GB2195217A

GB2195217A - Acoustic-optic transducer

Info

Publication number: GB2195217A
Application number: GB08622721A
Authority: GB
Inventors: Uist Urquhart Macdonald
Original assignee: University of St Andrews; University of Dundee
Current assignee: University of St Andrews; University of Dundee
Priority date: 1986-08-20
Filing date: 1986-08-20
Publication date: 1988-03-30
Also published as: GB8622721D0

Abstract

A ceramic base has a rim (14) supporting a polycarbonate film diaphragm (16). Aligned optical fibres (20,22) are secured to the base to define a gap in which moves an obturator (28) carried by the diaphragm (16) so as to modulate the amplitude of light supplied via one fibre (20) and passing via the other (22) to a detector. <IMAGE>

Description

SPECIFICATION Acoustic-optic transducer This invention relates to a transducer for converting acoustic waves into optical signals.

One field in which the present invention is particulariy but, not exclusively useful is a microphone for use in voice communication in helium saturation diving. It has hitherto been customary to use conventional electric microphones, but these have a number of disadvantages:: -the electric circuit components, which are necessarily metallic, are liable to corrode rapidly in the marine environment -metallic diaphragms are subject to embrittlement and consequent damage in a helium atmosphere -many plastics exhibit undesirable physical changes when subjected to high pressure helium -there is poor immunity to electrical noise from other electrical circuits associated with the diver It is therefore one object of the present invention to provide a transducer capable of overcoming or mitigating the above problems.

The invention is based on the use of an acoustic-optic, rather than an acoustic-electric, transducer.

Such transducers are known in principle in the prior art; see for example: GB 2 079 932 A GB 1 226 522 GB 1 584 048 EP 0 030 262 A This prior art, however, discloses arrangements which are relatively complex and are unsuitable for applications such as helium saturation diving.

The present invention resides in an acousticoptical transducer comprising a body, a resilient diaphragm mounted on the body for movement in response to applied acoustic energy, input light-transmissive means and output light-transmissive means for conducting light through the body along a predetermined path, said means being spaced to define a gap within the housing, and an obturator mounted on the diaphragm for movement therewith in the gap so as to modulate the amplitude of light passing from the input to the output means.

Preferably, the input and the output means each comprise a single optical fibre. Each fibre suitably has a core diameter less than 200 ; presently preferred fibres have a core diameter of about 100 ,u and a sheath diameter of about 140 C(.

In a preferred arrangement, the body has a planar base machined with aligned V-grooves for receipt of the input and output fibres.

Preferably also, the body is of a machinabie ceramic and the diaphragm is of a plastics film, suitably polycarbonate film, for low moisture absorption.

The obturator may comprise an element of highly opaque material preferably platinum of aluminium, secured to the diaphragm by adhesive. Preferably, the lower edge of the element is cut at an angle to the direction of diaphragm movement.

An embodiment of the invention will now be described, by way of example only, with reference to the drawings, in which: Figure 1 is a plan view of a microphone embodying the invention; Figure 2 is a similar view of the microphone with its diaphragm removed; Figure 3 is a cross-sectional view on the line 3-3 of Fig. 1; Figure 4 is a fragmentary perspective view illustrating a detail of the microphone; Figure 5 is a perspective view of an obturator used in the microphone; and Figure 6 is a perspective view of a modified obturator.

Referring particularly to Figs. 1 to 3 the microphone comprises a body 10 of circular form and having a base 12 and a raised rim 14. The body 10 is machined from "Corning" machinable ceramic. A resilient diaphragm 16 of polycarbonate film is stretched across the rim 14 and secured under a ring 18 by adhesive.

Input and output optical cables generally designated at 20 and 22 are provided. Each cable comprises a protective covering 24 and a single optical fibre 26. In each case the protective covering 24 terminates adjacent the inner surface of the rim 14 and is secured thereto by adhesive, and the optical fibre 26 extends to a position adjacent the centre of the body 10. An obturator 28 is secured to the centre of the diaphragm 16 and extends into a gap between the aligned ends of the fibres 26, such that in a rest condition the lower free edge of the obturator is positioned within a light beam passing between the fibres. Movement of the diaphragm 16 in response to incident acoustic energy thus causes movement of the edge of the obturator across the light beam to produce corresponding modulation of the amplitude of light energy passing through the microphone.

It has been established that to obtain a useful response the mass of the moving parts must be kept low and the cross-section of the light beam small. For this purpose the core diameter of the fibres 26 is suitably less than 200 , and the presently preferred optical fibres have a core diameter of 100 ,u and a sheath diameter of 140 8. It is also necessary for good performance for these fine fibres to be accurately aligned. Preferably this is achieved as shown in Fig. 4 by machining an accurate V-groove 30 across the surface of the base 12, the fibres 26 being seated in this groove. An anti-reflective coating is ap plied to the ends of the fibres to improve light throughout.As seen in Figs. 2 and 3 the base 12 is also provided with a bore 32 for accommodating movement of the obturator 28 and with a through aperture 36 for equalizing pressure within the microphone with the ambient pressure.

As seen in more detail in Fig. 5 the obturator 28 is suitably formed by cutting from and bending thin metal sheet or foil. Platinum would provide an optimum mix of mechnical characteristics and resistance to corrosion, but aluminium is an acceptable alternative. The obturator 28 of Fig. 5 is formed with oppositely extending portions 28a for securement by ad- hesive to the diaphragm 16 and a main body portion 28b terminating in a lower edge 28c.

The lower edge 28c is preferably formed as shown at an angle; this permits the degree of obstruction of the light beam in the rest position of the diaphragm to be adjusted during manufacture by small adjustments to the position of the obturator in the direction transverse to the fibres 26.

Fig. 6 illustrates a modified form of obturator having the form of a 3-sided box structure for greater rigidity.

Although described with particular reference :to a microphone for use by divers, the present invention is also of utility in other applications requiring the conversion of acoustic waves to optical signals. For example the invention could be applied to an underwater microphone (hydrophone) with the diaphragm in direct contact with water. In this case the interior of the body would be connected to a resilient air bag for the purpose of equalizing internal and external pressure, to maintain the obturator in the correct position.

It will be understood that the input cable 20 is provided with a constant level of illumination, and the output cable 22 is connected to any suitable means for further processing of the optical signal. A light-emitting diode has been found to be suitable as an illumination source. Means for processing such signals eg by conversion to electric signals followed by amplification, are well known in the art. It has been found that the microphone described in conjunction with commercially available optical fibres can give good quality voice reproduction over distances of at least hundreds of metres.

Claims

1. An acousto-optical transducer comprising a body, a resilient diaphragm mounted on the body for movement in response to applied acoustic energy, input light-transmissive means and output light-transmissive means for conducting light through the body along a predetermined path, said means being spaced to define a gap within the housing, and an obturator mounted on the diaphragm for movement therewith in the gap so as to moduiate the amplitude of light passing from the input to the output means.

2. A transducer of claim 1, in which the input and the output means each comprise a single optical fibre.

3. The transducer of claim 2, in which each fibre has a core diameter less than 200 It.

4. The transducer of claim 2, in which each fibre has a core diameter of about 100 It and a sheath diameter of about 140 IL.

5. The transducer of any preceding claim, in which the body has a planar base machined with aligned V-grooves for receipt of the input and output fibres.

6. The transducer of any preceding claim, in which the body is of a machinable ceramic and the diaphragm is of a plastics film.

7. The transducer of claim 6, in which said plastics film is polycarbonate.

8. The transducer of any preceding claim, in which the obturator comprises an element of highly opaque material secured to the diaphragm.

9. The transducer of claim 8, in which said opaque material is platinum or aluminium foil.

10. The transducer of claim 8 or claim 9, in which the obturator is a planar element and has a lower edge disposed at an angle to the direction of diaphragm movement.