PURPOSE: To form an orthogonal polarization type optical frequency shifter without using any half-wavelength plate by detecting a polarized beam by a photodetector through the 2nd polarization beam splitter, the 1st nonpolarization beam splitter, and the 3rd beam splitter, and equalizing a P and an S polarized light beam as primary-order diffracted light in intensity.

CONSTITUTION: Of a two-channel acousto-optic element 21, a P polarized beam 42 and an S polarized beam 23 corresponding to primary-order diffracted light beam 37 projected by a medium 23 under a transducer 22 and S polarized primary-order diffracted light beam 38 projected by a medium 23 under a transducer 27 are passed through the 2nd polarization beam splitter 25, the 1st nonpolarization beam splitter 28, and the 3rd polarization beam splitter 29, and detected by photodetectors 30 and 31 as monitors; and their light intensity values are converted into electric signals (voltage) and control is so performed as to eliminate their electric signal difference (voltage difference), the thereby equalizing a P polarized beam 37 and an S polarized beam 38 in light intensity to each other.