| 摘要 |
A scattered light separated from the optical transmission path is monitored, a part of the excitation light is separated and is monitored, a reflected light which passes in a direction opposite to the direction in which the signal light passes through the optical transmission path is monitored, and, when the power of the excitation light monitored reaches a predetermined determination value, it is determined whether or not any loss point occurs based on a ratio between the power of the scattered light monitored and the power of the reflected light monitored.
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