| 摘要 |
PROBLEM TO BE SOLVED: To provide a numerical aperture of an objective lens and a material for a transparent layer unaffected by the thickness of a transparent layer protecting a recording material. SOLUTION: An optical head device 3 has a laser element to emit a laser beam and an objective lens 36 to converge laser beams from the laser element at a prescribed position and a prescribed depth of a recording surface of an optical disk D, and a mumerical aperture NA of the objective lens is set to satisfy NA4×d<=0.0744 according to a thickness d of a transparent protective layer for protecting the recording layer 103 (113) of the optical disk, i.e., transparent substrates 101 and 111. As the result, without being affected by spherical aberration derived from the transparent substrate which is the protective layer, the recording density is enhanced. By specifying a material for the protective layer in accordance with the numerical aperture of the subjective lens, the recording density is enhanced without adding specified structure to the optical disk device. |
