| 摘要 |
PROBLEM TO BE SOLVED: To measure the distance between fixed targets and the thickness of a relatively thick surface film with the accuracy of an optical heterodyne interferometer by measuring the phase difference of the AC signal component. SOLUTION: Two beams 33 orthogonal to each other and f1 in the circumference difference are formed of the beam 31 of the single frequencyν1 emitted by a first laser 1 by an AOM (photoacoustic element) 3 excited by an oscillator 8 of f1 in oscillation frequency. Two beams 34 orthogonal to each other of f2 in frequency difference are formed of the beam ofν2 in frequency to be emitted from a second laser 2 by an oscillator 9 of f2 in oscillation frequency and an AOM 4. The synthesized beam 35 becomes the input beam of a heterodyne optical system 18 by synthesizing the beam 33, 34 by a translucent mirror 6 so that the azimuth of polarization coincides with each other. The output beam 36 is detected by a detector 7. All AC signals of the photoelectric current of the detector 7 have the spectra of f1 and f2 in frequency. The components of f1 -f2 in frequency are extracted from all AC signals. As for the components of f1 -f2 in frequency (signal components), the difference f1 -f2 in frequency of two transmitters of the frequency f1 -f2 of the AOM is obtained by a mixer 14, and the phase difference therebetween is obtained by a phase difference meter 16.
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