Three-Layer Structure Multiplexing Fading Elimination Method in Long-Haul Φ-OTDR

Phase fading is a critical issue that hinders phase retrieval in the phase-sensitive optical time-domain reflectometry (ϕ-OTDR). This paper introduces a novel approach for mitigating fading effects by implementing a three-layer structure multiplexing (TLSM) technique. The TLSM method is based on multiple pulses detection, trace decomposition and spatial multiplexing to create independent sub-components, which are then re-combined by rotated vector summation (RVS) method. Additionally, a Kalman filter is employed to eliminate the phase mismatching among these sub-components. Experimental results demonstrate a fading-free interrogation in a 40-km fiber sensing range and distorted vibration signal can be corrected. The demodulated amplitude trough could be improved by 35.2 dB. Large-sample testing confirms no occurrence of fading throughout the entire 40 km sensing range, indicating the high reliability of this method.