Compensation of laser source frequency drift in Φ-OTDR system using inter-pulse difference

The distributed acoustic sensing (DAS) system based on coherent optical time-domain reflectometry (COTDR) features many advantages, such as distributed sensing, low cost, wide dynamic range, and high spatial resolution. COTDR typically requires narrow-linewidth lasers to generate a pulsed probe. However, narrow-linewidth lasers exhibit frequency drift in the low-frequency range. This introduces slow phase drifts into the demodulation results, thereby degrading the system's detection performance in the sub-hertz frequency band. This paper proposes an inter-pulse differencing method based on multi-frequency for phase drift elimination. It employs two or more pulses with different central frequencies, which have a fixed delay time τ_d. There is a fixed delay time between their corresponding RBS. At the time of t, the RBS with different frequencies represent the information of different locations, while they beat with the same local light (LO). The beat signals include the same frequency drift and the different vibration information at different locations. By their spatial difference, the expected vibration information could be obtained, and the frequency drift could be suppressed. Experimental results show that noise near zero frequency is reduced by more than 20 dB, and the signal-to-noise ratio (SNR) of vibration signals at 1.0 Hz is improved by approximately 10 dB. A minimum of 0.1 Hz vibration signal waveform recovery is achieved, with its SNR remaining above 30 dB. The results indicate that the method exhibits high reliability.