Optical head apparatus and optical disc apparatus

An optical head apparatus irradiating a multi-layer optical disc having a plurality of information recording surfaces with laser light and detecting reflected light of the laser light, the optical head apparatus comprising:

• a laser light source;
• a diffraction means splitting laser light which is emitted from the laser light source into zero-order diffracted light, positive-first-order diffracted light, and negative-first-order diffracted light;
• a photodetector;
• and
• an optical system means collecting the zero-order diffracted light, the positive-first-order diffracted light, and the negative-first-order diffracted light, on an access-target information recording surface out of the plurality of information recording surfaces, and guiding on the photodetector, reflected light of the zero-order diffracted light, reflected light of the positive-first-order diffracted light, and reflected light of the negative-first-order diffracted light by the access-target information recording surface;
• wherein:
• the optical system means includes an adjusting means moving a light-collected position of the zero-order diffracted light, the positive-first-order diffracted light, and the negative-first-order diffracted light, in a focus direction and a tracking direction;
• the photodetector includes
• a main-beam light-receiving unit which is disposed at a position where the reflected light of the zero-order diffracted light by the access-target information recording surface is received,
• a first sub-beam light-receiving unit which is disposed at a position where the reflected light of the positive-first-order diffracted light by the access-target information recording surface is received, and
• a second sub-beam light-receiving unit which is disposed at a position where the reflected light of the negative-first-order diffracted light by the access-target information recording surface is received;
• the first sub-beam light-receiving unit is divided by a first division line in an x-direction being a direction on the photodetector corresponding to a tangential direction of a recording track on the optical disc, and includes a first divisional light-receiving element and a second divisional light-receiving element which are arranged in a y-direction being orthogonal to the x-direction;
• the second sub-beam light-receiving unit is divided by a second division line in the x-direction, and includes a third divisional light-receiving element and a fourth divisional light-receiving element which are arranged in the y-direction;
• Uy=Ty×N1 is satisfied,
• where Ty denotes a length in the y-direction of one period of an interference fringe which is formed on the photodetector by reflected light of the zero-order diffracted light by an information recording surface other than the access-target information recording surface, reflected light of the positive-first-order diffracted light by the information recording surface other than the access-target information recording surface, and reflected light of the negative-first-order diffracted light by the information recording surface other than the access-target information recording surface,
• N1 denotes a positive integer, and
• Uy denotes each length in the y-direction of the first divisional light-receiving element, the second divisional light-receiving element, the third divisional light-receiving element, and the fourth divisional light-receiving element.