Using an Auxiliary Mach–Zehnder Interferometer to Compensate for the Influence of Laser-Frequency-Drift in Φ-OTDR

In phase-sensitive optical time-domain reflectometry (Φ-OTDR), the frequency drift of the laser source induces the fluctuation of signal, which limits the measurement capacity of Φ-OTDR for low frequency severely. In this paper, we propose a method to compensate for the influence of the frequency drift by using an auxiliary Mach-Zehnder interferometer (MZI). In this method, the signal of the auxiliary MZI is received to monitor the frequency drift continuously, and then, to correct the signal obtained from the sensing path. In the experiment, a vibration with a 0.1-Hz frequency on a 6-km sensing fiber is detected with a 10-m spatial resolution and the sensitivity is estimated to be 5.9 nε. This method will expand the scope of application of Φ-OTDR in the fields that require high sensitivity and low frequency response.