GB2087545A - Interferometer - Google Patents

Abstract

An interferometer comprises generator means (21, 22, 23, 24, 25) for producing a beam of coherent light which alternates between two different frequencies at a predetermined rate. A beam splitter (28) is arranged to route the alternating beam over two different paths simultaneously. A fibre optic sensor (29) is arranged to receive the light beam in one path. A combining device (30) is arranged to receive the light beam after passage through the sensor (29) and also the light beam in the other path which bypasses the sensor (29). The relative length of the two paths is arranged such that two different frequency components are combined contemporaneously. An optical detector (31) is responsive to the combined beams to produce a difference frequency component and a phase detector means is provided which produces a signal variable in dependence upon the phase of the difference frequency component. While the invention is not limited to a particular field one useful application is in the production of a hydrophone. <IMAGE>

Description

SPECIFICATION Interferometer This invention relates to interferometers and more particularly to an interferometer in which a fibre optic sensor can be located remotely from a generator of coherent light and from an optical detector for the sensed signal.

According to the invention there is provided, an interferometer comprising generator means for producing a beam of coherent light which alternates between two different frequencies at a predetermined rate, a beam splitter arranged to route the alternating beam over two different paths simultaneously, a fibre optic sensor arranged to receive the light beam in one path, a light combining device arranged to receive the light beam after passage through the sensor and also the light beam in the other path which bypasses the sensor, the bypass length being arranged such that two different frequency components are combined contemporaneously, a non linear optical detector responsive to the combined beams to produce a difference frequency component and a phase detector means for providing a signal variable in dependence upon the phase of the difference frequency component.

In a particularly advantageous form of the invention the splitter and the combiner are positioned adjacent the sensor and remote from said generator means and the beam is coupled with the light splitter via an optical fibre. Such an arrangement enables the sensor to be located remotely from the generator means without substantial degradation of signal by environmental variations as temporally close frequency components experience similar changes and due to cancellation in the subtraction process these changes are not present on the difference frequency component.

Preferably, the bypass length is arranged so that consecutive periods of said two different frequencies are combined contemporaneously at the combiner.

The optical detector may also be positioned remotely from the combiner and be optically coupled therewith via an optical fibre line. Such an arrangement has the advantage that the sensor with its associated beam splitter and combiner can be separated from the processing and generating apparatus and can be linked therewith by just two optical fibre lines.

the generator means may comprise a Bragg cell having an input from a coherent light source at a first frequency and driven by a modulating frequency to provide a second frequency shifted relative to the first frequency.

The Bragg cell may be continuously modulated by the modulating frequency and said first and second frequencies may alternately be routed to the beam splitter by means of an optical switch operating at said predetermined rate.

Alternatively the frequency transmitted by the Bragg cell may be alternated between said first and second frequency by alternately connecting and disconnecting the modulating frequency to the cell.

In a particularly advantageous application of the invention a hydrophone comprises an interferometer as previously defined wherein the sensor is adapted to sense acoustic signals and phase displacements of the difference signal are representative of the acoustic signal.

In order that the invention and its various other preferred features may be understood more easily, an interferometer of known type and an interferometer constructed in accordance with the invention will now be described, by way of example only, with reference to the drawings in which: Figure 1 is a schematic diagram of a standard Heterodyne Mach Zehnder interferometer of known type, Figure 2 is a schematic diagram of an interferometer constructed in accordance with the invention, and Figure 3 is a schematic illustration of the signals passing through the sensor head of the interferometer of Figure 2.

The system described in Figure 1 comprises a high coherence laser 10 which supplies light at an angular frequency of WL to a beam splitter 11. The beam splitter directs the beams along two paths 12,13.

The light beam in path 12 is reflected by a mirror 1 4 into one end of a fibre optic light guide which is coiled and forms a fibre optic sensor 1 5.

The light beam after passing through the sensor 1 5 enters a beam combiner 16.

The light beam in path 13 enters a Bragg cell 1 7 which is modulated by a signal generator 1 8 of angular frequency WB to produce a band of frequencies centred on WL but spaced one from another by W,. A mirror 1 9 is arranged to direct the angular frequency component WL + W5 as a reference beam to the beam combiner 1 6.

The combined output of the beam combiner 1 6 is focused onto a photodiode 20 which is non linear and mixes the two signals to provide a difference frequency component W8 by optical heterodyning.

The sensor 1 5 is responsive to environmental changes and when these are oscillatory e.g.

pressure variations due to sound waves, then the light travelling through the sensor 1 5 is phase modulated by the sound field due to a combination of mechanical (changes in length) and optoelastic (changes of refractive index) effects on the fibre. Accordingly the difference frequency WB produced by the photodiode 20 is phase modulated by the acoustic signal and the information can be detected by standard phase modulation techniques in an f.m. receiver. The function of this system can be expressed mathematically as follows:--- Electric field of combined beam = E = A cos (wLt + A) + B cos (w, + WB)t Where the first term contains the phase modulation as seen by the sensing arm and the second term is due to the reference arm.

Output from the photodetector is proportional to EE* i.e. cc (A2 + B2) + 2AB cos (wBt + A) The second term contains the phase modulated radio frequency carrier and also contains the acoustic phase modulation, which can be recovered by conventional techniques.

In some applications of such an interferometer it is necessary for the fibre optic sensor to be located remotely from the active parts of the system i.e. the Bragg cell, laser, photodiode and phase detector. One such application is use as a hydrophone. Some possible ways of doing this are.

Positioning the fibre optic sensor 1 5 remotely from the rest of the interferometer and linking by means of two fibre optic lines. In such an arrangement however the cabies pick up noise from the environment. Even with acoustic screening of the lines the required signal can still be lost if the lines are long enough.

Theoretically, environmental insulation could be achieved by making both sensing and reference paths through optical fibres both of which are routed down to the hydrophone in a two core cable or in two cables mechanically coupled.

Environmental noise would affect the two fibres equaily and so should cancel out. However, only a slight difference between the two fibres would result in excess noise and signal loss.

An alternative approach is to have the complete system at the sensor coil and just the phase detector remotely situated. Here there are problems of supplying electrical power to the interferometer, and of encapsulating the system for protection from the environment, and of cost related to the problem of replacement or repair in the event of failure.

The arrangement of Figure 2 overcomes the problem of loss of signal due to noise pick up.

Figure 2 illustrates a hydrophone system in which a laser 21 provides a highly coherent source of light at an angular frequency of W, which is directed into a Bragg cell 22. The Bragg cell is modulated at an angular frequency of WB by a signal generator 23 to produce a band of frequencies centred on WL but spaced one from another by WB. An optical switch 24 is arranged to select WL or WL + WB from the output of the Bragg cell and is alternated therebetween at a predetermined rate by a switch 25. In this way a time division multiplex signal comprising frequency shifted beams and unshifted beams are produced. As shown by the inset waveform in Figure 2. The duration of each signal portion is T.

This multiplexed signal is launched into a downward fibre optic line 26 to a remote hydrophone head 27 which comprises a beam splitter 28 which directs the beam over two paths.

One of the paths is via a fibre optic sensor 29 of coiled form to a combiner 30 and the other path is directly to the combiner over a line bypassing the sensor 29. The path length of the bypass line is arranged such that the combiner receives contemporaneously signals of the same frequency WL 8 WL + WB and shifted and unshifted beam frequencies are superimposed (see Figure 3).

Preferably the differential delay in the paths from the beam splitter 28 to the combiner 29 via the sensor and via the bypass line is equal to the duration T of each signal portion. The delay is governed by the length L of the fibre line and the velocity V of light in the fibre.

The combined signals are routed to a sensing photodector 31 along an upward optic fibre line 32 having a square law characteristic which results in hetrodyning therebetween. In this way a difference frequency is obtained. The signal passing through the sensor 29 is phase modulated by acoustic signals due to mechanical and acoustic optic effects as previously described in relation to Figure 1 and the phase modulation can be detected by an f.m. receiver.

Environmental immunity of the downward line 26 is achieved as follows. The system effectively uses consecutive bursts of shifted and unshifted frequencies to form the heterodyne. As these are temporally short e.g. they effectively experience the same sound field with the result that phase changes caused by environmental noise are equal in both sections and effective relative modulation is zero.

In the upward line 32 the two aligned beams are accurately co-planar. By following the same path they are subject to the same sound fields and so again experience no overall relative modulation.

Expressing the operation in mathematical terms, the electric field after recombination of the beam is given by either: E1 = A cos (wLt + A1 + A2) + B cos (wL + WB)t + A1) Where At is due to the environmental noise and A2 is due to the sound field (unshifted beam in sensor coil), or E2 = A cos (WLt + A1) + B cos ((WL + WB)t + A1 + A2) (shifted beam in sensor coil).

Now the detector output is either V cc Elm1* or E2E2* i.e.. V cc (A2 + B2) + 2AB cos (WBt + A2) or ce (A2 + B2) + 2AB cos (WBt - A2) These are the signals in alternate pulses and can be demodulated using standard techniques.

The arrangement of Figure 2 has the following advantages (a) Time division multiplexing of the downward going signals permits transmission over a single optical fibre line (b) Environmental insepsitivity (c) Low cost and simplicity.

Instead of modulating the Bragg cell 22 continuously and alternately selecting the angular frequencies W, a WL + WB by means of an optical switch, the modulating signal WB from the generator 23 may be gated at the input to the Bragg cell at the predetermined rate so that the cell passes the frequencies W, and W, + WB alternately into the downward line 26.

Although the interferometer of the present invention has been described as applied to hydrophones it will be appreciated that it is applicable to other applications where rapidly alternating environmental conditions are to be sensed e.g. force and pressure and can be extended to radar and microwave processing applications. Such applications are intended to fall within the scope of this invention.

Claims (9)

1. An interferometer comprising generator means for producing a beam of coherent light which alternates between two different frequencies at a predetermined rate, a beam splitter arranged to route the alternating beam over two different paths simultaneously, a fibre optic sensor arranged to receive the light beam in one path, a light combining device arranged to receive the light beam after passage through the sensor and also the light beam in the other path which bypasses the sensor, the relative length of the two paths being arranged such that two different frequency components are combined contemporaneously, an optical detector responsive to the combined beams to produce a difference frequency component and a phase detector means for providing a signal variable in dependence upon the phase of the difference frequency component.

2. An interferometer as claimed in claim 1, wherein the splitter and the combiner are positioned adjacent the sensor and remote from said generator means and the beam is coupled with the light splitter via an optical fibre.

3. An interferometer as claimed in claim 1 or 2, wherein the relative path lengths are arranged so that consecutive periods of said two different frequencies are combined contemporaneously at the combiner.

4. An interferometer as claimed in any one of the preceding claims, wherein the optical detector is positioned remotely from the combiner and is optically coupled therewith via an optical fibre line.

5. An interferometer as claimed in any one of the preceding claims, wherein the generator means comprises an optical modulator cell having an input from a coherent light source at a first frequency and driven by a modulating frequency to provide a second frequency shifted relative to the first frequency.

6. An interferometer as claimed in claim 5, wherein the modulator cell is continuously modulated by the modulating frequency and said first and second frequencies are alternately routed to the beam splitter by means of an optical switch operating at said predetermined rate.

7. An interferometer as claimed in claim 5, wherein the frequency transmitted by the modulator cell is alternated between said first and second frequency by alternately connecting and disconnecting the modulating frequency to the cell.

8. An interferometer substantially as described herein with reference to Figures 2 and 3 of the drawings.

9. A hydrophone comprising an interferometer as claimed in any one of the preceding claims wherein the fibre optic sensor forms an acoustic sensor for the hydrophone.