| 摘要 |
PURPOSE:To obtain a focus detecting signal which is not influenced by the variation of a collimated state, based on a differential output, by providing the first focus detecting means for detecting a focal state of a going path going toward an optical disk from a collimated light source, and the second focus detecting means for detecting a focal state of a light of a return path. CONSTITUTION:Light of a going path, which has been collimated is divided into a disk 5 side and the first detecting means side by a beam splitter 3, and led to a 4-split photodetector 8 being the first focus detecting means through an image forming lens 6 and a cylindrical lens 7. On the other hand, as for light of a return path, which has been reflected by the disk 5, a part of the light is returned to a light source side, and the remaining light is divided to the second focus detecting means side, and led to a 4-split photodetector 11 being the second focus detecting means through an image forming lens 9 and a cylindrical lens 10. In this way, when a differentiation F of a focus detecting signal C and F+C is derived by providing an output of the 4-split photodetector 8 to a minus input of a differentiating circuit 12, and providing an output of the 4-split photodetector 11 to a plus input of the differentiating circuit 12, a correct focus detecting signal which is not influenced by a collimated state can be obtained. |
