| 摘要 |
PURPOSE:To detect the focusing error amounts symmetrical between front and rear focuses based on a focused position by distributing four photodetecting elements forming a photodetector approximately in proportion to the lengths of long and short axes of an oval beam spot whose photoelectric output is projected on the photodetector. CONSTITUTION:The luminous flux given from a laser diode 21 is projected in the form of microspots on a recording medium 25 where information pits are formed via a polarized beam splitter 22, etc. The reflected light of the medium 25 is made incident on a cylindrical lens 26 serving as an astigmatic element from the splitter 22 and then receives a lens function in the direction X vertical to a cylinder axis Y of the lens 26 to be turned into an oval beam. This oval beam is made incident on a photodetector 27. The detector 27 contains four rectangular photodetecting elements 27a-27d. A circular beam spot 30 is projected on the detector 27 when the medium 25 is set in a focused state; while oval beam spots 32 and 31 are projected on the detector 27 in the front and rear focus states respectively. |
