Patent ID: 6925046

Claim:
An optical disk unit that emits a light beam to an optical disk, which includes an information recording layer and a light transmission layer for protecting the information recording layer, and that causes the light beam to fall the optical disk from the light transmission layer, thereby recording information or reproducing information, comprising: a light emitting unit that emits a light beam with a predetermined wavelength; a focusing unit configured to focus the light beam from the light emitting unit at a predetermined position of the information recording layer of the optical disk; a wavefront transforming optical element provided along an optical axis defined between the light emitting unit and the focusing unit, to transform a wavefront of the light beam guided to the focusing unit to the light emitting unit; a thickness difference detection unit to detect a thickness difference of the light transmission layer of the optical disk positioned between the focusing unit and the information recording layer of the optical disk; a wavefront transforming optical element control unit to control the wavefront transforming optical element, the wavefront transforming optical element is reducing a spherical aberration based on a thickness error of the light transmission layer of the optical disk: a beam splitter provided between the wavefront transforming optical element and the focusing unit, the beam splitter reflecting in a predetermine direction a part of the light beam oriented from the wavefront transforming optical element to the focusing unit; a light detector that receives a light beam split from the light beam oriented from the wavefront transforming optical element to the focusing unit by the beam splitter, and outputs an electrical signal corresponding to the light amount of the received light beam; and an output control unit that controls the light amount of the light beam radiated by the light emitting unit based on the magnitude of the electrical signal outputted from the light detector, wherein the wavefront transforming optical element includes a first lens and a second lens either one is a convex lens and the other is a concave lens, and wherein an opening restriction mechanism free of an effect of change in beam diameter on the focusing unit caused when either of the first and second lenses of the wavefront transforming optical element is moved is provided at the light incidence side of the light detector.