| 摘要 |
PURPOSE:To ensure the stable detection of both the out-of-focus and a shifted tracking, by detecting the travelling direction of a beam spot on a photodetector which is formed, by an objective lens reflected from an information forming layer, and a cylindrical lens out of the optical axis, as well as the image formation patterns in these beam spots respectively. CONSTITUTION:The laser light 11 is condensed on an information forming layer 4 via a coupling lens 13, a polarized beam aplitter 15, a lambda/4 plate 16 and an objective lens 17. The reflected light travels backward to pass through the splitter 15 and is formed into an image on a photodetector 26 via a cylindrical lens 23, a spherical lens 24 and a mirror 25. An out-of-focus detecting beam spot 11a' which did not pass through the lens 23 and an oval beam spot 11b' for detection of track shift are formed on the photodetecting cells 26a-26c. Then the shifts of these spots toward an arrow A and in the direction orthogonal to the arrow A are detected. In such way, the stable detection is done for both the out-of- focus and a track shift. |
