Fading-free DAS with multifrequency pulses modulated via a fiber frequency-shifted loop

This paper proposes and demonstrates an arbitrarily tunable multi-frequency pulse modulation scheme based on an active fiber loop, which effectively suppresses the interference fading effect in distributed acoustic sensing (DAS) systems and significantly improves sensing fidelity. The proposed scheme innovatively employs an active fiber loop structure to achieve flexible multi-frequency pulse modulation. It integrates frequency-division multiplexing (FDM) to suppress interference fading in DAS systems effectively. In the experimental demonstration, up to 15 modulation frequencies were precisely generated simultaneously, with each frequency component exhibiting excellent stability (maximum amplitude fluctuation of only 1.401dB). In a long-distance sensing test over 74.5 km with a spatial resolution of 20m, fading-free detection was achieved using 13-frequency multiplexing, exhibiting an outstanding linear response to vibration amplitude. Compared to existing FDM schemes based on multiple acoustic-optic modulators, multi-laser multiplexing, or phase modulators, the proposed approach offers three key advantages: (i) an arbitrarily selectable number of multiplexed frequency components; (ii) significantly reduced the required modulation bandwidth and improved modulation efficiency; and (iii) a compact and reliable system architecture. This study provides an innovative and practical solution for fading-free long-range DAS systems.