| 摘要 |
PURPOSE:To stabilize focus servo operation and to simplify initial leading-in operation at the start of the servo system by executing efficient defocusing detection symmetrical around a focused point and free from plural zero crosses. CONSTITUTION:A light flux separating element 14 is formed to separate reflected light flux 6 separated by a beam splitter 7 into reflected light flux 6', 6'' each of which is about a half light flux having a different optical path in space. A photodetector 8 consisting of two photodetecting elements 8', 8'' detecting the light flux 6', 6'' is arranged on a position far from one focused point of the two reflected light flux 6', 6'' separated by the element 14 and near from the other focused point. A signal corresponding to defocusing is obtained from the outputs of the elements 8', 8''. Consequently, efficient defocusing detecting characteristics symmetrical around the focused point free from plural zero crosses are obtained, so that the focus servo operation is stabilized and the initial leading-in can be easily attained at the start of the servo system. |
