Patent ID: 7778140

Claim:
An optical head device, comprising: a light source for emitting a light beam; a focusing optical system for focusing the light beam emitted from the light source into a convergent light to be incident on an information recording medium having tracks; a diffraction optical system for diffracting a part of the light beam reflected and diffracted by the information recording medium; and a photodetector for receiving the light beam diffracted by the diffraction optical system and the light beam having passed through the diffraction optical system without being diffracted, wherein: the diffraction optical system is divided into a plurality of regions by first and second dividing lines extending in a first direction and third and fourth dividing lines extending in a second direction intersecting with the first direction and uses regions at the outer sides of the first and second dividing lines as first and second subregions and regions at the outer sides of the third and fourth dividing lines as first and second main regions; the photodetector includes a 0 th -order light receiving portion group for receiving the light beam having passed through the diffraction optical system without being diffracted, a main region light receiving portion group for receiving the light beam diffracted by the first and second main regions and a subregion light receiving portion group for receiving the light beam diffracted by the first and second subregions; the information recording medium includes a plurality of information layers; light receiving portions of the main region light receiving portion group are arranged between projection lines of the third and fourth dividing lines on the photodetector by stray lights from the information layer adjacent to the one information layer, on which the light beam is focused, out of the plurality of information layers; and light receiving portions of the subregion light receiving portion group are arranged between projection lines of the first and second dividing lines on the photodetector by the stray lights from the information layer adjacent to the one information layer, on which the light beam is focused, out of the plurality of information layers.