| 摘要 |
PURPOSE:To detect focus error signal without lowering the condensing efficiency, by separating the parallel beams given from a collimator lens into a main light flux and a secondary light flux with use of a prism having a specific form. CONSTITUTION:A prism 9 having a trapezoidal section is provided between a collimator lens 7 and an objective lens 2. This prism 9 can be formed also into a conical trapezoid. A light flux 1 made parallel by the lens 7 is separated into a main light flux 10 and a secondary light flux 11 through the prism 9. These light fluxes are irradiated on a disk 3 through the lens 2, and the flux 11 forms an image at a place apart from the flux 10. The reflected beams of these fluxes are made incident to a photodetector 5 via a convex lens 8. Accordingly the flux 11 is received by a 2- split optical detector to obtain a focus error signal. At the same time, the flux 10 is made incident to the lens 2 up to its full aperture to be stopped down to the limit of diffraction on a disk 3. In such way, the focus error signal can be detected without lowering the condensing efficiency. |
