| 摘要 |
PURPOSE:To measure a difference in optical path length with high precision by measuring a low and a high frequency twice successively. CONSTITUTION:A modulation frequency f1 is so selected that laser light beams with wavelengths lambda1 and lambda2 from LE light sources 3 and 5 have a phase difference theta1 of <=180 deg. after propagating in a single-mode fiber 7 having length L. Then, the 1st measurement is taken at the sufficiently low frequency f1 and the phase difference theta1 is obtained from a rough standard; and the frequency is raised to f2 to read a position meter within a range of + or -180 deg., and the read value is corrected by an integral multiple of 360 deg. estimated from the phase difference theta2=theta1Xf2/f1 expected at the frequency f2 to calculate a phase difference theta2. Thus, the phase difference is measured precisely even on a condition where the phase difference exceeds + or -180 deg.. |
