Identification and observation of the phase fading effect in phase-sensitive OTDR

We report a numerical analysis and direct measurement of the phase fading effect in phase-sensitive OTDR (Φ-OTDR) acoustic sensors. By demodulating the acoustic-induced phase information using a dual-pulse heterodyne Φ-OTDR, we show that the amplitude of the phase signal appears with ripple-like fluctuations over the fiber distance, even though the applied phase modulation has a constant amplitude. A generalized interference model for a Φ-OTDR acoustic sensor is presented, which identifies that this phase fading effect is mostly contributed by the random phase retardant (rather than the scattering amplitude) introduced through the distributed Rayleigh scattering process. Our analysis provides insight on the potential suppression of the phase fading noise in a phase-retrieved Φ-OTDR system.