Patent ID: 7564762

Claim:
An optical head device, comprising: a diffractive optical element for dividing a light emitted from a light source into a main beam and at least one sub-beam in an outward path; an objective lens for focusing the main beam and the sub-beam divided by the diffractive optical element in the outward path on a disk-shaped optical recording medium with spiral tracks; and a photodetector for receiving the main beam and the sub-beam reflected from the optical recording medium in an inward path, wherein at least a part of the diffractive optical element includes a diffraction grating divided into a plurality of areas, which are four or more areas, by a plurality of lines, which are three or more lines, in parallel with a direction corresponding to a tangential direction of the optical recording medium, and in two adjacent areas among the plurality of areas, phases of lattices are shifted by π to each other, wherein focused spots of the main beam and the sub-beam are located on a same track of the optical recording medium, wherein the photodetector is configured so as to detect a push-pull signal from each of the main beam and the sub-beam, wherein the diffractive optical element has a diffraction grating divided into eight areas, which are first to eighth areas from one end to another end, by a line passing through an optical axis of an incident light and in parallel with the direction corresponding to the tangential direction of the optical recording medium and by six lines symmetrical with respect to the optical axis of the incident light and in parallel with the direction corresponding to the tangential direction of the optical recording medium, and wherein phases of lattices in the first, third, fifth and seventh areas and phases of lattices in the second, fourth, sixth and eighth areas are shifted by π to each other.