GB2125957A - Interferometers - Google Patents

Abstract

An optical fibre interferometer comprising generator means (1-4) for producing coherent light pulses at first and second frequencies simultaneously, optical fibre reference delay means (5) arranged for receiving the light pulses at the second frequency and delaying said pulses relative to the first frequency pulses prior to the combination of the pulses and onward transmission over optical fibre means 8 to a beam splitter device 9 which routes the first and second frequency signals through an optical fibre sensor 10 and a by-pass path 11 (e.g. optical fibre) the length of which is such that the pulses at the first and second frequencies are then combined by a combining device 12 to produce coincident pulses from which a frequency difference signal is provided by non-linear optical detector means, 14 phase detector means 15 also being provided to detect any phase displacement of the frequency difference signal which occurs when acoustic pressure waves impinge on the optical fibre sensor. <IMAGE>

Description

SPECIFICATION Improvements relating to interferometers This invention relates to optical fibre interferometers eminently suitable for use in hydrophones for the detection of underwater acoustic pressure waves.

One embodiment of an optional fibre interferometer forming the subject of our co-pending Patent Application No. 8034178 comprises a highly coherent light source which generates light at a first frequency (unshifted frequency) which is fed to a Bragg cell driven by a modulating frequency to provide a second frequency (shifted frequency) shifted relative to the first frequency, optical switch means operable at a predetermined rate sequentially to apply light at said first and second frequencies to a light beam splitting device over a first optical fibre, said beam splitting device routing respective beam parts over separate paths through an optical fibre sensor and a by-pass path to a beam combining device, the by-pass length being such in relation to the sensor that the first and second frequencies occur contemporaneously in the output from the combining device.This output from the combining device is fed to a non-linear optical detector (e.g. photodiode) which produces a difference frequency signal which after being applied to a phase detector means gives rise to a signal which is dependent upon the phase of the difference frequency signal. The impinging of acoustic pressure waves on the optical fibre sensor produces as a result of mechanical and opto-elastic characteristics effects on the fibre sensor phase modulation of the light transmitted by the optical fibre sensor and this modulation is detected by the phase detector means referred to.

Since the pulses of light emanating from the highly coherent light source at the said first frequency need to be of a sufficient duration to facilitate the generation of the sequential pulses at the said first and second frequencies by the operation of the Bragg cell and the optical switch means the coherent length of the light source must necessarily be relatively long, requiring the use of a gas laser as the light source.

According to the present invention there is provided an optical fibre interferometer which enables the coherent length requirement of the light source to be reduced, said interferometer comprising generator means for producing coherent light pulses at the first and second frequencies simultaneously, optical fibre reference delay means arranged for receiving the light pulses at the second frequency and delaying said pulses relative to the first frequency pulses prior to the combination of the pulses and onward transmission over optical fibre means to a beam splitter device which routes the first and second frequency signals through an optical fibre sensor and a by-pass path (e.g. optical fibre) the length of which is such that the pulses at the first and second frequencies are then combined by a combining device to produce co-incident pulses from which a frequency difference signal is provided by non-linear optical detector means, phase detector means also being provided to detect any phase displacement of the frequency difference signal which occurs when acoustic pressure waves imping on the optical fibre sensor.

As will readily be apparent from the foregoing light pulses at the first and second frequencies are derived from the same coherent light pulse produced by the coherent light source. As a consequence a semiconductor laser having a relatively short coherent length may be used as the coherent light source. Such solid state semiconductor lasers are much more compact and rugged than gas lasers having a relatively long coherent length. Moreover, an interferometer with significantly lower attenuation results from the use of a semiconductor laser which generate radiation in the infra-red band, since optical fibres generally have a much lower attenuation in this part of the light spectrum.

In carrying out the present invention the generator means referred to may comprise a semiconductor laser which may be pulsed to provide light pulses of short coherent length which are fed to a beam splitter arranged to route the first frequency pulse over two alternative paths, one of which includes a Bragg cell which accordingly produces a second frequency pulse having a predetermined frequency shift from the first frequency and a reference coil which delays the second frequency pulse by an amount equal to the delay produced in the optical fibre sensor, the other path of which provides a by-pass path.

Both paths are terminated by a combining device which routes the two sequential pulses towards the sensor coil and first mentioned by-pass path.

It may here be mentioned that the beam splitter of the generator means could be dispensed with and the Bragg cell utilized to effect switching between the first and second frequency pulses in which case the laser may be run continously and the Bragg cell used to perform the switching.

By way of example an embodiment of the present invention will now be described with reference to the accompanying single figure drawing.

Referring to the drawing, the interferometer depicted comprises a semiconductor laser 1 which produces light pulses, such as the pulse 2, having a frequency W1 in response to pulsing of the laser.

Each of the pulses at frequency W1 is applied to a beam splitter 3 which routes the pulses over alternative paths. One of these paths includes a Bragg cell 4 which is modulated at a frequency AWl so that it produces a light pulse of frequency W1 +AW1 which then passes through an optical fibre reference coil 5 which acts to delay the pulse with respect to the light pulse of frequency W1 travelling through by-pass path 6 so that the pulses received by a combining device 7 are displaced in time or multplexed in a downward optical fibre 8. The multiplexed pulses of frequencies W1 and W1 +AW1 are routed by a beam splitter 9 through an optical fibre sensor coil 10 and through a by-pass optical fibre 11, respectively.The sensor coil delays the pulse of frequency W1 by the same amount as the reference coil 7 so that as the pulse of frequency W1 is combined with the pulse of frequency W1 +AW1 these pulses are coincident or at least contemporaneous with one another in the output of combining means 12 which applied to an upward optical fibre 13. These coincident or contemporaneous pulses are then fed to a photo detector 14 which provides an output signal dependent upon the difference frequency between the two co-incident pulses and any phase shift of this difference signal due to the incidence of acoustic pressure waves on the sensor coil 10 is detected by phase detector 15.

As will readily be seen from the foregoing description of one embodiment of this invention, the two light pulses of frequencies of W1 and Wl+hW1 are both derived at the same time from the same pulse from semiconductor laser 1.

Consequently, the semiconductor laser need only have a short coherence length pulse output.

Claims (4)

Claims

1. An optical fibre interferometer comprising generator means for producing coherent light pulses at first and second frequencies simultaneously, optical fibre reference delay means arranged for receiving the light pulses at the second frequency and delaying said pulses relative to the first frequency pulses prior to the combination of the pulses and onward transmission over optical fibre means to a beam splitter device which routes the first and second frequency signals through an optical fibre sensor and a by-pass path (e.g. optical fibre) the length of which is such that the pulses at the first and second frequencies are then combined by a combining device to produce co-incident pulses from which a frequency difference signal is provided by non-linear optical detector means, phase detector means also being provided to detect any phase displacement of the frequency difference signal which occurs when acoustic pressure waves impinge on the optical fibre sensor.

2. An optical fibre interferometer as claimed in claim 1, in which the generator means comprises a semiconductor laser device.

3. An optical fibre interferometer as claimed in claim 2, in which the semiconductor laser is arranged to provide light pulses of short coherent length which are fed to a beam splitter arranged to route the first frequency pulse over two alternative paths, one of which includes a Bragg cell which accordingly produces a second frequency pulse having a predetermined frequency shift from the first frequency and a reference coil which delays the secondary frequency pulse by an amount equal to the delay produced in the optical fibre sensor and the other path of which provides a by-pass path, both paths being terminated by a combining device which routes the two sequential pulses towards the sensor coil and the first-mentioned by-pass path.

4. An optical fibre interferometer substantially as hereinbefore described with reference to the drawing.