GB2152689A - Optical fibre sensing apparatus - Google Patents

Abstract

An interferometer comprises a laser (21) which generates coherent light pulses and launches them via a first optical fibre to an optical switch (23). The optical switch (23) is arranged to provide selected connections between the first optical fibre and/or a second optical fibre (24) and a third optical fibre connected to the switch thereby enabling selective routing of predetermined optical pulses between predetermined optical fibres according to requirements. The optical switch (23) may be in the form of an integrated cross-over optical switch or a Bragg cell. <IMAGE>

Description

SPECIFICATION Optical fibre sensing apparatus This invention relates to optical fibre sensing apparatus and relates more especially, but not exclusively, to interferometers which include means for combining and/or directing predetermined optical signals in the form of pulses along predetermined optical paths.

Previously proposed interferometers include one or more beam splitters for combining and/or directing optical pulses along optical fibre paths. For example, in our co-pending UK Patent Application No. 8220793 there is described, an optical sensing system for sensing changes in length of an optical fibre due to the impingement thereon of acoustic waves. In this system, a beam splitter is used for focusing incoming optical pulses from one optical fibre into another optical fibre and then directing optical pulses reflected back along the latter optical fibre into yet another optical fibre leading to a photodetector. Again, in our co-pending UK Patent Application No.

8207961, optical pulses having two different frequencies are passed over two optical fibre paths and then received by a beam combiner which routes the two different frequency pulses for transmission down another optical fibre path.

Such beam splitters and beam combiners, as used in these previously proposed interferometers, may consist of half-silvered mirrors.

One disadvantage of using half-silvered mirrors is that the combining or directing process involves the loss of light power which leads to a reduction in the efficiency of the interferometer which is more serious in cases where optical pulses or reflections thereof pass more than once through the beam splitter or combiner.

According to the present invention, there is provided an optical fibre sensing apparatus, such as an interferometer, comprising means for generating coherent light pulses, a plurality of optical fibres defining light transmission means of said apparatus and an optical switch arranged to provide selective connections between input and/or output optical fibres connected to said switch thereby enabling selective routing of predetermined optical pulses between predetermined optical fibres according to requirements.

In a first embodiment of the present invention, a first optical fibre is arranged to be subjected along its length to fibre deforming forces during operation of the interferometer.

The coherent light pulses are transmitted along a second optical fibre, via the optical switch, to the first optical fibre, the first optical fibre being provided along its length with a plurality of equally spaced discontinuities which effectively divide the first optical fibre into a plurality of discrete fibre elements.

The discontinuities are such that a small proportion of each light pulse being transmitted along the fibre will be reflected back along the first optical fibre from each of the discontinuities whereby each reflected light pulse after the first interferes with either the previously reflected pulse from the preceding discontinuity or a reference light pulse of the same frequency or a frequency with a constant difference frequency to the said transmitted light pulse to produce an interference signal.

A third optical fibre is coupled between the optical switch and photo-detection means to permit routing of the interference signal and the production of an electrical signal representative of the deforming forces to be sensed, the difference between respective electrical signals corresponding to successive fibre elements being dependent upon the length of the fibre elements so that changes in length of these elements produced by the incidence of deforming forces will result in changes in the electrical signals which will be detected.

In this embodiment, a heterodyne system may be used in which two-pulse signals each comprising two pulses of slightly different frequencies (F and F + AF) and of predetermined duration and time relationship are transmitted along the first optical fibre so that small proportions of the pulses are reflected back at each fibre discontinuity. The signal reflected from the second discontinuity of the first fibre is caused to interfere with that reflected from the first discontinuity (i.e. the pulse of frequency F of the second reflected signal is heterodyned with the pulse of frequency F + AF of the first reflected signal) This heterodyning produces a detectable electrical beat frequency signal the modulation of which will vary with changes in length of an element of the first optical fibre between the first and second optical fibre discontinuities.It will be appreciated that signals reflected from the third, fourth and fifth and last discontinuities will similarly interfere with those signals reflected from the preceding discontinuity.

Thus, by detecting and measuring phase modulation of the electrical beat signals corresponding to the respective optical fibre elements between discontinuities any changes in length of such elements due to their being stressed can be determined.

In this first embodiment, an alternative heterodyne arrangement to that just described is envisaged. In this arrangement a single pulse light signal of frequency F is transmitted down the first optical fibre for reflection from the fibre discontinuities whilst a two-pulse signal comprising consecutive pulses of frequencies F and F + AF, respectively, is used as a continuous reference at the photodetection means to beat with the reflected signals of frequency F. In this case, however, it is necessary to make comparison between the difference frequencies arising from consecutive reflections and this will require some means of electronically delaying or storing the information from the preceding reflection in order to compare electrical phase relationships.

In an alternative to the arrangement just described, reflected signals from the optical fibre discontinuities of the first fibre may be homodyned by arranging that one or two light pulses in predetermined time relationship and of the same frequency are transmitted along the first optical fibre and reflected signals from the respective discontinuities except the first are caused to interfere with the signals reflected from the preceding discontinuities to produce ampiitude modulated electrical signals in dependence upon the lengths of the optical fibre elements. The detection means will detect and/or measure any changes in modulation due to deformation of the fibre elements.

In this embodiment, the optical switch may be an integrated crossover switch or a Bragg cell switch.

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

In a second embodiment of the present invention there is provided an optical interferometer, wherein the generator means produces coherent light signals in the form of light pulses at first and second frequencies simultaneously. The interferometer comprises optical fibre reference delay means disposed in a first optical fibre, the delay means being arranged to receive the light pulses at the second frequency and to delay said pulses relative to the first frequency pulses which are passed along a second optical fibre, the light pulses at the first and second frequencies then being received by the optical switch which routes the sequential pulses into a third optical fibre path.The third optical path then transmits the pulses 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.

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 a 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 generates 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 second embodiment. 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 along the first and the second optical fibres. The first optical fibre 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 and the second optical fibre path provides a by-pass path. Both paths are terminated by the optical switch which routes the two sequential pulses towards the sensor coil and the above mentioned by-pass path which by-passes the sensor coil. In this embodiment.

the optical switch may take the form of an integrated optic crossover switch.

The invention will now be described. by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagram of an integrated optic crossover switch of a known construction, Figure 2 is a diagram to illustrate the function of a Bragg cell switch; Figure 3 shows a schematic diagram of an optical fibre deformation detection system including an interferometer in accordance with the invention; Figures 4 and 5 show pulse diagrams relating to the operation of the system of Figure 3; and Figure 6 is a schematic diagram of another optical detection system including an interferometer in accordance with the present invention.

Referring to Figure 1, there is shown a known type of integrated optic crossover switch 1 which comprises a lithium niobate substrate (LiNbO3). The switch 1 has two pairs of ports 2,3 and 4,5 the ports of each pair being linked by a stripe of titanium 6 and 7 respectively, which is diffused into the substrate of the switch 1. The stripes 6 and 7 form optical waveguides since they are of different refractive index from the remainder of the substrate. The stripes 6 and 7 converge at one point on the substrate, so that they are separated by a small distance. At this point, a pair of electrodes 8 and 9 are printed into the substrate to form a junction. By applying a voitage across the electrodes 8 and 9, the refractive index of the substrate between the stripes 6 and 7 at the junction can be varied as a result of the electro-optic effect.Hence, by applying suitable voltages across the electrodes 8 and 9, light launched into, for example, port 2 may be selectively routed to either of ports 3 or 5.

An alternative optical switch is illustrated in Figure 2, in which a Bragg cell 10 is illustrated. By applying a frequency w to one side of the Bragg cell, light entering the cell can be deviated through an angle 8 proportional to w.

Hence, by selectively switching the frequency on and off, light passes through the cell 10 in one direction without being deviated (when the frequency w is off), and light which returns through the cell 10 in the opposite direction can be deviated through and angle 8 when the frequency w is switched on. Therefore, the deviated light can be launched into another optical path (not shown).

Referring to Figure 3 of the drawing a pulsed laser 21 produces an output pulse of coherent light of frequency F which is fed into a modulator 22 wherein a modulated pulse of frequency F + AF is produced which by the inclusion of delay means in the modulator 22 lags behind the pulse of frequency F by a predetermined time interval T. This two-pulse light signal passes through either an integrated optic crossover switch or a Bragg cell switch 23 and is focussed into an optical fibre 24.

Equispaced discontinuities 25 to 31 are provided along the optical fibre and these discontinuities may, for example, be formed by suitable joints in the optical fibre. The fibre is effectively divided by these discontinuities into six sensing elements and variations in the lengths of these fibre elements, such as due to the impingement thereon of acoustic waves, can be detected and measured in the manner now to be described.

As each two-puise light signal reaches the first optical fibre discontinuity 25 a small proportion of the signal will be reflected back along the fibre 24 to the switch 23 which directs the signal to a photodetector 32. The remaining part of the two-pulse signal travels on to discontinuity 26 at which a further small proportion thereof will be reflected back along the optical fibre 24 to the detector 32. This procedure continues until that part of the twopulse signal remaining reaches the last of the optical fibre discontinuities 31 and a small proportion of this signal is again reflected back along the optical fibre to the detector 32. A further two-pulse optical transmission is then made and the cycle repeated.

In previously proposed optical interferometer arrangements, a beamsplitter has been used for directing pulses reflected from the respective discontinuities to the photodetector 32. The use of a beamsplitter has the disadvantage that the two-pulse light signal loses 3 dB of power when passing through the beamsplitter and into the optical fibre 24. In addition to this loss, the directed pulses lose 3 dB when they are directed towards the photodetector 32. This loss of 6 dB can be eliminated or at least significantly reduced by replacing the beamsplitter with the switch 23. The modulator 22 may be connected to the port of the switch 23 corresponding to the port 2 of the switch 1 of Figure 1, the optical fibre 24 may be connected to the port corresponding to the port 3, and the optical fibre leading to the photodetector 32 may be connected to the port corresponding to the port 5.In this case, it is clear that the switching of the switch 23 must be synchronised with the time of arrival at the switch of the two-pulse light signal and the light signal after it has been reflected from the discontinuities. If the switch 23 is a Bragg cell switch, an appropriate value of w must be applied to the cell to cause the reflected light pulses to be directed into the optical fibre which leads to the photodetector 32, and the switching of the frequency w between off and on states is synchronised so that light to be launched into the optical fibre 24 is not deviated by the width 23, but light returning from the discontinuities is deviated so that it is directed to the photodetector 32.

Referring now to Figure 4 of the drawing this shows by way of example reflections of the two-pulse signals from the discontinuities 25, 26 and 27. As can be seen from the drawing the reflection from the second discontinuity 26 in the present example is delayed with respect to the reflection from the first discontinuity 25 by time T.

T = 2L/Cc where L = the length of each optical fibre element and Cc = velocity of light in the optical fibre.

By the appropriate choice of length L the delay between the reflections is such that there is total coincidence or at least some overlap between the reflected pulse of frequency F of a later reflected signal with the pulse of frequency F + AF of the preceding reflected signal. The reflectcd pulses are heterodyned in the square law photodetector 32 to produce beat or modulated signals as shown and the phase modulation of these signals will vary in dependence upon variations in length of the optical fibre elements. Accordingly, by detecting and measuring the phase modulation of the beat signals by means of a phase detector 33 changes in length of the optical fibre elements and thus deformation forces acting on these elements can be measured.

Referring now to Figure 5 of the drawings this shows the pulse diagram of an alternative sensing system in which the pulsed laser will produce at predetermined intervals one or two closely spaced pulses of the same frequency which constitute the signals fed to the optical fibre 24 (Figure 3) without the intervention of the modulator 22 (Figure 3). Assuming singlepulse signals are transmitted to the optical fibre the signals reflected from the discontinuities 25, 26 and 27 will be as shown in Figure 5. The reflected signals are homodyned and the changes in amplitude of the electrical signals produced by changes in length of the optical fibre elements will be detected by the phase photodetector 32 (Figure 3). The phase detector 33 is not required for this embodiment.

When the embodiments just above described are used in a hydrophone the free end of the optical fibre including the discontinuities 25 to 31 will be trailed through the water and will provide a beamforming acoustic wave sensor array which will respond to acoustic waves impinging on the optical fibre sensing elements to produce variations in the lengths thereof which will be measured in the manner described.

Referring now to Figure 6, a second embodiment of the present invention is shown.

The detection system comprises a semiconductor laser 71 which produces light pulses, such as the pulse 72 having a frequency W1 in response to pulsing of the laser.

Each of the pulses at frequency W1 is applied to a beam splitter 73 which routes the pulses over alternative paths. One of these paths includes a Bragg cell 74 which is modulated at a frequency AW1 so that it produces a light pulse of frequency AW1 + aW1 which then passes through an optical fibre reference coil 75 which acts to delay the pulse with respect to the light pulse of frequency W1 travelling through by-pass path 76 so that the pulses received by a combining device 77 are displaced in time or multiplexed in a downward optical fibre 78.

The combining device 77 is an integrated optic crossover switch as illustrated in Figure 1. The switch 77 must be synchronised with the arrival of pulses at the switch from both paths. In this case, the ports 2 and 4 can be respectively connected to the paths having the reference coil 75 and the by-pass path 76, and the port 3 or 5 can be connected to the downward optical fibre 78. This switch 77 operates in the manner as described earlier with reference to Figure 1.

The multiplexed pulses of frequencies W1 and W1 + AW1 are routed by a beam splitter 79 through an optical fibre sensor coil 80 and through a by-pass optical fibre 81, respectively.

The sensor coil delays the pulse of frequency W1 by the same amount as the reference coil 75 so that as the pulse of frequency W1 is combined with the pulse of frequency W1 + aW1 these pulses are coincident with one another in the output of combining means 82 which applied to an upward optical fibre 83. These coincident pulses are then fed to a photo detector 84 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 80 is detected by phase detector 85.

As will readily be seen from the foregoing, the two light pulses of frequencies of W1 and W1 + AWl are both derived at the same time from the same pulse from semiconductor laser 71. Consequently, the semiconductor laser need only have a short coherence length pulse output.

Although the present invention has been described with reference to apparatus which includes either an integrated optic crossover switch 1 or a Bragg Cell switch 1 0, it should be understood that other forms of switch could be used, for example. a mechanical switch or a switch in which optical polarisation states could change upon application of an electrical signal thereby to enable selective routing of predetermined optical pulses between predetermined optical fibres, Alternatively, a passive device could be used for selectively routing the light, which device does not switch in time but has different characteristics for light passing through the device in two different directions Such a device can be made using polarisers and the device uses the characteristics of polarised light so that it can selectively route the light.

Claims (9)

1. An optical fibre sensing apparatus, comprising means for generating coherent light pulses, a plurality of optical fibres defining light transmission means of said apparatus and an optical switch arranged to provide selective connections between input and/or output optical fibres conected to said switch thereby enabling selective routing of predetermined optical pulses between predetermined optical fibres.

2. An optical fibre sensing apparatus according to claim 1, comprising first, second and third optical fibres, wherein the first optical fibre is arranged to be subjected along its length to fibre deforming forces during operation of the apparatus, the second optical fibre receives the coherent light pulses from the generating means and transmits the pulses to the first optical fibre via the optical switch, the first optical fibre is provided along its length with a plurality of equally spaced discontinuities which effectively divide the first optical fibre into a plurality of discrete fibre elements, the discontinuities being such that a small proportion of each light pulse transmitted along the fibre will be reflected back along the first optical fibre from each of the discontinuities whereby each reflected light pluse after the first interferes with either the previously reflected pulse from the preceding discontinuity or a reference light pulse of the same frequency or a frequency with a constant difference frequency to the said transmitted light pulse to produce an interference signal, and the third optical fibre is coupled between the optical switch and a photo-detection means thereby to permit routing of the interference signal to the photodetection means, which photo-detection means produces an electrical signal representative of the deforming forces to be sensed, the difference between respective electrical signals corresponding to successive fibre elements being dependent upon the length of the fibre elements so that changes in length of these elements produced by the incidence of deforming forces will result in changes in the electrical signals which will be detected.

3. An optical fibre sensing apparatus according to claim 2, in which two-pulse signals each comprising two pulses of slightly different frequencies (F and F + AF) and of predetermined duration and time relationship are transmitted along the first optical fibre so that small proportions of the pulses are reflected back at each fibre discontinuity, in which the signal reflected from the second discontinuity of the first fibre is caused to interfere with that reflected from the first discontinuity to produce a detectable electrical beat frequency signal the modulation of which will vary with changes in length of an element of the first optical fibre between the first and second optical fibre discontinuities and signals reflected from the third, fourth and fifth and last discontinuities, as the case may be, will similarly interfere with those signals reflected from the preceding discontinuity.

4. An optical sensing system according to claim 2, in which a single pulse light signal of frequency F is transmitted down the first optical fibre for reflection from the fibre discontinuities whilst a two-pulse signal comprising consecutive pulses of frequencies F and F + AF, respectively, is used as a continuous reference at the photo-detection means to beat with the reflected signals of frequency F, and in which means are provided to electroni cally delay or store the information from the preceding reflection in order to compare electrical phase relationships.

5. An optical sensing system as claimed in claim 3, in which reflected signals from the optical fibre discontinuities of the first fibre are homodyned by arranging that one or two light pulses in predetermined time relationship and of the same frequency are transmitted along the first optical fibre and reflected signals from the respective discontinuities except the first are caused to interfere with the signals reflected from the preceding discontinuities to produce amplitude-modulated electrical signals in dependence upon the lengths of the optical fibre elements, the detection means detecting and/or measuring any changes in modulation due to deformation of the fibre elements.

6. An optical sensing system as claimed in claim 1, comprising first, second and third optical fibres, in which system the generator means produces coherent light signals in the form of light pulses at first and second frequencies simultaneously, the interferometer comprises optical fibre reference delay means disposed in the first optical fibre, the delay means being arranged to receive the light pulses at the second frequency and to delay said pulses relative to the first frequency pulses which are passed along the second optical fibre, the light pulses at the first and second frequencies then being received by the optical switch which routes the sequential pulses into the first optical fibre path which then transmits the pulses to a beam splitter device which routes the first and second frequency signals through an optical fibre sensor and a by-pass path 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.

7. An optical fibre sensing system as claimed in claim 6, in which the generator means is a semiconductor laser which is pulsed to provide light pulses of short coherent length, which pulses are fed to a beam splitter arranged to route the first frequency pulse along the first and the second optical fibres, the first optical fibre 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 and the second optical fibre path provides a by-pass path, both paths being terminated by the optical switch which routes the two sequential pulses towards the sensor coil and the firstmentioned by-pass path.

8. An optical sensing system substantially as herein described with reference to Figures 1 and 3 of the accompanying drawings.

9. An optical sensing system substantially as herein described with reference to Figures 1 and 6 of the accompanying drawings.