GB2298276A - Sonar or geophonic signal bearing determining apparatus - Google Patents

Abstract

A sonar or geophonic signal bearing determining apparatus includes a generator (10) of coherent light and an array of optical transducers (14) arranged to receive light from the generator (10). The transducers each include a light transmitting fibre the transmission properties of which are responsive to pressure variation. A light sensor (16) is responsive to light transmitted by the light transmitting elements to produce a sensing signal. Interferometer arrangements employing Bragg cell modulators are described. A processor (18) is arranged to determine from the sensing signal the bearing of a sonic or geophonic signal.

Description

SONAR OR GEOPHONIC SIGNAL BEARING DETERMINING APPkAtTS This invention relates to an apparatus for determining the bearing of a sonar or geophonic signal which apparatus employs transducers having a light transmitting element which is responsive to pressure variations.

Experiments have proved that the microphony of some light transmitting devices, e.g. optical fibres, can be employed as an under water acoustic sensor. This invention seeks to provide a sonar or geophonic signal bearing determining apparatus employing an array of such transducers.

According to the invention there is provided a sonar or geophonic signal bearing determining apparatus comprising a generator of coherent light, an array of optical transducers arranged to receive light from the generator, which transducers have a light transmitting element the transmission properties of which are responsive to pressure variation, a light sensor responsive to light transmitted by said elements for producing a sensing signal and processing means for determining from the sensing signal the bearing of a sonic or geophonic source.

The processing means can employ conventional beamforming techniques as are known for sonar or radar systems.

The processing means may be responsive to the phase or the amplitude of the sensing signal.

In a particularly advantageous form of the invention the light transmitted by the transducers is coupled to an interferometer in which, prior to sensing by the light sensor, the transmitted light is combined with a coherent reference light signal.

Preferably, the generator is arranged to produce a source of coherent light which alternates between two different frequencies at a predetermined rate, the interferometer comprises a beam splitter arranged to route the alternating beam over two different paths simultaneously, the transducer comprises a fibre optic sensor which is located in one path and a light combining device is arranged to receive the light after passage through the sensor and also the light in the other path which bipasses the sensor, the relative length of the two paths being arranged such that two different frequency components are combined contemparaneously to produce a difference frequency component which is detected by the light sensor to provide said sensing signal.Each tranducer may be provided with a beam splitter and a light combining device which are positioned adacent the sensor and remote from the generator means so that light from the generator can be coupled with the light splitter via an optical fibre.

In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings, in which Figure 1 is a basic block diagram of bearing determining apparatus constructed in accordance with the invention, Figure 2 is a embodiment of the invention employing parallel processing and heterodyning interferometers, Figure 3 illustrates the wave fronts of a signal approaching an array to facilitate understanding of the processing employed in Figure 2, Figure 4 is an embodiment of the invention employing serial processing and heterodyning interferometers, Figure 5 illustrates the principle of the beamforming store to facilitate understanding of the processing employed in Figure 4.

Figure 6 is an embodiment of the invention employing serial processing differential delay heterodyne interferometers and holographic beamforming.

Figure 7 shows waveform diagrams for the embodiment of Figure 6.

Figure 8 is an embodiment of the invention employing serial processing, differential delay heterodyne interferometers and time domain beamforming and Figure 9 is an embodiment of the invention employing parallel processing and differential delay inteferometers in which different frequencies are employed for each transducer.

The same reference numerals are employed in the various embodiments to indicate similar components or processes.

The embodiment of Figure 1 employs a generator of coherent light formed by a laser 10 which feeds a serial or parallel optical multiplexer 12 which is arranged to route light from the generator to an array of optical sensors 14 either sequentially or simultaneously. The sensors are arranged to transmit light to a photodetector circuit 16 and their transmission properties are influenced by pressure so that the light signal emerging therefrom is modulated by any received sonar or geophonic signal. A suitable sensor is an optical fibre light guide. Such fibres have been found to be responsive to pressure variation due to sound waves in that light passing therethrough is phase modulated by the sound field due to a combination of mechanical (changes in length) and optoelastic (changes of refractive index) effects on the fibre.The output from the photodetector circuit is fed to a processing unit 18 which demodulates the detected signals and determines therefrom the bearing from which the signal is received by any suitable means e.g. conventional beamforming techniques such as are employed in sonar or radar systems.

In the embodiment of Figure 2 the output of the laser 10 is coupled to a parallel multiplexer in the form of a splitter 20 which routes light to each optical sensor 14 simultaneously along separate paths. Connected in parallel with each sensor there is a Bragg cell frequency shifter 22 having a modulating input 24 fed from a common frequency generator (not shown). The frequency shifted light from the Bragg cell and the modulated light coming from the optical sensor is combined in a combiner 26 the arrangement thereby forming an interferometer and producing by heterodyning a difference frequency component. The combined signal from each combiner is fed to a different photodiode 28 the output from which is amplified by an individual 9 amplifier 30. The output from each amplifier is fed to a processor 18 where they are demodulated each by an individual demodulator 32.

The demodulated signals which are the acoustic signal received by each sensor are further processed in the processor 18 to provide bearing information by feeding each via a different variable delay device 34 to a common output.

The delay devices can be adjusted to provide a signal output which is an extreme value e.g. a maximum value whereupon the delays set are related to the direction from which a signal is received. This can be seen more easily from the illustration in Figure 3 where 14a 14b 14c represent three spaced sensors and 36 indicates a wave from approaching the sensors. It will be seen that when the wavefront is incident on 14c it is displaced from 14b by a time element x and from 14a by a time element y and these elements are a function of angle of incidence of the wavefront. In order to provide a maximum combined output signal the delays must be adjusted so that the signals before combination are in phase i.e. introduce delay x and y into the appropriate lines.

Accordingly the delays provide an indication of the bearing of the received signal.

In the embodiment of Figure 4 the beam from the laser 10 is fed to a serial optical multiplexer 38 which is triggered to route light from the laser sequentially to the sensor/Bragg cell arrangement 14/22. The heterodyned outputs from the combiners 26 are added and fed via a photodiode 28, amplifier 30 to a processor 18 which in this case includes a demodulator 32 and a store 40. The store 40 is a matrix store as indicated schematically in Figure 5 columns corresponding to sensor channels and rows corresponding to time intervals. The stored information can be processed in several different ways to provide bearing information. In the illustration of Figure 5 and as a simplified illustration of bearing determination the "x" indicates storage locations with a maximum value of storage signal indicating that a wave front was incident on that sensor channel.Joining the "x"s to form a straight line gives the orientation of the wave front 36.

The embodiment of Figure 6 employs a serial optical multiplexer 38 operating at a switching rate of not greater than twice the acoustic frequency e.g. of the order of 200 KHz and which is arranged to route a coherent beam from the laser 10 which has been modulated by a switched modulator 42, e.g. a Bragg cell, at a rate of the order of 2MHz to provide blocks of one optical frequency alternating with blocks of another optical frequency. The signals from the multiplexer 38 are coupled via lines 1,2,3 each to a different interferometer 44 of the differential delay heterodyning type. This system is described in greater detail in our copending British Patent Application No. 8034178 the disclosure of which is hereby incorporated by reference.The relative path length between beam splitter 46 and beam combiner 48 is arranged such that the combiner receives contemporaneously signals of the two frequencies. Preferably the differential delay in the two paths is equal to the duration of each block of optical frequency. The outputs from each of the combiners 48 are combined in a further combiner 50 the combined output of which is detected by a photodiode 28. The detected signal is fed via an amplifier 30 phase demodulator 32 and compressor 52 to a spectrum analyser 54 where the bearing is determined in dependance upon the frequency of the combined signal. The spectrum analysor provides an output which is analagous to the signal bearing in the frequency domain. This form of processing is described in detail in or copending British Patent Application Number 8028583 the disclosure of which is hereby incorporated by reference.Typical values for a high frequency sonar are 100 KHz for the acoustic frequency, 200 KHz or lower for the optical multiplexer, 2MHz for the switched modulator and 40MHz for the frequency shifted sub carrier. Waveform diagrams for this embodiment can be seen in Figure 7. This system is especially suitable for receiving a single frequency sonar signal.

The Figure 8 embodiment is particularly suitable for passive sonar or geophonic roles. The system is similar to Figure 6 except for the processing. in general passive signals are of a low frequency nature i.e. less than 2KHz.

In this case the optical multiplexer 38 is required to switch fast enough to ensure that successive outputs from the sensors are apparently taken in parallel. After demodulation the serially multiplexed signals are placed in a matrix store 56 whose columns correspond to sensor channels, and whose rows correspond to time intervals. If necessary the multiplexed stream can be heterodyned before insertion into the store to reduce the data processing bandwidth requirements. The matrix store can provide beamforming in a number of ways as described in relation to Figure 4.The system of Figure 8 could be employed with active signals but care would be necessary to ensure that the acoustic frequency and multiplexer and modulator switch rates are compatible at high frequencies Figure 9 is an embodiment which employs the switched modulator 42 as in Figure 6 but which employs a Bragg cell multiplexer 58 instead of the serial optical multiplexer.

The Bragg cell 58 provides shifted beams at different frequencies which are derived from the switch modulated laser source. The beams are fed simultaneously one to each inteferometer 44 and the outputs therefrom are combined in combiner 50, detected by photodiode 28 and then amplified in amplifier 30. The combined signals are then fed to a processor 18 whereby they are separated by selective demodulators 60 to provide signals from which the direction can be determined by beamformer 62 in accordance with methods previously described. This method is disadvantageous as compared with Figures 6 and Figures 8 in view of the higher laser power required to feed the sensors simultaneously and as it requires many switched Bragg mdoualtors and as many demodulators. However no optical multiplexer is required.

this system could be used for broad band or single frequency operation.

It will be appreciated that the embodiments of Figures 4, 6 and 8 which employ serial optical multiplexers 38 could have the multiplexer positioned after the sensors and the sensors would then be fed continously with the coherent light beam. However, this would be wasteful of laser power.

It is also appreciated that the different processing possibilities could be interchanged in some of the embodiments. Such constructions are also intended to fall within the scope of this invention.

Although for ease of description and illustration the invention has been described employing three sensors it is to be understood that a larger number of sensors and associated devices can be employed to provide an arrangement capable of greater resolution in determination of bearing.

Claims (16)

WHAT WE CLAIM IS

1. A sonar or geophonic signal bearing determining apparatus comprising a generator of coherent light, an array of optical transducers arranged to receive light from the generator, which transducers have a light transmitting element the transmission properties of which are responsive to pressure variation, a light sensor responsive to light transmitted by said elements for producing a sensing signal and processing means for determining from the sensing signal the bearing of a sonic or geophonic source.

2. An apparatus as claimed in claim 1, wherein said processing means is responsive to the phase of the sensing signal.

3. An apparatus as claimed in claim 1, wherein said processing means is responsive to the amplitude of the sensing signal.

4. An apparatus as claimed in claim 1 or 2, wherein the light transmitted by the transducers is coupled to an inteferometer in which, prior to sensing by the light sensor, the transmitted light is combined with a coherent reference light signal.

5. An apparatus as claimed in claim 4, wherein the generator is arranged to produce a source of coherent light which alternates between two different frequencies at a predetermined rate, the interferometer comprises a beam splitter arranged to route the alternating beam over two different paths simultaneously, the transducer comprises a fibre optic sensor which is located in one path and a light combining device is arranged to receive the light after passage through the sensor and also the light in the other path which bipasses the sensor, the relative length of the two paths being arranged such that two different frequency components are combined contemporaneously to produce a difference frequency component which is detected by the light sensor to provide said sensing signal.

6. An apparatus as claimed in claim 5 wherein each tranducer is provided with a beam splitter and a light combining device which are positioned adjacent the sensor and remote from the generator means and the light from the generator is coupled with the light splitters via an optical fibre.

7. An apparatus as claimed in claim 6, wherein the relative path lengths are arranged so that consecutive periods of said two different frequencies are combined contemporaneously at the combiners.

8. An apparatus as claimed in any one of the preceding claims including multiplexing means for routing sequentially to the processing means, sensing signals derived from each of the sensors.

9. An apparatus as claimed in claim 8, wherein the multiplexing means is located in the light path between the generator of coherent light and the optical transducers and is arranged to route light from the generator sequentially to the optical transducers.

10. An apparatus as claimed in any one of claims 5 to 9, wherein the processing means comprises means for determining from the combined sensing signals a bearing indication in dependence upon the frequency component of the combined signal or upon the time displacement of the combined signal relative to a predetermined time value.

11. An apparatus as claimed in claim 10, wherein the processing means comprises a spectrum analyser.

12. An apparatus as claimed in claim 8 including a store for receiving and storing a predetermined duration of said sensing signal for assessment by said processing means.

13. An apparatus as claimed in any one of claims 1 to 7, wherein the generator of coherent light is arranged to route light from the generator simultaneously to the optical transducers.

14. An apparatus as claimed in claim 13, wherein the light sensor is arranged to detect light from each transducer and to route detected signals from each transducer via separate lines to a combining means, and the processing means comprises a light intensity indicating device and variable signal delay means provided in each of said separate lines whereby the delay necessary to provide an extreme intensity indication is representative of bearing of the signal.

15. An apparatus as claimed in claim 13 or 14 wherein the generator is coupled via a switched modulator to a Bragg cell which provides a group of outputs at spaced frequencies each of which outputs forms a light source for a particular one of the transducers and demodulating means is provided for demodulating each of said spaced frequencies prior to routing to the processing means.

16. An apparatus substantially as described herein with reference to and as illustrated in any one of the drawings.