| 摘要 |
PURPOSE:To perform a highly accurate measurement of length with a high resolution, by changing a delay in the phase of one or both of the two lights so that the difference thereof will be always a fixed value in the phase delay between the two lights on an interfering surface of an interference optical system to obtain an absolute length measurement output. CONSTITUTION:An AO modulator 2 and a VCO3 compose a phase varying means for two lights to change a wavelength difference (frequency difference) between two lights by varying driving frequency of the AO modulator 2, causing a change in the phase delay between the respective lights. The output signal Vbeta of a phase detection circuit 15 is fed back to the VCO3 to control the oscillation frequency of the VCO3 in the manner that the difference in the phase delay between the two lights will be a desired fixed value DELTAbeta. Thus, wavelength difference (frequency difference) or the like of the two lights is changed so that the difference in the phase delay on an interfering surface will be a fixed value thereby facilitating an arithmetic processing to be performed later.
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