Laser-Frequency-Drift Compensation in Φ-OTDR Using Linear Fitting Intercepts Method

With many significant advantages, the phase-sensitive optical time-domain reflectometry (Φ-OTDR) system has become a popular monitoring instrument, especially in geophysical research, such as seismic wave observation. In some scenarios, low-frequency vibration detection provides important clues for analyzing changes in target behavior. However, the ever-present frequency drift in lasers poses a challenge for sub-hertz vibration detection. In this paper, we propose a linear fitting method for taking intercepts to reduce the influence of laser frequency drift in heterodyne Φ-OTDR systems. Experimental results show that the method can effectively improve the signal-to-noise ratio (SNR) of 0.5 Hz sinusoidal signals by 23.909 dB over a 5 km long sensing fiber. Additionally, the variance of the compensation results at 40 different positions is 0.666 (dB)², which is 15.6 times better than the variance of 10.397 (dB)² obtained using the conventional compensation method under the same conditions, indicating good stability in compensation at different positions. Using this method, we also succeeded in improving the SNR of the perturbed signals of 0.1 Hz and 0.05 Hz by 8.575 dB and 6.056 dB, respectively, at 2 km, effectively reducing the distortion of the original demodulation curves. Benefiting from this, the proposed method can further improve the low-frequency detection performance of the Φ-OTDR system.