Patent ID: 6418098
Filing Date: 2002-07-09
Classification: G11B

Abstract:
An optical head apparatus for reproducing information from an optical recording medium, comprising:an objective lens that receives a light beam from a light source and condenses the light beam onto the optical recording medium; a photo-detector that has plurality of photo-detecting planes disposed orthogonal to an optical axis of the reflected light beam and symmetrical with respect to the optical axis, and detects the light beam reflected by the optical recording medium, wherein each of the photo-detecting planes is divided into a plurality of divided areas by a plurality of dividing lines which symmetrically divide the photo-detecting planes with respect to the optical axis; a diffractive optical element that is disposed upstream to the photo-detector for diffracting the reflected light beam, wherein the diffracting optical element diffracts the reflected light beam such that the reflected light beam is divided at least in a direction parallel to a track of the optical recording medium, and only a +Nth order diffracted light beam and a âˆ’Nth order diffracted light beam (N is an integer greater than or equal to 1) in the photo-detecting plane are independently received by the divided areas which are disposed in positions symmetrically opposite with respect to the optical axis respectively; an arithmetic circuit that calculates an error signal with respect to a position of the objective lens along the optical axis, by using outputs from the photo-detector of both the +Nth order diffracted light beam and the âˆ’Nth order diffracted light beam; and a driver that generates a signal for controlling the position of the objective lens based on an output from the arithmetic circuit; and wherein the diffractive optical element diffracts the reflected light beam such that the reflected light beam is further divided in a direction orthogonal to a track of the optical recording medium.