Patent ID: 8891342
Filing Date: 2014-11-18
Classification: G11B

Abstract:
1. An optical head device comprising: a laser light source for emitting laser light; an objective lens for focusing the laser light onto an information track in an information recording layer of an optical disc and focusing reflected light diffracted by the information track; a diffractive element for generating diffracted light from the reflected light focused by the objective lens; an optical element for producing astigmatism in the reflected light; and a photodetector for receiving the reflected light; wherein the diffractive element includes a plurality of diffraction regions separated by a first line intersecting, at a +45-degree angle, a line extending in a radial direction of the optical disc and a second line intersecting, at a −45-degree angle, the line extending in the radial direction of the optical disc, the plurality of diffraction regions including first and second diffraction regions having first grating patterns extending in mutually identical directions and third and fourth diffraction regions having second grating patterns extending in mutually identical directions; the photodetector has first to eighth light receiving surfaces bounded by a third line intersecting, at a +45-degree angle, a line extending in a direction corresponding to the radial direction, a fourth line intersecting, at a −45-degree angle, the line extending in the direction corresponding to the radial direction, a fifth line extending in the direction corresponding to the radial direction, a sixth line extending in the direction corresponding to a tangential direction of the optical disc, and the first to eighth light receiving surfaces being arranged so as to receive +1-order light in the diffracted light from the first and second diffraction regions, and ninth to sixteenth light receiving surfaces bounded by a seventh line intersecting, at a +45-degree angle, a line extending in a direction corresponding to the radial direction, an eighth line intersecting, at a −45-degree angle, the line extending in the direction corresponding to the radial direction, a ninth line extending in the direction corresponding to the radial direction, and a tenth line extending in the direction corresponding to the tangential direction, and the ninth to sixteenth light receiving surfaces being arranged so as to receive +1-order light in the diffracted light from the third and fourth diffraction regions; the first to sixteenth light receiving surfaces constitute a light receiving surface for detecting focus error, and a plurality of light receiving surfaces having a side being in contact with the line extending in the direction corresponding to the radial direction among the first to eighth light receiving surfaces and a plurality of light receiving surfaces having a side being in contact with the line extending in the direction corresponding to the tangential direction among the ninth to sixteenth light receiving surfaces constitute the light receiving surface for detecting the tracking error; and the optical element, the diffractive element, and the photodetector are configured in such a way that +1-order light or −1-order light in the diffracted light generated by the diffractive element from reflected light from an information track in an information recording layer where information is to be recorded or reproduced strikes inside the light receiving surface for detecting the tracking error signal, +1-order light or −1-order light in the diffracted light generated by the diffractive element from reflected light from an information track in an information recording layer one layer deeper than the information recording layer where information is to be recorded or reproduced strikes outside the light receiving surface for detecting the tracking error signal, and +1-order light or −1-order light in the diffracted light generated by the diffractive element from reflected light from an information track in an information recording layer one layer shallower than the information recording layer where information is to be recorded or reproduced strikes outside the light receiving surface for detecting the tracking error signal.