| 摘要 |
PURPOSE:To improve the focus error detecting sensitivity of the title device by arranging a convergent optical system to convert a light flux divergent angle into a smaller divergent angle between a light source and an optical element to generate an astigmatism. CONSTITUTION:A light flux 9 emitted from a light source 7 of a semiconductor laser, etc., is converted into a light flux 8 with the smaller divergent angle. The divertent light flux 8 is reflected by a half mirror formed on a first surface 19 of parallel plates 10, passes through an objective lens 16, is made into a convergent light flux, and made incident on an optical disk 1. The light flux including information reflected by an information recording surface 2 transmits a half mirror surface 19 of the parallel plates 10, thereafter, reflected by mirror arranged at a rear surface 18, made into a light flux 8' possessing the astigmatism, and made incident on a photo detector 13. That is, by the function of the convergent lens 6, the divergent angle of the light flux 8 is set at 1/2 of the divertent angle of the light flux 9, and a desired lead-in range can be obtained even with the parallel plates 10 of the same width. Thus, the accuracy of the focus error detection can be increased. |
