Patent ID: 7426222

Claim:
A laser apparatus, comprising: an optical element which receives at least a part of laser light emitted from a laser generation source and generates interference fringes; a first two-divided detector having two detectors arranged in the direction of which the interference fringes appear, each of the detectors detecting an amount of light of the interference fringes; a second two-divided detector having two detectors arranged in the direction of which the interference fringes appear, each of the detectors detecting an amount of light of the interference fringes, the second two-divided detector being spaced apart from the first two-divided detector for an odd-number multiple of nearly ¼ period of the interference fringes, the second two-divided detector and the first two-divided detector being disposed on a plane perpendicular to an optical path of light of the interference fringes; first calculation means for calculating a first difference signal of detection signals of the two detectors of the first two-divided detector; second calculation means for calculating a second difference signal of detection signals of the two detectors of the second two-divided detector; selection means for selecting one of the first difference signal and the second difference signal; and determination means for detecting a discontinuous variation of a level of one difference signal selected from the first difference signal and the second difference signal.