| 摘要 |
PROBLEM TO BE SOLVED: To suppress spherical aberration caused by unevenness in the thickness of a transparent substrate of a recording and reproducing medium. SOLUTION: A transparent substrate side 9 of an optical disk 11 is irradiated with a light beam emitted from a semiconductor laser 2 through an object lens 5. On the front face of the object lens 5, 1st and 2nd regions AR1, AR2 different in the curvatures are formed. A recording and reproducing plane 10 of the optical disk 11 is irradiated with a spot light beam from the 1st region AR1, the surface of the transparent substrate 9 is irradiated with a pattern light beam from the 2nd region AR2 and the 1st and 2nd return light beams reflected and diffracted by each of the planes 10, 9a are condensed by the object lens 5 again and are irradiated on the 1st and 2nd light-receiving regions 12, 13 of a photo-detector 7 via a hologram element 12. Since the 1st and 2nd return light beams contain the positional information of each of the planes 10, 9a in the direction of the optic axis, it is possible to determine unevenness in the thickness of the transparent substrate 9 by detecting and comparing these positional information based on the outputs detected by the light-receiving regions 12, 13. Spherical aberration is corrected, based on this information of unevenness in the thickness. |
