A distributed optical fiber system used to detect pipeline leakage and lawless excavation is put forward in this paper. This system is based on Mach-Zehnder optical fiber interferometer theory, which uses three monomode fibers in one optical fiber cable to compose two Mach-Zehnder interferometers. Vibrations from leakage point and lawless excavation along the pipeline can be acquired by the optical fibers, so the vibrations occurred on the pipeline can be detected in real time. In this paper, the principle and the system construction are introduced, and the way of the fiber cable to influence the sensitivity is studied. And also, the polarization on the optical path is studied in this paper, and a new technology to eliminate “Polarization Debilitating” is put forward. With principle analysis and experimental results, it is demonstrated that the detection system’s measuring sensitivity and location accuracy for detecting leakage and lawless excavation are greatly improved when adopting this technology, and furthermore the cost is very low.

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