GB2190262A - Optical fibre sensor - Google Patents

Abstract

An optical fibre sensing arrangement has an optical fibre 2 which has a number of equally-spaced reflection points R0, R1 .... Rn+1 along its length. A pulsed light signal is sent along the fibre and at each reflection point a portion of the light signal is reflected back. The back-reflected signals after conversion into electrical form are applied to the detection means via a delayed and an undelayed path, the delay being equal to the return time of flight between adjacent reflection points. The outputs of the two paths are compared in phase to sense any change in the lengths of fibre between Ri and Ri+1. Application to hydrophenes is mentioned. <IMAGE>

Description

SPECIFICATION Sensor This invention relates to an optical fibre sensor which monitors optical path length changes between spaced apart partially reflecting discontinuities in a sensor fibre.

In one arrangement of such a sensor described in UK Patent Specification No.

2126820A, to which attention is directed, double pulses of light differing slightly in frequency are launched into the sensor fibre. The interval between the two pulses is equal to the time taken for light to make a round trip path between consecutive partially reflecting discontinuities, and so the proportion of light of the first pulse reflected back up the sensor fibre by the (n+1)'h discontinuity is overlapped in time with, and hence interferes with, the proportion of light of the second pulse reflected by the nth discontinuity. The reflected light is fed to a photodetector which performs heterodyne detection. At this photodetector the reflections from the first and second discontinuities are first heterodyned, and then a little later, those from the second and third discontinuities, and so on.The phase of the IF output is dependent upon the relative phases of the two interfering hyeterodyned optical signals, and hence the phase of the IF changes with change of optical path length between the two discontinuities.

A drawback of this arrangement is that both of the optical signals being heterodyned are relatively low power signals, since both of them are derived from partially reflecting discontinuities. The reflectivity of these discontinuities is necessarily relatively low, firstly to allow a reasonable number of such discontinuities to be included in the one sensor fibre, and second to suppress unwanted interference from light suffering three reflections rather than a single reflection.

Problems of noise in the sensor system, and problems of dynamic range, would be reduced if one weak reflected signal from the sensor fibre were heterodyned with a strong local oscillator signal rather than a second weak reflected signal from the sensor fibre.

The disadvantage of this approach is that it takes away one of the advantages of firstmentioned approach, namely that the two optical paths are identical except for the section between the two discontinuities in question.

The separation of the two optical paths is liable to make the system also sensitive to changes in optical path length elsewhere than in the section between the two discontinuities in question. The present invention is concerned to cancel out this unwanted sensitivity by processing of the electrical output signals from the heterodyne detection. In this way a requirement for source phase coherence over the total transit time is also avoided.

According to the invention there is provided a method of monitoring the condition of an optical fibre which has a number of points of reflection spaced along its length by transmitting pulses of light along the fibre and monitoring the reflected pulses at the sending end, in which the pulses as received back at the sending end are converted into electrical form and processed by a delay processor, and in which the processing involves delaying the pulses by the time between adjacent reflections and mixing the delayed pulses with the undelayed versions of these pulses, so that the output signal after said combination is a measure of the optical phase over the individual fibre sections between the reflection points.

This arrangement enables coherent detection to be achieved to attain high sensitivity, without requiring the local oscillator to be phase coherent over the transit time of the downlead. This eases source coherence requirements, and eliminates downlead sensitivity in sensors.

An embodiment of the invention will now be described with reference to the accompanying highly schematic drawings, in which Fig. 1 illustrates the basic principles of the sensor, Fig. 2 shows a design of OTDR which may be used as a component of the sensor, and Figs. 3, 4 and 5 show the delay arrangements used in the delay processor of the sensor.

The basic arrangement is shown in Fig. 1, where we see the components of a coherent light OTDR (Optical time domain reflectometer) 1 used to launch light into a sensor fibre 2.

The filtered IF output of the heterodyne detector of the OTDR is fed to a processor 3. The sensor fibre consists of a downlead portion of single mode fibre which connects the OTDR optical output to a sensor portion, also of single mode fibre, where the fibre is divided into sections by a cascade of partially reflecting discontinuities Ro, R1, R2, etc. These partially reflecting discontinuities may conveniently take the form of special fused fibre splices as described in our co-pending patent application No. 8611408 (R.E. Epworth-A.J. Robertson 36-9).

The OTDR consists essentially of a coherent optical source 4 (Fig. 2), a frequently shifting modulator 5, optical power splitting/combining elements 6, 7 and 8, and a photodetector 9 and IF filter 10. Light from the optical source 4, which may conveniently be constituted by an injection laser 4a coupled to a length of optical fibre 4b used for line narrowing purposes, is directed to element 6, which like elements 7 and 8 is conveniently constituted by a fused single mode fibre directional coupler, to tap off a proportion of the source output for later use as LO (local oscillator) power for heterodyning purposes.The remainder of the optical power is directed to the modulator 5, which is conveniently constituted by an acousto-optic Bragg optical frequency shifting modulator pulsed with power from an IF frequency offset signal generator 11, which is typically designed to operate at a frequency of 40 MHz. The frequency offset optical output of the Bragg modulator 5 is collected by a lens 12 and directed through more single mode fibre to element 7 and on to the sensor fibre 2. The reflected signals pass back through element 7 where half their power is directed to element 8 for heterodyne mixing at the detector 9 with the local oscillator signal derived from element 7.

The phase of the 40 MHz IF output from the detector 9 depends upon the relative phases of the optical inputs. Thus the phase of the IF signal provided by heterodyning the LO signal with the signal reflected by R0 will depend upon the difference in optical path length between that of the local oscillator pathway direct from element 6 to element 8, and that of the signal pathway from element 6 via the Bragg modulator and element 7 to the OTDR optical output 13, down and up the downlead and back through element 7 to element 8.If this IF phase is On, and similarly the phases produced by heterodyning the reflection from R ,with LO and Rn with LO are respectively 1!), and On, then the function of the processor 3 is to form signals representative of the differences OiOo, D 0, 2 O-O2,... )n-ç)a-l Under the assumption that no significant change in relative optical path length occurs upstream of Rn occurs in the time interval elapsing between heterodyning the signal reflected by Rn with LO and heterodyning the signal reflected by Rn with LO, each of these differences produced by processor 3 is determined solely by the optical path length between the relevant pair of partially reflecting discontinuities.

In Fig. 2, the processor applies the incoming IF signals to both inputs of a mixer M1. One is applied direct while the other is applied via a delay T1, which introduces the delay corresponding to the round trip optical path length between consecutive discontinuities. The output base band signal thus obtained is a measure only of the optical phase over the individual sections between adjacent discontinuities, and is no longer sensitive to the downlead phase. Further, fluctuations in phase of the local oscillator over the double transit time of the downlead are automatically cancelled leaving merely the requirement for low local oscillator phase noise over a time equal to the delay between adjacent reflections. Thus the output from the mixer M1 changes sinusoidally with change in optical phase between two optical reflectors.

With the arrangement of Fig. 2 there is no output when the reflections are in quadrature; this may be overcome by introducing a frequency shift in one of the paths as indicated in broken lines at FS.

Another way to overcome the above-mentioned difficulty is to apply the delay output via another delay T2, to give a quadrature phase shift to another mixer M2. This gives I and Q quadrature outputs which are then processed to assess the reflection points and the condition of the fibres at those reflection points.

If multiple reflectors are used with short pulses then the same demodulation method may be used with the addition of a demultiplexer/sampler arrangement at each I and 0 output. This is shown in Fig. 4, where the demultiplexer/samplers are represented by rotating multiposition switches. These separate the I and Q signals appropriate to each sensing length of fibre between adjacent reflectors.

The optical phase, and hence the information to be sensed, may be derived from the I and Q components by well-known methods.

Such interferometric arrangements can be used in a variety of distributed optical sensor arrangements, e.g. in hydrophones.

Claims (5)

1. A method of monitoring the condition of an optical fibre which has a number of points of reflection spaced along its length and monitoring the reflected pulses at the sending end, in which the pulses as received back at the sending end are converted into electrical form and processed by a delay processor, and in which the processing involves delaying the pulses by the time between adjacent reflections and mixing the delayed pulses with the undelayed versions of these pulses, so that the output signal after said mixing of said pulses is a measure of the optical phase over the individual fibre sections between the reflection points.

2. An optical sensing arrangement, which includes an optical fibre which may be subjected along its length to fibre deforming forces when the arrangement is in use, and a light source from which coherent light signals are transmitted along the optical fibre, in which the fibre has along its length equallyspaced discontinuities which divide the fibre into a number of discrete sections, so that when the arrangement is in use a small proportion of each light signal transmitted along the fibre is reflected back along the fibre from each of the discontinuities, whereby each reflected light signal apart from the first interferes with the previously-reflected signal from the preceding discontinuity, in which the reflected light signals are applied to detection means whereas they are converted into electrical form for processing by processing means, in which the difference between respective electrical signals corresponding to successive fibre elements is dependent on the length of the fibre elements so that changes in length of said elements due to said deform ing forces cause corresponding changes in the electrical signals, in which the detected electrical signals are processed by a delay processor, and in which the processing by said delay processor involves delaying the electrical pulses by the time between adjacent ones of said reflections, applying the undelayed pulses to one input of a mixer and applying the de iayed pulses to another input of the mixer so that the output signal from said mixer is a measure of the optical phase over the individual fibre sections between the reflecting points, and hence of the deforming forces to which the fibre is subjected.

3. An arrangement as claimed in claim 2, in which a frequency shifting circuit is included in the non-delayed signal input to the mixer.

4. An arrangement as claimed in claim 2, in which the delayed signal is further delayed, the further delayed signal being applied to one input of a second mixer, and in which the non-delayed signal is applied to another input of the second mixer, so that the two mixers provide I and 0 quadrature outputs which are then processed to assess the reflection points and the condition of the fibres at those reflection points.

5. An optical sensing arrangement substantially as described with reference to Figs. 1, 2 and 3, or Figs. 1, 2 and 4, or Figs. 1, 2 and 5 of the accompanying drawings.