Patent ID: 8452135

Claim:
A method of measuring fiber twist in a multi-core optical fiber comprising a center core and a plurality of peripheral cores at a distance radially from said center core and running parallel to said center core, said center core comprising at least one fiber Bragg grating with polarization-dependent reflectivity at a specified distance from a launch end of the multi-core fiber, said multi-core optical fiber being operably located to receive launched light, the launched light comprising a state of polarization (“SOP”) and a polarization axis, the fiber Bragg grating comprising a fast birefringent axis and a slow birefringent axis, said method comprising: calibrating intrinsic fiber birefringence in the center core and birefringence produced in the center core by bending and twisting; varying the SOP of the launched light while measuring the wavelength of light reflected from an FBG until a maximum polarization-dependent reflectivity wavelength of the FBG is detected; measuring the SOP of the launched light when a maximum polarization-dependent reflectivity wavelength of the FBG is detected, the launched light entering the multi-core fiber becoming propagated light, wherein while said launched light has said SOP, said propagated light reaching the FBG is linearly polarized thereby having a polarization axis, and the polarization axis of the propagated light at the FBG is aligned with the slow birefringent axis of the FBG; determining a local bending of the multi-core optical fiber based on strain sensed in the plurality of peripheral cores; calculating a bend-induced birefringence produced by the determined bending of the multi-core optical fiber; calculating a derived SOP at the FBG based on the SOP of the propagated light, which evolves over the specified distance in a manner determined by birefringence produced by the determined local bending, the calibrated intrinsic fiber birefringence, and a candidate level of fiber twist; and adjusting the candidate level of fiber twist until the derived SOP at the FBG is linear and aligned with the FBG rotated by the candidate level of fiber twist to derive an estimated level of fiber twist.