Frequency multiplexed dual-pulse fiber-optic Ф-OTDR for fading-free distributed fiber sound sensor

• Yiluo Jiang, Xilun Gao, Wenping He, and Yonghang Shen
• received 03/01/2024; accepted 05/01/2024; posted 05/02/2024; Doc. ID 522731
• Abstract: Distributed fiber-optic sensor utilizing optical interference signal detection of the Rayleigh backscattering (RBS) lightalong the fiber under test (FUT) has been greatly progressed over the past twenty years. It can now realize real-time soundsignal recovery along the FUT with good fidelity and high sensitivity. However, the performance of the practicaldistributed fiber-optic sound sensor is still limited by the fading problem, as invalid spatial channels at unknownpositions arise due to the low signal intensity at these positions and the resulted near-zero signal-to-noise ratio (SNR). Inthis paper, we propose a novel fading-free fiber sensor structure by incorporating an acousto-optic frequency shift (AOFS)loop into the light source in a dual-pulse phase optical time domain reflectometry (Ф-OTDR). As a result of stablefrequency shifting and feedback within the loop, the loop source can output high-repetition-rate narrow pulse trainconsisting of multiple separated frequencies. This pulse train is further shaped into two pulse bursts by an acousto-opticmodulator (AOM) and an in-balanced fiber-optic Mach-Zehnder interferometer (MZI). The pulse bursts are then sent intothe sensing fiber while the RBS light is detected using an Avalanche photo-diode after an erbium doped fiber amplifier.Experimental results show that the proposed structure is capable of vibration signal recovery with quite good SNR alongall the FUT. Fading free performance is evaluated under different AOFS loop gains, showing significant potential inpractical fiber sensor network application.