| 摘要 |
PURPOSE:To simplify a measuring operation and to maintain the high stability of electric signals, by using two electric signals obtained from the same oscillator for reference and a third electric signal having a frequency equivalent to a frequency difference between those signals. CONSTITUTION:A driving circuit 42 composed of a PLL circuit 42A having a frequency divider incorporated therein and an amplifier 42C, and of a PLL circuit 42B having a frequency divider of a variable frequency division ratio incorporated therein and an amplifier 42D, is provided for a reference oscillator 41 having a quartz vibrator incorporated therein, and a construction is made so that acoustooptic elements 301 and 302 of an optical frequency shifter 3 of an orthogonal polarization type are driven by an output of said driving circuit. On the other hand, a PLL circuit 43 having a frequency divider of a variable frequency division ratio incorporated therein is provided for the oscillator 41, the frequency division ratio is set to be the difference between the frequency division ratios of the circuits 42A and 42B, and an output signal i0' of a frequency equialent to the difference between the output frequencies of these two circuits is obtained. Using this signal i0' as a reference, electric signals i1 and i1' obtained from a photodetector 30 and having a phase difference are compared in a phase detector 31 to measure phase variation, and an optical path difference is determined from the result of the measurement in an arithmetic unit 32.
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