We propose a method for monitoring branched sections in passive optical networks (PONs), where identification fibers are used to distinguish each branched section. We clarify the required characteristics for the separation of the Brillouin frequency shifts of each identification fiber. We design and fabricate identification fibers with well-controlled ν<sub>B</sub> values for PON monitoring based on the required characteristics. The realized Brillouin frequency shift difference is less than 40 MHz from the target, and we achieve a 1-GHz Brillouin frequency shift separation and a 0.06-dB splice loss for standard single-mode fiber. We also demonstrate fault location in a PON using identification fibers connected to a 4-branched splitter. The bending loss in the branched section is successfully located by using the designed identification fibers and a Brillouin optical time domain reflectometer.
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