Measurement of polarization fading sensitivity in FBGs-assisted Phase-OTDR

Distributed optical fiber vibration sensors (DOFSs) such as those based on phase-sensitive optical time-domain reflectometry (φ-OTDR) have contributed to improve structural health monitoring (SHM). φ-OTDR allows distributed vibration sensing by analysing the interference properties of the backscattered/reflected signal when an optical pulse is launched into the sensing fiber. As the Rayleigh backscattered light is relatively weak, fiber Bragg grating (FBG) arrays can be inscribed in the sensing fiber to increase the signal-to-noise ratio. However, the interference of the signals reflected by two consecutive FBGs (containing information about the locally applied vibration) is subject to a polarization fading effect as the sensing fiber presents some birefringence. Therefore the states of polarization (SOP) of the interfering reflected signals are no longer fully aligned. The present work proposes a measurement setup to quantify the polarization effects in a direct detection phase-OTDR scheme through the polarization fading sensitivity (PFS) parameter.