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Research on composite interferometric distributed optical fiber vibration sensing technology

In this paper, the composite interferometric distributed optical fiber vibration sensing technology based on a two-path unbalanced Mach-Zehnder/Sagnac linear interferometric structure is proposed. Firstly, the laser interference mechanism and vibration positioning principle of this system are theoretically analyzed, and the vibration positioning algorithm with the time-delay estimation based on the improved generalized cross-correlation principle is proposed. Secondly, the experimental system is built and double photodetectors are used to obtain the two-path interferometric signal with a certain time delay, and the high-precision time delay signal is obtained with the help of conjugate multiplication and digital filtering algorithm. The experimental results show that the system can accurately locate different types of vibration signals with a 65 km sensing fiber, and the location limitation of broadband vibration signals for a single-channel interferometric structure can be resolved. This system is expected to be popularized and applied in fields such as leakage monitoring of coalbed methane pipelines.