Patent ID: 8080773

Claim:
A method of measuring backward light, in an optical head that configures a laser processing apparatus, together with a light source part emitting a processing laser beam, that has a light incident end for capturing the laser beam from the light source part and an light emission end for outputting the captured laser beam toward an object to be processed, and that is provided with an isolator disposed on an optical path of the laser beam propagating from the light incident end to the light emission end, the method measuring the power of backward light passing through the isolator from the light emission end toward the light incident end, in an direction opposite to a propagation direction of the laser beam, and comprising the steps of: disposing a measurement light source for emitting light of large numerical aperture on the light emission end side of the optical head; emitting reference light from the measurement light source into the optical head via the light emission end of the optical head, while changing an emission position of the measurement light source serving as an emitting position of the reference light, relative to the light emission end of the optical head; measuring a power of the reference light passing through the isolator, for each emitting position of the reference light; and comparing the obtained reference light power with a previously-set base power for each emitting position of the reference light.