Patent ID: 8068403
Filing Date: 2011-11-29
Classification: G11B

Abstract:
1. An optical head for performing reading and/or writing on an optical disc that has a plurality of storage layers including a first storage layer and a second storage layer, the optical head comprising: a light source; a first optical system for converging light emitted from the light source on either the first storage layer or the second storage layer; a second optical system for receiving and transmitting light reflected from the first storage layer and stray light reflected from the second storage layer when the light is converged on the first storage layer; a photodetector that has a plurality of photosensitive areas, which output respective signals representing intensities of the light that has been incident there by way of the second optical system; and a calculating section for calculating the difference between sum of a first group of signals that have been supplied from a first group of photosensitive areas among the plurality of photosensitive areas and that of a second group of signals that have been supplied from a second group, thereby generating a focus error signal, wherein the reflected light and the stray light that have been incident on the second optical system interfere with each other, and the first and second groups of photosensitive areas are arranged so as to be equally affected by a variation in light intensity caused by the interference, wherein the second optical system includes an astigmatism element, wherein the astigmatism element gives the reflected light and the stray light astigmatism in a predetermined direction on a photosensitive plane of the photodetector, wherein the photosensitive areas are arranged asymmetrically with respect to the predetermined direction, and wherein the first group of photosensitive areas consists of at least two photosensitive areas that are arranged in the predetermined direction, while the second group of photosensitive areas consists of at least two photosensitive areas that are arranged perpendicularly to the predetermined direction.