Optical pickup device with dual grating element

Abstract

In an apparatus for recording and reproducing optical information, light emitted from a laser beam source is irradiated to an optical information recording medium to record the optical information and the light reflected from the recording medium is guided to a signal detecting optical system to detect tracking and focusing error signals and a reproducing signal. The apparatus comprises a dual grading arranged through a detecting lens on an optical path on which the light reflected from the recording medium is guided to the optical system, the dual grating having diffraction gratings formed at an equal pitch on the front and rear faces of a substrate, a refractive index of the substrate being greater than the refractive indexes of the diffraction gratings; and a photodetector for detecting light transmitted through the dual grating. The dual grating may have diffraction gratings respectively formed at equal and irregular pitches on both faces of a substrate. The dual grating may have a front face grating formed on a first incident side of the reflected light and a rear face grating formed on the side of a substrate opposite to the first incident side; the front face grating having a pitch smaller than that of the rear face grating; an incident angle of the reflected light with respect to the front face grating being set to be greater than a Bragg angle with respect to the front face grating.