| 摘要 |
PURPOSE:To easily detect information recorded with high density, by providing an objective lens which condenses only a part out of reading beams passing a ring area coming off the optical axis of a pupil surface on an information recording medium. CONSTITUTION:The objective lens 5 is provided with a shielding plate 5a which shields a center part including the optical axis of a pupil surface. Therefore, after a reading beam emitted from a laser beam source 1 is condensed on an object 3 by a condenser lens once, only a component passing through the ring belt shaped aperture part of the pupil surface of the lens 5 is condensed on a disk plane 6 via a beam splitter 4. And the reading beam reflected on the disk plane 6 returns to the lens 5 again, and only the component passing the ring belt shaped aperture part is made incident on a detector 8. By constituting a device in such way, the expansion of the center part of a beam spot formed on the disk 6 can be compressed. Therefore, a spot size used for the reading of the information can be decreased, and it is possible to perform the reading of the information recorded with high density easily. |
