Patent ID: 7596061

Claim:
An optical disk apparatus comprising: a light source; a spherical aberration corrector thereby adding a spherical aberration to the light flux from the light source; an astigmatism correcting element thereby adding astigmatism to the light flux; an objective lens converging a light flux passed through the spherical aberration corrector and the astigmatism correcting element to an optical disk, and converging a reflection light from the optical disk; an actuator to drive the objective lens in the focus direction and in the vertical direction thereto; a photo-detector detecting the reflected light; a focus error signal generator to generate a focus error signal from the output of the photo-detector; a push-pull signal generator to generate a push-pull signal from the output of the photo-detector; a reproduction signal generator to generate a reproduction signal from the output of the photo-detector; and a controller to control the spherical aberration corrector, the astigmatism correcting element, and the actuator; wherein the controller adjusts the objective lens to focus by controlling the actuator based on the focus error signal, and controls and tracks the actuator based on the push-pull signal, and reads the push-pull signal and the reproduction signal by varying an amount of spherical aberration to be added to the light flux by the spherical aberration corrector, furthermore, if an amount of spherical aberration yielding a maximum amplitude of the push-pull signal, and an amount of spherical aberration yielding a maximum amplitude of the reproduction signal do not agree, the controller also adjusts the astigmatism correcting element to make the both amounts agree, and makes correction for astigmatism in the direction of 0 degree or 90 degrees, defining the astigmatism direction forming a focal line in parallel to the track direction near the objective lens as zero degree.