Patent ID: 8400899

Claim:
An optical pickup device comprising: a laser light source; an irradiation light focusing optical system that focuses a laser beam from the laser light source on one selected recording layer of a multi-layer optical information storage medium; and a detection optical system that detects reflection light reflected from the selected recording layer of the multi-layer optical information storage medium; wherein the detection optical system comprises: a detection lens; a semiconductor detector comprising a plurality of sensing regions; and a multi-region diffraction grating that is divided into a plurality of regions and located between the detection lens and the semiconductor detector; wherein an irradiation position, on the semiconductor detector, of a diffracted light beam from, of the plurality of regions of the multi-region diffraction grating, a center region including an optical axis is further from the optical axis than an irradiation position, on the semiconductor detector, of a diffracted light beam from another divided region; wherein the center region of the multi-region diffraction grating is divided into a first region and a second region by a straight line gassing through the optical axis; wherein the number of diffracted light beams from the first region and the second region is one each; and wherein diffraction directions of the first region and the second region are defined in such a manner that diffracted light beams of, among stray light from recording layers other than the selected recording layer of the multi-layer optical information storage medium, stray light that travels through the first region and the second region and becomes largest in shape on the semiconductor detector would be of a shape whose protruding part faces the opposite direction to the optical axis on the semiconductor detector.