Patent ID: 8036092
Filing Date: 2011-10-11
Classification: G11B

Abstract:
1. An optical head device comprising a light source; an objective lens for collecting emitting light from the light source on a disc shaped optical recording medium; a diffractive optical element, arranged on an optical path of a reflection light from the optical recording medium, for generating a diffracted light from the reflected light; and a photodetector for receiving the diffracted light generated by the diffractive optical element, wherein the optical recording medium includes an optical recording medium with at least a signal layer of first layer and second layer; the photodetector includes a plurality of positive component light receiving sections for receiving the diffracted light and outputting as a positive component of a focus error signal, and a plurality of negative component light receiving sections for receiving the diffracted light and outputting as a negative component of the focus error signal; and when the signal layer of the optical recording medium is far from the objective lens or close to the objective lens by a defocusing quantity corresponding to an interval between the first layer and the second layer from a just focus position, the diffraction direction of the diffracted light is set and the plurality of positive component light receiving sections and the plurality of negative component light receiving sections are arranged so that the diffracted light generated from the reflected light of the signal layer spreads to the positive component light receiving section and the negative component light receiving section, wherein the diffractive optical element is divided into four diffracting regions in a plane vertical to an optical axis of the reflection light by a first line passing through the optical axis and corresponding to a radial direction of the optical recording medium and a second line passing through the optical axis and corresponding to a tangential direction of the optical recording medium; the four diffracting regions each generate the diffracted light of a specific order from the reflection light; the photodetector includes four dual-divided light receiving sections divided into two of the positive component light receiving section and the negative component light receiving section by a third line parallel to the first line; and when the two diffracting regions positioned on one side with the first line among the four diffracting regions are set as first and second diffracting regions, the two diffracting regions positioned on the other side are set as third and fourth diffracting regions, and the four dual-divided light receiving sections are set as first to fourth dual-divided light receiving sections, the four dual-divided light receiving sections are arranged in an order of the first dual-divided light receiving section, the third dual-divided light receiving section, the second dual-divided light receiving section, and the fourth dual-divided light receiving section along the third line, wherein a linear diffraction grating is formed in the four diffracting regions each, the direction of the grating being parallel to the second line and the pitch of the grating being equal; and the pitch of the diffraction grating becomes wider or narrower in the order of the first diffracting region, the third diffracting region, the second diffracting region, and the fourth diffracting region.