PURPOSE:To simplify configuration of an electric circuit by frequency sweeping frequency of reference laser light for making optical heterodyne detection using a optical frequency modulator. CONSTITUTION:Laser light 2 emitted from a laser light source 1 is branched into two 2A, 2B by a light branching device 3. One of branched 3 laser light 2A is supplied to an optical path that constitutes an object to be measured. Another laser light 2B is fed to a optical frequency modulator 5. Assuming that frequency of laser light is fL, frequency deviation received by the object to be measured 4B is fX, and frequency to be modulated is fB, optical frequency passed through the object 4B becomes fL+fX, and frequency of modulated 5 signal goes to fL+fB. Accordingly, the output voltage of counted value of a counter 7B and a converter 7C can be taken in circuits 13, 14 by detecting signals when the signals are outputted from a band pass filter 11, then applied with waveform shaping and supplying them to a latch circuit 13 and an analog holding circuit 14. Further, it is possible to find frequency deviation fX received by the object 4B by finding the counted value or voltage value.
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