GB2289331A

GB2289331A - Distributed sensing in a fibre optic cable

Info

Publication number: GB2289331A
Application number: GB9507989A
Authority: GB
Inventors: David Alfred Jackson; David John Webb; Christopher Pannell
Original assignee: Furukawa Research & Engineerin
Current assignee: Furukawa Research & Engineerin
Priority date: 1994-04-28
Filing date: 1995-04-19
Publication date: 1995-11-15
Anticipated expiration: 2015-04-19
Also published as: AU691573B2; GB2289331B; GB9507989D0; JPH0854257A; GB9408502D0; AU1656895A

Abstract

Apparatus for sensing temperature or strain in an optical sensing fibre 6 comprises a laser 1; first means 3 for modifying light from the laser by a Brillouin frequency shift; second means 7 for converting light from the laser into pulses. Modified light from the first and second means is directed to the first and second ends of the sensing fibre 6. Light entering the fibre 6 from the respective two ends interacts to amplify the light entering from the first fibre end by a Brillouin gain or reduce its power by a Brillouin loss, which can be measured to give an indication of temperature or strain in the fibre. Further embodiments include threshold temperature and strain sensing, means for transmitting communications signals along the fibre and enhanced sensing utilising a second sensing fibre. <IMAGE>

Description

DISTRIBUTED SENSING APPARATUS This invention relates to a distributed sensing apparatus. Particularly, the invention relates to an apparatus for measuring temperature or strain along an optical fibre.

Distributed sensors can be used to determine the temperature or strain and their variations along the length of a continuous uninterrupted optical fibre, thereby offering a powerful and economical means of monitoring the local environment at a large number of individual locations.

The most developed conventional system is based upon Raman scattering, where the ratio of the Stokes to anti-Stokes backscattered intensity signals gives the absolute temperature. The level of this Raman scattered signal is approximately 10-3 times lower than that of the Rayleigh backscattered light signal which is normally used for OTDR measurements in optical fibres. For a typical optical peak launch power of 100mW, the Raman signal will be at the level of 10-8 W for a few hundred meters of fibre length and this may be further reduced by the optical attenuation of the fibre. In order to achieve a reasonable signal to noise ratio for such a weak signal, it is necessary to use long integration times. For example, a Raman system based upon multimode fibre with a 10km sensing length can require at least 65536 sample averages to achieve a temperature resolution of 20C with a spatial resolution of 2m.

Very recently, distributed sensors based on Brillouin scattering in standard communication fibre have been developed. These offer a considerable improvement in sensing length, when compared with the Raman system.

The temperature sensitive mechanism exploited in such a system is known as "Brillouin gain" and is an interaction between two counter-propagating light waves and an acoustic wave.

If beams of light from two lasers are launched into opposite ends of an optical fibre, it turns out that one of the beams (the signal beam) can be amplified as it passes along the fibre at the expense of the other (the pump beam), so long as the optical frequencies of the two beams differ by a certain amount, known as the Brillouin frequency shift" (about 10 GHz). The Brillouin frequency is the frequency of an acoustic wave travelling in the fibre which has a wavelength half that of the light. Brillouin gain is this amplification process.

The Brillouin frequency shift depends on the temperature (and strain) of the fibre, so if one adjusts the frequency difference between the two lasers and notes the frequency at which the Brillouin gain is maximum by monitoring the power of the amplified beam as it emerges from the fibre, the temperature or strain may be deduced.

This system provides information on the temperature or strain of the fibre but there is no positional information. To obtain this it is necessary that the pump beam should be in the form of a short pulse of light, the signal beam still being continuous. In operation, the signal beam power emerging from the fibre is monitored following the launch of a pump pulse. An increase in the signal beam power will occur whenever the laser frequency difference matches the Brillouin frequency. The time delay between the launch of the pump pulse and the observed increase in the signal power corresponds to the round trip time for light travelling from the pump end of the fibre to the region of interaction and back.

In order to recover the temperature or strain from a point at a given position along the fibre, it is necessary to measure the signal level at the appropriate time following the launch of the pulse.

This process must be repeated many times as the laser frequency difference is changed, thereby allowing the temperature or strain of that point on the sensing fibre to be determined.

The foregoing describes a system based on Brillouin gain - where the signal is amplified at the expense of the pulsed beam. This occurs when the signal beam has an optical frequency less than that of the pulsed beam by an amount equal to the Brillouin frequency. When the signal beam frequency is greater than that of the pulsed beam by the Brillouin frequency, the signal power is reduced, with power being transferred to the pulse. This is known as "Brillouin loss" and offers certain advantages (in particular a longer range) than Brillouin gain based systems.

The present invention sets out to improve the above described Brillouin scattering distributed sensor and is equally applicable to systems based on Brillouin gain or Brillouin loss.

The above conventional system is able to recover the temperature (or strain) from anywhere along the sensing fibre. However, in order to do this it is necessary to both vary and measure the frequency difference between the two lasers and to determine the Brillouin gain at each measurement point along the fibre for each frequency. This process is complex and costly in terms of time and processing power. There are applications where it is necessary merely to know when the temperature (or strain) at any point reaches a certain threshold: examples include fire detection, where it might be sufficient to know that the temperature at a point had reached say 700C, and strain monitoring in a large structure such as a bridge, where it might be sufficient to know that the strain had reached an unsafe level at a particular place.

The first aspect of the present invention sets out to provide a device which is capable of monitoring temperature or strain cheaply and efficiently, whilst enabling detection that predetermined limits of strain or temperature experienced in a monitored structure have been reached.

According to a first aspect of the invention there is provided a device for detecting temperature or strain in an optical fibre, comprising means for launching a first laser beam into an end of the fibre and a second laser beam into an end of the fibre, wherein one of the beams is pulsed and the respective optical frequencies of the beams differ by a Brillouin frequency shift; the device further comprising means for monitoring a Brillouin gain or loss resulting from interaction between the two beams, which monitoring means is arranged and adapted particularly to detect when the Brillouin gain or loss reaches a specified value at any point in the fibre.

Using this arrangement, it is unnecessary to scan the frequency of one of the lasers. Under these circumstances, in normal operation, the Brillouin gain signal from the fibre is generally small. However, when the temperature (or strain) rises and approaches the set threshold at a particular place along the fibre, the Brillouin gain signal from that point will also rise, reaching a maximum at the threshold value.

So by monitoring the Brillouin gain signal from the fibre and looking for any tell tale increases in the signal level, it is possible to know when and where the threshold value has been reached and, if necessary, the rate of change of temperature or strain. As a result, frequency scanning apparatus is rendered otiose, thereby simplifying the device and significantly reducing its cost.

Preferably, the laser source is the only laser source in the apparatus.

Preferably, the first means modify the light by altering its frequency by Brillouin frequency shift arising due to a non linear interaction of the light beam in the fibre known as Stimulated Brillouin Scattering (SBS), with this interaction being sufficiently strong to generate a second laser source.

The conventional distributed sensing system requires two laser sources - which are currently very expensive.

A second aspect of the invention sets out to reduce significantly the expense of a sensing system, by eliminating one of the lasers.

According to a second aspect of the invention there is provided a sensing apparatus for sensing temperature or strain in an optical sensing fibre, comprising a laser source and means for directing light from the laser source down first and second paths; the first path comprising first means for receiving light from the laser source and modifying it by altering its frequency by a Brillouin frequency shift; and the first or second path comprising second means for converting light into light pulses; wherein light from the laser source is transmitted into the sensing fibre via the first path to define a first beam and light from the laser source is transmitted into the sensing fibre via the second path to define a second beam; the arrangement being such that, during use, the two beams are caused to propagate in the sensing fibre in opposite directions and light from the two beams interacts to reduce the power of one of the beams by a Brillouin'loss or increase the power of one of the beams by a Brillouin gain, which gain or loss can be measured to give an indication of temperature or strain in the sensing fibre.

In a preferred embodiment the first means is a stimulated Brillouin scattering generator, comprising a length of optical fibre into one end of which light from the laser source is launched. The frequency shift made by the stimulated Brillouin scattering generator may be adjusted by controlling the temperature or strain of its length of fibre. Preferably the frequency of the stimulated Brillouin scattering generator is shifted by means of some form of optical modulator or modulators such as an acoustic optic modulator.

The second means may be an optical switch. The optical switch may be an acousto-optic modulator; an integrated optic switch, a Pockel cell or a Kerr cell.

The fibre of the stimulated Brillouin scattering generator, may be situated in heating means provided with a temperature measuring device such as a thermistor. This apparatus may be employed in a method of temperature measurement in which the fibre of the stimulated Brillouin scattering generator is heated to a certain temperature, such that the light it produces will only experience Brillouin gain or Brillouin loss in those regions of the sensing fibre which are also at that certain temperature, thereby enabling the temperature profile of the sensing fibre to be determined by temperature tuning the stimulated Brillouin scattering generator. A similar method may be employed where the sensor is to test strain. In this case, the fibre of the stimulated Brillouin scattering generator is subjected to strain by a strain inducing device and the strain profile of the sensing fibre may be determined.

Embodiments of the invention in accordance with the second aspect of the invention overcome the abovediscussed problems in that only one laser is needed, thereby significantly reducing costs.

Further advantages are also provided: the arrangement, when provided with means for heating the fibre of the stimulated Brillouin scattering generator or for putting it under a strain, does not require fast detection electronics (which are needed to determine the beat frequency between the two lasers in the prior art). This is because only regions of the sensing fibre having the same temperature or strain as the stimulated Brillouin scattering generator fibre will experience gain or loss. Furthermore, if the two fibres are the same, no calibration is required.

Furthermore, in the conventional arrangement, the frequencies of the lasers had to be very stable because it was necessary accurately to control their difference. In practising with the second aspect of the invention by using a stimulated Brillouin scattering generator, it is possible for the device to generate a constant frequency shift, for example, by holding the stimulated Brillouin scattering generator at constant temperature.

An optical fibre amplifier may be provided either before or after the sensing fibre in order to boost signal levels.

According to a third aspect of the invention there is provided a sensing apparatus for sensing temperature or strain in an optical sensing fibre; comprising means for generating a signal laser beam, the propagation of which may be monitored to give an indication of temperature or strain in the sensing fibre; wherein a transmitter is provided for transmitting communications signals along the sensing fibre and a receiver is provided for receiving communications signals transmitted along the sensing fibre.

According to a further aspect of the invention there is provided a sensing apparatus for sensing temperature or strain in an optical sensing fibre; comprising means for generating a signal laser beam, means for generating a pulsed laser beam and means for causing the two beams to propagate in the sensing fibre in opposite directions in such a manner as to cause the beams to interact to produce a Brillouin loss or a Brillouin gain in one of the beams which may be detected to give an indication of temperature or strain in the sensing fibre; wherein the apparatus comprises a further sensing fibre into which a signal beam is also transmitted, and the pulsed beam is amplified by a Brillouin loss in the signal beam in the first sensing fibre and the amplified pulsed beam emerging from the first sensing fibre is directed into the further sensing fibre to interact with the signal beam in the further sensing fibre in such a manner that one of the beams undergoes a Brillouin loss or Brillouin gain, which gain or loss can be detected to give an indication of strain or temperature in the further sensing fibre.

According to a fifth aspect of the invention there is provided a method of sensing temperature or strain at a number of spatially distributed positions; comprising (i) providing light from a laser source; (ii) splitting the light into first and second paths; (iii) altering the frequency of the light in the first path; (iv) converting the light in the first or second path into pulses; (v) launching the light from the first path into an end of a sensing fibre and light from the second path into an end of the sensing fibre; and (vi) measuring a Brillouin gain or loss caused by interaction of the two beams of light in the sensing fibre.

According to a sixth aspect of the invention there is provided a communications network according to Claim 16.

Further preferred features of the various aspects of the invention are set out in the respective dependent claims.

Preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an embodiment of a distributed sensing device in accordance with the invention and which employs Brillouin gain; Figure 2 is a schematic diagram of an alternative embodiment of a distributed sensing device in accordance with the invention, which employs Brillouin loss; Figure 3 is a schematic diagram of a further embodiment a distributed sensing system in accordance with the present invention, which employs a pair of coupled sensing fibres; and Figure 4 is a schematic diagram of a still further embodiment of the present invention in which a distributed sensing system also acts as a communication channel.

Figure 1 shows a single laser source 1 which launches light continuously into an optical fibre 5. A fibre directional coupler 2 splits the power into two and directs it into a stimulated Brillouin scattering generator 3 and an optical switch 7 respectively. The stimulated Brillouin scattering generator 3 produces light of a different frequency from that generated by the laser source 1 and which travels in the opposite direction 4. This reverse beam forms a signal beam input to the sensing fibre 6. The optical switch 7 produces pulses of light which form a pump beam 9 which is input to the opposite end of the sensing fibre 6.

The signal beam is amplified by interaction with the pump beam 9, as described in relation to the conventional arrangement. The amplified signal beam is split off by a second directional coupler 8 and fed to a photodetector 10 which monitors the amplified signal beam.

The stimulated Brillouin scattering generator 3 comprises a long length of optical fibre into one end of which light from the laser source 1 is launched.

The length of the fibre and the optical power are sufficiently large to enable stimulated Brillouin scattering to occur. Shorter lengths of fibre may be used if the ordinary fibre is at least partially replaced by an optical fibre amplifier.

Stimulated Brillouin scattering is a process relating to Brillouin gain which serves to generate light in the fibre which is travelling in the opposite direction to the light that was originally launched into the fibre.

Importantly, the stimulated Brillouin scattering light has an optical frequency that is different from the light generated by the laser source by an amount equal to the Brillouin frequency associated with the fibre of the stimulated Brillouin scattering generator 3. In particular, as the Brillouin frequency is temperature sensitive, the frequency shift may be adjusted by controlling the temperature (or strain) of the fibre in a stimulated Brillouin scattering generator.

The optical switch 7 is required in order to produce pulses in order to obtain spatial information from the continuous light beam. The switch could be an acoustooptic modulator, an integrated optic switch or a Pockel or Kerr cell.

If a very rapid frequency tuning rate is required, an acousto-optic modulator is best employed as the optical switch 7. This may be used to produce the light pulses required and also to shift the optical frequency by a controllable amount in the 10's of MHz range. In this way, the Brillouin gain may be rapidly scanned over a frequency shift corresponding to several OC (one degree changes the Brillouin frequency by about 1 MHz). It may be advantageous to have an acousto-optic modulator as a frequency shifter in addition to the optical switch, if it turns out that a single device is not able to carry out both functions sufficiently.

If the fibre producing the stimulated Brillouin scattering is heated to a certain temperature, T, then the light it produces will only experience gain in the sensing fibre 6 in those places which are also at temperature T. This means that the temperature profile of the sensing fibre 6 can be found by temperature tuning the stimulated Brillouin scattering generator 3.

The result of this is that, instead of having to measuring the laser beat frequency, one simply has to determine the temperature of the fibre in the stimulated Brillouin scattering generator 3, which may be done with a simple thermistor or similar device.

Rather than determining the temperature from the relationship between Brillouin gain and laser frequency difference, one would be provided with the gain as a function of the temperature of the stimulated Brillouin scattering generator 3. Furthermore, if the fibre in the stimulated Brillouin scattering generator 3 is the same as the sensing fibre 6, no calibration is required in that the temperature of the stimulated Brillouin scattering generator 3 that maximises the Brillouin gain at a particular point on the sensing fibre will be the temperature of that point. This applies equally to a distributed strain sensor, where the stimulated Brillouin scattering generator 3 could either be temperature tuned, or the fibre could be mounted on a strain inducing device - which might provide a more rapid tuning rate.

In the above embodiment, th fibre of the stimulated Brillouin scattering generator 3 has essentially the same composition as the sensing fibre 6. However, in a further embodiment, the fibre of the stimulated Brillouin scattering generator 3 has a different composition to the sensing fibre 6. The Brillouin frequency shift depends upon a number of parameters associated with the material of the fibre as well as the temperature and strain environment. The fibre materials are selected to ensure that the Brillouin frequency shift produced by the fibre in the stimulated Brillouin scattering generator 3 at room temperature corresponds to the Brillouin frequency shift in the sensing fibre 6 at a critical temperature in the sensing range. This means that the device can be used as a threshold detection system and there is no need to maintain the stimulated Brillouin scattering generator fibre at the threshold temperature. A strain sensing device can be constructed in a similar manner.

Figure 2 shows an arrangement suitable for monitoring Brillouin loss. This arrangement is generally similar to that shown in Figure 1 and the same reference numerals are used to denote the same components. The arrangement differs, however, in that the part of the laser light forming the signal beam is fed directly into the sensing fibre 6 via the directional coupler 8, rather than entering the sensing fibre 6 from the stimulated Brillouin scattering generator 3, as in Figure 1. Light from the laser source 5 is still fed to the stimulated Brillouin scattering generator 4, but the modified light produced is fed to the optical switch 7 to form the pulsed beam - which is subsequently fed to the sensing fibre 6 at the opposite end to the signal beam. This arrangement is necessary because the Brillouin scattering generator is only able to reduce the frequency of an incident wave.

In practical terms, the Brillouin loss arrangement shown in Figure 2 may otherwise be constructed in a similar fashion to that described in relation to the Brillouin gain arrangement. That is to say a similar switch or acousto-optic modulator arrangement can be employed. Furthermore, the fibres employed can be of similar types and the fibre'of the stimulated Brillouin Scattering generator can be subjected to similar ambient conditions.

Although the above described arrangements each transmit a beam into each end of the sensing fibre 6, single ended operation is possible. To achieve this, both the signal beam and the pump beam are launched into the same end of the sensing fibre 6 and use is made of a reflection from the far end of the fibre (which may be silvered to provide a good reflection) to provide the counter propagating signal beam.

When a distributed sensor employs Brillouin loss, the pulsed beam is amplified as it propagates through the fibre; this amplification more than compensates for any loss incurred in the transit of the pulse through the fibre. Thus, when the pulse emerges from the fibre its amplitude will remain virtually unchanged and it can be used as the pulsed source for a second sensor. In order to achieve this the CW part of the laser is split into two sensing fibres as shown in Figure 3. The pulsed beam is injected into sensing fibre 100, where interactions occur when the frequency matching condition is satisfied. The emergent pulsed beam is then coupled into sensing fibre 102 - and once again interactions will occur for similar frequency matching conditions. Sensing ranges in excess of 100 km should be obtainable by this technique.

It is possible to modify any of the above described distributed sensors such that it can also support a conventional long distance high bandwidth communication channel. An embodiment of this is illustrated schematically in the accompanying Figure 4. Standard Wavelength Division Multiplexers (WDM) are incorporated into the system at any desired position. The wavelength of the source for the distributed sensor is set at 1.3 m and the communications wavelength at 1.5 m. The WDM's are selected for these wavelengths. The output signal of a standard digitally encoded optical transmitter at 1.5 m is coupled into one port of the WDM at location 201 and the output signal from the second WDM at location 202 is used to couple the transmitted signal into the'receiver. Bi-directional communication is also possible.

In principle, any fibre deployed in a communication network can be used as part of a distributed sensor.

Claims

1. A sensing apparatus for sensing temperature or strain in an optical sensing fibre, comprising a laser source and means for directing light from the laser source down first and second paths; the first path comprising first means for receiving light from the laser source and modifying it by altering its frequency by a Brillouin frequency shift; and the first or second path comprising second means for converting light into light pulses; wherein light from the laser source is transmitted into the sensing fibre via the first path to define a first beam and light from the laser source is transmitted into the sensing fibre via the second path to define a second beam; the arrangement being such that, during use, the two beams are caused to propagate in the sensing fibre in opposite directions and light from the two beams interacts to reduce the power of one of the beams by a Brillouin loss or increase the power of one of the beams by a Brillouin gain, which gain or loss can be measured to give an indication of temperature or strain in the sensing fibre.

2. A sensing apparatus according to Claim 1, wherein one of the beams is directed to a first end of the sensing fibre and the other beam is directed into a second end of the sensing fibre.

3. A sensing apparatus according to Claim 1, wherein the first and second beams are directed into the same end of the sensing fibre.

4. A sensing apparatus according to Claim 3, comprising reflecting means for reversing the direction of propagation of one of the beams.

5. A sensing apparatus according to any preceding Claim, wherein the said first means is a stimulated Brillouin scattering generator, comprising a length of optical fibre into one end of which light from the laser source is launched.

6. A sensing apparatus according to Claim 5, wherein the frequency shift made by the stimulated Brillouin scattering generator is determined by controlling the temperature and/or strain of the said length of fibre which it comprises.

7. A sensing apparatus according to Claim 6 wherein the fibre of the stimulated Brillouin scattering generator is heated by heating means, and its temperature is measured by a temperature measuring device.

8. A sensing apparatus according to Claim 6, wherein the fibre of the stimulated Brillouin scattering generator is subjected to strain by a strain inducing device.

9. A sensing apparatus according to any one of Claims 5 to 8, wherein the sensing fibre and the fibre provided in the stimulated Brillouin scattering generator are of the same type.

10. A sensing apparatus according to any one of Claims 5 to 8, wherein the sensing fibre and the fibre provided in the stimulated Brillouin scattering generator are of different types.

11. A sensing apparatus according to any preceding claim, wherein the said second means is an optical switch.

12. A sensing apparatus according to Claim 11, wherein the optical switch is an acousto-optic modulator; an integrated optic switch; a Pockel cell or a Kerr cell.

13. A sensing apparatus for sensing temperature or strain in an optical sensing fibre; comprising means for generating a signal laser beam, the propagation of which may be monitored to give an indication of temperature or strain in the sensing fibre; wherein a transmitter is provided for transmitting communications signals along the sensing fibre and a receiver is provided for receiving communications signals transmitted along the sensing fibre.

14. A sensing apparatus according to Claim 13 wherein means are provided for generating a pulsed laser beam and means are provided for causing the two beams to propagate in the sensing fibre in opposite directions in such a manner as to cause the beams to interact to produce a Brillouin loss or a Brillouin gain in one of the beams.

15. A sensing apparatus according to Claim 13 or 14, wherein the beam used for transmitting communications signals has a wavelength different from that of the beam or beams used for sensing temperature or strain.

16. A sensing apparatus according to Claim 15, comprising a wavelength division multiplexer.

17. A communications network comprising an optical fibre used as a sensing fibre of a distributed sensor in accordance with any of the preceding claims.

18. A sensing apparatus for sensing temperature or strain in an optical sensing fibre; comprising means for generating a signal laser beam, means for generating a pulsed laser beam and means for causing the two beams to propagate in the sensing fibre in opposite directions in such a manner as to cause the beams to interact to produce a Brillouin loss or a Brillouin gain in one of the beams, which may be detected to give an indication of temperature or strain in the sensing fibre; wherein the apparatus comprises a further sensing fibre into which a signal beam is also transmitted, and the pulsed beam is amplified by a Brillouin loss in the signal beam in the first sensing fibre and the amplified pulsed beam emerging from the first sensing fibre is directed into the further sensing fibre to interact with the signal beam in the further sensing fibre in such a manner that one of the beams undergoes a Brillouin loss or Brillouin gain, which gain or loss can be detected to give an indication of strain or temperature in the further fibre.

19. A sensing apparatus according to Claim 18 wherein the signal beam transmitted into the first sensing fibre and the signal beam transmitted into the further sensing fibre originate from the same laser source.

20. A sensing apparatus substantially as hereinbefore described with reference to'Figure 1, 2, 3 or 4 of the accompanying drawings.

21. A method of sensing temperature or strain at a number of spatially distributed positions; comprising (i) providing light from a laser source; (ii) splitting the light into first and second paths; (iii) altering the frequency of the light in the first path; (iv) converting the light in the first or second path into pulses; (v) launching the light from the first path into an end of a sensing fibre and light from the second path into an end of the sensing fibre; and (vi) measuring a Brillouin gain or Brillouin loss caused by interaction of the two beams of light in the sensing fibre.

22. A method according to Claim 21, wherein light from the first path is launched into a first end of the sensing fibre and light from the second path is launched into a second end of the sensing fibre.

23. A method according to Claim 21, wherein light from the first and second paths are launched into the same end of the sensing fibre.

24. A method according to Claim 21, wherein a reflector is provided in the sensing fibre for reversing the direction of a beam of light from either the first path or the second path.

25. A method according to any one of Claims 21 to 24, wherein the frequency of the light in the first path is adjusted by means of a stimulated Brillouin scattering generator comprising an optical fibre.

26. A method according to Claim 25, wherein the temperature or strain detected is determined by controlling the temperature or strain of the fibre in the stimulated Brillouin scattering generator.

27. A method according to Claim 26, wherein the fibre in the stimulated Brillouin scattering generator and the sensing fibre are of the same type and the fibre in the stimulated Brillouin scattering generator is caused to experience a temperature or strain which is to be detected by the sensing fibre at that time.

28. A method according to Claim 26, wherein the fibre in the stimulated Brillouin scattering generator and the sensing fibre are of different types and the fibre in the stimulated Brillouin scattering generator is caused to experience a temperature or strain which is different from that to be detected by the sensing fibre at that time.

29. A method of sensing temperature or strain at a number of spatially distributed positions substantially as hereinbefore described with reference to Figure 1, 2, 3 or 4 of the accompanying drawings.

30. A device for detecting temperature or strain in an optical fibre, comprising means for launching a first laser beam into an end of the fibre and a second laser beam into an end of the fibre, wherein one of the beams is pulsed and the respective optical frequencies of the beams differ by a Brillouin frequency shift; the device further comprising means for monitoring a Brillouin gain or loss resulting from interaction between the two beams, which monitoring means is arranged and adapted particularly to detect when the Brillouin gain or loss reaches a specified value at any point in the fibre.

31. A sensing apparatus according to any one of claims 1 to 12 wherein said laser source is the only laser source in the apparatus.

32. A sensing apparatus according to claim 6 wherein the frequency of the stimulated Brillouin scattering generator is shifted by optical modulation means.