Long-Range Distributed Vibration Sensing Based on Phase Extraction From Phase-Sensitive OTDR

Distributed fiber-optic vibration sensors based on phase-sensitive optical time-domain reflectometry ( $\phi$-OTDR) have found many applications in various fields. In this paper, we analyze the phase noise of $\phi$-OTDR, which is the main limiting factor of the measurement range. We found that the laser phase noise and phase extraction error caused by the intensity noise in photodetection contribute to the total phase noise. By introducing a series of auxiliary weak reflection points along the fiber, we develop a phase-noise-compensated $\phi$-OTDR and realize a long-range distributed vibration sensing based on the phase extraction. Furthermore, a statistical analysis was proposed to maintain the vibration measurement sensitivity along the whole fiber. In the experiment, vibrations at 30 km were measured with a linear response, which confirmed the validity of our proposed system.