Dual-pulse phase-OTDR-based distributed optic-fiber acoustic sensor with active laser frequency compensation.

We report a novel, to the best of our knowledge, distributed optic-fiber acoustic sensor in which active laser frequency compensation is proposed to improve the performance of an in-balanced Mach-Zehnder interferometer (IMZI)-based dual-pulse phase-sensitive optical time-domain reflectometry (ø-OTDR). By reversely monitoring the interferometric signal from the IMZI, the laser frequency of the sensor system is actively adjusted to compensate both the interferometer arm length jitter and the laser frequency drift itself. Long-term stable interferometric signal is demonstrated, together with 60% reduction in its short-term noise. Distributed fiber sensing performance is evaluated for this frequency actively compensated sensor system, showing significant improvement in both long-term stability and signal-to-noise ratio (SNR). Compared with the system of a free-running laser, the proposed system eliminates the waveform distortion and enables a 50% noise fluctuation reduction in the retrieved waveform, showing its high signal fidelity and facilitating complex waveform reconstruction.