Active Compensation Method for Light Source Frequency Drifting in $\Phi $ -OTDR Sensing System

Phase-sensitive optical time-domain reflectometry ( $\Phi $ -OTDR) has the ability to make a measurement on external disturbance dynamically along the sensing fiber by monitoring the interference fading pattern variance of the Rayleigh backscattering waveform through continuous probe pulse injection. However, even if there is no disturbance, the received $\Phi $ -OTDR trace will still show slow but noteworthy distortion due to light source frequency drifting (LSFD). This trace-to-trace slow distortion makes it challenging to capture low-frequency event. In this letter, an active compensation method based on laser frequency sweep and cross-correlation calculation has been proposed to suppress the influence of LSFD. With the proposed active compensation method, the tracking of LSFD can be realized, and the trace-to-trace distortion can be greatly suppressed. The experimental result has shown that a vibration event of 0.5 Hz has been identified successfully under mean frequency drifting speed of 1.68 MHz/s, which extends the application realm of the $\Phi $ -OTDR sensing system to quasi-static measurement.