Optical disk system having objective lens with a numerical aperture related to thickness of protective layer
 Disclosed is a magneto-optical disc system comprising a magneto-optical disc 40 with a light-transmitting cover 41, an objective lens 2 for bundling or focusing a laser beam on a magnetic recording layer 43 of the magneto-optical disc 40 in order to perform recording and/or reproduction, and a magnetic field generating unit 9 obtained by forming a coil pattern 7 in an optical glass 8. In the above system, the thickness t.sub.2 of the light-transmitting cover 41 is set to fall within the range of 0.6 to 0.1 mm, and the numerical aperture (NA) of the objective lens 2 is set to fall within the range of 0.55 to 0.70, so as to obtain high-density recording and/or reproduction. 
Sony Corporation
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Tetsu Yoshio
Watanabe Aoki