| 摘要 |
PURPOSE:To improve the sensitivity of a focus detecting signal by providing a photodetecting element at a position opposite to a photodetecting element divided into four parts centering on the optical axis of a reflected luminous flux and also at a position away from the optical axis by an amount larger than the beam radius of the reflected luminous flux in a focused mode. CONSTITUTION:The luminous flux delivered from a laser light source 1 is condensed on the surface of an optical disk 6 via an objective lens 5 for recording reproduction of information. A part of the luminous flux reflected from the disk 6 is received by a photodetecting element for focus detection. A photodetecting element 10 is provided at the position opposite to photodetecting elements 82 and 83 centering on the axis of the reflected luminous flux and to a 4-split photodetecting element 8 with distances secured to photodetecting elements 81 and 84 respectively. If a focus shift is produced in the direction where the surface of the disk 6 approaches from the focused position, the reflected luminous flux is turned into dispersed beams. Then some dispersed beams are detected by the element 10, and therefore a focus signal increased in the negative direction is obtained. Thus the sensitivity is improved for a focus detecting signal. |
