Distributed fibre optic sensing for event detection
View Patent ↗A fiber optic sensing method and apparatus for determining location and direction information of disturbances occurring in the environment of a sensor optical fiber are provided. The method comprises launching optical pulses into at least one polarisation eigenmode of a polarisation maintaining fiber as the sensor optical fiber, detecting temporal speckle patterns of light backscattered from the at least one polarisation eigenmode of the fiber, comparing the temporal speckle patterns to determine the location and direction information of a disturbance in the environment of the sensor optical fiber. The location information may be a distance along the fiber, and the direction information may be a direction radially from the axis of the fiber. 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.
1. A distributed fibre optic sensing method for determining location and direction information of disturbances occurring in the environment of a sensing optical fibre, the method comprising:
launching optical pulses into first and second polarisation eigenmodes of polarisation maintaining sensing optical fibre;
detecting temporal speckle patterns of the optical pulses backscattered from the polarisation eigenmodes of the sensing optical fibre, said temporal speckle patterns being intensity variations resulting from self-interference of backscatter within the sensing optical fibre;
comparing the temporal speckle patterns detected from a first or second polarisation eigenmode to determine information of the location of a disturbance in the environment of the sensor optical fibre,
wherein the location information corresponds to a distance along the sensing optical fibre, and
the method further comprising comparing between changes in the temporal speckle patterns of the first and second polarisation eigenmodes to determine direction information of the disturbance, the direction information corresponding to a radial direction from the axis of the sensing optical fibre, and refractive index in the sensing fibre being changed in the vicinity of the disturbance.
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 said direction information is determined from a comparison of the temporal speckle patterns at a range specific timing for two polarisation eigenmodes.
4. The method of claim 1 , wherein the step of detecting comprises splitting the backscattered light into constituent polarisation eigenmodes and detecting the intensity of backscattered light in each eigenmode.
5. The method of claim 1 , wherein the optical pulse is polarised.
6. The method of claim 1 , 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.
7. The method of claim 1 , 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 polarisation maintaining optical fibre; and
determining an angular direction of the disturbance from the comparison for each eigenmode.
8. 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.
9. The method of claim 8 , further comprising calculating the location and direction of the disturbance about the fibre using the relationship determined in the step of calibrating.
10. The method of claim 8 , wherein the step of calibrating comprises initiating a disturbance event at a known position in the environment of the sensing 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.
11. The method of claim 1 , wherein the step of launching comprises launching optical pulses into two lengths of polarisation maintaining optical fibre arranged parallel to each other and having a fixed angular relationship between first polarisation eigenmodes of the polarization maintaining optical fibres along their length; and
the step of detecting comprises detecting a temporal speckle pattern of backscattered light from the two polarisation maintaining fibres.
12. 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 .
13. 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 .
14. A distributed fibre optic sensor apparatus for determining location and direction information on disturbances occurring in the environment of a sensing optical fibre, the apparatus comprising:
the sensing optical fibre comprising a length of polarisation maintaining optical fibre, having first and second polarization eigenmodes;
a light source arranged to launch optical pulses into first and second polarisation eigenmodes of the sensing optical fibre;
a photodetector for detecting temporal speckle patterns of the optical pulses backscattered from first and second polarisation eigenmodes of the length of optical fibre, said temporal speckle patterns being intensity variations resulting from self-interference of backscatter within the sensing optical fibre;
an analyser arranged to compare said temporal speckle patterns detected from a first or second polarisation eigenmode to determine information of the location of a disturbance in the environment of the sensing optical fibre,
wherein the location information corresponds to a distance along the sensing optical fibre, and the analyser further arranged to compare between changes in the temporal speckle patterns of the first and second polarisation eigenmodes to determine direction information of the disturbance, the direction information corresponding to a radial direction from the axis of the sensing optical fibre, and refractive index in the sensing fibre being changed in the vicinity of the disturbance.
15. The apparatus of claim 14 , wherein the analyser is configured to determine location information of the disturbance from a range dependent timing of changes in the temporal speckle patterns.
16. The apparatus of claim 14 , 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.
17. The apparatus of claim 14 , 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.
18. The apparatus of claim 14 , wherein the light source is configured to provide polarised optical pulses.
19. The apparatus of claim 14 , wherein the light source is arranged to launch the optical pulse at substantially 45° to the polarisation eigenmodes of the polarisation maintaining optical fibre such that the amplitudes launched into the two eigenmodes are substantially equal.
20. The apparatus of claim 14 , 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 polarisation maintaining optical 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 polarisation maintaining optical fibre; and
determining the angular direction of the disturbance from the comparison for each eigenmode.
21. The apparatus of claim 14 , 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.
22. The apparatus of claim 21 , wherein the analyser is arranged to calculate location and direction information of the disturbance using the calibration information.
23. The apparatus of claim 14 , arranged for launching optical pulses into two lengths of polarisation maintaining optical fibre arranged parallel to each other and having a fixed angular relationship between first polarisation eigenmodes of the polarisation maintaining optical 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 optical fibres.
24. Apparatus for determining location and direction information of disturbances occurring in the environment of a boundary, the apparatus comprising the apparatus of claim 14 , wherein the polarisation maintaining optical fibre is provided along the boundary.
25. 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 14 , wherein the polarisation maintaining optical fibre is provided in or adjacent to the pipeline, pipe, well, or hole.
26. The apparatus of claim 14 , wherein the environment being measured is adjacent to the sensing optical fibre.
27. The apparatus of claim 14 , wherein the refractive index in the sensing fibre is changed in the vicinity of the disturbance.
28. A distributed fibre optic sensor apparatus for determining location and direction information on disturbances occurring in the environment of a sensing optical fibre, the apparatus comprising:
first and second sensing optical fibres, each comprising a length of polarisation maintaining optical fibre;
a light source arranged to launch optical pulses into at least one polarisation eigenmode of each of the first and second sensing optical fibres, the at least one polarisation eigenmode of the sensing optical fibres in which light is launched oriented at an angle to each other;
a photodetector for detecting temporal speckle patterns of the optical pulses backscattered from the polarisation eigenmodes of the sensing optical fibres; said temporal speckle patterns being intensity variations resulting from self-interference of backscatter within the sensing optical fibres;
an analyser arranged to compare between said temporal speckle patterns detected from a polarisation eigenmode of the first or second sensing optical fibre to determine information of the location of a disturbance in the environment of the sensing optical fibres, wherein the location information corresponds to a distance along the first or second sensing optical fibre, and the analyser further arranged to compare between changes in the temporal speckle patterns of the first and second sensing optical fibres to determine direction information, the direction information corresponding to a radial direction from the axes of the sensing optical fibres, and the refractive index in the sensing optical fibres being changed in the vicinity of the disturbance.
29. A distributed fibre optic sensing method for determining location and direction information on disturbances occurring in the environment of a sensing optical fibre, the method comprising:
launching optical pulses into first and second sensing optical fibres, each comprising a length of polarisation maintaining optical fibre and the optical pulses being launched into at least one polarisation eigenmode of each of the first and second sensing optical fibres, the at least one polarisation eigenmode of the sensing optical fibres in which light is launched oriented at an angle to each other;
detecting temporal speckle patterns of the optical pulses backscattered from the polarisation eigenmodes of the sensing optical fibres, said temporal speckle patterns being intensity variations resulting from self-interference of backscatter within the sensing optical fibres;
comparing the temporal speckle patterns detected from a polarisation eigenmode of the first or second sensing optical fibre to determine information of the location of a disturbance in the environment of the sensing optical fibres,
wherein the location information corresponds to a distance along the first or second sensing optical fibre, and
the method further comprising comparing between changes in the temporal speckle patterns of the first and second sensing optical fibres to determine direction information, the direction information corresponding to a radial direction from the axes of the sensing optical fibres, and refractive index in the sensing optical fibres being changed in the vicinity of the disturbance.