Fading-free and bandwidth-enhanced TGD-OFDR system with a resource-efficient fiber frequency-shifting loop

This paper proposes a time-gated distributed optical frequency domain reflectometry (TGD-OFDR) system based on a fiber frequency-shifting loop (FFSL) to address the critical challenges of signal fading and limited response bandwidth (RBW) in distributed acoustic sensing (DAS). The core innovation leverages a polarization-maintaining FFSL to generate broadband frequency-encoded pulse sequences from a single initial pulse through cyclic modulation, significantly relaxing the requirements for high-speed modulation hardware and system cost by using a conventional acoustic-optic modulator (AOM). The AOM is further utilized to pre-correct the pulse train, suppressing transient effects from optical amplifiers and enhancing system stability. A polarization-frequency joint diversity scheme is used to mitigate interference and polarization fading simultaneously. Experimental results over 11.5 km of sensing fiber demonstrate a tenfold enhancement in RBW (up to 43.5 kHz vibration detection), successful demodulation of broadband chirped signals, excellent long-term stability, high sensitivity down to 0.1 Hz, and a highly linear response. This work presents a resource-efficient and scalable solution for fading-free, bandwidth-enhanced DAS.