LIGHT BEAM PATH MONITORING DEVICE

摘要

PROBLEM TO BE SOLVED: To provide a light beam path monitoring device capable of highly accurately measuring a position of a reflection point existing in a light beam path.SOLUTION: A light beam path monitoring device comprises: a semiconductor laser 5 oscillating at a single wavelength; a phase adjuster 6 in which output light of the semiconductor laser 5 adjusts a phase with reflected return light from a reflection position in a light beam path to be measured; an optical branching filter 7 splitting the output light of the semiconductor laser into a light beam path 16 to be measured and a light beam path 12 for monitoring; and a frequency spectrum observer 9 observing a frequency spectrum of propagation light from the light beam path 12 for monitoring. The output light of the semiconductor laser 5 and the reflected return light resonate with each other to generate modes at a plurality of frequencies. The light beam path monitoring device has a function to identify a reflection position in the light beam path to be measured on the basis of an interval of the modes of the frequency spectrum.