of cadmium telluride (CdTe) with a cubic crystalline structure, and the polarised light is directed to pass through the crystal in the direction perpendicular to the (~Ϊ10) crystallographic plane.

7. An optical sensor as claimed in claim 6 wherein the wavelength of the light is 1300nm.

8. An optical sensor as claimed in claim 6 or claim 7 wherein said predetermined polarisation is circular polarisation.

9. An optical sensor as claimed in any one of claims 1 to 5 wherein the sensing element is a magneto-optic sensing element comprising a said crystal of manganese-doped cadmium telluride (Cd, Mn Te) in which the proportion of manganese is in the range x = 0.43 to 0.48, and preferably x = 0.45.

10. An optical sensor as claimed in claim 9 wherein the sensing element is cylindrical in shape.

11. An optical sensor as claimed in claim 9 or claim 10 wherein the wavelength of the light is 650nm.

12. An optical sensor as claimed in any one of claims 9 to 11 in which the predetermined polarisation is a linear polarisation.

13. An optical sensor as claimed in any one of claims 9 to 12 wherein the polarised light is caused to pass through the -sensing element more than once.

14. An optical sensor as claimed in any one of claims 1 to 13 wherein said analyser means resolves the polarised light into two, mutually orthogonal plane polarised components which form said output from the analyser means, and the processing means includes a respective photodetector for detecting each said plane polarised component in the output of the analyser.

15. An optical sensor as claimed in any one of claims 1 to 14 comprising collimator means for forming light from said source into a beam, and said polariser means comprises means for converting light in said beam to polarised light having said predetermined polarisation.

16. An optical sensor as claimed in claim 15 wherein light is guided to the polariser means by an optical fibre, and said collimator means comprises a ball lens and means for mounting the ball lens centrally with respect to the longitudinal axis of the fibre and in spaced relationship to an end face of the fibre.

17. An optical sensor as claimed in claim 16 wherein the mounting means comprises a first cylindrical holder for holding the fibre and a second cylindrical holder housing the ball lens and said first holder.

18. An optical sensor as claimed in any one of claims 15 to 17 wherein said polariser means comprises a polarising element for converting light from the light source to plane polarised light and a retardation element for converting the plane polarised light to polarised light having said predetermined polarisation.

19. An optical sensor as claimed in claim 18 wherein the sensing element is an electro-optic sensing element and said retardation element is a quarter-wave plate, the predetermined polarisation being circular polarisation.

20. An optical sensor as claimed in claim 18 wherein the sensing element is a magneto-optic sensing element and said retardation element is a half-wave plate, the predetermined polarisation being a linear polarisation.

21. An optical sensor as claimed in claim 19 or claim 20 wherein said polarising element has an

4 extinction ratio of better than 1:10 and said retardation element has a phase retardation accuracy of better than lmrad.

22. An optical sensor as claimed in any one of claims 1 to 21 wherein the light source comprises a laser-diode which is stabilised by an output from the processing means.

23. An optical sensor comprising an electro-optic sensing element in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure, means for directing a beam of polarised light through said crystal of cadmium telluride in a direction perpendicular to the (110) crystallographic plane of the crystal, the polarised light having a predetermined polarisation and the sensing element being effective to modify the predetermined polarisation according to an applied electric field, and means responsive to the polarisation of the polarised light after the polarised light has passed through the sensing element to produce an output representative of the applied electric field.

24. An optical sensor as claimed in claim 23 comprising a light source, a collimator for forming light from the light source into a beam and a polariser for converting light in said beam to polarised light having said predetermined polarisation.

25. An optical sensor as claimed in claim 24 wherein said polariser comprises a polarising element for converting light in said beam to plane polarised light and a quarter-wave plate for converting the plane polarised light to circularly polarised light.

26. An optical sensor as claimed in claim 25 wherein the polariser has an extinction ratio better than 1:10⁴ and said quarter-wave plate has a phase retardation accuracy of better than lmrad.

27. An optical sensor as claimed in any one of claims 1 to 26 wherein the wavelength of the light is 1300nm.

28. An optical sensor as claimed in claim 24 comprising an optical fibre for guiding light from the light source to the polariser, and said collimator comprises a ball lens and means for mounting the ball lens centrally with respect to the longitudinal axis of the fibre and in spaced relationship to an end face of the fibre.

29. An optical sensor as claimed in claim 28 wherein the optical fibre is a highly, linearly birefringent (Hi-Bi) fibre.

30. An optical sensor comprising, a magneto-optic sensing element in the form of a crystal of manganese-doped cadmium telluride (Cd X,^—XMn Te) with a cubic crystalline structure, means for directing a beam of polarised light through said crystal, wherein the beam passes through the crystal more than once, the polarised light has a predetermined polarisation, and the sensing element modifies said predetermined polarisation according to an applied magnetic field, and means responsive to the polarisation of the polarised light after the beam has passed through the sensing element to produce an output representative of the applied magnetic field.

31. An optical sensor as claimed in claim 30 wherein the proportion of manganese in the crystal (Cd, Mn Te) is in the range x = 0.43 to 0.48, and is preferably x = 0.45.

32. An optical sensor as claimed in claim 30 or claim 31 wherein said predetermined polarisation is a linear polarisation

33. An optical sensor as claimed in claims 30 to 32 wherein the wavelength of the light is 650nm.

34. An optical sensor comprising, a laser-diode light source, a sensing element comprising either an electro-optic sensing element in the form of a crystal of cadmium telluride (CdTe) with a cubic crystalline structure or a magneto-optic sensing element in the form of a crystal of manganese-doped cadmium telluride (Cd X,™~XMnXTe) with a cubic crystalline structure, polarising means for converting light from the laser-diode light source to polarised light having a predetermined polarisation, wherein said polarised light passes through the sensing element and said electro-optic element is effective to modify said predetermined polarisation according to an applied electric field and said magneto-optic element is effective to modify said predetermined polarisation according to an applied magnetic field, analyser means for producing an output in response to the polarisation of the polarised light after the polarised light has passed through the sensing element, and processing means responsive to said output from the analyser means to produce an electrical output representative of the applied electric or magnetic field and a further output for stabilising the output of light from the laser-diode.

35. An optical sensor as claimed in claim 34 wherein said processing means has two channels and said further output is derived by averaging respective low frequency signal components in the channels.

36. An optical sensor as claimed in claim 35 wherein said analyser means resolves the polarised light into two, mutually orthogonal, plane polarised components which form said output from the analyser means, and the processing means includes a respective photodetector for detecting each said plane polarised component in the output from the analyser and said further output is derived by averaging respective low frequency components in the outputs from the photodetectors.

37. An optical sensor substantially as herein described with reference to the accompanying drawings.