| 摘要 |
PROBLEM TO BE SOLVED: To reduce the change in ±1st-order diffracted light intensity and to realize a stable automatic track control even when zero-order diffracted light intensity is changed at the recording time of information and at the reproducing time. SOLUTION: An optical head contains a composite diffraction grating 35 arranging in parallel reflection type diffraction gratings 35a, 35b, and the diffraction grating 35b is possible to relatively move in the grating pitch direction to the diffraction grating 35a. A luminous flux from an irradiation optical system is made incident on a composite diffraction grating 35 through a polarizing beam splitter 31, a 1/4 wavelength plate 32, a mirror 33, the 1/4 wavelength plate 32, the polarizing beam splitter 31 and the 1/4 waveform plate 34, and the zero-order diffracted light and the ±1st-order diffracted light formed here are made incident on an optical card 1 as a light spot through the 1/4 wavelength plate 34, the polarizing beam splitter 31, the 1/4 wavelength plate 36 and the objective lens 37. The luminous flux from the light spot on the optical card 1 is led to a detection optical system through the objective lens 37, the 1/4 wavelength plate 36 and the polarizing beam splitter 31. |
