Improving Distributed Acoustic Sensing Optical Phase Measurement by Local Oscillator Closed-Loop Frequency Tuning

Distributed acoustic sensing systems based on phase-sensitive optical time-domain reflectometry have been used in several applications, especially in oil and gas fields and seismic applications, to continuously monitor dynamic strain and temperature variations. The magnitude of these events can be retrieved from the phase changes of the backscattered beam through in-phase and quadrature demodulation. Systems based on heterodyne coherent detection generally perform demodulation in the digital domain because of the greater ease of implementation and the lack of need for changes in the optoelectronic system. However, fluctuations in frequency shift induced by the acoustical-optical modulator can introduce errors if the carrier of the sine and cosine reference signals is assumed to be static. These errors may result in spectral leakage, which degrades the signal-to-noise ratio of the recovered phase signals. In this letter, we propose a method to track the most significant frequency of each backscattered trace using the power spectral density to tune the local oscillator in a closed loop of in-phase and quadrature demodulation. This method ensures that the local oscillator coincides with the actual carrier of each backscatter measured by the system. The proposed method shows significant potential for enhancing the signal-to-noise ratio of phase signals, achieving a 13.12 dB improvement in optical phase recovery and enabling effective aliasing correction for the case analyzed in this study.