Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Patent
1991-09-16
1998-07-21
Nguyen, Hoa T.
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
360114, G11B 1114, G11B 1304
Patent
active
057843430
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a magneto-optical recording and/or reproducing apparatus for recording information signals in and/or reproducing information signals from a magneto-optical recording medium due to magneto-optical effect such as magnetic Kerr effect or Faraday effect.
BACKGROUND ART
The optical modulation scheme and the magnetic field modulation scheme are known as to recording in magneto-optical recording media. An overwrite operation can be performed according to the magnetic field modulation scheme.
A schematic arrangement of a conventional magneto-optical recording and/or reproducing apparatus due to the magnetic field modulation scheme is shown in FIG. 1.
Referring to FIG. 1, the magneto-optical recording and/or reproducing apparatus is arranged such that when a magneto-optical recording medium 50 is mounted, an optical system including a laser unit 58 and an objective lens 59 is arranged below the magneto-optical recording medium 50, and a magnetic field generating unit 60 serving as a magnetic system is arranged above the magneto-optical recording medium. The objective lens 59 has a numerical aperture (to be referred to as an NA hereinafter) falling within the range of about 0.5 to 0.53.
In the magnetic field modulation scheme, a high-speed reverse control of a magnetic field is required in accordance with information signals supplied to a magnetic field generating unit, so that a sufficiently large excitation current cannot be obtained, and the intensity of the generated magnetic field is limited. For this reason, the magnetic field generating unit 60 is arranged near a magnetic recording layer 53 provided in the magneto-optical recording medium 50. In general, the magnetic field generating unit 60 is positioned near a side of the magneto-optical recording medium 50, where a protective cover 56 is formed, and face the objective lens 59 arranged near the other side of the recording medium 50 as shown in FIG. 1.
The magneto-optical recording medium 50 is formed as follows. A dielectric layer 52, a magnetic recording layer 53 having a large magneto-optical effect and made of, e.g., a rare earth element-transition metal alloy amorphous thin film, a dielectric film 54, a reflecting layer 55, a protective cover 56 are sequentially stacked on one surface of a transparent substrate 50 made of, e.g., polycarbonate. The thickness of the substrate 51 is generally set to be 1.2 mm.
An operation of the magneto-optical disc system will be described below.
The magneto-optical recording medium 50 is placed on and driven by a rotary disc (not shown), and a magnetic field from the magnetic field generating unit 60 is applied to the magnetic recording layer 53 of the magneto-optical recording medium 50. A high-speed reverse control of the applied magnetic field is performed in response to information signals to be recorded as described before. A laser beam emitted by the laser unit 58 is bundled or focused on the magnetic recording layer 53 by the objective lens 59, where the magnetic field is applied. The direction of magnetization is changeable in a region of the magnetic recording layer 53 irradiated by the focused laser beam, so that an overwrite operation of information signals can be performed in real time.
In order to record information in high density in the magneto-optical recording medium and, thereby, increase its recording capacity, it is assumed that a laser beam is focused on the magneto-optical recording medium by an objective lens for the spot to be made as small as possible. For this purpose, the NA of the objective lens in the optical system must be increased. The increase of the NA, however, poses the following problem.
The objective lens in the optical system is subjected to correction of spherical aberration W.sub.40 represented by the following equation: N: refractive index of transparent substrate of optical storage recording medium)
The spherical aberration of an objective lens having a large NA is also corrected due to equation (1). The correction value is d
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Watanabe Tetsu
Yamagami Tamotsu
Frommer William S.
Nguyen Hoa T.
Sony Corporation
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