GB1584048A - Optical transducers - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL TRANSDUCERS (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company of 190 Strand, London W.C.2, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to transducers in which an effect to be monitored is directly transduced into light for transmission to a remote point.

Conventionel transducers may be expensive or otherwise inconvenient, e.g. on the score of weight, and the present invention seeks to provide transducers in which these difficulties are overcome.

According to the present invention there is provided an optical transducing arrangement for producing an electrical representation of pressure waves, in which light to be modulated in accordance with the pressure waves to be converted into electrical representations is passed from a light source via a first optical fibre to a surface spaced from one end of the first optical fibre, the light being reflected back from said surface to an end of a second optical fibre, in which the second optical fibre is separate from the first optical fibre while its said end is adjacent to said one end of the first optical fibre, the arrangement comprising a single said pair of optical fibres, in which during its transmission from said one end of said first optical fibre to said surface and back to said end of the second optical fibre the light is modulated in accordance with changes in the parameters of the said pressure waves, and in which the light thus modulated passes via the second optical fibre to photo-detection means.

In certain cases the photo-detector, and possibly also the light source, may be remote from the point at which the modulation is effected.

Embodiments of the invention will now be described with reference to the drawings accompanying the Provisional Specification, in which Fig. 1 shows in principle how a microphone embodying the invention may be constructed, Fig. 2 shows a geophone embodying the invention, and Fig. 3 shows a transducer (e.g. a microphone) also embodying the invention.

The microphone shown schematically in Fig. 1 has a diaphragm 1 with at least its lower surface of optically reflective material.

This can be provided by a thin silvering on the surface of a conventional diaphragm made of, for instance, a thermoplastics material. Located adjacent to the diaphragm are two optical fibres 2 and 3 whose upper ends are close to the diaphragm, as- can be seen in Fig. 1.

Light from a source, e.g. a laser, at the remote end of the fibre 2 passes therealong, to be emitted from its uppermost end towards the diaphragm. Vibrations of the diaphragm due to sound waves impinging on the diaphragm modulate the light from the lower end of the fibre 2 so that a modulated light beam travserses the fibre 3 to a remote point where a photo-detector is mounted.

Note that the microphone structure is relatively simple and very light.

Fig. 2 shows the application of the principle of the invention to a geophone or seismometer. Here the light from the source fibre 5 is collimated by a lens 6 so as to fall on a fixed optical plane 7. This reflects part of the light back via the lens 6 to a transmitting fibre 8. In addition, some of the light passes through the fixed plane 7 so as to fall on the surface of a vibration-sensing fluid 9 such as mercury. Reflection from this depends on vibration to which the fluid 9 is subjected, and the dimensions are such that the light thus reflected sets up interference patterns with the light reflected from the plane 7. Hence a light beam modulated in accordance with the vibration to which the fluid 9 is subjected passes along the fibre 8.

In this case much of the modulation imparted to the light which leaves the device via the fibre 8 is due to interference effects.

Fig. 3 shows in essence the use of the Schlieren system, based on the original Fou cault knife-edge test for telescope mirrors.

In essence this is also simple, and uses the same paired arrangement of fibres 10 and 11 as the other two arrangements. The light from the fibre 10 is reflected by a spherical mirror 12 past a knife edge 13 to the other fibre 11. This operates essentially by detect ing -local refractive index changes in the medium between the mirror 12 and the knife-edge 13, and is capable of high sensi tivity.

An optical microphone or hydrophone can be constructed in this way, either by allowing the acoustic waves to directly modulate the light, or if this does not give adequate sensitivity by using the diaphagm as the Schlieren mirror 12. Alternatively the diaphragm could be coupled to the mirror to drive the latter to thereby modulate the light.

WHAT WE CLAIM IS:- 1. An optical transducing arrangement for producing an electrical representation of pressure waves, in which light to be modulated in accordance with the pressure waves to be converted into electrical representa tions is passed from a light source via a first optical fibre to a surface spaced from one end of the first optical fibre, the light being reflected back from said surface to an end of a second optical fibre, in which the second optical fibre is separate from the first optical fibre while its said end is adjacent to said one end of the first optical fibre, the arrangement comprising a single said pair of optical fibres, in which during its transmission from said one end of said first optical fibre to said surface and back to the said end of the second optical fibre the light is modulated in accordance with changes in the parameters of said pressure waves, and in which the light thus modulated passes via the second optical fibre to photo-detection means.

2. An arrangement as claimed in claim 1, in which the two optical fibres have their said ends facing a diaphragm the surface of which is rendered highly optically reflective, e.g.. by silvering, and in which movements of the diaphragm due to the action of pres sure waves thereon modulates the light between its departure from one of said fibres and its entry into the other of said fibres.

3. An arrangement as claimed in claim 1, in which the two optical fibres have their ends facing a collimating lens which derives from the light received from the light source a parallel light beam which is applied to a fixed optical plane, in which said plane reflects part of the light incident thereupon from the collimating lens back through that lens to the other optical fibre, and in which some of the light incident on said plane passes therethrough to the surface of a vibration-sensing fluid from which it is reflected back to said other fibre, so that vibration to which said fluid is subjected can be caused to modulate the light beam which reaches the other fibre.

4. An arrangement as claimed in claim 1, in which light from said one fibre is passed to a concave mirror from which it is reflected past a Foucault knife edge to the second optical fibre, and in which modulation is imparted to the light beam as a result of changes in the refractive index of the light transmissive medium between the fibre ends and the mirror.

5. A microphone, which includes a diaphragm formed by a sheet of plastics material having a silvered area on its inner surface, a first optical fibre having one of its ends closely adjacent to said silvered area and via which light from a light source passes to and is reflected from said silvered area, and a second optical fibre one end of which is also closely adjacent to said silvered area so as to receive light reflected from said silvered area, the light thus received by said second optical fibre being passed therefrom to utilisation means, said received light being subject to modulation due to movement of the diaphragm due to sound waves incident thereupon.

6. An optical transducing arrangement substantially as described with reference to Fig. 1 of the drawing accompanying the Provisional Specification.

7. An optical transducing arrangement substantially as described with reference to Fig. 2 of the accompanying Provisional Specification.

8. An optical transducing arrangement substantially as described with reference to Fig. 3 of the drawing accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. Schlieren system, based on the original Fou cault knife-edge test for telescope mirrors. In essence this is also simple, and uses the same paired arrangement of fibres 10 and 11 as the other two arrangements. The light from the fibre 10 is reflected by a spherical mirror 12 past a knife edge 13 to the other fibre 11. This operates essentially by detect ing -local refractive index changes in the medium between the mirror 12 and the knife-edge 13, and is capable of high sensi tivity. An optical microphone or hydrophone can be constructed in this way, either by allowing the acoustic waves to directly modulate the light, or if this does not give adequate sensitivity by using the diaphagm as the Schlieren mirror 12. Alternatively the diaphragm could be coupled to the mirror to drive the latter to thereby modulate the light. WHAT WE CLAIM IS:-

1. An optical transducing arrangement for producing an electrical representation of pressure waves, in which light to be modulated in accordance with the pressure waves to be converted into electrical representa tions is passed from a light source via a first optical fibre to a surface spaced from one end of the first optical fibre, the light being reflected back from said surface to an end of a second optical fibre, in which the second optical fibre is separate from the first optical fibre while its said end is adjacent to said one end of the first optical fibre, the arrangement comprising a single said pair of optical fibres, in which during its transmission from said one end of said first optical fibre to said surface and back to the said end of the second optical fibre the light is modulated in accordance with changes in the parameters of said pressure waves, and in which the light thus modulated passes via the second optical fibre to photo-detection means.

2. An arrangement as claimed in claim 1, in which the two optical fibres have their said ends facing a diaphragm the surface of which is rendered highly optically reflective, e.g.. by silvering, and in which movements of the diaphragm due to the action of pres sure waves thereon modulates the light between its departure from one of said fibres and its entry into the other of said fibres.

3. An arrangement as claimed in claim 1, in which the two optical fibres have their ends facing a collimating lens which derives from the light received from the light source a parallel light beam which is applied to a fixed optical plane, in which said plane reflects part of the light incident thereupon from the collimating lens back through that lens to the other optical fibre, and in which some of the light incident on said plane passes therethrough to the surface of a vibration-sensing fluid from which it is reflected back to said other fibre, so that vibration to which said fluid is subjected can be caused to modulate the light beam which reaches the other fibre.

4. An arrangement as claimed in claim 1, in which light from said one fibre is passed to a concave mirror from which it is reflected past a Foucault knife edge to the second optical fibre, and in which modulation is imparted to the light beam as a result of changes in the refractive index of the light transmissive medium between the fibre ends and the mirror.

5. A microphone, which includes a diaphragm formed by a sheet of plastics material having a silvered area on its inner surface, a first optical fibre having one of its ends closely adjacent to said silvered area and via which light from a light source passes to and is reflected from said silvered area, and a second optical fibre one end of which is also closely adjacent to said silvered area so as to receive light reflected from said silvered area, the light thus received by said second optical fibre being passed therefrom to utilisation means, said received light being subject to modulation due to movement of the diaphragm due to sound waves incident thereupon.

6. An optical transducing arrangement substantially as described with reference to Fig. 1 of the drawing accompanying the Provisional Specification.

7. An optical transducing arrangement substantially as described with reference to Fig. 2 of the accompanying Provisional Specification.

8. An optical transducing arrangement substantially as described with reference to Fig. 3 of the drawing accompanying the Provisional Specification.