| 摘要 |
PURPOSE:To perform an accurate focus servo action in a simple constitution by setting the width of the center area of a three-split photodetector at the prescribed value and at the same time controlling the gains of the detection signal, etc., received from said center area to eliminate the influence of the diffracted light caused by tracks on a disk and the offset. CONSTITUTION:The light beams of a light source are reflected by a half mirror of a parallel plane plate 11 and project on a disk 4. The reflected light beams are made incident on three divided areas of a photodetector 6 respectively via the plate 11. The detection outputs of both areas A and C are added together by an adder circuit 8 and processed by a differential together with the detection output of the center area B received via a gain control circuit 12. Thus, a focus error signal is outputted. In this case, the influence of the track diffracted light caused by the comma aberration, etc., excepting astigmatism of the plate 11 is offset with selection of the width of the area B. This offset value is corrected by the gain control of the circuit 12. Thus, an accurate focus servo is possible in a simple constitution. The same effect is also obtained by controlling the gains of the addition output. |
