Optical head apparatus

Abstract

A light beam reflected from an optical disc 1 is passed through an objective lens and a beam splitter to a detective lens. Two diffraction gratings having the same diffraction pitch, i.e., a first diffraction grating and a second diffraction grating, are disposed so that they are relatively inclined at a given angle to each other. Signal beams are detected by a photodetector. When the focusing position of the objective lens relative to the optical disc 1 is shifted in the focusing direction, the focusing position of a light beam from the objective lens is changed, resulting in changed inclination angle .alpha. of moire fringes on the photodetector. Thus, provision of two gratings relatively inclined at a given angle to each other enables detecting signals to be obtained based on the angle signals of the moire fringes even from minute errors with respect to focusing errors on an optical disc or multi-value of pit.

Dependencies are inferred from the claim text.