Optical time-domain reflectometry based on a Brillouin dynamic grating in an elliptical-core two-mode fiber.

Optical time-domain reflectometry based on a Brillouin dynamic grating (BDG-OTDR) in an elliptical-core two-mode fiber (e-core TMF) is proposed and experimentally demonstrated for the discriminative measurement of strain and temperature distributions. Acoustic gratings are generated by the spontaneous Brillouin scattering of a pump pulse, which is used to reflect a probe pulse. Two orthogonal polarizations of the LP01 mode are sequentially used as the pump, while single polarization of the LP11 mode is used as the probe for intermodal BDG operation. Distribution maps of intermodal and polarization birefringence are acquired by analyzing the two BDG spectra, which are used to separately measure the temperature and strain distributions. In experiments, distributed measurement of intermodal BDG spectra along a 95 m e-core TMF is performed with a 2 m spatial resolution, where the strain and temperature coefficients of the BDG spectral shift are measured to be -0.085  MHz/με, +7.6  MHz/°C for the pump-probe of the LP01y-LP11x mode, and -0.093  MHz/με, +4.3  MHz/°C for the LP01x-LP11x mode, respectively. Distribution maps of the strain and temperature variation along the TMF are separately obtained by matrix calculation using the four coefficients with an error of ±105  με in strain and ±1.6°C in temperature.