| 摘要 |
PURPOSE:To prevent a reproducing signal SNR from deteriorating, by detecting complementarily the size of a beam with two-piece photodetectors located apart with the same distance at the front ad rear of an image forming point, obtaining a linear focus dislocation signal, and using effectively all of the reflected light quantities. CONSTITUTION:Light beams emitted from a semiconductor laser 1, after being changed to a parallel beams with a collimator lens 3 through a beam splitter 2, are condensed on a disk 5 through an objective lens 4. Those light flux are reflected on a direction to interset orthogonally with an incident light flux, then being made incident on a four-piece photodetector 6. The photodetector 6 is recognized as that two-piece photodetectors C and D are positioned at the back of the photodetectors A and B, looking from an optical axis direction, and when it is watched from a direction in parallel with the optical axis, the photodetectors A and B, and C and D are arranged apart with the same distance from the image-forming point. And by adjusting a device so as to coincide the outputs of the photodetectors A, B, C, and D when focusing is obtained, the light receiving quantities in the photodetectors B and C increase when the disk goes away. In this way, the linear focus dislocation signal can be obtained, and also, all of the reflected light quantities can be used effectively. |
