Optical disk system having an objective lens with a numerical aperture related to the thickness of the protective layer
 A magneto-optical disc system includes a magneto-optical disc 40 with a light-transmitting layer 41, an objective lens 2 for bundling or focusing a laser beam onto a magnetic recording layer 43 of the magneto-optical disc 40 for recording information thereon and/or reproducing information therefrom, and a magnetic field generating unit 9 having a coil pattern 7 arranged on an optical glass element 8. The thickness t.sub.2 of the light-transmitting layer 41 falls within the range of 0.6 to 0.1 mm, and the numerical aperture (NA) of the objective lens 2 falls within the range of 0.55 to 0.70. 
Sony Corporation
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Yoshio Tetsu
Aoki Watanabe