Interference-Fading-Free <inline-formula> <tex-math notation="LaTeX">$\Phi$ </tex-math> </inline-formula>-OTDR Based on Differential Phase Shift Pulsing Technology

Abstract

In order to mitigate the interference fading problem in a phase-demodulated phase-sensitive optical time-domain reflectometer ( $\Phi $ -OTDR) based on heterodyne coherent detection, a novel scheme based on differential phase shift pulsing technology is proposed in this letter. A 0– $\pi $ binary phase shift process is applied on the second half of probe pulses trace by trace, and thus interference fading, which arises from spatial non-uniform distribution of the refractive index, becomes controllable. Phase will be demodulated from traces of no fading by an amplitude threshold. The theoretical analysis and experimental results show that the probability of interference fading can reduce greatly. Then, a phase stitching method is introduced to demodulate phase in both no fading areas, without sacrificing vibration response bandwidth. The results provide a simple and effective approach to an interference-fading-free phase-demodulated $\Phi $ -OTDR system.