Mitigation of Polarization Fading in Phase OTDR by Sequential Dual Polarization Reception

Abstract

Phase-sensitive Optical Time-Domain Reflectometry (phi-OTDR) is inherently limited by polarization-induced fading caused by random state-of-polarization evolution of Rayleigh backscattered fields in optical fibers. These fluctuations lead to deep signal attenuation and degraded phase estimation in coherent detection systems. Conventional mitigation approaches rely on polarization-diversity receivers employing parallel coherent detection channels, which increases system complexity and cost. This paper presents a coherent phi-OTDR architecture that suppresses polarization fading using a single coherent receiver. The proposed method is based on the sequential acquisition of orthogonal polarization components of the backscattered field. Identical probe pulses are launched into the sensing fiber, while the returned signal is split into two orthogonal polarization states and time-multiplexed into a single coherent detection chain. The complex responses corresponding to both polarization states are subsequently combined using Maximum-Ratio Combining (MRC), enabling statistically efficient phase reconstruction without parallel receiver hardware. Experimental results demonstrate effective fading suppression and robust vibration detection. Compared to selecting only the stronger polarization channel, the proposed MRC-based approach provides an improvement in the Signal-to-Noise Ratio (SNR) of approximately 3 dB. The architecture preserves polarization diversity gain while significantly reducing receiver complexity, offering a cost-efficient solution for polarization-independent distributed acoustic sensing.