Ultralow-Frequency Vibration Sensing in Phase-Sensitive OTDR Using Multiscale VMD

Laser frequency drift (LFD) restricts the low-frequency vibration sensing ability of phase-sensitive optical time-domain reflectometer ( $\Phi $ -OTDR). In this study, we propose a self-diagnosis structure combining Mach–Zehnder interference and $\Phi $ -OTDR is first designed to realize LFD measurement and vibration sensing simultaneously. A multiscale variational mode decomposition (VMD) with the judgment at different amplitude scales, time scales, and modal scales is proposed to suppress the influence of LFD at low-frequency vibration demodulation. Experimental results show a long-time continuous LFD measurement and vibration sensing along the sensing fiber of 10040 m. After the parameter optimization of iteration and mode decomposition with multiscale VMD, infrasonic and sub-Hertz vibration signals can be detected with improved SNR. The ultralow-frequency vibration signal as low as 0.04 Hz is also demodulated under a long-time continuous detection of more than 40 min. The fitting coefficient between the recovered vibration frequency and the actual vibration frequency is 0.99966. This self-diagnostic structure, based on multiscale VMD, effectively suppresses the influence of LFD on $\Phi $ -OTDR and reliably recovers ultralow-frequency vibrations during long-term detection.