Patent ID: 7542385

Claim:
An optical disk unit having reproducer for reproducing information recorded in an information recording layer of an optical disk, comprising: a laser beam source; an aberration corrector to correct a spherical aberration by adjusting the diverging or converging angle of a laser beam emitted from the laser beam source; an objective lens to condense the laser beam and form a condensed beam spot on the information recording layer; a focus controller having a moving mechanism to move the objective lens along an optical axis of the laser beam, the focus controller moving the objective lens so that the condensed beam spot focuses on the information recording layer; a detector capable of allowing the focus controller to move the objective lens by a predetermined distance from an in-focus position in a first direction, allowing the reproducer to reproduce a random signal having a plurality of amplitudes and periods from an area of the information recording layer, extracting a specific portion having a specific amplitude or period from the reproduced random signal or an interpolated signal thereof, finding a first amplitude value in the specific portion, allowing the focus controller to move the objective lens by the predetermined distance from the in-focus position in a second direction that is opposite to the first direction, allowing the reproducer to reproduce a random signal having a plurality of amplitudes and periods from the area of the information recording layer, extracting a specific portion having a specific amplitude or period from the reproduced random signal or an interpolated signal thereof, and finding a second amplitude value from the specific portion; a control unit capable of controlling the aberration corrector so that the difference between the first amplitude value and the second amplitude value approaches zero; a determiner capable of determining whether or not the information recording layer has a record to reproduce a random signal by the detector; and a recorder capable of recording a random signal having a plurality of amplitudes and periods in the area of the information recording layer if the determiner determines that the information recording layer has no record to reproduce a random signal by the detector.