GB2117993A - Electro-optical transducers - Google Patents

Abstract

A liquid crystal modulator (22), receiving optical energy from a remote source (50) through a fibre optic cable (48) modulates the optical energy in response to an electrical signal from transducer(s) 10. The optical signal is transmitted to utilisation apparatus (62) through a fibre optic cable (52, 60). In a preferred embodiment a piezoelectric hydrophone transducer (10-16) generates an electrical signal in response to acoustic stimulus. The optical signal may be multiplexed, using passive optical techniques, with other optical signals produced by like piezoelectric-liquid crystal hybrid devices. <IMAGE>

Description

SPECIFICATION Electro-optical transducers The present invention relates to hybrid electrical-optical transducers and more particularly, although not exclusively, to a hydrophone transducer utilising a piezoelectric sensor in combination with a liquid crystal light modulator and fibre optic transmission system.

Present day underwater acoustic transducers or hydrophones employ piezoelectric crystals to transform acoustic signals into electric signals by converting pressure variations at the crystal into corresponding voltage variations across electrodes positioned on opposite sides of the crystal. These crystals typically supply very small voltages at very high impedance levels. To overcome impedance matching problems, each hydrophone conventionally requires transimpedance amplifiers which convert the high output impedance of the crystal into a low output impedance for coupling to a transmission line such as a wire or coaxial cable. The failure rate for transimpedance amplifiers is high, so they are often employed in redundant sets, each requiring a supply of electrical energy.

In an underwater target locating system, large numbers of hydrophones, typically 100--1,000, are utilised in a one or two dimensional array which can be electronically combined to produce directional acoustic resolution in the vertical and horizontal planes. Each hydrophone must communicate electrically with a receiver, often through its own dedicated pair of transmission wires. In a system employing 1,000 such hydrophones, for example, it will be readily appreciated that the bulk, weight, and cost of all these transmission wires is enormous.

Furthermore, such transmission wires can be relatively long and the system is subject to electromagnetic interference.

Electrically active multiplexing techniques are sometimes used to transmit the signals of a plurality of hydrophones along a single coaxial transmission cable. However, this requires submerging the multiplexing equipment near the array, adding weight and bulk to the underwater package and increasing the likelihood of electronic failure.

It is therefore apparent that the present day acoustic sensor arrays are complicated systems comprising acoustical signal-to-electrical signal transducers, electronics for impedance conversion, electronics for multiplexing, cables for carrying power lines, transmission lines for the propogation of electrical signals, and mechanical members for maintaining the various elements within a specified package.

Fibre optic techniques have been suggested which can overcome many of the problems attendant to conventional piezoelectric hydrophone technology. For instance, fibre optic cables are not subject to electromagnetic interference and are much lighter than their electrical counterparts. Moreover, fibre optic cables have extraordinary bandwidths compared with coaxial cables and are compatible with passive optical multiplexing techniques. Many of these fibre optic approaches are based on some variation of either a single mode interferometric approach or upon one of a number of intensity modulation mechanisms, the latter involving physical motion or mechanical effects such as micro-bending.

One such mechanical effect is taught in U.S.

Patent Specification No. 4,300,813 and this fibre optical transducer includes two optical fibres each cut to have end faces substantially perpendicular to an axis and positioned with a small gap between the end faces of the fibres. One fibre is mounted to maintain its end face stationary, for example, while the other is cantilevered to permit displacement of its end face. When the fibre axes lie on a common straight line, light propagating in one fibre will couple with maximum intensity to the other fibre.

Another example of an acousto-optic transducer is described in U.S. Patent Specification No. 4,293,1 88 and this fibre optic transducer employs a first optical fibre guide disposed with its end face stationary, while a second optical fibre guide is disposed so that its free end may be laterally displaced from the axis ol the first guide in proportion to the displacement being measured.

While these acousto-optic transducers are a significant improvement over the present day piezoelectric crystal hydrophone in terms of lower bulk, weight, and cost, the piezoelectric technology remains desirable since the properties of that technology are predictable and well understood. Therefore, it is an object of this invention to combine the advantages of fibre optic technology with the desirable properties of piezoelectric technology. The intention, which is defined in the appended claims, results in a predictable and reliable hydrophone which is easily implemented, has no moving parts requires only normal electrical power, and retains the benefits of fibre optic technology.

According to a preferred embodiment, a hydrophone transducer for sensing a stimulus, which may be acoustical energy, and for converting the stimulus first into an electrical signal, and then into an optical signal comprises a piezoelectric sensor which produces electrical signals under the influence of pressure variations present within an acoustic medium in which the hydrophone transducer is submerged. Optical energy is provided to the transducer by means of an optical source, such as a laser diode or light emitting diode (LED) which may be located in a remote location, such as on board ship, and coupled to the transducer via a multimode fibre optic cable. A liquid crystal modulator receives the electrical signal from the piezoelectric transducer and modulates the optical energy in response to the electrical signal.The modulated optical signal, which is representative of the pressure variations sent by the piezoelectric transducer, is coupled to utilisation apparatus via a second fibre optic cable, or via the first fibre optic cable in a multiplexed arrangement. A photoelectric diode may be utilised to convert the modulated optical signal into an electrical signal for connection to the utilisation apparatus in the conventional fashion.

The hybrid electro-optical transducer exhibits similar properties to the conventional piezoelectric transducer, and is therefore compatible with existing sonar equipment. One advantage over the prior art technology is the ability of the present invention to interface with weight-saving, passive optical multiplexing devices. In this regard, it will be appreciated that the bandwidth of a fibre optic cable is much greater than its coaxial electrical transmission line equivalent. Furthermore, by converting to optical energy, electromagnetic interference is all but eliminated. The need for transimpedance amplifiers is eliminated, as well as the need to supply electrical power to each amplifier.

A hydrophone embodying the present invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block schematic diagram of the embodiment.

Figure 2 is a cross sectional view of a tunable birefringent liquid crystal modulator of the embodiment, Figure 3 is an exploded view of the liquid crystal modulator illustrating by graphical effect or representation the electric field characteristics of the optical energy as it progresses through the liquid crystal modulator, Figure 4 is a graphical representation of the relationship between transmitted optical power through the liquid crystal modulator in response to applied electrical bias voltage, and Figure 5 is an alternative liquid crystal modulator of the twisted nematic type.

With reference to Figure 1, the invention in its presently preferred form comprises one or more piezoelectric crystals 10 which affixed to a sensitivity enhacing plate 12, and electrically connected in series, although they could equally well be connected in parallel. In Figure 1 each piezoelectric crystal is modelled as conceptually comprising an internal voltage source 14 and source impedance 1 6 in series therewith. It will be understood that this model is presented as a means of teaching the principles of the invention, other models being equally applicable, such as an equivalent current source and shunt impedance.

Furthermore, while a plurality of piezoelectric crystals 10 are shown connected so their respective outputs contribute in unison to the total output signal, as carried on leads 1 8 and 20, it will be understood that in some applications, only one piezoelectric crystal might be employed. In the general case, the piezoelectric crystal may be replaced by any of a number of transducers capable of responding to a number of physical variables, such as pressure, temperature, displacement strain, flow, for example.

The preferred embodiment further comprises a liquid crystal modulator 22. As is known, a liquid crystal modulator is a device containing a mesomorphic material which exhibits optical activity, such as birefringence, that is sensitive to relatively weak external stimuli. The stimuli include electric fields, magnetic fields, heat enegy, and acoustical energy all of which may be used to induce optical effects in the material. The presently preferred embodiments uses the electrooptic effect to modify the passage of light through the crystal, as will be more fully explained herein.

Among the electro-optic effects which may be utilised in practising the invention are induced birefringence, twisted nematic effects, electrohydrodynamic turbulence that leads to wide angle light scattering, and cholesteric-nematic phase change in which optical absorption arises from orientable dichroic dye molecules added to the liquid crystal. Induced birefringence is presently preferred.

A liquid crystal modulator configured to operate in the induced birefringence mode is shown in Figure 2. The modulator comprises a pair of parallel glass plates 24 each coated with a transparent electrically, conductive film 26 on their interior surfaces. The transparent conductive film 26 may be indium-tin oxide, for example. A pair of electrodes 28 are electrically connected to the respective conductive films 26 providing first and second terminals 30 and 32 for applying a voltage across the two conductive films. A surfactant layer 34 of SiO is deposited on each of the conductive films 26 at a 600 incidence angle, according to the usual practice, to provide uniform parallel ordering to the liquid crystal.A cylindrical spacer 36 is attached between the parallel plates as with heat curable epoxy adhesive, for example, and the internal region defined between the plates and the spacer is filled with a liquid crystal material such as phenylcyclohexane, a mixture which exhibits optical birefringence properties.

The birefringeent material is depicted in Figure 2 as a plurality of planar ordered, elliptical cells 38.

It will be appreciated that the cells 38 and the coatings deposited on the parallel glass plates 24 have been greatly exaggerated as to scale for illustration purposes.

With continued reference to Figure 2, the embodiment further comprises a polariser 44 and analyser 46 which are affixed to the respective outer faces of the parallel glass plates 24.

Considering the optical axis of the liquid crystal to be in axial alignment with the cells 38, as denoted by reference arrow X, the polariser 44 is oriented so its axis of polarisation is rotated 450 with respect to the optical axis X of the liquid crystal.

The analyser 46 is positioned so its polarisation angle is 450 with respect to the optical axis X of the liquid crystal and orthogonal to the polarisation axis of the polariser 44. In other words, the polariser 44 and analyser 46 are configured as crossed polarisers. Figure 3 shows the axial relationship between the polariser 44, analyser 46, and optical axis X of the liquid crystal.

Returning to Figure 1 but with continued reference to Figure 2, the transducer further comprises first and second graded index rod lenses 40 and 42 attached directly to the polariser 44 and analyser 46, respectively. As is well known, graded index rod lenses can receive light rays emanating from a point source, such as from the end of a tiny fibre optic cable, expand and collimate those rays into an optical beam; or by reversing the process, it can take a collimated expanded beam, decollimate it, and focus it a single point. As is known, the graded index rod lens performs its expanding and collimating or decollimating and focusing functions without the necessity of providing an air gap or space between the lens and the focal point.Thus it will be appreciated that the graded index rod lens is suitable for attachment directly to the surface of the polariser 44 and anaylser 46, as by bonding.

For this reason, the graded index rod lens is presently preferred; however, other types of lenses may be utilised in practising the invention.

A first fibre optic cable 48 having a proximal end 48a and a digital end 48b is attached at its proximal end to the graded index rod lens 40, as by bonding. The distal end 48b of the fibre optic cable 48 is connected to a source of optical energy such as a laser 50. The laser 50 may be located conveniently on board a marine vessel. A second fibre optic cable 52 having a proximal end 52a and a distal end 52b is attached at its proximal end to the graded index rod lens 42. The distal end 52b is connected to a passive optical multiplexing device 54. Such a multiplexing device is described in U.S. Patent Specification No.

4,302,835.

The optical multiplexing device 54 may be adapted to receive additional fibre optic input cables, such as denoted by reference numerals 56 and 58. It provides a multiplexed output via a fibre optic cable 60 having a distal end 60b which is coupled to utilisation apparatus 62. As taught in U.S. Patent Specification No. 4,302,835, a plurality of optical signals, introduced through fibre optic cables 52, 56 and 58 are time delay multiplexed for transmission along the output fibre optic cable 60, in such a manner that the signals may be separated on the basis of time delay differences at the utilisation apparatus 62.

The utilisation apparatus 62 includes an optical energy-to-electrical energy converter such as a photo diode 64 to which the cable 60 is connected. The output of photo diode 64 is carried via electrical leads 66 and 68 to a demultiplexer 70 which electrically separates the time delay multiplexed signals once they have been converted to electrical energy. In some applications, for instance where a small hydrophone array is utilised, the optical multiplexer may be eliminated. In this instance, the distal end 52b of the fibre optic cable 52 might be coupled directly to the photo diode 64, and the optical multiplexer 64 and demultiplexer 70 may be omitted.

As will be further explained, the liquid crystal modulator 22 modulates the amplitude or intensity of an applied optical signal in accordance with the voltage applied across terminals 30 and 32. Figure 4 is a graph showing the relative optical power or signal intensity transmitted through a typical liquid crystal modulator of this type as a function of electrical bias voltage as applied across terminals 30 and 32. Referring to Figure 4, it will be seen that very little optical power is transmitted when zero volts is applied. Above zero volts there are peak and valley regions of the curve, such as the region between reference numeral 1 and 2, in which the transmitted power varies from maximum to minimum in response to relatively small voltage changes.Biasing the liquid crystal modulator at the midpoint between these peaks and valleys allows the liquid crystal to function as a light valve. For instance, a bias voltage Vb yields an operating region between the point of maximum optical power transmission 1 to the point of minimum transmission 2 for applied signal excursions both above and below the bias point Vb.

In order to supply bias voltage to the liquid crystal modulator 22 in accordance with Figure 4, a voltage source 72 is coupled, preferably in parallel, to the terminals 30 and 32 of the liquid crystal modulator. The voltage source 72 may be embodied in a number of ways. For example, a small battery may be disposed within the submersible portion 1 3 of the hydrophone assembly and connected to terminals 30 and 32 of the liquid crystal modulator 22. An alternative method would be to transmit optical power through a fibre optic cable, such as the fibre optic cable 48, or through a separate fibre optic cable to an opto-electric energy converting device such as photo diode, for example.The bias voltage source 72 may supply dc voltage, such as in the case of a battery, or it may supply alternately current, both dc and ac being usable to bias the liquid crystal modulator 22. It is known that a dc bias, if applied for long periods of time, will cause electrochemical reactions in the liquid crystal. Therefore, where the period of operation is long, ac bias is preferred. The ac bias may be any conveniently produced waveform including, square waves and sine waves. Where maximum sensitivity of the liquid crystal modulator 22 is desired, square wave bias is preferred over sine wave bias.

While the voltage tunable or induced birefringence light modulator 22 is preferred, it will be appreciated that other light modulating devices may be employed. For instance, a moduator in the 900 twisted nematic configuration may be used as the liquid crystal modulator 22 of Figure 1. A typical cell structure used in the twisted nematic device is shown in Figure 5. It will be noted that the structure of this device is similar to the tunable birefringent device 22 of Figure 2. A primary difference is that the surfactant layers 34 which cause the uniform alignment in one direction of the surface layer molecules of the liquid crystal are orthoganal to one another instead of parallel. The molecules in each surface layer of the liquid crystal are thus uniformly aligned in one direction, but with a twist angle of 900 between the preferred direction for the two surfaces.With no applied voltage, the bulk fluid distorts so as to provide a graduai rotation of the molecular alignment from one cell wall to the other. With a nematic fluid of positive dielectric anisotropy, voltages exceeding the threshold voltage cause the nematic director to become untwisted and to tend to align parallel to the applied field. The optical properties of the twisted nematic field effect are such that linearly polarised light propagating perpendicular to the cell is rotated by approximately 900 as it passes through the fluid when there is no applied voltage.With the crossed polariser and analyser in place as described in conjunction with the birefringent modulator 22, maximum light transmission is obtained for the zero applied voltage case by orienting the crossed polariser optical axis parallel to one of the preferred surface alignment directions in the cell. The transmitted light decreases when the applied voltage exceeds the threshold voltage.

Another suitable liquid crystal modulator might be, for example, an electro-optic total internal reflection device (TIR). Such devices are well known and discussed in the literature. For example, Richard A. Soref"Liquid-Crystal Fiberoptic Switch", OPTIC LETTERS, Volume 4, Page 155, May, 1979.

To demonstrate the principles of operation, it will be assumed that the liquid crystal modulator 22 is biased with the appropriate ac or dc voltage such that the modulator operates in the region as depicted in Figure 4. It will be understood, however, that the device may be biased to operate along other portions of the transmission versus bias curve. The bias point denoted as Vb is a good choice of bias voltage because the curve is relatively linear between the peak denoted by reference numeral 1 and reference numeral 2 and because a relatively small change in applied voltage produces a large change in transmissive qualities.

Consider the hydrophone array 1 3 immersed in an acoustic medium when a mechanical stimulant, in the form of an acoustic shock wave travelling in the medium, impinges upon the hydrophone transducer. It is assumed that the plurality of piezoelectric crystals 10 are disposed sufficiently close to one another within the transducer that all of the crystals experience substantially simultaneously the same acoustic shock wave pressures. The shock wave causes the piezoelectric crystals 10 to emit electrical signals which are applied to the terminals 30 and 32 of the liquid crystal modulator 22. Being connected in series or parallel, the electrical signals of the piezoelectric crystals are summed at terminals 30 and 32. Whereby the crystals may be said to contribute in unison to the signal voltage applied to the liquid crystal modulator 22.For instance, in the series connected case, modelling the piezoelectric crystals as comprising individual interval voltage sources 14, the applied voltage is the sum of the individual voltage sources less any voltage drop attributable to the source impedances 1 6. In the parallel connected case, modelling the piezoelectric crystals as equivalent current sources and shunt impedances, the signal applied at the terminals 30 and 32 will comprise a sum of the individual current sources less any current diverted through the shut impedances.

It will be understood that in a dispersed array, a large number of individual transducers may be placed in a number of spatially remote locations and, therefore, the electrical signals produced by the piezoelectric crystals of these separate transducers would not necessarily experience the acoustic shock wave simultaneously. In such applications, it is anticipated that each spatially distinct transducer, perhaps comprising a multiplicity of individual piezoelectric crystals, would utilise its own dedicated liquid crystal modulator packaged as an integral part of the transducer. A multiplicity of these transducers might then be connected to an optical multiplexing device, such as the optical multiplexer 54, through individual fibre optic cables such as the cables 52, 56 and 58.

Focusing attention on the'operation of one transducer, for the moment, reference is made to Figure 3, with continued reference to Figure 1. The fibre optic cable 48 receives optically energy hv from the laser 50 and transmits that energy to the graded index rod lens 40 where the optical beam is expanded and collimated accordinqto the normal operation of a graded index rod lens. The collimated beam is then polarised in polariser 44.

In Figure 3, the polariser direction, denoted by the arrow on the surface of the polariser 44, will be seen to be 450 with respect to the optical axis X of the liquid crystal modulator 22. In Figure 3, the graded index rod lenses 40, 42 have been omitted and the liquid crystal modulator 22, polariser 44, and analyser 46 are shown in exploded view for illustration purposes. Also shown in Figure 3 is a graphical vector representation of the electric field characteristic of the optical energy as it progresses through the polariser 44, liquid crystal modulator 22, and the analyser 46. The electric field of the optical signal thus depicted is not to be confused with the applied voltage across the terminals 30 and 32 of the liquid crystal.

The optical energy incident upon the polariser 44 emerges polarised at 450 with respect to the optical axis of the liquid crystal modulator 22. The electric field vector of the emerging light is shown in inset A of Figure 3. The electric field denoted by tmay be broken down into ints vector components Ep, the component parallel to the optical axis X of the crystal, and Es, the component orthogonal to the optical axis X. The linearly polarised emerging light then passes through the liquid crystal. Because the liquid crystal has birefringent properties, the two orthogonal components Ep and E5 must travel through the liquid crystal material at different rates, depending on the orientation of the E vector with respect to the crystalline structure of the anistropic material.

Thus, one orthogonal component is delayed with respect to the other and superposition of the two components results in an elliptically polarised beam. Insets B, C, D and F of Figure 3 depict the electric field of various possible elliptically polarised beams, it being understood that the circular polarisation of inset C and the linear polarisation of inset B are two special cases of elliptical polarisation. The elliptically polarised beam emerging from the liquid crystal modulator 22 illuminates the analyser 46 which is polarised orthogonally to the polariser 44, as already explained. Thus only one component of the electric field vector which is parallel to the polarisation axis of the analyser 46 will be transmitted. Insets B', C', D' and F' of Figure 3 depict this component of the electric field corresponding to the respective elliptically polarised beams of inserts B, C, D and F.As is well known, the birefringence, or the degree to which an incoming linearly polarised beam is elliptically polarised, is dependent upon the applied voltage V across the liquid crystal. For liquid crystal material exhibiting positive dielectric anistropy, the birefringence of the device is maximum when the applied voltage V = O. Above the threshold voltage, the birefringence decreases as the applied voltage increases. For materials exhibiting negative dielectric anisotroy, birefringence is at a minimum when the applied voltage is zero and increase to maximum as the applied voltage is increased above a threshold. Either type may be utilised in practising the present invention.

It will be seen that the amplitude or intensity of the optical energy emerging from the analyser 46 is thus dependent in a predictable way upon the voltage applied across the liquid crystal modulator 22. In this fashion, the emerging optical energy is modulated or encoded in accordance with the electrical signal produced by the piezoelectric crystal. This modulated optical energy is then transmitted through the fibre optic cable 52 to the optical multiplexer 54 where it may be time domain multiplexed, for example, and then transmitted through the optical cable 60 to the utilisation device 62. While intensity modulation is preferred, it will be recognised that phase modulation may also be employed, in which case a single mode, polarisation preserving fibre optic cable would be used.

In order to interface with electrical equipment, the modulated optical beam is converted into electrical signals by the photo diode 64; if time domain multiplexing is employed, the electrical signal from the photodiode 64 would be time demultiplexed according to well known electronic techniques. It will thus be appreciated that the present invention provides an electro-optical hybrid hydrophone transducer in which a piezoelectric sensing device, whose properties are well known and understood, is interfaced with an optical modulator. The combination results in a hydrophone that is compatible with multimode optical multiplexing techniques, has no moving parts, requires only nominal power, and eliminates troublesome transimpedance amplifiers of the prior art.

Claims (23)

1. A transducer for sensing a stimulus and transmitting a signal to utilisation apparatus characterised in that it comprises sensing means (10) responsive to said stimulus for producing a first electrical signal, liquid crystal modulator means (22) receptive of incoming optical energy and responsive to the first electrical signal for modulating the incoming optical energy and for producing an optical signal, and output means (54) receptive of the optical signal for transmitting the optical signal to the utilisation apparatus (62).

A transducer according to claim 1, characterised in that it further comprises a source (50) of optical energy coupled to the liquid crystal modulator means (22).

3. A transducer according to claim 2, characterised in that the source of optical energy is a laser (50).

4. A transducer according to any of the preceding claims, characterised in that if further comprises means (40) for linearly polarising the incoming optical energy at a first polarity.

5. A transducer according to any of the preceding claims, characterised in that the sensing means (10) is responsive to mechanical energy.

6. A transducer according to any of claims 1 to 5, characterised in that the sensing means (10) is responsive to acoustical energy.

7. A transducer according to any of the preceding claims, wherein the sensing means (10) comprises piezoelectric means.

8. A transducer according to any of the preceding claims, characterised in that the sensing means (10) is a hydrophone.

9. A transducer according to any of the preceding claims, characterised in that the sensing means comprises a plurality of mechanical energy-to-electrical energy converting means (10) coupled so as simultaneously to receive said stimulus and to produce, in unison, the first electrical signal.

10. A transducer according to claim 6, characterised in that it further comprises pressure amplifying means (12) coupled to said sensing means (10) for increasing the sensing means responsiveness to acoustical energy.

11. A transducer according to claim 2 and any claim appended thereto, characterised in that it further comprises first optical waveguide means (48) coupled to the source (50) of optical energy and to the liquid crystal modulator means (22).

12. A transducer according to claim 11, characterised in that the first optical waveguide means (48) comprises fibre optic means.

13. A transducer according to any of the preceding claims, characterised in that the liquid crystal modulator means (22) produces intensity modulation of the optical energy.

14. A transducer according to any of the preceding claims, characterised in that the liquid crystal modulator means (22) comprises tunable birefringement means.

15. A transducer according to claim 4, and any claim appended thereto, characterised in that it further comprises analyser means (42) for polarising the optical signal at a second polarity other than said first polarity.

1 6. A transducer according to claim 1 characterised in that it further comprises means (44) receptive of the incoming optical energy for producing linearly polarised optical energy, tunable birefringement means (22) receptive of the linearly polarised optical energy and responsive to the electrical signal for producing an elliptically polarised optical signal, and analyser means (46) receptive of the elliptically polarised optical signal for transmitting a linearly polarised component thereof.

17. A transducer according to claim 16, characterised in that the linearly polarised component is substantially orthogonal to the linearly polarised optical energy.

1 8. A transducer according to claim 1 6 or 1 7, characterised in that the tunable birefringent means (22) produces elliptically polarised optical energy having first and second orthogonal electric field components, the magnitudes of the first and second components being relatively variable in accordance with the electrical signal.

1 9. A transducer according to any of the preceding claims, characterised in that the output means includes second optical waveguide means (52).

20. A transducer according to claim 19, characterised in that the second optical waveguide means comprises fibre optic means (52).

21. A transducer according to any of the preceding claims, characterised in that the output means includes means (62) for converting the optical signal into a second electrical signal.

22. A transducer according to claim 21, characterised in that the means for converting the optical signal into a second electrical signal comprises photodiode means (64).

23. A transducer according to claim 11 and any claim appended thereto, characterised in that the first optical waveguide means (48) has a first cross sectional area, and in that the transducer further comprises beam expander means (40) coupled to the first optical waveguide means for illuminating a second area of the liquid crystal modulator means (22), the second area being greater than the first cross sectional area.