OPTICAL TRANSDUCER SYSTEMS

Abstract: An optical transducer system for use with an optical transducer of the kind which produces a wavelength multiplexed optical output signal comprises a broadband optical source, and a tunable filter in the form of a Fabry-Perot interferometer connected to receive a broadband optical signal from the source. The interferometer is repeatedly swept through its pass band by a control signal of triangular or sawtooth waveform, producing a swept wavelength optical signal for application to the transducer. In one embodiment, the optical output signal from the transducer is compared with the output signal from the interferometer to decode it, while in another embodiment, a control loop maintains the output amplitude (intensity) of the broadband source substantially constant over the range of wavelengths used by the transducer, enabling a reference channel provided in the transducer to be used for decoding purposes.

Description: The present invention relates to optical transducer systems, and is more particularly concerned with optical transducer systems of the kind involving an optical transducer which produces a wavelength multiplexed output signal, such as the transducer described in our United Kingdom Patent Application No. 8720156 (Publication No. 2 209 101A). A wavelength multiplexed optical position transducer is described in our aforementioned United Kingdom patent application. This transducer uses a graded-index rod lens and a glass wedge grating assembly to disperse and recombine a broad spectrum optical signal generated by an LED. The dispersed light is reflected by a coded strip and the combination of wavelengths present in the recombined spectrum is a function of the position of the lens/grating assembly relative to the coded strip. The recombined signal from a transducer is transmitted along a single optical fibre to a detector system. The detector system wavelength demultiplexes the received signal in a further wedge/grating assembly. Typically a ten bit coded strip is used in the transducer to reflect the various wavelengths in the dispersed optical signal. A further channel is used to provide a continuous ON signal for use as a reference signal channel. This reference channel is used to compensate for wavelength-independent variable system losses such as poor optical connections, bent fibres, and ageing of the LED's. However, there may be further, wavelength-dependent, variations in intensity caused by factors such as temperature variations of the LEDs. The LEDs could have their temperatures stabilised by using PELTIER coolers but this is not a practical solution in many applications due to the low lifetime of Peltier coolers at elevated temperatures (eg 10,000 hours at 125°C). Another drawback with the prior detector system is the fact that eleven data channels have to be dealt with simultaneously. This means that each channel requires a separate photodiode, pre-amplifier and phase-sensitive detector, which makes it difficult to reduce the size and weight of the detector system for applications such as aerospace, where these are important factors. It is an object of the present invention to alleviate at least some of the drawbacks of the prior optical transducer and its detector system. According to the present invention, there is provided an optical transducer system comprising a broadband optical signal source, an optical transducer arranged to receive an optical input signal from the source and to produce therefrom a wavelength multiplexed digital optical output signal representative of a parameter being monitored by the transducer, and detector means coupled to receive and monitor the optical output signal from the transducer, characterised in that said input signal is a swept wavelength signal derived from the source by means of an interferometer coupled to receive a broadband optical signal from the source and means for varying the wavelength of the optical signal transmitted by the interferometer so as to produce said swept wavelength signal, whereby said digital optical output signal is a serial signal. In a preferred embodiment of the invention, the detector means is coupled to receive and compare the optical output signal from the transducer and the varying wavelength optical signal from the interferometer. Preferably, the varying wavelength optical signal from the interferometer is used to provide a threshold level with which the output signal from the transducer is compared, whereby to compensate automatically for any wavelength drift in the source and any changes in the overall transmission characteristics in the interferometer. The interferometer is preferably a Fabry-Perot interferometer, and the broadband source preferably comprises at least two light emitting diodes with different by partially overlapping output spectra. Furthermore, there is preferably provided control means for maintaining the amplitude of the optical signal transmitted by the interferometer substantially constant, which control means preferably comprises a photo-sensitive detector coupled to receive the optical signal transmitted by the interferometer, a differential amplifier coupled to receive the output of the photo-sensitive detector and a reference level so as to produce a control signal dependent upon the difference therebetween, a variable current source for supplying drive current to the light-emitting diodes, and a control circuit coupled to the variable current source and responsive to the control signal to vary the drive current to the light emitting diodes so as to tend to reduce said difference. The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows a typical modulated LED spectrum after transmission through an optical fibre leading from an optical transducer; Figure 2 shows a schematic cross-section through a Fabry-Perot interferometer; Figure 3 shows the spectral transmission characteristics of the Fabry-Perot interferometer; Figure 4 shows an optical transducer system using a Fabry-Perot interferometer to demultiplex a wavelength multiplexed signal; Figure 5 shows the scan voltage applied to the Fabry-Perot interferometer and a typical output spectrum obtained from the system of Figure 4; Figure 6 shows an optical transducer system which embodies the invention; Figure 7 shows the transducer and source spectra for the system of Figure 6; Figure 8 shows a comparator to generate an output when the signal spectrum is greater than the source spectrum and shows the comparison graphically; Figure 9 shows another optical transducer system in accordance with the invention; and Figure 10 to 12 are explanatory diagrams useful for explaining the operation of the system of Figure 9. The spectrum shown in Figure 1 is the type that would be received from an optical position transducer which uses a coded reflective strip to reflect different wavelengths of a dispersed spectrum according to the position of the coded strip. The bandwidth of the spectrum illustrated is 100nm. Each channel is received as a '1' or a '0'. The non-zero levels 2 of the '0' state are present after transmission through an optical fibre and are caused by back reflections in the fibre. The envelope of the spectrum is indicated by the dotted line. The ratio of the on/off states is typically in the range of 4-6dB. A Fabry-Perot interferometer (FP) may be used to decode such a spectrum and a schematic cross-section through an FP is shown in Figure 2. This shows the passage of light through the FP for a given separation d of the two parallel, partially transmitting mirrors 4 and 6 which have reflectivities R1 and R2 respectively. Figure 3 shows the spectral transmission characteristics of an FP. This consists of a series of peaks (or orders) of width δλ, separated by the free spectral range (fsr), Δλ. The ratio Δλ/δλ is called the finesse and for a high-quality FP this is in the range 40-100. For an ideal FP the peak transmission would be 100%, but in practice this will be in the range 50-70%. The wavelength position of the transmitted peaks is dependent upon the spacing d of the mirrors of the FP. Changing d by λ/2 will move the order n peak to the position of the order n+1 (or n-1) peak. Therefore, if the fsr is in the range 150-200nm, the transmission peak of order n can be scanned across the full LED spectrum of Figure 1. If the fsr is 200 nm and the finesse is 50 then the peak width will be 4 nm. This then enables the peak to be scanned through and to resolve the spectrum of Figure 1 by varying the gap between the mirrors. An optical transducer system which uses an FP in the detector is shown in Figure 4. Two source LEDs 8 direct broad band light down two optical fibres 10. The fibres are connected to a fibre optic coupler 12 which transmits light from the two LEDs down a single optical fibre 14 connected to one of its outputs. This fibre 14 is connected to the input of an optical position transducer 16. No details of this transducer are given: it may be as described in the aforementioned application. The output of the transducer 16 is connected to a further optical fibre 18, which is in turn connected to the input of an FP 20. The output of the FP 20 illuminates a photo-sensitive detector 21 which provides an output signal proportional to intensity. Changing the gap width of the FP enables the whole spectrum output by the transducer 16 to be scanned through to detect the peaks representing each '1', so that the position can be determined from the output of the photo-sensitive detector. Piezo-electric crystals may be used to alter the positions of the mirrors in an FP (such as in the WD1 FP wavelength demultiplexer produced by Queensgate Instruments Ltd), and the shape of the voltage waveform applied to them will correspond to the variation in d with time, as shown in Figure 5. This is a simple triangular waveform. The resultant output of the FP as picked up by the photo-sensitive detector 21 is shown beneath the triangular waveform. Thus the intensities representing data values of '1' or '0' are detected by the photo-sensitive 21. For simplicity, a six bit code is shown in Figure 5, and it can be seen that the output on the negative-going slope of the triangular waveform is the mirror image of that on the positive-going slope. Large humps 3 may be produced between the groups of data channels due to imperfect mirror coatings, but these do not have any serious effect on the data output. In order to overcome the drawbacks of the simple system of Figure 4, and at the same time to provide a reference signal from the source for comparison with the signal from the transducer 16 while minimising the number of optical couplers required, the optical transducer system of the present invention, as shown in Figure 6, is provided. This system comprises the two source LEDs 8 connected via the optical fibres 10 to the coupler 12, but the output of the coupler 12 transmits light from the two LEDs to the input of an FP 50 (instead of to the input of the transducer 16). The FP 50 is driven by (ie has the gap d between its mirrors varied by) the triangular waveform of Figure 5, and its output is connected to an input port of a second coupler 52. The coupler 52 has two output ports 54, 56 between which light at the input of the coupler is preferably divided in the ratio 100:1 (although the use of lower ratios is feasible). The portion of the light leaving the coupler 52 via its output port 54 is directed via the optical fibre 14 to the transducer 16, whose output is connected via the optical fibre 18 to a first photo-sensitive detector 58, while the portion of the light leaving the other output port 56 of the coupler 52 is directed via a variable attenuator 60 to a second photo-sensitive detector 62. The combination of the LEDs 8 and the FP 50 operates as a tunable broadband source, so that the light arriving at the transducer 16 has its wavelength periodically swept first in one direction and then in the other. The output of the transducer 16 is therefore time multiplexed as well as wavelength multiplexed. The photo-sensitive detectors 28 simultaneously monitor the time-varying output spectrum from the transducer 16 and the simultaneously time-varying spectrum from the FP 50 (ie the input spectrum to the transducer 16). These two spectra are illustrated in Figure 7, the transducer spectrum (a) being formed from large and small peaks representing '1' or '0' values respectively, and the source spectrum (b) being a continuous broadband signal. Since both spectra take the same optical path through the FP 50, they both see the same optical characteristics in terms of mirror spacing, mirror reflectivities etc. This is in practice very important since the mirrors may not be perfectly flat and parallel. The spectrum received by detector 62 is used as a threshold level for the transducer signal, as shown in Figure 8. The outputs from the two detectors 58, 62 are connected to the inputs of a comparator 66. When the level of the transducer spectrum exceeds the level of the source spectrum, which is set to a suitable amplitude by the attenuator 60 as shown in the graph of Figure 8(a), the output C of the comparator 66 goes high as shown in the graph of Figure 8(b). Using this type of detection, any wavelength drift of the source LEDs is automatically compensated for, since source and transducer spectra will both drift by the same amount and in the same direction. As is apparent from Figures 1 and 8, the signal-to-noise ratio for the channels at the two extremes of the spectrum is significantly worse than for those in the central region of the spectrum. This problem can be substantially alleviated by the improved system of Figure 9, which comprises many of the elements of the system of Figure 6, together with a control circuit for maintaining the amplitude (or intensity) of the optical signal at the output of the FP 50 substantially constant for all eleven channels. Thus the system of Figure 9 still comprises the two source LEDs 8 connected via the optical fibres 10, the coupler 12, the FP 50 and the coupler 52 to the transducer 16 on the one hand and to the photo-sensitive detector 62 on the other hand, but the output of the photo-sensitive detector 62 is connected to one input of a differential amplifier 70 whose other input is connected to receive a reference voltage V R . The output of the differential amplifier 70 is connected to the input of an integrator 72, whose output is connected via a switch 74 to the control input of a variable current source 76 which provides the drive current to the two source LEDs 8. The integrator 72 is provided with a reset switch 78, the switches 74 and 78 being controlled by a comparator 80 connected to receive a sawtooth voltage waveform, which is also applied to the FP 50 in place of the triangular waveform mentioned earlier: this sawtooth waveform is produced by a suitable sawtooth waveform generator 82, and is illustrated at 84 in Figure 9. In operation, the comparator 80 momentarily closes the reset switch 78, and then closes the switch 74, just after the beginning of an up-going ramp of the sawtooth voltage waveform produced by the generator 82. The output of the integrator 72 immediately begins to ramp up, causing a rapid rise in the current supplied by the current source 76 to the LEDs 8. However, as the transmission frequency of the FP 50 approaches the centre frequency of the first of the source LEDs 8, the output of the photo-sensitive detector 62 increases sufficiently for the output of the differential amplifier 70 to change polarity. This causes the output of the integrator 72 to ramp back down, so reducing the drive current to the LEDs 8. Thus the drive current to the LEDs 8 tends to follow a curve which is the inverse of the spectrum they would produce with constant drive current, with the result that the amplitude (or intensity) of the optical signal at the output of the FP 50 tends to be uniform over the useful part of its frequency range. This can best be seen in Figures 10 and 11, in which Figure 10 shows at 90 the variation with time of the amplitude of the optical output signal from the FP 50 at constant drive current (shown at 91) to the source LEDs 8, over half a cycle of operation of the FP 50, while Figure 11 shows at 92 the varying drive current to the LEDs 8 produced by the circuit of Figure 9, and at 93 the resulting variation with time of the amplitude of the optical output signal from the FP 50, again over half a cycle of operation of the FP 50. Similarly, Figure 12 illustrates the improvement between the amplitudes of the bits of the various channels for the FP 50 output signal of Figure 10, shown in the upper half of Figure 12, and the amplitudes of Figure 11, shown in the lower half of Figure 12: the improved signal-to-noise ratio of the channels 95, 96 at the two extremes of the spectrum is readily apparent in the lower half of Figure 12. Just before the end of the up-going ramp of the sawtooth voltage waveform produced by the generator 82, the comparator 80 opens the switch 74, momentarily closes the reset switch 78 to reset the integrator 72, and then closes the switch 74 again just after the sawtooth voltage waveform starts its next up-going ramp, and a sequence of events identical to those of the preceding paragraph is repeated. The comparator 80 and the switch 74 together ensure that the current source 76 is not caused to supply excessively high drive current to the two source LEDs 8. The output of the photo-sensitive detector 58 can be connected to the comparator 66, as in the embodiment of Figures 6 and 8, or to a comparator having its other input connected to receive a reference voltage which is a predetermined proportion, eg 50%, of the reference voltage V R : alternatively and preferably, it is connected to the input of a peak detecting sample-and-hold circuit, shown at 86 in Figure 9. The circuit 86 has a reset input which is connected to receive a reset signal from the comparator 80 simultaneously with the operation of the reset switch 78, and its output is connected via a divide-by-two voltage divider 87 to one input of a comparator 88. The other input of the comparator 88 is connected to the output of the photosensitive detector 62. With this arrangement, the peak amplitude of the signal produced by the photodetector 58 for the first channel, which is chosen to be the continuously ON reference channel mentioned earlier, is stored in the circuit 86 near the beginning of each cycle of the sawtooth waveform produced by the generator 82, and half this amplitude, produced by the voltage divider 87, is used by the comparator 88 as the reference threshold for detecting the bit values of the remaining ten data channels. FP interferometers (or filters) may also scan through a range of wavelengths by altering the angle of the mirrors with respect to the direction of travel of the light. Such a filter has been described in a paper in Electronics Letters Vol. 24, No. 3, February 1988 entitled "Inline tunable Etalon filter for optical channel selection in high density wavelength division multiplexed fibre systems" by A Frenkel and C Lin.

Claims: An optical transducer system comprising a broadband optical signal source (8), an optical transducer (16) arranged to receive an optical input signal from the source and the produce therefrom a wavelength multiplexed digital optical output signal representative of a parameter being monitored by the transducer, and detector means (58) coupled to receive and monitor the optical output signal from the transducer, characterised in that said input signal is a swept wavelength signal derived from the source by means of an interferometer (50) coupled to receive a broadband optical signal from the source and means (82) for varying the wavelength of the optical signal transmitted by the interferometer so as to produce said swept wavelength signal, whereby said digital optical output signal is a serial signal. An optical transducer system as claimed in claim 1, wherein the detector means (58, 62) is coupled to receive and compare the optical output signal from the transducer (16) and the varying wavelength optical signal from the interferometer (50). An optical transducer system as claimed in claim 2, wherein the varying wavelength optical signal from the interferometer (50) is used to provide a threshold level with which the output signal form the transducer is compared, whereby to compensate automatically for any wavelength drift in the source and any changes in the overall transmission characteristics in the interferometer. An optical transducer system as claimed in claim 3, wherein the detector means comprises first and second photo-sensitive detectors (58, 62) respectively coupled to receive the optical signal from the transducer (16) and the optical signal from the interferometer (50), and a comparator coupled to compare the respective outputs of the photo-sensitive detectors. An optical transducer system as claimed in any preceding claim, wherein the interferometer (50) is a Fabry-Perot interferometer. An optical transducer system as claimed in any preceding claim, including control means (62, 70, 72, 74, 76, 78, 80) for maintaining the amplitude of the optical signal transmitted by the interferometer (50) substantially constant. An optical transducer system as claimed in claim 6, wherein the broadband source (8) is current-controlled, and the control means (62, 70, 72, 74, 76, 78, 80) comprises a photo-sensitive detector (62) coupled to receive the optical signal transmitted by the interferometer (50), a differential amplifier (70) coupled to receive the output of the photo-sensitive detector and a reference level (V R ) so as to produce a control signal dependent upon the difference therebetween, a variable current source (76) for supplying drive current to the broadband source, and a control circuit (72, 74, 78, 80) coupled to the variable current source and responsive to the control signal to vary the drive current to the broadband source so as to tend to reduce said difference. An optical transducer system as claimed in claim 7, wherein the control circuit (72, 74, 78, 80) comprises an integrator (72) coupled to receive the control signal, and switches (74, 78) for applying the output of the integrator to the variable current source (76) during a predetermined part of the wavelength range of the wavelength varying means and for resetting the integrator when the wavelength varying means reaches the end of said part of its range. An optical transducer system as claimed in any one of claims 6 to 8, wherein the optical output signal produced by the transducer includes a reference signal, and wherein the detector means (58, 86, 87, 88) includes a sample and hold circuit (86) for periodically sampling and storing the magnitude of the reference signal and is arranged to compare the remainder of the optical output signal from the transducer with the stored reference signal. An optical transducer system as claimed in any preceding claim, wherein the broadband source (8) comprises at least two light emitting diodes (8) with different but partially overlapping output spectra.

Inventors: KREIT, DARRAN, WILLSON, JOLYON PETER
Applicants: SCHLUMBERGER INDUSTRIES LIMITED
Application year: 1989
Main classification: G01D5
IPC codes: G01B9, H01L33, G01D5