| 摘要 |
PURPOSE:To prevent an adverse effect due to repeated diffraction by providing an optical device with size of 1/2 that of a lens pupil which diffracts reflected light from a disk in a tracking direction in the neighborhood of an objective lens to converge a laser beam from a laser beam source. CONSTITUTION:A diffraction grating 5 with size of 1/2 that of the pupil of the objective lens 6 and with the tracking direction in a diffracting direction is formed on a diffraction grating substrate 4. Incident laser beam from the laser beam source 1 is diffracted along the tracking direction on the disk 7, and around two sub beams generated by the diffraction are projected on positions before and behind a main beam in the tracking direction. In such a case, light transmitting a part where the diffraction grating with size less than around the half of the size of the objective lens 6 is provided is diffracted in the tracking direction, and is reflected on a disk plane. The reflected light transmits a part where no such diffraction grating is provided in the objective lens 6. Therefore, an error signal equivalent to the one by a three-beam system can be obtained. |
