In many practical application scenarios, especially power grids and communication lines, the lines that need to be monitored will have many branches, and their distribution is not linear, but tree or star shaped. However, most of the existing distributed fiber optic sensing systems have sensing fibers that can only be laid along one line, and a few systems are capable of monitoring two lines. This paper proposes a distributed fiber optic sensing system that can monitor multiple branches of a non-linear network. The system uses a single-core Sagnac structure with branch extensions in the sensing fiber optic cable section and employs the twice-FFT localization algorithm to accurately locate the external disturbances and identify the specific disturbed branch. Compared with the traditional null-frequency method, the twice-FFT method can distinguish the specific branch of the disturbance action while locating the specific location of the disturbance by only one operation. This improves the positioning efficiency of the system and avoids the large errors caused by the null-frequency miscalculation. We have also verified the system's performance and feasibility through experiments.
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