Mitigating the impact of laser phase noise and frequency drift in Φ-OTDR with delay-beat differential phase demodulation

The demodulated phase in heterodyne coherent phase-sensitive optical time-domain reflectometry (Φ-OTDR) is mainly affected by interference fading and laser phase noise. Generally, the impact of interference fading can be easily suppressed by fading suppression algorithm, but the impact of laser phase noise and frequency drift is challenging to mitigate. A delay-beat differential phase demodulation method is proposed to mitigate the impact of laser phase noise and frequency drift on phase demodulation in delay-beat coherent Φ-OTDR. The Rayleigh backscattered light is split into two beams with a time delay between them, and then beaten with two local light beams with the same optical path. The phase difference of the two beat signals is used instead of the traditional range-differential phase to extract the disturbance information within the differential interval. Since the local lights originate at the same moment, the demodulated phase difference can avoid the impact of laser phase noise and frequency drift induced by the local light delay. In the experiment, the proposed method greatly improves the accuracy and stability of phase demodulation. Compared with the traditional demodulation method, demodulated phase noise floor is reduced by 11.37 dB, and the phase drift is reduced by 12.69 dB.