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Generally, phase-sensitive optical time-domain reflectometer (φ-OTDR) adopts a single-channel sensing structure, which makes it vulnerable to random interferences and increases the probability of vibration misjudgment in practical applications. In this paper, a dual-channel φ-OTDR based on a two-mode fiber (LP01 mode and LP11 mode) is constructed, and a simple demodulation algorithm is designed accordingly to locate pencil-break vibrations. The purpose of using dual-channel scheme is that the probability of false detection, simultaneously happened in double channels at the same position and at the same time, would be greatly reduced. In signal processing, both the conventional amplitude differential accumulation algorithm (DAA) and the standard variance algorithm (SVA) are employed to process the Rayleigh scattering traces of LP01 and LP11 channels to detect the pencil-break. The results show that the DAA is highly dependent on the parameters of the algorithm and not suitable to be directly used in practical. Due to the strong randomness of Rayleigh scattering, it is found that the pencil break cannot be detected just by the SVA. Thus, a simple method of producing two decision signals is proposed for vibration detection by combining the DAA and SVA, in which the DAA signals of one channel are crossmultiplied with the SVA signals in another channel. The results show that this method shows reliable performance of locating the pencil-break.
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