| 摘要 |
PURPOSE:To reduce measurement time considerably and to improve the accuracy of measurement by making modulating frequencies variable in such a way that wavelengths are made integer times the optical path length. CONSTITUTION:A differential frequency f3 between a specific frequency f1 and a variable frequency f2 is generated and the light wave which is subjected to light intensity modulation by said frequency is emitted from an intesity modulator 4 for light source and the light wave reflectd by a reflecting mirror 5 is detected with a photodetector 6. The optical path length in this time is 2L. The received output of the photodetector 6 is applied to a phase detector 8 after amplification and with f3 as a reference signal, the output is subjected to synchronous detection. Here the wavelength of the frequency f3 is c/f[(c) is velocity of light]and therefore the phase difference phi of the photodetected light wave with respect to the emitted light wave is 4piLf3/c. Now if arrangement is so made that the f3 changes from the low frequency side where there are none of the nodes of the light wave in the path 2L, the change in the differential frequency f3 is stopped where the output from the detector 8 attains phi=pi, and the f3 is read with an operator 11, which operates L=c/(4f3) and determines the distance L. Said distance is displayed on a display 12. |
