Distributed optical fibre sensors

1. A distributed optical fibre sensor for measuring acoustic vibration, as a concurrent function of position along each of a plurality of sensing optical fibres, from properties of probe light backscattered within the sensing optical fibres, the sensor comprising:

a probe light source arranged to generate pulses of probe light for transmission to the sensing optical fibres;

a detector arranged to receive and detect probe light backscattered within the sensing optical fibres;

an optical switch arranged to switch between the sensing optical fibres, so as to direct each generated probe light pulse to a currently selected one of the sensing optical fibres, and to direct backscattered probe light from a currently selected one of the sensing optical fibres to the detector, the sensor being configured to control the optical switch such that all of the sensing optical fibres can be used concurrently to detect acoustic vibration by controlling the switch to return to each of the sensing optical fibres at a frequency of at least 2 kHz; and

an analyser arranged to measure acoustic vibration, as a concurrent function of position along each of the sensing optical fibres, from the detected backscattered probe light.

2. The distributed optical fibre sensor of claim 1 further comprising a controller arranged to control the optical switch as to interleave the generated probe light pulses to, and the direction of backscattered light to the detector from, the plurality of sensing optical fibres, such that all of the sensing optical fibres can be used concurrently to detect acoustic vibration.

3. The distributed optical fibre sensor of claim 1 wherein each subsequent one of the generated probe light pulses is directed to a different selected one of the plurality of the sensing optical fibres to the previously generated probe light pulse.

4. The distributed optical fibre sensor of claim 1 wherein each sensing optical fibre is deployed, to measure acoustic vibration in an environment, within a separate fibre optic cable.

5. The distributed optical fibre sensor of claim 4 wherein each fibre optic cable is deployed, at least partly, along a different path through the environment than the other fibre optic cables, so as to measure acoustic vibration in different parts of the environment than each of the other fibre optic cables.

6. The distributed optical fibre sensor of claim 5 , wherein the majority of the length of each sensing optical fibre is spaced by at least one metre from each other sensing optical fibre.

7. The distributed optical fibre sensor of claim 5 , wherein the majority of the length of each sensing optical fibre from which an acoustic signal is detected is spaced by at least one metre from each other sensing optical fibre.

8. The distributed optical fibre sensor of claim 1 wherein the lengths of at least two of the sensing optical fibres are substantially different.

9. The distributed optical fibre sensor of claim 1 comprising at least three said sensing optical fibres.

10. The distributed optical fibre sensor of claim 1 configured to control the optical switch such that the duration of detection of backscatter of a probe light pulse received from a first of the sensing optical fibres is longer than the duration of detection of backscatter of a probe light pulse received from a second of the sensing optical fibres, and the analyser is correspondingly arranged to determine vibration along a greater length of the first of the sensing optical fibres than along the second of the sensing optical fibres.

11. The distributed optical fibre sensor of claim 10 configured such that a dwell time of the optical switch at the first of the sensing optical fibres is greater than a dwell time of the optical switch at the second of the sensing optical fibres.

12. The distributed optical fibre sensor of claim 1 configured to control the optical switch such that detection of backscatter from a second of the sensing optical fibres at least partly occurs between directing a probe light pulse to a first of the sensing optical fibres and at least partly detecting backscatter of the probe light pulse from the first sensing optical fibre.

13. The distributed optical fibre sensor of claim 1 configured such that the probe light pulses are directed sequentially to the sensing optical fibres such that a probe light pulse is directed to each one of the sensing optical fibres before a further probe light pulse is directed to any one of the sensing optical fibres.

14. The distributed optical fibre sensor of claim 1 configured such that following a probe light pulse being directed a first time to one of the sensing optical fibres, a probe light pulse is then directed a second time to the same sensing fibre before probe light pulses have been directed to all of the other sensing optical fibres.

15. The distributed optical fibre sensor of claim 14 whereby backscattered light is detected from one of the sensing optical fibres at least twice as often as from one of the other sensing optical fibres.

16. The distributed optical fibre sensor of claim 1 wherein probe light pulses are directed into at least two different ones of the sensing optical fibres at different repetition rates for each of the at least two different ones of the sensing optical fibres.

17. The distributed optical fibre sensor of claim 1 arranged to measure vibration as a concurrent function of position along each of the sensing optical fibres using a coherent Rayleigh profile of the backscattered probe light.

18. The distributed optical fibre sensor of claim 1 arranged for the optical switch to switch between the sensing optical fibres at a sufficient rate that acoustic signals of at least 5 kHz can be detected concurrently in each of the sensing optical fibres.

19. The distributed optical fibre sensor of claim 1 wherein the optical switch comprises a cascade of switch elements including a first layer switch element arranged to receive and direct probe light pulses to one or more further layers each of one or more switch elements, to thereby direct each probe light pulse to any one of the sensing optical fibres.

20. An optical sensor for measuring one or more parameters, at one or more positions along each of a plurality of optical fibres, from properties of probe light returned from said one or more positions, the sensor comprising:

a probe light source arranged to generate probe light pulses for transmission to the optical fibres;

a detector arranged to receive and detect the returned probe light received from the optical fibres;

an optical switch arranged to switch between the optical fibres, so as to direct each generated probe light pulse to a currently selected one of the optical fibres, and to direct returned probe light from a currently selected one of the optical fibres to the detector, the optical sensor being arranged to control the optical switch to return to each of the optical fibres at a frequency of at least 2 kHz; and

an analyser arranged to determine one or more parameters at said one or more positions from the detected returned probe light,

wherein the optical switch comprises a cascade of switch elements including a first layer switch element arranged to receive and direct probe light pulses to one or more further layers each of one or more switch elements, to thereby direct each probe light pulse to one of the optical fibres.

21. The optical sensor of claim 20 further comprising a delay element between the first layer switch element and at least one switch element of a further layer, the delay element being arranged to delay reflection of a probe light pulse from the switch element of the further layer back to the first layer switch element until the first layer switch element has completed transmission of the probe light pulse.

22. The optical sensor of claim 21 arranged to redirect the first layer switch element, to block transmission of the reflection, after completing transmission of the probe light pulse.

23. A method of measuring acoustic vibration as a concurrent function of position along each of a plurality of sensing optical fibres, from properties of probe light backscattered within the sensing optical fibres, comprising:

generating pulses of probe light for transmission to the sensing optical fibres;

detecting probe light backscattered within the sensing optical fibres;

switching between the sensing optical fibres, so as to direct each generated probe light pulse to a currently selected one of the sensing optical fibres, and to direct backscattered probe light from a currently selected one of the sensing optical fibres to the detector, such that all of the sensing optical fibres can be used concurrently to detect acoustic vibration, by switching to return to each of the sensing optical fibres at a rate of at least 2 kHz; and

measuring acoustic vibration, as a concurrent function of position along each of the sensing optical fibres, from the detected backscattered probe light.

24. The method of claim 23 further comprising deploying each sensing optical fibre, to measure acoustic vibration within an environment, housed within a separate fibre optic cable deployed at least partly along a different path through the environment to the other fibre optic cables.

25. The method of claim 23 comprising switching between the sensing optical fibres such that the duration of detection of backscatter of a probe light pulse received from a first of the sensing optical fibres is longer than the duration of detection of backscatter of a probe light pulse received from a second of the sensing optical fibres, and the analyser is correspondingly arranged to determine vibration along a longer portion of the first of the sensing optical fibres than along the second of the sensing optical fibres.

26. The method of claim 23 comprising:

controlling the optical switch such that detection of backscatter from at least a portion of a second of the sensing optical fibres occurs between directing a probe light pulse to a first of the sensing optical fibres and detecting backscatter of the probe light pulse from at least a portion of the first sensing optical fibre; and

determining vibration for the portions of the first and second sensing optical fibres from the detected backscatter.

27. The method of claim 23 comprising:

directing a probe light pulse a first time to one of the sensing optical fibres; and then

directing a probe light a second time to the same sensing fibre before directing probe light pulses to all of the other sensing optical fibres.

28. The method of claim 23 comprising:

using a cascade of optical switch elements to provide the optical switch;

providing at least one optical delay element between a first and second layer of the cascade to delay reflection of probe light pulse energy from the second to the first layers; and

redirecting the first layer of the cascade before arrival of reflection of probe light pulse energy from the second layer.

29. The method of claim 23 wherein the vibration signals are determined from coherent Rayleigh backscatter of the probe light pulses from the sensing optical fibres.

30. The distributed optical sensor of claim 19 further comprising a delay element between the first layer switch element and at least one switch element of a further layer, the delay element being arranged to delay reflection of a probe light pulse from the switch element of the further layer back to the first layer switch element until the first layer switch element has completed transmission of the probe light pulse.