| 摘要 |
PROBLEM TO BE SOLVED: To make it possible to interchangeably reproduce optical disks different in the thickness of a substrate by selectively generating a first laser beam having a first wavelength and a second laser beam having a second wavelength different from the first wavelength. SOLUTION: A laser beam transmitted from semiconductor lasers 1e, 1b in an optical means 202 having a wavelength 480 (allowable range: 350-550)nm or a wavelength 635 (allowable range ±15)nm is selectively diffracted by a hologram 1g provided on the surface of the optical means 202, selectively shielded by an numerical aperture changing means 203 and made incident on an objective lens 204. The laser beam converted by the objective lens 204 passes through a substrate 205 made of transparent polycarbonate and projected on the signal recording surface 205a of an optical disk. The laser beam reflected by the signal recording surface 205a returns through the substrate 205, the objective lens 204 and the numerical aperture changing means 203 and is detected by a photodetector 1c provided in the optical means 202 through the hologram 1g. |
