GB2242267A

GB2242267A - Sonar detector

Info

Publication number: GB2242267A
Application number: GB8902981A
Authority: GB
Inventors: Michael Laurence Henning
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1989-02-10
Filing date: 1989-02-10
Publication date: 1991-09-25
Also published as: GB8902981D0; FR2664783A1

Abstract

A sonobuoy comprises a vane means 20 coupled to a central member, the central member 12 having a significant inertial mass relative to the vane means and including a coil 14 comprising a multiplicity of turns of optical fibre. The vane means are mechanically coupled to the optical fibre coil whereby pressure gradients created across the sonobuoy by sonar waves cause flexing of the vane means relative to the central member, thereby creating stress in the optical fibre of the coil, thereby to after the optical path distance within the fibre coil. Changes in the distance are detected by interferometric techniques. A number of devices may share a single optical fibre. <IMAGE>

Description

SONAR DEtIECI OR This invention relates to a sonar detector of the type known as a Sonobuoy.

A known type of sonobuoy is shown in figures 1A and IB. The Sonobuoy comprises a heavy inertial mass 2 with planar vanes 4 extending from opposite sides of mass 2. Piezoelectric strips 6 are bonded to the inner parts of vanes 4. The whole is encapsulated in a very soft polyurethane compound 8. In use, in the presence of sonar waves from an under water object, the entire buoy will be subject to sonar pressure waves, and since these are longitudinal waves, a pressure gradient will be created over the dimension of the buoy.

Thus the pressure at one side of the buoy will be different to the pressure on the other side of the buoy and this will create a force which acts on vanes 4 (naturally the extent of the force will depend on the direction and magnitude of the pressure gradient) and will cause the vanes 4 to move or bend relative to the inertial mass 2.

This bending is detected by piezoelectric strips 6 and the output from strips 6 is electrically processed in order to derive a signal representative of the sonar pressure waves incident on the Sonobuoy. The sensitivity of the Sonobuoy to incident sonar waves is determined by the disposition of the vanes in the soft rubber encapsulant 8. The advantage of such a hydrophone is that its dipole response can be combined with an omnidirectional pressure amplitude hydrophone response to provide directional beam patterns for very low frequency sonar waves which are difficult to achieve by other means within a compact instrument package.

With a principal view of reducing the overall cost of sonobuoys, the present invention provides a sonobuoy comprising a vane means coupled to a central member, the central member having a significant inertial mass relative to the vane means and including a coil comprising a multiplicity of turns of optical fibre, the vane means being mechanically coupled to the optical fibre coil whereby pressure gradients created across the sonobuoy by sonar waves cause flexing of the vane means relative to the central member, thereby creating stress in the optical fibre of the coil, thereby to alter the optical path distance within the fibre coil.

Thus, in accordance with the invention a means is provided of measuring sonar waves with a Sonobuoy employing the known principle that optical fibre exposed to mechanical stress will alter its optical path distance. This optical path distance change can be measured preferably by means of an interferometric arrangement wherein for example light pulses are fed through the multi-turn coil and are compared in phase with light pulses which have been fed through a reference coil which has not been exposed to mechanical stress. A preferred method of interferometric detection is described in our British Patent 2126820B.

Advantages of employing an optical fiber technique include the possibility of multiplexing a number of hydrophones economically onto a single fibre remote from the electronic processing, thus introducing the possibility of cost reduction.

As preferred any convenient method may be employed for fixing the vane means to the fibre coil, but in one preferred arrangement, the vane means comprises a planar reinforced plastic sheet member, the sheet member being attached to an encapsulant material for the multi-turn coil. The plastic sheet is coupled to the encapsulant material by a secure bond so that forces exerted on the sheet are transmitted via the metallic-rods to the encapsulant material and thereby create strains on the multi-turn coil within the encapsulant material. The reinforcement may for example comprise rigid metallic-rods; as an alternative method of securing the sheet to the coil the metallic rods may project into the encapsulant material and may if desired be bonded to the optical fibre itself.

In another arrangement, the optical fibre of the coil is employed to form part of the vanes, i.e. a multiplicity of extraneous loops which form a matrix within the vanes.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings whereip: Figures 1A and lB are schematic elevation and side views of a known type of Sonobuoy; and, Figures 2A and 2B are elevation and side views of a sonobuoy in accordance with the invention.

Referring now to figures 2A and 2B, there is shown a sonobuoy in accordance with the invention comprising a central rectangular member 12 comprising a relatively heavy metallic material, for example Brass. The member is hollow and contains a multi-turn optical fibre coil 14 encapsulated in a suitable plastics material or a epoxy resin and firmly secured to the member 12. The length of the fibre coil is typically between 50 and 200 metres. A slot 16 is provided in the sides of member 12 to enable a vane means 18 to be firmly secured by a adhesive bond to the fibre coil 14. The vane means 18 comprises one rectangular sheet 20, extending at from the inertial mals and having embedded within longitudinal metallic rod members which are indicated in dotted lines as at 24 (or alternative stiffening members). The whole assembly is encapsulated in a very soft rubber material as at 26.

Thus in operation when the sonobuoy is placed under water and experiences incident sonar radiation, a pressure differential is created across the buoy which results in forces being exerted on the vanes to flex the vanes relative to the inertial mass 12. This flexing is transmitted to the optical fibre coil and the stress exerted on the optical fibres create a change in optical path length. Such a change in optical path length can be detected by an interferometric method of detection such as for example, described in our Patent No. 2126820B.

The measurement of phase displacement in an optical fibre can be very accurate, down to the order of micro radians. It is thereby possible to obtain a sufficiently sensitive arrangement for measuring the sonar radiation incident on the sonobuoy.

Thus as shown in figure 2 vanes are used, in the form of a sheet attached to a coil of optical fibre, suitably encapsulated. This coil is also attached to an inertial mass. Pressure gradients across the vanes will cause a flexing of the sheet, the magnitude and nature of which will be determined by the coil construction and materials, and the nature of the connection between the coil and the sheet. The flexing of the sheet will cause a strain in the fibre coil, with a direction determined by the direction of the pressure gradient. If coherent light is transmitted through the fibre, the optical phase will change, providing a linear relation with the strain and hence an indication of the pressure gradient. This phase change can be detected by any of a variety variant of well known means.

The essential nature of the device is the provision of a mechanical pressure gradient sensor (the vanes) which act to strain optical fibre. It will be appreciated, there are many embodiments of such a device.

Claims

CLAIMS:

1. A sonobuoy comprising a vane means coupled to a central member, the central member having a significant inertial mass relative to the vane means and including a coil comprising a multiplicity of turns of optical fibre, the vane means being mechanically coupled to the optical fibre coil whereby pressure gradients created across the sonobuoy by sonar waves cause flexing of the vane means relative to the central member, thereby creating stress in the optical fibre of the coil, thereby to alter the optical path distance within the fibre coil.

2. A sonobuoy as claimed in claim 1, wherein the vane means and central member are encapsulated in a bulky relatively soft encapsulant.

3. A sonobuoy as claimed in claim 1, wherein the vane means comprises one or more sheet members.

4. A sonobuoy as claimed in claim 3, wherein each sheet member is formed of plastics with reinforcement embedded therein, either of metal wine or glass.

5. A sonobuoy as claimed in any preceding claim, wherein the central member is hollow containing said coil firmly secured therein.

6. A sonobuoy as claimed in claim 5, wherein the coil is encapsulated in a plastics material and the vane means is firmly secured thereto.

7. A sonobuoy substantially as described with reference to figure 2 of the accompanying drawings.