| 摘要 |
PURPOSE:To eliminate the need to adjust the photodetection surface of a photodetector in a focusing direction by detect a focus error from the mean quantity of reflected light from a flank of a reflecting body, and eliminating the need to work the reflecting body with high precision and also selecting a desired reflecting body. CONSTITUTION:A focus error signal is obtained as SB-(SC1+SC2) by photodetection areas B and C (C1 and C2), and focus control is performed on the basis of the focus error signal. When a disk D is displaced approaching an objective lens 4, the SB-(SC1+SC2) becomes negative. When the disk D moves away from the objective lens 4, the SB-(SC1+SC2) becomes positive. Then, an optical system is moved into an in-focus state so that the output difference SB-(SC1+ SC2) is eliminated, performing the focus control. Further, the reflecting body 5 differs in height in the optical-axis direction of return luminous flux and is selected and replaced easily, so the outer circumferential part of the luminous flux is projected and adjusted, eliminating the need to adjust the photodetection surface of the photodetector P in the focus direction. |
