Patent ID: 6418107
Filing Date: 2002-07-09
Classification: G11B

Abstract:
An optical head device, comprising:a light source for emitting at least one of a coherent beam and a quasi-monochromatic beam; a collection optical system for collecting the beam emitted by the light source to an information memory medium having tracks having a mark and a space selectively arranged or tracks having prescribed grooves; a diffraction element for receiving a beam diffracted by the information memory medium and generating a diffraction beam; and a light detector having a plurality of detection areas for receiving the beam diffracted by the diffraction element and outputting a signal in accordance with a light amount of the beam received, wherein: the diffraction element includes a plurality of areas, a diffraction beam of a desired order generated by an area group A included in the plurality of areas form a first spherical wave, a diffraction beam of a desired order generated by an area group B included in the plurality of areas but excluded from the area group A form a second spherical wave, which has a light collection point farther than the light collection point of the first spherical wave with respect to the diffraction element, a focusing error signal generator is provided for generating a focusing error signal based on the difference between the cross sections of the first spherical wave and the second spherical wave on the light detector, the diffraction element has at least one division line perpendicular to the tangent to the tracks, and either one of portions sandwiching the at least one division line is included in the area group A and the other portion is included in the area group B.