| 摘要 |
PROBLEM TO BE SOLVED: To reduce an offset amount of focus error signals whether a double refraction amount of an information memory medium is large or small by setting a circular opening between a diffraction grating for diffracting light beams converged by a converging optical system and a photodetector having a plurality of photodetect areas for detecting a diffraction light diffracted by the diffraction grating. SOLUTION: A circular opening 31 limits to pass only an effective light of light beams 2 projected from a semiconductor laser 1. One light beam 2a of the effective grating light passing the opening 31 enters a polarizing anisotropic hologram 3, is diffracted by a linearly polarization light having a plane of polarization within an xz plane, passes a quarter wave plate 4 to be a circularly polarized light, penetrates a collimator lens 5, is condensed by an objective lens 6 to an information record face of an optical disk 7 and reflected. The reflected light becomes a circularly polarized light advancing by a half wavelength phase than the light reflected by the optical disk 7 because of a double refraction of approximately half the wavelength at a disk base reciprocatory optical path. |
