GB2312502A

GB2312502A - Vibration compensation for a balanding coil assembly

Info

Publication number: GB2312502A
Application number: GB9608655A
Authority: GB
Inventors: Michael Laurence Henning
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1996-04-26
Filing date: 1996-04-26
Publication date: 1997-10-29
Also published as: GB9608655D0; KR970071038A; FR2748108A1

Abstract

A time-multiplexed pulsed heterodyne optical fibre sensing system has two optical fibre arms along which modulated light signals are fed. The first arm has a balancing coil 24 which provides it with a greater optical path than the second arm. The two light pulses which are spaced in time are recombined and fed to a sensor array which can be an optical fibre hydrophone, or it might detect changes in pressure, temperature, stress and/or strain along a fibre. Acoustic and vibrational noise can cause the optical path length of the balancing coil to change, thus introducing noise into the system and reducing its sensitivity. By providing a compensating coil 32 in the second arm, such that the compensating coil is sensitive to the same environmental noise as affects the balancing coil and its optical path length is changed by an equivalent amount, there is no net change in the desired optical path length difference between the arms and so sensitivity is not affected by environmental noise.

Description

IMPROVEMENTS RELATING TO OPTICAL FIBRE SENSING SYSTEMS This invention relates to optical fibre sensing systems and relates more specifically to optical fibre coil assemblies for incorporation in such Systems.

In many coherent optical fibre sensing systems, sensor and balancing optical fibre coils are provided. The sensor coil or coils are sensitive to the measurand, such as pressure in the case of hydrophone based sensing systems. The balancing coil acts to delay a light pulse or a modulated light signal.

In the case of heterodyne optical fibre sensing system the balancing coil causes a predetermined optical path length difference in order to delay a first modulated light signal with respect to a second modulated light signal. In this way balanced interferometric techniques can be used.

In a typical time-multiplexed pulsed heterodyne optical fibre sensing system two light pulses which are spaced in time are transmitted along a sensor optical fibre. The sensor optical fibre has partially reflective discontinuities or junctions along its length Small proportions of the light pulses (effectively return signals) are reflected back along the fibre from each fibre discontinuity. The signals reflected back from the successive discontinuities are caused to interfere with the signals reflected from the preceding discontinuities in order to produce heterodyning. This provides a detectable beat signal, the rnodulation of which is dependent upon the optical path lengths of optical fibre sensor coils between the discontinuities. As the optical path lengths change the modulation changes.

A problem in such systems is that the balancing coils are sensitive to acoustic and vibrational noise. In environments where such disturbances are present, they may apply force to the balancing coil which can change its optical path length which in turn may introduce spurious effects in the system. This can cause, for example, optical phase noise.

A solution which has been adopted is to provide optical fibre balancing coils which are insensitive to environmental influences thus reducing the spurios effects in the system.

Such a balancing coil is shown in British Patent No. 2 250 588 which shows the use of two coils each of which is affected by environmental influences. By causing the effect generated in each coil to be opposite, the net effect can be cancelled out. However, it is very difficult to remove completely the effects of environmental influences and some spurious signals usually remain.

It is an object of the invention to provide a system which is less sensitive to environmental influences. It is another object of the invention to provide such a system which is simpler than known systems.

According to a first aspect the invention provides an optical fibre sensing system comprising a first optical fibre arm and a second optical fibre arm, the first optical arm having a balancing coil and the second optical fibre arm having compensating means in which the compensating means changes an optical characteristic in response to environmental influences by an amount which is substantially equivalent to a change caused by the environmental influences in the optical characteristic of the balancing coil.

An advantage of this system over the prior art is that it is not necessary to attempt to eliminate the effects of environmental influences, nor is it necessary to attempt to balance equal and opposite effects in a single balancing coil assembly. The compensating means can be affected by the environmental influences.

Preferably the optical characteristic is optical path length.

Preferably the compensating means is a compensating coil. Preferably the optical path length of the balancing coil is greater than the optical path length of the compensating coil.

Preferably the environmental influences are vibration and/or acoustic noise.

Preferably a first end of each of the first and second optical fibre arms are optically linked by a coupler. Light may be fed through the coupler into each arm. Preferably a second end of each of the first and second optical fibre arms are optically linked by a coupler. Light pulses may be fed through the coupler to sensor means. Accordingly the system may comprise a balancing circuit. Preferably light in each arm is modulated into pulses by a respective modulator. The modulators may produce modulated light pulses having a slightly different frequency in each arm.

The sensor means may comprise a coil or coils. It may be an array of coils. The coils may be connected in series. Alternatively the sensor means may be one or more non-coiled lengths of optical fibre. In an embodiment having a plurality of sensors, they may be in individual zones, a zone being defined at a first end and a second end by partially reflective discontinuities or junctions.

One aspect of the present invention is directed to optical fibre hydrophones, in which the balancing coil may be exposed to environmental influences such as vibration and airborne acoustic pressures. It is also applicable to other optical fibre sensing systems in which a degree of insensitivity to environmental influences is required. The sensing system may be a terrestrial based system. Light pulses may be fed into a length of optical fibre extending around the circumference or perimeter of an area, such as a perimeter fence. The length of optical fibre may be buried.

The sensing system may be an interferometric reflectometric sensor system.

Preferably the compensating means is sensitive to any of the environmental influences which affect the balancing coil. The sensitivity of the compensating means may be adjustable.

The system may detect pressure or temperature applied to a fibre. It may detect stress and/or strain along a fibre.

According to a second aspect the invention provides a fibre optic sensing system comprising a first optical fibre arm and a second optical fibre arm the first arm having a balancing coil, the system further comprising a sensor which is adapted to detect vibration and to input a signal to a cancellation system which can apply a counter-vibration to the balancing coil to cancel the effect of the vibration on the balancing coil.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying figures in which: Figure 1 shows part of a known sensor system; Figure 2 shows part of a new sensor system; ar.d Figure 3 shows detail of a compensating coil.

Figure 1 shows an optical circuit 10 which is part of a known sensing system. The circuit 10 has a first arm 12 and a second arm 14. Ends of the first and second arms are joined by couplers 16 and 18. Each arm 12, 14 has a respective modulator 20, 22 which modulates light passing along that arm and through the modulator.

The arm 12 has a path balancing coil 24 which is present to delay a light signal passing through the arm 16 with respect to the arm 18.

Light from a laser is fed into a leg 26 of the coupler 16 where it is split into two signals, each of which goes into a respective arm 12, 14. On passing through the modulators 20, 22 the signals are modulated into pulses having frequencies which are similar but not identical. The modulated signal in arm 12 is delayed as it passes through the balancing coil 24.

The modulated signals are recombined by the coupler 18 and are fed from leg 28 of coupler 18 to a sensor array (not shown). In the case of a hydrophone system, the sensor array is a set of optical fibre sensor coils which are connected together in series. Each sensor coil is separated from an adjacent sensor coil by a partially reflective junction, adjacent junctions defining separate zones of the sensor array.

Light signals which are fed to the sensor array from the optical circuit 10 are present as two sets of pulses which are separated in time. As the pulses travel through the array they are partially reflected by successive junctions and travel back towards the circuit 10. Before entering the circuit 10 they pass through a coupler 30 from which they are tapped off and fed to a receiver. At the receiver return signals generate an interference pattern which is converted to electrical signals which are then analysed by signal processing means to produce a output indicative of pressure which has been detected by the sensor coil. Since rapid pressure changes can be detected, sounds transmitted underwater can be detected and processed.

A problem is caused by exposure of the balancing coil to environmental influences such as vibration or acoustic noise. Such influences can change optical characteristics of the balancing coil, for example the optical path length of the coil. Such a change can cause noise in the system and reduce its sensitivity. Figure 2 shows part of a sensing system which alleviates this problem to some degree. This shows an optical circuit very similar to that of Figure 1 and in Figure 2 corresponding reference numerals have been applied to corresponding parts. A compensating coil 32 has been introduced into arm 14. The compensating coil 32 is susceptible to environmental influences to substantially the same extent as the balancing coil 24. Accordingly any optical changes induced in the balancing coil 24 will also be induced in the compensating coil. For example the optical path length of both coils may increase by an equivalent amount which would result in no net change to the desired optical path length difference between arms 12 and 14.

Of course the optical path length of the compensating coil 32 is much less than that of the balancing coil 24 since the balancing coil 24 must introduce extra "length" in to arm 12.

Therefore the compensating coil should be more sensitive per fibre length to vibration than the balancing coil.

Fibre used in the compensating coil 32 could be coated with a polymer which increases its sensitivity. A more sensitive fibre would allow the compensating coil 32 to be shorter than the balance coil whilst still providing the same degree of sensitivity. Acceleration or vibration sensitivity may be increased by increasing the mass of the compensating coil, for example by mounting it in a more massive encapsulent.

The sensitivity of the compensating coil 32 may be tunable so that its response to vibration matches that of the balancing coil. Figure 3 shows such a tunable compensating system 38. A coil 40 is wound onto a former 42 comprising a base 44 and a spool 46. The coil is wound around the spool 46 and rests on a compressible layer 48 which sits on the base 44.

The coil 40 is held onto the former 42 by a washer 50 and adjustable nut 52. The adjustable nut 52 cooperates with a threaded portion 54 of the spool 46.

The compensating system 38 is tuned by changing the vibrational coupling between the coil 40 and the former 42. This is achieved by tighening or loosening the nut 52. As the nut 52 is tightened vibrational coupling is increased. As the nut 52 is loosened the vibrational coupling is reduced.

The compressible layer 48 provides a soft bed into which the coil can be pressed as the nut 52 is tightened. Using a compressible layer 48 enables a gradual increase in pressure to be applied to the coil 40 rather than an abrupt increase.

It is preferred that the compensating system 38 is orientated such that vibrational motion occurs along the longitudinal axis of the spool 46, since the compensating system is balanced for motion particularly along this axis.

The invention provides a straightforward and inexpensive solution to the problem of susceptibility to environmental influences. Of course these may include factors other than noise and vibration.

Claims

1. An optical fibre sensing system comprising a first optical fibre arm and a second optical fibre arm, the first optical arm having a balancing coil and the second optical fibre arm having compensating means in which the compensating means changes an optical characteristic in response to environmental influences by an amount which is substantially equivalent to a change caused by the environmental influences in the optical characteristic of the balancing coil.

2. A system according to Claim 1 in which the optical characteristic is optical path length.

3. A system according to Claim 1 or Claim 2 in which the environmental influences are vibration and/or acoustic noise.

4. A system according to any preceding claim in which the compensating means is a compensating coil.

5. A system according to any preceding claim which comprises a balancing circuit.

6. A system according to any preceding claim in which the sensitivity of the compensating means is adjustable.

7. A system according to any preceding claim which detects pressure or temperature applied to a fibre.

8. A system according to any preceding claim which detects stress and/or strain along a fibre.

9. A system according to any preceding claim which is an interferometric reflectrometric sensor system.

10. A system according to any preceding claim which comprises an optical fibre hydrophone, in which the balancing coil may be exposed to environmental influences such as vibration and airborne acoustic pressures.

11. A system according to any of Claims 1 to 9 which is a terrestrial based system.

12. An optical fibre sensing system substantially as described herein with reference to Figures 1 to 3 of the accompanying drawings.

12. An optical fibre sensing system comprising a first optical fibre arm and a second optical fibre arm the first arm having a balancing coil, the system further comprising a sensor which is adapted to detect vibration and to input a signal to a cancellation system which can apply a counter-vibration to the balancing coil to cancel the effect of the vibration on the balancing coil.

13. An optical fibre sensing system substantially as described herein with reference to Figures 1 to 3 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. An optical fibre sensing system comprising a first optical fibre arm and a second optical fibre arm, the first optical arm having a balancing coil and the second optical fibre arm having compensating means in which the compensating means changes an optical characteristic in response to environmental influences by an amount which is substantially equivalent to a change caused by the environmental influences in the optical characteristic of the balancing coil.