Patent ID: 6292441
Filing Date: 2001-09-18
Classification: G11B

Abstract:
An optical head device comprising:a light source for emitting a light beam having a predetermined wavelength;an optical system for converging the light beam emitted from the light source on an information medium;a holographic optical element for converting the light beam reflected from the information medium into diffracted light; anda light detector including a plurality of pairs of photodetectors, for detecting a focus error based on the diffracted light,wherein each of the plurality of pairs of photodetectors has a boundary line dividing one photodetector from the other photodetector of the pair,the holographic optical element changes an angle of the diffracted light in response to a deviation of the predetermined wavelength of the light beam so as to cause an offset in the position of light spots corresponding to the diffracted light formed on the boundary line of each of the plurality of pairs of photodetectors,the plurality of pairs of photodetectors are arranged so that a direction of the offset in the position of the light spot crosses the boundary line, andthe light detector generates a focus error signal indicating the focus error based on differential signals output from each pair of the plurality of pairs of photodetectors by canceling the fluctuations in the respective differential signals output from the plurality of pairs of photodetectors due to the offset in the position of the light spots by one another.