Distributed Fibre Optic Sensing for Event Detection

• US 20110216996A1
• Filed: 11/12/2009
• Published: 09/08/2011
• Est. Priority Date: 11/12/2008
• Status: Active Grant

• Current Assignee: Viavi Solutions, Inc. (Lumentum Holdings Inc.)
• Original Assignee: Fotech Solutions Limited
• Inventors: Rogers, Alan John
• Granted Patent: US 9,002,149 B2
• Time in Patent Office: Days
• US Class Current: 385/12
• CPC Class Codes: G01D 5/353 influencing the transmission properties of an optical fibre
• CPC Class Codes: G01D 5/35358 using backscattering to detect the measured quantity
• CPC Class Codes: G01M 11/3109 Reflectometers detecting the back-scattered light in the time-domain, e.g. OTDR
• CPC Class Codes: G01M 11/3181 Reflectometers dealing with polarisation

Abstract

A fibre optic sensing method and apparatus for determining location and direction information of disturbances occurring in the environment of a sensor optical fibre are provided. The method comprises launching optical pulses into at least one polarisation eigenmode of a polarisation maintaining fibre as the sensor optical fibre, detecting temporal speckle patterns of light backscattered from the at least one polarisation eigenmode of the fibre, comparing the temporal speckle patterns to determine the location and direction information of a disturbance in the environment of the sensor optical fibre. The location information may be a distance along the fibre, and the direction information may be a direction radially from the axis of the fibre. The apparatus or instrument may be used to detect disturbance over long distances such as pipes, pipelines, or wells. Other applications include detecting intruders entering a controlled area.

Claims

1. A fibre optic sensing method for determining location and direction information of disturbances occurring in the environment of a sensor optical fibre, the method comprising:

launching optical pulses into at least one polarisation eigenmode of a polarisation maintaining sensor optical fibre;

detecting temporal speckle patterns of light backscattered from the at least one polarisation eigenmode of the fibre;

comparing the temporal speckle patterns to determine the location and direction information of a disturbance in the environment of the sensor optical fibre.

2. The method of claim 1, wherein the location information of the disturbance is determined from a range dependent timing of changes in the temporal speckle patterns.

3. The method of claim 1, wherein the direction information is determined by comparing between one or more temporal speckle patterns detected from each polarisation eigenmode.

4. The method of claim 1, wherein the direction information is a radial direction from the axis of the fibre.

5. The method of claim 1, wherein said direction information is determined from a comparison of the temporal speckle patterns at a range specific timing for two polarisation eigenmodes.

6. The method of claim 1, wherein the step of launching comprises launching optical pulses into two polarisation eigenmodes of a length of polarisation maintaining fibre;

and the step of detecting comprises detecting temporal speckle patterns of backscattered light from the two eigenmodes.

7. The method of claim 6, wherein the step of detecting comprises splitting the backscattered light into constituent polarisation eigenmodes and detecting the intensity of backscattered light in each eigenmode.

8. The method of claim 6, wherein the optical pulse is polarised.

9. The method of claim 6, wherein the optical pulse is launched at substantially 45°

to the polarisation eigenmodes of the polarisation maintaining fibre such that the amplitudes launched into the two eigenmodes are substantially equal.

10. The method of claim 6, wherein the step of comparing comprises:

comparing the temporal speckle pattern of backscattered light from a first eigenmode component of a first pulse with the temporal speckle pattern of backscattered light from a first eigenmode component of a second pulse to determine a first change in refractive index of the fibre at a location along the length of the fibre;

comparing the temporal speckle pattern of backscattered light from a second eigenmode component of the first pulse with the temporal speckle pattern of backscattered light from a second eigenmode component of a second pulse to determine a second change in refractive index at a location along the fibre; and

determining a radial direction of the disturbance from the comparison for each eigenmode.

11. The method of claim 1, further comprising calibrating the relationship between changes in the speckle pattern for the polarisation eigenmodes and location and direction information.

12. The method of claim 11, further comprising calculating the location and direction of the disturbance about the fibre using the relationship determined in the step of calibrating.

13. The method of claim 11, wherein the step of calibrating comprises initiating a disturbance event at a known position in the environment of the sensor optical fibre, the step of detecting comprises detecting temporal speckle patterns of light backscattered from the polarisation eigenmodes, and further comprising determining a relationship between the known position and changes in the detected temporal speckle patterns.

14. The method of claim 1, wherein the step of launching comprises launching optical pulses into two lengths of polarisation maintaining fibre arranged parallel to each other and having a fixed angular relationship between first polarisation eigenmodes of the fibres along their length;

and

the step of detecting comprises detecting a temporal speckle pattern of backscattered light from the two polarisation maintaining fibres.

15. A method of determining location and direction information of disturbances occurring in the environment of a boundary, the method comprising providing a polarisation maintaining fibre along the boundary and performing the method of claim 1.

16. A method of determining location and direction information of disturbances occurring in the environment of a pipeline, pipe, well, or hole, the method comprising providing a polarisation maintaining fibre in or adjacent to the pipeline, pipe, well, or hole and performing the method of claim 1.

17. The use of a polarisation maintaining single mode optical fibre in determining external disturbance directionality from partially coherent optical pulses backscattered to a detector.

18. An apparatus for determining location and direction information on disturbances occurring in the environment of a sensor optical fibre, the apparatus comprising:

a sensor optical fibre comprising a length of polarisation maintaining optical fibre having two polarization eigenmodes;

a light source arranged to launch optical pulses into at least one polarisation eigenmode of the sensor fibre;

a photodetector for detecting temporal speckle patterns of the optical pulses backscattered from at least one polarisation eigenmode of the length of fibre;

an analyser arranged to compare said temporal speckle patterns to determine information of the location and direction of a disturbance in the environment of the sensor fibre.

19. The apparatus of claim 18, wherein the analyser is configured to determine location information of the disturbance from a range dependent timing of changes in the temporal speckle patterns.

20. The apparatus of claim 18, wherein the analyser is configured to determine location information by comparing between one or more temporal speckle patterns detected from each polarisation eigenmode.

21. The apparatus of claim 18, wherein the direction information is a radial direction from the axis of the fibre.

22. The apparatus of claim 18, wherein the analyser is arranged to determine said direction information from a comparison of the temporal speckle patterns at a range specific timing for two polarisation eigenmodes.

23. The apparatus of claim 18, wherein the light source is arranged to launch an optical pulse into two polarisation eigenmodes of the length of polarisation maintaining fibre;

and

the photodetector is arranged to detect temporal speckle patterns of backscattered light from the two eigenmodes

24. The apparatus of claim 23, further comprising:

a polarisation splitter to split the backscattered light into constituent polarisation eigenmodes; and

the photodetector comprises two photosensitive elements, each arranged to detect the amplitude of backscattered light in each eigenmode.

25. The apparatus of claim 23, wherein the optical source is configured to provide polarised optical pulses.

26. The apparatus of claim 23, wherein the optical source is arranged to launch the optical pulse at substantially 45°

to the polarisation eigenmodes of the polarisation maintaining fibre such that the amplitudes launched into the two eigenmodes are substantially equal.

27. The apparatus of claim 23, wherein the analyser is arranged to perform the steps of:

comparing the temporal speckle pattern of backscattered light from a first eigenmode component of a first pulse with the temporal speckle pattern of backscattered light from a first eigenmode component of a second pulse to determine a first change in refractive index of the fibre at a location along the length of the fibre;

comparing the temporal speckle pattern of backscattered light from a second eigenmode component of the first pulse with the temporal speckle pattern of backscattered light from a second eigenmode component of a second pulse to determine a second change in refractive index at a location along the fibre; and

determining the angular direction of the disturbance from the comparison for each eigenmode.

28. The apparatus of claim 18, wherein the analyser comprises a memory for storing calibration information relating changes in the speckle pattern for the polarisation eigenmodes to location and direction information.

29. The apparatus of claim 28, wherein the analyser is arranged to calculate location and direction information of the disturbance using the calibration information.

30. The apparatus of claim 18, arranged for launching optical pulses into two lengths of polarisation maintaining fibre arranged parallel to each other and having a fixed angular relationship between first polarisation eigenmodes of the fibre along their length;

and

the photodetector comprises two photosensitive elements each arranged to detect a temporal speckle pattern of backscattered light from one of the two polarisation maintaining fibres.

31. Apparatus for determining location and direction information of disturbances occurring in the environment of a boundary, the apparatus comprising the apparatus of claim 18, wherein the polarisation maintaining fibre is provided along the boundary.

32. Apparatus for determining location and direction information of disturbances occurring in the environment of a pipeline, pipe, well, or hole, the apparatus comprising the apparatus of claim 18, wherein the polarisation maintaining fibre is provided in or adjacent to the pipeline, pipe, well, or hole.