| 摘要 |
PROBLEM TO BE SOLVED: To eliminate an adverse effect due to useless reflected light from the other side recording layer for reflected light of reproducing laser light simultaneously emitted on plural tracks on one side recording layer to be reproduced by specifying an optical condition of a focus detection system. SOLUTION: A quadripartite optical sensor 10 detecting a focus error is made P<2.β.NA0 .df-H (where, P: a length from an outer periphery c/2 of a reflected light spot to the maximum position of the reflected light spot in the direction that anisotropy exists at an optimum operation point time based on the size C of the reflected light spot s1 on the quadripartite optical sensor 10 at the optimum operation point time, NA0 : a numerical aperture(NA) of an objective lens,β: mean image forming magnification from an optical disk to a multi-division sensor, df: 1.2.dDL/n, dDL: a gap between recording layers of optical disk, (n): a refractive index between recording layers of optical disk, H: a half of a variable of a width of the reflected light spot in the two directions that the anisotropy on the quadripartite sensor 10 exists).
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