Real-time distributed acoustic sensing based on phase-sensitive optical time-domain reflectometer with fading suppression

Distributed acoustic sensing (DAS) based on a phase-sensitive optical time-domain reflectometer (Φ-OTDR) system has been widely investigated, but it is still limited by reliability and real-time issues due to signal fading and large amounts of data. A phase modulation scheme is applied to Φ-OTDR to generate sensing probes with multiple frequency components, using optimal modulation depth and frequency determined by analyzing the power ratios and correlations among these components. These signals propagate through the fiber with independent intensity fluctuations but carry the same external disturbance information. Synthesizing these signals effectively suppresses signal fading by reducing intensity fluctuations, leveraging the uncorrelated nature of fading across different components. Experimental results show that the minimum intensity of the synthesized signal is enhanced by 2.7 dB above the noise floor, and the signal fading probability is significantly reduced from 2.1577% to 0.0426%. To further enhance performance, a field-programmable gate array-based signal processing system is employed, achieving real-time DAS with high-precision phase demodulation within a 50-km range. The system is not only capable of recovering vibration signals of different frequencies in the laboratory environment but has also successfully completed vibration signal reduction experiments at several locations in the field. In addition, through the 4-day stability test, the system’s fading probability only fluctuates between 0.01% and 0.1%, and the daily average fading probability remains stable. These results fully validate the effectiveness of the proposed method in accurately recovering vibration disturbances.