Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse...
Reexamination Certificate
1998-11-16
2001-04-17
Dinh, Tan (Department: 2651)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Reexamination Certificate
active
06219308
ABSTRACT:
TECHNICAL FIELD
The present invention relates to optical and magneto-optical disc systems for performing recording and/or reproduction of an information signal, and optical and magneto-optical disc used in these systems.
BACKGROUND ART
A magneto-optical disc system utilizing a magneto-optical effect is shown in
FIG. 1
as an optical disc system for performing recording in and/or reproduction from an optical recording medium such as an optical disc or a magneto-optical disc.
A conventional magneto-optical disc system shown in
FIG. 1
is arranged such that when a disc-like one-sided magneto-optical recording medium
50
is mounted, an optical system including a laser unit
58
, an objective lens
59
, and the like is arranged above the one-sided magneto-optical recording medium
50
, and a magnetic field generation unit
60
serving as a magnetic system is arranged below the one-sided magneto-optical recording medium
50
. A numerical aperture (to be referred to as an NA hereinafter) of the objective lens
59
is set to fall within the range of 0.50 to 0.53.
In order to drive the optical system in focusing and tracking directions with respect to the magneto-optical recording medium
50
, a drive system (not shown) is arranged. Another drive system (not shown) is arranged to drive the magnetic system in a direction indicated by an arrow in FIG.
1
and the tracking direction.
In this magneto-optical disc system, a magnetic field modulation scheme is employed to perform recording. In this magnetic field modulation scheme, high-speed reverse control of a magnetic field must be performed in accordance with an information signal to be recorded. For this reason, a sufficiently large excitation current cannot be obtained. The intensity of the magnetic field generated by the magnetic field generation unit is limited. The magnetic field generation unit
60
is located near a magnetic recording layer
53
(to be described later) in the one-sided magneto-optical recording medium
50
. An overwrite operation can be performed according to the magnetic field modulation scheme.
The one-sided magneto-optical recording medium
50
is formed as follows. A dielectric layer
52
, a magnetic recording layer
53
having a large mangeto-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 light-transmitting transparent substrate
50
made of, e.g., polycarbonate. The transparent substrate
51
has a predetermined thickness t
1
. The thickness of a conventional substrate is set to be 1.2 mm.
An operation of the magneto-optical disc system will be described below.
The one-sided magneto-optical recording medium
50
is placed and driven on a rotary disc (not shown), and a magnetic field is applied from the magnetic field generation unit
60
to the magnetic recording layer
53
of the one-sided magneto-optical recording medium
50
. High-speed reverse control of the magnetic field to be applied is performed on the basis of an information signal to be recorded. A laser beam emitted from the laser unit
58
is focused through the objective lens
59
on the magnetic recording layer
53
applied with this magnetic field. A change in direction of magnetization occurs in a region of the magnetic recording layer
53
irradiated with the focused laser beam in accordance with the direction of the magnetic field applied from the magnetic field generation unit
60
. Therefore, an overwrite operation of an information signal can be performed in real time.
In order to obtain a more compact magneto-optical pickup, which is constituted by the optical system, the magnetic system, and the drive systems for driving the optical and magnetic systems, it is assumed that the optical system may be formed integrally with the magnetic system and both the systems are arranged on the one side of the magneto-optical recording medium. More specifically, the magnetic field generation unit
60
of
FIG. 1
is located on the side where the objective lens
59
is located (i.e., the side of the transparent substrate
51
). However, since a distance between the magnetic field generation unit
60
and the magnetic recording layer
53
becomes larger than before, a sufficient high magnetic field cannot be applied to the magnetic recording layer
53
.
Along with an increase in information volume in recent years, a two-sided magneto-optical recording medium has been developed, in which the magnetic recording layers are formed respectively on both surfaces of one magneto-optical recording medium so as to be capable of recording information signals on each of the surface.
In order to perform recording in and/or reproduction from such two-sided magneto-optical recording medium, it is very difficult to apply a sufficiently high magnetic field to each of the magnetic recording layers by means of the magneto-optical pickup of
FIG. 1
, constituted by the optical and magnetic systems. The reason for this is such that in the magnetic field generation unit of a magnetic field modulation scheme, a high-frequency current corresponding to a high-frequency data signal that is an information signal to be recorded must be supplied to an electromagnetic coil. A current tends to become difficult to flow through an electromagnetic coil as the frequency of the current increases, so that the generated magnetic field is limited. In addition, it is the another reason that the distance the magnetic field generation unit and each magnetic recording layer is rather large. Therefore, in state-of-the-art techniques, it is very difficult to perform two-side optomagnetic recording under the magnetic field modulation scheme.
In order to cope with an increase in information volume, more information signals must be recorded in, e.g., a recording portion such as the magnetic recording layer of an optical disc.
DISCLOSURE OF INVENTION
It is, therefore, an object of the present invention to provide a large-capacity optical disc system capable of performing recording and/or reproduction in higher density, and an optical disc used in this optical disc system.
According to an aspect of the present invention, there is provided an optical disc system for recording an information signal in an optical disc and/or reproducing the information signal from the optical disc, the optical disc being provided with a recording layer irradiated with a laser beam to perform recording or reproduction and a light-transmitting cover which is formed to cover the recording layer and through which the laser beam is transmitted, and the optical disc system comprising laser beam generating means for generating the laser beam and an objective lens for bundling or focusing the laser beam on the recording layer through the light-transmitting cover, characterized in that a thickness of the light-transmitting cover of the optical disc falls within a range of 0.6 to 0.1 mm, and a numerical aperture of the objective lens falls within a range of 0.55 to 0.70.
When the laser beam is bundled or focused by the objective lens, the convergent rays have a minimum diameter (2&ohgr;
0
) represented by equation (1) below:
2&ohgr;
0
=0.82
×&lgr;/NA
(1)
(&lgr;: wavelength of laser beam)
Since the numerical aperture NA of the objective lens is larger than a value falling within the conventional NA range of 0.50 to 0.53, the minimum diameter of the convergent rays is decreased, so that the recording density is increased. Therefore, reproduction corresponding to this high recording density can be performed.
As the NA is increased, the thickness of the objective lens is increased, but the thickness of the light-transmitting cover of the optical disc is smaller than the conventional thickness (1.2 mm), so that the objective lens having a larger thickness will not be brought into contact with the optical disc.
As the NA of the objective lens and the thickness t of the light-transmitting cover of the optical disc are changed, the aberrati
Aoki Yoshio
Watanabe Tetsu
Dinh Tan
Maioli Jay H.
Sony Corporation
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