On the Signal Distortion in a Sampling-Rate Enhanced DAS System With Mode Division Multiplexing

In this paper, signal distortion in a sampling-rate enhanced distributed acoustic sensing (DAS) system with mode division multiplexing is studied. Infidelity effect of the phase response of a Rayleigh backscattered signal (RBS) to an external perturbation is theoretically analyzed, and the expression of the reconstructed signals in the sampling-rate enhanced system is rigorously derived. The expression reveals that differences in the direct-current (DC) offsets of the phase responses contribute to the spurious signals at frequencies associated with the repetition rate of the probe pulses, while differences in the alternating-current (AC) amplitudes lead to the spurious signals at frequencies associated with both the pulse repetition rate and external perturbations. Based on theoretical analysis, a novel method named Active Offset‑Amplitude Correction (AOAC) is proposed to suppress the spurious signals. It eliminates the offsets and compensates the amplitudes of the phase responses retrieved from the RBSs of different optical modes before phase splicing. The validity of the proposed method is confirmed through high-fidelity detection of diverse perturbations and a maximum suppression of spurious signal by 38 dB in both simulations and experiments. Benefiting from the proposed method, a high-performance DAS system is implemented along a 497 km fiber link, featuring a high frequency response of 240 Hz and a low noise level of -40 dB at that frequency. This study is crucial for simultaneously achieving long-distance, broad-bandwidth, and high-fidelity sensing in DAS systems.