| 摘要 |
PURPOSE:To improve an S/N near a focusing point and to increase a permissible amount of position shifting of a photodetector by reflecting the quantity of light which illuminate one convergence point on the photodetector by a recording medium, and making the reflected quantity less than half the total quantity of light passed through an objective, during focusing. CONSTITUTION:A parallel laser beam converges onto the recording medium 14 through the objective 40 and its reflected light converges onto the two-split photodetector 58 through an objective 40, light shield plate 52, and projection lens 54. When laser light 70 illuminates both photodetection cells 58a and 58b, the photodetector operates possibly as if an in-focus state were detected electrically even in an out-of-focus state due to dust or a flaw on an optical path, and dither and inclination of the surface of the information recording medium 14, a diffraction pattern of track grooves, etc. When the light shield plate 52 cuts off more than the half of the reflected light passed through the objective 40, the light strikes only either of the photodetection cells 58a and 58b, so the SN ratio increases almost at the focusing point to increase the permissible amount of the position shifting of the photodetector 58. |
