PURPOSE: To simplify a control system and to prevent a malfunction by arranging a lens which is locally different in shape and focal length in front of a detector which detects the quantity of surface undulations of a disk.
CONSTITUTION: The focus position controller is constituted by arranging lens which is locally different in shape and focal length, e.g. convex lens 19 in front of the detector 20 which detects the quantity of surface undulations of the disk. Namely, the convex lens 19 is plane where a reflected beam 21 enters the detector 20 without being bent and convex where the beam is not incident on the detector as they are. Consequently, the reflected beam which is not incident conventionally enters the detector 20, so the detection range of surface undulation of the disk becomes wide. Consequently, neither a circuit which elevates an objective lens forcibly when control is off nor a detecting circuit for an optimum point is necessary, so the control system is simplified and malfunction is prevented.
SAKATA AKIHIRO
TSUJI SEIZO
TAKAHASHI TSUNEO
| JPS54124704A | 1979-09-27 | |||
| JP55061838B |
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