PURPOSE: To improve measurement precision by shifting one wave orthogonally to the other wave and analyzing the shift state of an output beam before the waves are combined. CONSTITUTION: A beam splitter 2 spits an incoming beam, which is polarized horizontally linearly into a reference beam 3 and a measurement beam 4. The beams 3 and 4 are returned by mirrors 5 and 6 to the splitter 2 and recombined into a beam 7. In this case, the beam 3 before being returned to the splitter 2 is converted by a 1/4-wavelength plate 10 into orthogonal polarized light. Then the polarization state of the recombined beam 7 is analyzed by a polarized light analyzing device 11. Consequently, the phase shift between the two light beams due to a difference in the optical path is measured to improve the measurement precision.
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