| 摘要 |
PURPOSE:To correct an error due to a difference of the plane of polarization, etc., and to accurately measure a distance by making light which is equal in wavelength on an interference optical system and using the detected phase at this time as an initial phase to correct a subsequent measured value. CONSTITUTION:The value of the initial phase corresponding to the state where the distance is zero is measured first. At this time, only a laser light source 11 is driven, an acoustooptic modulator AOM2 is not driven, and a lambda/2 plate P2 is put in an optical path. At this time, a phase difference detected by a phase detector 3 is an error caused by the difference in the plane of polarization and used as the initial phase. Then when the modulator AOM2 is put in operation, two light beams which have an optional frequency difference are incident on the interference optical system. Then a laser light source 12 is driven in addition to the laser light source 11, the modulator AOM2 is stopped from being driven, and the plate P2 is taken out of the optical path, so that light beams emitted by the light sources 11 and 12 enter the interference optical system. The detector 3 detects the phase difference with the two light beams with different wavelengths obtained as mentioned above and this phase difference output is corrected with the value of the initial phase to accurately measure the distance.
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