Optical disk and optical disk system with numerical aperture of objective lens related to protective layer thickness of optical disk
 An optical disc system uses a rewritable optical disc with a light-transmitting cover and an objective lens for bundling or focusing a light beam on a recording layer of the optical disc in order to perform recording and/or reproducing of information. The recording layer is formed of a phase-change material. The thickness of the light-transmitting cover falls within the range of 0.05 mm to 0.6 mm, the numerical aperture (NA) of the objective lens is set to fall within the range of 0.55 to 1.10, and the wavelength of the light beam is selected to be between 100 nm to 780 nm. 
Sony Corporation
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Yoshio Tetsu
Aoki Watanabe