| 摘要 |
On an optical disk in a recording and/or playback apparatus, the position at which the thickness error of a light transmission layer is in the optical axis of an objective lens 11, and the position at which the recording/playback of information is performed is in the optical axis of a two-group objective lens 20, so that they are spaced apart from each other, making it impossible to accurately detect the thickness error at the position where the recording/playback of information is performed. Further, to chronologically compensate for the space between them, an excess circuit is required. In view of this, a first reflection light 107 a reflected by a medium surface 106 a of the optical disk is received to detect the focal error with respect to the medium surface, and a second reflection light 107 b reflected by the surface of a light transmission layer 106 b formed on the medium surface 106 a to detect the focal error with respect to the surface of the light transmission layer 106 b. And, in correspondence with the focal error with respect to the surface of the light transmission layer 106 a, the collimator lens 103 is moved.
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