Variable Gain Optical Sensing System

This invention relates to optical sensing systems, such as for example are used in the detection of acoustic pressure waves e.g. hydrophone applications or temperature sensing, and more particularly to such a system in which the gain is variable.

There are a range of multiplexed optical fibre sensor arrays which work on the principle of pulsed heterodyne interferometry for example the hydrophone array described in British Patent Specification No. 212682OB. In such arrangements the sensor signals are carried as a phase modulation on a high frequency carrier, and the dynamic range of the sensor, assuming linearity of the transducer itself, is determined by the bandwidth available around this carrier. The bandwidth is determined by a number of factors including number of sensors and operating frequency and is fixed for a particular type of system.

Certain applications can require a very large dynamic range of the sensors, for example in the case of optical hydrophones where high level and low level signals may be present for detection. The present invention seeks to

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provide an optical sensing system in which the gain is variable to permit sensing of low or high level signals.

The invention may provide an optical fibre sensor which is entirely passive in operation, but which, by suitable changes in the format of the sampling light pulses can effectively change its sensitivity to environmental effects.

In a standard reflecting interferometric sensor system the transducer consists of a length of optical fibre and two reflecting elements at either end of the fibre. The transducer sensitivity is directly proportional to the length of fibre in the transducer. If a third reflector is now placed in the transducer to split the fibre into two unequal parts, effectively two transducers of differing sensitivities are formed. For example, if the ratio of the two parts is 9:1, the ratio of the two sensitivities will be 20 dB.

An Electro-optic system can now be designed to either interrogate the high gain sensor, or the low gain sensor by adjusting the separation of the two interrogating pulses accordingly. With use of a suitable detection system the electronics can be controlled to switch between the two

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sensitivities depending upon the incoming signal level.

According to the invention there is provided an optical sensing system comprising an optical fibre provided with at least three reflecting elements spaced along the fibre, a coherent light source arranged to supply a light pulse train to the fibre, means for comparing reflected light pulses from two of the reflecting elements to provide a signal representative of fibre deforming forces occurring on the fibre between the two reflecting elements and means enabling selection for comparison of reflections from a chosen two of the reflectors thereby determining the length of fibre used to provide the signal which is sensed.

In one advantageous refinement of the invention at least three reflecting elements are unequally spaced apart along the fibre.

The means enabling selection may comprise means for altering the spacing between pulses in the pulse train so that reflections from the required pair of reflectors occur at the same time at the means for comparing the light pulses.

In order that the invention and its various other

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preferred features may be understood more easily, embodiments thereof will now be described, by way of example only, with reference to the drawings, in which:-

Figure 1 is a schematic diagram of one optical sensing system constructed in accordance with the invention;

Figure 2 is a pulse diagram illustrating on a time axis the launch and reflected pulses for low sensitivity sampling and

Figure 3 is a pulse diagram illustrating on a time axis the launch and reflected pulses for high sensitivity sampling.

Referring to Figure 1 of the drawings a laser 1 produces an output of coherent light of frequency F which is fed into an optical switch means 2 wherein a modulated pulse of frequency F +4F is produced which by the inclusion of delay means in the optical switch means lags behind the pulse of frequency F by a time interval determined by the output of level detector 6 as will be described. This two-pulse light signal passes through a beam splitter 3 and is focused into an optical fibre 7.

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Discontinuities A,B,C,D,E are provided along the optical fibre and these discontinuities may, for example, be formed by suitable joints in the optical fibre or other forms of partial reflector. The fibre is effectively divided by these discontinuities into several sensing elements and variations in the lengths of these fibre elements, such as due to the impingement thereon of acoustic waves, can be detected and measured in the manner now to be described.

As each two-pulse light signal reaches the first optical fibre discontinuity A a small proportion of the signal will be reflected back along the fibre 7 to the beam splitter 3 which directs the signal to a photodeteσtor 4.

The remaining part of the two-pulse signal travels on to discontinuity B at which a further small proportion thereof will be reflected back along the optical fibre 7 to the detector 4. This procedure continues until that part of the two-pulse signal remaining reaches the last of the optical fibre discontinuities and a small proportion of this signal is again reflected back along the optical fibre to the detector 4. A further two-pulse optical transmission is then made and the cycle repeated.

Referring now to Figure 2 of the drawing this shows by

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way of example reflections of the two-pulse signals from the discontinuities A,B,C,D and E. As can be seen from the drawing the reflection from the second discontinuity B in the present example is delayed with respect to the reflection from the first discontinuity A.

By the appropriate choice of time interval between pulses fed to the optical fibre the delay between the reflections may be arranged to be such that there is total coincidence or at least some overlap between the reflected pulse of frequency F of a later reflected signal with the pulse of frequency F +ΔF of the preceding reflected signal. This can be seen in Figure 2 where there is overlap of the reflected signals from A and B, C and D. The reflected pulses are heterodyned in the square law photodetector 4 to produce beat or modulated signals as shown and the phase modulation of these signals will vary in dependence upon variations in length of the optical fibre elements. Accordingly, by detecting and measuring the phase modulation of the beat signals by means of a phase detector 5 changes in length of the optical fibre elements and thus deformation forces acting on these elements can be measured.

By varying the time interval between pulses fed to the optical fibre it can be arranged that overlap occurs between

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reflected pulses from different spaced reflectors e.g. A and C,B and D,C and E (see Figure 3) so that effectively the length of fibre which is operating as a sensor is changed thereby changing the sensitivity of the system. It will be appreciated that the time interval could be changed such that reflections from any two of the reflectors overlap, e.g. A and D.

In the embodiment described the output of the square law detector 4 is fed to level detector 6 which serves to measure the magnitude of the detected sensor signal. When the magnitude falls below one or more preset references the level detector causes the optical switch to change the spacing between pulses so that coincidence of pulses reflected from reflectors spaced at greater distances occurs so that the length of fibre forming the sensor is effectively increased thereby increasing the sensitivitity.

Although the embodiment described employs automatic selection of the sensor pair dependent on an assessment of amplitude of deforming forces it will be appreciated that such selection could be manually switched. Although the description relates to switching between two alternative

"4 pairings of reflector it will be appreciated that the same principles can be applied to any desired numbers of pairings

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either automatically or manually. Such arrangements fall within the scope of this invention.

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