Patent ID: 7260028

Claim:
An optical head unit, comprising: a light source configured to supply light of a predetermined wavelength; an objective lens configured to focus light from the light source to a recording layer of a recording medium; a defocus detecting system configured to detect a defocus generated when light converged to the recording medium by the objective lens is not focused at a predetermined position; a thickness unevenness detecting system, including at least two photo detecting regions, and configured to detect a thickness unevenness in a transparent resin layer of the recording medium provided nearest the objective lens; a thickness unevenness correcting mechanism configured to change a focusing characteristic of light impinging upon the objective lens from the light source based on a change in thickness of the transparent resin layer of the recording medium detected by the thickness unevenness detecting system; a defocus correcting mechanism configured to correct a defocus detected by the defocus detecting system; wherein a thickness unevenness detecting signal output from the thickness unevenness detecting system is used to detect an objective lens erroneous move when the objective lens for focusing light is erroneously moved from a currently focused recording layer to another recording layer; and wherein the objective lens erroneous move is detected after: moving the objective lens in a predetermined direction while a voltage whose magnitude changes gradually is applied to the thickness unevenness correcting mechanism, with the defocus correcting system turned off; detecting that the objective lens is located in the vicinity of a focusing position by referring to the level of a sum signal of the defocus detecting signal outputted form individual photo detecting regions of the defocus detecting system; activating the defocus correcting system when the defocus detecting signal outputted from the defocus detecting system reaches a reference voltage; moving the thickness unevenness correcting mechanism in the direction of decreasing the thickness unevenness correction amount detected by the thickness unevenness detecting system with respect to the recording layer on which the objective lens is focused; activating the thickness unevenness correcting mechanism when the thickness unevenness detecting signal outputted by the thickness unevenness detecting system is substantially a reference voltage; and wherein the thickness unevenness correcting mechanism being operated with a smaller gain relative to a servo gain of a DC level of the frequency characteristic of a defocus correcting control circuit.