Distributed fibre optic sensing for event detection

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.

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.

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.