IMPROVEMENTS RELATING TO OPTICAL SENSING SYSTEMS

Abstract: An optical sensing system comprises a single continuous wave light source (1), a series of phase shifting switching devices (6) arranged to receive light from said light source (1) and driving means (4) individually associated with the phase shifting switching devices (6) for driving said devices (6) with pulses at different frequencies whereby each device (6) produces output pulses of different frequencies in different deflected directions. The devices (6) are sequentially operated so that the light from the continuous wave light source (1) passes straight through a first device (6) to a second device (6) when the first device (6) has produced its light output and is no longer being driven by the driving means (4) individual to it whereupon the second device (6) will be driven to generate output pulses after which the second device (6) ceases to be driven and allows the light from the continuous wave light source (1) to pass through both the first and second devices (6) to a third device (6) and so on until the last device has produced output pulses whereupon the first device will be re-operated for the generation of pulses.

Description: This invention relates to optical sensing systems and relates especially, but not exclusively, to such systems for use in hydrophones for detecting the impingement of acoustic waves on optical sensor arrays. In our British Patent No. 2126820B there is described an optical sensing system in which electrical pulses of the same frequency or slightly different frequencies are launched into an optical fibre sensor array having distributed therealong a plurality of partially-reflective discontinuities whereby a small proportion of each of the light pulses propagating along the sensor array will be reflected back along the optical fibre sensor array by the discontinuities and is caused to interfere with other reflected signals or with reference light signals on photo-detector means for detecting changes in length of the optical fibre elements and thus the acoustic waves impinging on such elements. The optical sensing system may be a heterodyne system in which pairs of time-displaced pulses of the same duration but of slightly different frequencies F and F+ΔF are transmitted down the sensor fibre in which case signals reflected back along the sensor fibre from a discontinuity other than the first discontinuity interfere with signals reflected back from the preceding discontinuity. Alternatively, a single pulse light signal of frequency F may be transmitted down the optical fibre sensor for reflection by the discontinuities whilst a continuous light signal of frequency F+ΔF is used as a reference signal at the photo-detector means to interfere with the reflected signals of frequency F. In our published European Patent Application No. 0183502, to which attention is hereby directed, there is described a pulse generating arrangement for producing time-displaced coherent pulses of two slightly different frequencies from a continuous wave light source. These pulses may be launched into an optical fibre sensor array having discontinuities as described above so that a proportion of the light pulses is reflected back from the discontinuities and is heterodyned with other reflected signals or with reference signals as previously described on photo-detector means in order to produce beat or modulated signals. The phase modulation of the signals will vary in dependence upon variations in length of the optical fibre elements between discontinuities due to the impingment of acoustic waves on the optical fibre sensor. The coherent pulses of slightly different frequencies may be generated by a Bragg cell arranged to be driven alternately by signals of the two frequencies concerned so that the cell switches to produce time-displaced output pulses from a continuous wave light input to the cell. The emergent angles of the output pulses is sufficiently close to enable both pulses to be launched into the same optical fibre. With the cell in its unswitched condition before, after or between pulses the continuous wave light input passes directly through the cell without deflection. The light reflected back along the optical fibre sensor array from the discontinuities therealong also passes back through the Bragg cell without deflection and may be arranged to fall on to a mirror which directs the reflected light on to photo-detector means which heterodynes the reflected signals. It will be appreciated therefore that the Bragg cell serves both as a frequency shifting and switching device. Other frequency shifting devices could be used instead of the Bragg cell in optical sensing systems of the form described. For example, our British Patent No. 2167574B describes a frequency shifting device or modulator comprising a piezo-electric transducer (e.g. PZT cylinder) which produces mechanical stressing of an optical fibre wound around the cylinder in response to an electrical sawtooth waveform signal applied to the transducer so that the light signals propagating along the optical fibre are frequency shifted or modulated. The present invention is directed to a reflectometric optical sensing system which is of the general form described in our British Patent No. 2126820B and co-pending published European Patent Application No. 0183502 but which enables a multiplicity of optical sensor arrays comprising optical fibres having partially-reflective discontinuities therealong to be supplied from a common light source with pulses of different frequencies which will be partially reflected at each of the discontinuities and heterodyned with other reference signals or with reflected signals at photo-detector means. According to one aspect of the present invention continuous wave light produced by a single light source is arranged to be applied to the first of a series of Bragg cells or equivalent phase shifting and switching devices each of which has individually associated with it means for driving the cell or the equivalent with pulses at different frequencies whereby each cell or the equivalent device produces pulses of different frequencies in different deflected directions whereby the cells are operated in time sequence so that the light from the continuous wave light source passes straight through the first Bragg cell, or the equivalent, to the second Bragg cell when the first cell has produced its light pulse output and is no longer being driven whereupon the second Bragg cell can then be driven to generate output pulses after which the second cell ceases to be driven and allows the light from the continuous wave light source to pass through both the first and second cells to the third Bragg cell and so on until the last Bragg cell has produced output pulses whereupon the first Bragg cell will be re-operated for the generation of pulses. The Bragg cells may each produce pairs of pulses of slightly different frequency at different time intervals and these pairs of pulses which are of very close frequency may be launched into individual optical fibre sensor arrays of the kind already described. The reflected signals from each sensor array will pass through the appertaining Bragg cell without deflection during the time that the particular cell is not being driven and will impinge on the photodetector means individual to that cell. In another contemplated arrangement the Bragg cells may be driven in sequence with a multiplicity of pairs of pulses of slightly different frequency so that each cell produces a pair of optical pulses of slightly different frequency along respective directions which can then be launched into respective optical fibre sensor arrays. In a further optical sensing system according to the present invention a multiplicity of pairs of pulses of slightly different frequencies derived from each of the Bragg cells in turn are launched into respective optical fibres each of which is terminated by a plurality of optical sensor arrays in parallel and suitably time-displaced relationship. By way of example the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a single sensor array optical fibre sensing system of the kind described in our published European Patent Application No. 0183502; Figure 2 shows a multi-sensor optical fibre array sensing system according to the invention; Figure 3 shows a further multi-sensor array optical sensing system according to the present invention; Figure 4 shows a still further multi-sensor array optical sensing system according to the invention; and, Figure 5 shows a general switching and frequency shifting arrangement according to the invention for producing time multiplexed pulsing of a multiplicity of optical fibre sensor arrays. Referring to Figure 1 of the drawings, a relatively long coherent length laser light source 1 produces a continuous wave light output which is directed by means of two mirrors 2 and 3 on to a Bragg cell frequency shifting device 4. When the cell 4 is not being driven (i.e. switched off) the continuous wave light input passes through the cell without deflection, as shown. However, when the cell 4 is being driven by a pair of short duration pulses of very close frequencies F1 and F1+ΔF the continuous wave light input to the cell is switched and modulated to produce time-displaced optical pulses of frequencies f₁ and f₁+ΔF (f₁ refers to the optical carrier frequency and F refers to the sub-carrier frequency applied to the Bragg Cell) which, by virtue of their very close frequencies, are produced with a very small angular deviation between them which enables both output pulses to be launched into one end of the same optical fibre 5 having coupled thereto an optical fibre sensor 6 with a multiplicity of partially-reflective discontinuities 7 equally spaced along the sensor. In operation of the optical sensor system which may be used in hydrophones for detecting acoustic waves impinging on the sensor array the Bragg cell 4 will be driven at time intervals to produce pulses of frequencies f1 and f1+ΔF which propagate along the optical sensor array 6. A proportion of these pulses is reflected back along the optical sensor 6 and the optical fibre 5 to the Bragg cell 4 from the discontinuities 7 and it will be arranged that the light reflected from the second and later discontinuities 7 will interfere with the light reflected from the preceding discontinuities and this reflected light will be heterodyned by means of a photodetector 8 after passing straight through the cell 4 without being deflected when the cell 4 is in the switched off condition. This photodetector 8 will produce signals at the difference frequency ΔF from which phase modulations due to impingement of acoustic wave signals on the elements of the optical fibre sensor 6 can be detected and measured. Referring now to Figure 2 of the drawings, this shows an optical fibre sensing arrangement according to the present invention which provides for the pulsing of four optical fibre sensor arrays 9, 10, 11 and 12 in time sequence from a single continuous wave light laser 13. Each of the sensor arrays 9, 10, 11 and 12 may be of the same form as the array 6 in Figure 1 and the sensor arrays are connected to the outputs from respective Bragg cells 14, 15, 16 and 17. These Bragg cells are arranged to operate in the manner described with reference to the cell 4 in Figure 1 but they are driven in sequence by time-displaced pairs of pulses of frequencies F1 and F1+ΔF. With the Bragg cell 14 being driven, the pair of light pulses produced by the cell, as indicated at P14, will be launched into the sensor array 9 and the reflected light from the array 9 will pass straight through the cell on to photodetector 18 with the cell 14 in its non-driven or switched off condition. The light from the laser 13 will also pass through the cell 14 to the next Bragg cell 15 which is driven next by a pair of pulses of frequencies F1 and F1+ΔF so that it provides a pair of output pulses P15. This procedure is then repeated for the remainder of the Bragg cells 16 and 17 after which the Bragg cell 14 will be re-operated and so on. Figure 3 shows another optical sensing system similar to that depicted in Figure 2 in which each of the Bragg cells 22, 23 and 24 is arranged to receive light from a laser source 25 when the preceding cell is not being driven. The cells are driven in turn by pulse pairs of different frequencies, F1 and F1+ΔF, F2 and F2+ΔF, F3 and F3+ΔF, F4 and F4+ΔF, generated in series or parallel so that output pulse pairs (eg. f₁ and f₁ + ΔF) are produced in four different directions from the Bragg cells, as indicated. Each of these pulse pairs of different frequencies will be launched into different optical fibre sensor arrays with the appropriate time delays and the light reflected from the discontinuities along the sensor arrays will be caused to interfere with other reflected signals on respective photodetectors 26, 27 and 28. In the Figure 4 arrangement four different frequency output pulse pairs (eg. f₁ and f₁ + ΔF) from the Bragg cells 29, 30, 31 and 32 which are driven sequentially are launched into optical fibres each of which is terminated by time-displaced parallel optical sensor arrays such as the arrays 33, 34, 35 and 36 with the appropriate delays. As will be appreciated the number of sensor arrays served by a single laser light-source 57 is increased substantially by this arrangement. The operation of the sensing system is similar to that described with reference to Figure 3. Finally in Figure 5 the continuous wave light output from a laser 37 is fed to a coupler 38 the output from which is fed to frequency shifters 39 and 40 which are driven continuously by alternate pulses of frequency F1 and F1+ΔF from pulsers 41 and 42. The pulse outputs from the frequency shifters are fed into respective couplers 43 and 44 which couple the alternative pulses of frequency f₁ and f₁+ΔF to switched couplers 45, 46, 47, 48 and 49, 50, 51 and 52 which are switched under the control of respective timing means 53 and 54. As each of the switched couplers is switched on by the timing means appertaining thereto pulses of frequencies f₁ and f₁+ΔF are fed to the optical fibre sensor array connected to the output of the coupler concerned and the reflected light (ie. R1, R2, R3, or R4 or R1', R2', R3', or R4') passes back through the coupler to a photoconductor receiver 55 or 56. As will readily be appreciated from the foregoing the optical sensing system according the present invention enables relatively large numbers of sensor arrays to be operated from a single laser light source.

Claims: An optical sensing system comprising a single continuous wave light source (1), a modulator (4) arranged to receive light from said source, drive means for switching the modulator to produce pulses of different frequencies, and an optical fibre sensor array whereto the pulses of modulated light are fed and wherefrom reflections of the pulses may be received for detection by detection means (8), characterised in that a plurality of said modulators (14, 15, 16, 17; 22, 23, 24; 29, 30, 31, 32; 39, 40), each serving to modulate the continuous wave light source, are provided, respective drive means serving to direct pulses of modulated light of different frequencies from each modulator to a respective sensor array. A system as claimed in claim 1 wherein each modulator comprises a frequency shifting device characterised in that each modulator (14, 15, 16, 17; 22, 23, 24; 29, 30, 31, 32) provides a first path, when driven, to direct modulated pulses to the respective sensing array, and a second path, when not driven, allowing unmodulated continuous wave light to pass through the modulator to a next one of the modulators. A system as claimed in claim 1 or 2 characterised in that each drive means is arranged to drive its respective modulator (22, 23, 24; 29, 30, 31, 32) to produce a plurality of pairs of different frequency pulses, each modulator having, for each pair, a respective sensor array whereto the pulses of each said pair are directed. A system as claimed in claim 3 characterised in that each sensor array comprises a plurality of time spaced sensors (33, 34, 35 and 36). A system as claimed in claim 1 characterised in that the drive means includes switching couplers (43; 45, 46, 47, 48; 44; 49, 50, 51, 52) and timing means (53, 54) for directing the modulated light of different frequencies to respective sensor arrays (1, 2, 3, 4; 1¹, 2¹, 3¹, 4¹).

Inventors: HENNING, MICHAEL LAURENCE, LAMB, CHRISTOPHER
Applicants: GEC-MARCONI LIMITED
Application year: 1988
Main classification: G01H9
IPC codes: G01H9