GB2197953A - Acoustic sensor - Google Patents

Abstract

An acoustic sensor comprising a sensor element (1) and a compensation element (4), the two elements having different sensitivities and being positioned together such that they will both be subject to the same spectrum of disturbing phenomena, outputs from the two elements (1, 4) being applied to suitable circuit means to provide an acoustic output signal in which noise due to unwanted vibration effects is substantially reduced. The sensor element may be an optical fibre core surrounded by a jacket of a plastics material and the compensation element may be a similar core with a rubber jacket. The construction is suitable for a linear or a planar optical fibre sensor array.

Description

ACOUSTIC SENSOR This invention relates to an acoustic sensor. It relates particularly to a sensor construction that can be used to form a linear or a planar arrangement of sensitive pattern and in which the effect of one source of interference with the possible output of the sensor can be reduced.

In many applications of an acoustic sensor, the device is required.to detect acoustic signals in an environment where there is a high ambient level of vibration, for example in the presence of machinery noise.

One example of this is in the construction of a hydrophone which is intended to be towed behind a marine vessel or to be mounted in a planar arrangement as a flank array on a hull surface of the vessel. The output from such a hydrophone will be heavily influenced by vibration from the vessel's own machinery and this will make it difficult for any weaker signals to be detected. Sometimes it has been proposed that the sensing system which receives signals from the hydrophone will have a built-in capability for cancelling out the unwanted noise. The present invention provides an alternative approach in which the sensor itself has an inherent ability to reject the unwanted noise.

According to the invention, there is provided an acoustic sensor comprising a sensor element and a compensation element, the two elements having different sensitivities and being positioned together such that they will both be subject to the same spectrum of disturbing phenomena, outputs from the two elements being applied to suitable circuit means to provide an acoustic output signal in which noise due to unwanted vibration effects is substantially reduced.

Preferably, the elements comprise similar core constructions which are encapsulated in jacket materials having differing sensitivities to mechanical loads, such that one element is sensitive and the other element is insensitive to ambient pressure changes. The two elements may be located together in an interleaved arrangement whereby they will be subjected to similar acoustic and mechanical stresses when in operation.

The elements may be constructed in linear form such as an optical fibre, a length of piezoelectric plastics material or a piezoelectric rubber strip. Where the final sensor shape is required to be in a planar rather than a linear form, the shape nay be formed from a spirally sound or folded arrangement of the linear form.

By way of example, some particular embodiments of the invention will now be described with reference to the acconpanying drawing, in which: Figure 1 shows an acoustic sensor element comprising an optical fibre in a jacket of a plastics material; Figure 2 is a cross-sectional view on an enlarged scale of the element of Figure 1; Figure 3 shows the element interleaved with a similar element and formed into a spiral transducer; Figure 4 shows a different arrangement where the elements are interleaved into a curvilinear transducer; Figure 5 shows a further arrangement where the elements have been positioned one above the other as a stacked transducer; Figure 6 shows sensor elements arranged in a line to forr.. a towable array, and; Figure 7 is a circuit diagram of the optical and electronic circuits of the acoustic sensor.

As shown in Figures 1 and 2, an acoustic sensor element 1 comprises an optical fibre core 2 which is surrounded by a jacket 3 of a plastics material. In this example, the jacket 3 is of a thermoplastic plastics material and this has resulted in the element 1 becoming a hydrostatically pressure-sensitive sensor.

A compensation element 4 may be constructed by taking an identical core 2 and forming the jacket 3 of a rubber composition. This results an the element becoming a hydrostatically pressure-insensitive sensor.

The sensitivity of an optical fibre to mechanical strain depends upon the nature of the strain and the encapsulant in which the fibre is embedded. As a generalisation, an encapsulant with high Young's Modulus will always produce low sensitivity. However, an encapsulant with low Young's Modulus and low Poisson's ratio produces high sensitivity to hydrostatic stress whilst a high Poisson's ratio produces virtually zero sensitivity to hydrostatic pressure. Thus two coils of fibre identical in every respect save the nature of their secondary jackets can be made to have very different sensitivity to various mechanical loads.

The two forms of element may be fastened alongside one another so that in operation they will both be subject to the same spectrum of disturbing phenomena. It is convenient if the two forms of element are joined together by a semi-reflecting splice 6 at one end so that the elements are effectively connected in series. This construction is then capable of being towed behind a marine vessel for use as a hydrophone.

In order to obtain an output signal from the resulting hydrophone, a coherent light pulse or a light pulse pair is launched into the fibre which in the marine environment is being subjected to deforming forces such as acoustic waves. A suitable sensing system for sensing acoustic waves is disclosed in our published United Kingdom Patent Application No. 2126820A. This system thus acts to control the production of the light pulses and it receives an output light signal at an output end of the fibre as well as any small proportion of the original pulse that may be transmitted back to the input end by reflection from the splice located between the two element bodies.The sensing system then is capable p providing an output signal and, by use of the sensing and compensating elements of the present invention, any noise in this signal due to unwanted vibration effects has been substantially reduced.

Figure 3 shows a different construction of acoustic sensor where the twin pair of elements comprising a sensor element 1 and compensation element 4, joined at one end by a semi-reflecting splice 6, have been coiled into a flat spiral. After a suitable encapsulation process to give mechanical protection to the sensors, this construction forms a planar pattern of sensor which could for example be mounted as a flank array on the hull surface of a marine vessel.

Figure 4 shows a different arrangement where the interleaved elements haçe been folded to fonn a curvilinear transducer. This arrangement would be also suitable for mounting on a flat surface such as the hull of a vessel.

Figure 5 is a side view of a stacked transducer where the semi-reflecting splice 6 has been located within the thickness of the two layers of the device and the construction is supported in a body 7 of an encapsulation material.

Figure 6 shows a number of linear optical fibre sensor elements arranged in a line to form a towable array. The sensor elements 1 have references S1, S2, 53, etc. whilst the insensitive vibration compensation elements 4 with references S1', S2', S3' etc. are arranged to be interleaved with the sensor elements 1. A reference sensor 2 is provided to cancel out the effects of system phase noise as disclosed in our copending United Kingdom Patent Application No. 8525924. Semi-reflecting splices rl, r2, r3, etc. are located at the junctions between the sensor and compensation elements.

When the towed array is being used under water, it receives, in operation, acoustic signals al, a2, a3, etc.

which impinge on the sensor elements S1, 52, S3, of the array. Of course, the acoustic signals al, a2, a3, etc.

similarly impinge on the compensation elements S1', S2', S3' etc. but these elements have a built-in lack of sensitivity to hydrostatic pressures and they are not affected.

In this construction, the light pulses enter the array from the left hand end and pass through the first semi-reflecting splice rl, then the reference sensor R and enter the second semi-reflecting splice r2. At each semi-reflecting splice, a small proportion of the signal passing along the fibre is reflected back to the beginning of the fibre whilst the remainder of the signal passes through the splice and enters the next length of the optical fibre in the array.

After the second splice r2, the entering light pulses pass into the first sensor element S1 which is capable of being disturbed by the acoustic signal al that may be present in the aquatic environment where the towable array is being used. At the end of the sensor element S1, the entering light pulses pass through the third splice r3 and enter a further sensor element S1' which is arranged to be in a vibration compensation situation with the sensor element S1 as has been already described.

The light pulses entering the array at the left hand end as shown in Figure 6 are produced by an optical circuit as depicted at the left hand side of Figure 7.

Similarly, the pulses reflected back from the splices in the array are returned to the optical circuit. The optical circuit depicted is similar to that disclosed in our aforementioned copending patent application, where a laser 8 produces light pulses that are directed through a Bragg cell 9 and into a downlead 11 leading to the sensor array.

Light pulses returned from the array pass back along the downlead 11 and into the Bragg cell 9 where they are deflected via a mirror 12 onto a photo detector 13. The photo detector 13 forms part of the electronic circuit depicted at the right hand side of Figure 7.

The electronic circuit shown is similar to that disclosed in the aforementioned copending patent application where the photo detector 13 is connected through an amplifier 14 to demultiplexing means 16. The demultiplexed signals together with a reference signal are fed via band-pass filters 17 to demodulators 18. Phase noise and microphony compensation is provided by difference amplifiers 19 and further difference amplifiers 21 give vibration and acceleration compensation. Finally, electrical output signals al and a2 are produced and these are proportional to the acoustic signals which impinged on the sensor elements of the array as described in connection with Figure 6.

It will be apparent that the arrangement in the towable array of Figure 6 of a sensor element together with a compensation element, arranged such that they will both be subject to the same spectrum of disturbing phenomena, has given the resulting acoustic sensor a built-in ability to reject noise due to unwanted vibration effects since the two elements react differently to hydrostatic pressures. The electronic circuit can of course be used to provide compensation against further disturbances such as microphony and sensitivity to inputs other than acoustic.

The foregoing description of embodiments of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, although the principle of the invention has been described in connection with an optical fibre hydrophone, the invention will also be suitable for other types of linear array such as a cable made from a polyvinylidene fluoride plastics material, a piezoelectric ceramic material or a ceramic loaded rubber. Other configurations of sensor such as a planar shape which may be attached to a surface are also possible. Since the principle of the invention relies on the use of different coatings to provide different sensitivities in the two parts of the sensor, the invention may also be applied to other devices such as an optical fibre magnetometer.

Claims (7)

1. An acoustic sensor comprising a sensor element and a compensation element, the two elements having different sensitivities and being positioned together such that they will both be subject to the same spectrum of disturbing phenomena, outputs from the two elements being applied to suitable circuit means to provide an acoustic output signal in which noise due to unwanted vibration effects is substantially reduced.

2. A sensor as claimed in Claim 1, in which the elements comprise similar core constructions which are encapsulated in jacket materials having differing sensitivities to mechanical loads, such that one element is sensitive and the other element is insensitive to ambient pressure changes.

3. A sensor as claimed in Claim 2, in which the two elements are located together in an interleaved arrangement whereby they will be subjected to similar acoustic and mechanical stresses when in operation.

4. A sensor as claimed in any one of Claims 1 to 3, in which the said elements are fibre optic devices.

5. An optical fibre hydrophone or magnetometer comprising an acoustic sensor as claimed in Claim 4.

6. An acoustic sensor substantially as hereinbefore described with reference to the accompanying drawing.

The preceding claims have been superseded by the following claims: CLAIMS 1. An acoustic sensor comprising a sensor element and a compensation element. the two elements having different sensitivities and being positioned together such that they will both be subject to the same spectrum of disturbing phenomena, outputs from the two elements being applied to suitable circuit means to provide an acoustic output signal in which noise due to unwanted vibration effects is substantially reduced. in which the elements comprise similar core constructions which are encapsulated in jacket materials having differing sensitivities to mechanical loads, such that one element is sensitive and the other element is insensitive to ambient pressure changes.

2. A sensor as claimed in Claim 1, in which the two elements are located together in an interleaved arrangement whereby they will be subjected to similar acoustic and mechanical stresses when in operation.

3. A sensor as claimed in Claim 1 or 2 in which the said elements are fibre optic devices.

4. A sensor as claimed in any one of Claims 1 to 3 in which the two elements are shaped so as to form a spirally wound transducer.

5. A sensor as claimed in any one of Claims 1 to 3, in which the elements are shaped in a spirally wound or folded arrangement to give a planar form of transducer.

6. An optical fibre hydrophone or magnetometer comprising an acoustic sensor as claimed in any one of Claims 1 to 5.

7. An acoustic sensor substantially as hereinbefore described with reference to the accompanying drawing.