Compensating for influence of laser-frequency-drift in phase-sensitive OTDR with twice differential method

The frequency-drift of laser source is a crucial factor for influencing the performance of the phase-sensitive optical time domain reflectometer (Φ-OTDR). It induces signal fluctuation and severely limits the measurement capacity for low frequency. In this paper, a twice differential method is proposed to compensate for the influence of the laser-frequency-drift in Φ-OTDR. It uses the differential signal between two points on the sensing fiber as a reference signal and then subtracts the reference signal from the main signal to obtain the final result. In the experiment, the signal fluctuation induced by laser-frequency-drift is decreased by more than 95%. A vibration with a 0.1 Hz frequency on a 6 km sensing fiber is detected with 10 m spatial resolution and sensitivity is estimated to be 5.9 nε. With this method we also successfully measured the process of a stepper motor stretching a fiber section. This method will expand the scope of application of Φ-OTDR in the fields, which require high sensitivity and low frequency response.