Dynamic information storage or retrieval – Control of storage or retrieval operation by a control... – Mechanism control by the control signal
Reexamination Certificate
1999-09-09
2002-07-09
Hindi, Nahil (Department: 2653)
Dynamic information storage or retrieval
Control of storage or retrieval operation by a control...
Mechanism control by the control signal
C369S047500, C369S121000
Reexamination Certificate
active
06418103
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for initializing a phase-changing optical recording medium, and a phase-changing optical recording medium. More specifically, the present invention relates to an apparatus for initializing a phase-changing optical recording medium wherein a laser beam is irradiated to a recording layer, in an amorphous state, of an optical recording medium to be initialized, which is a pre-stage member of the phase-changing optical recording medium, for changing the amorphous state of the recording layer into a crystalline state with heat of the laser beam, and also relates to a phase-changing optical recording medium.
2. Description of the Related Art
One example of erasable and rewritable optical recording mediums is a phase-changing optical recording medium. A general schematic construction of the phase-changing optical recording medium will be described with reference to
FIGS. 7 and 8
which are schematic sectional views of different types of phase-changing optical recording mediums.
As shown in
FIG. 7
, in a phase-changing optical recording medium of the type that a laser beam emitted from a semiconductor laser enters the medium from the side of a transparent base plate
7
, a land
7
a
and a groove
7
b
are formed on the principal surface of the transparent base plate
7
in advance. On the transparent base plate
7
, a first dielectric layer
8
, a recording layer
9
made of a phase-changing material, a second dielectric layer
10
, a reflecting layer
11
, and a protective film
12
are formed successively in the order named. Also, as shown in
FIG. 8
, in a phase-changing optical recording medium of the type that a laser beam emitted from a semiconductor laser enters the medium from the side of a transparent cover layer
13
, a land
7
a
and a groove
7
b
are formed on the principal surface of a transparent base plate
7
in advance. On the transparent base plate
7
, a reflecting layer
11
, a second dielectric layer
10
, a recording layer
9
made of a phase-changing material, a first dielectric layer
8
, and the transparent cover layer
13
formed of a resin or film, for example, are formed successively in the order named. The thickness of the transparent base plate
7
or the transparent cover layer
13
is dependent on the NA (Numerical Aperture) of a condensing lens. Correlation between them is such that the thickness decreases as the NA of the condensing lens increases.
In the above structure, the recording layer
9
, the first and second dielectric layers
8
,
10
, and the reflecting layer
11
are generally formed by film forming steps using, e.g., sputtering or vapor deposition. The recording layer
9
is in an amorphous state after the film forming steps, and is then initialized from the amorphous state into a crystalline state. When information is recorded on the phase-changing optical recording medium, a recording mark portion is changed into the amorphous state with the intensity of the laser beam irradiated, while a non-recording portion remains in the crystalline state. In other words, before the user employs a phase-changing optical recording medium, an area of the medium which takes part in recording and reproduction of an information signal is entirely held in the crystalline state.
In the above initializing step, there are two typical methods shown in
FIGS. 9 and 10
, which conceptually illustrate the initializing process, for changing the recording layer
9
from the amorphous state into the crystalline state. An initializing apparatus is employed to carry out any of the two typical methods. The method shown in
FIG. 9
is called a fusion crystallizing process. With this process, the crystalline state is produced by raising the temperature of the recording layer
9
to a level higher than the melting point of the phase-changing material for change from the amorphous state into a molten state, and then slowly cooling it. The method shown in
FIG. 10
is called a solid-phase crystallizing process. With this process, the crystalline state is produced by raising the temperature of the recording layer
9
in the amorphous state to a level higher than the crystallizing temperature of the phase-changing material, holding the raised temperature during a period necessary for crystal growth, and then slowly cooling it. In other words, regardless of which one of the methods is employed, the initializing apparatus requires a means for raising the temperature of the recording layer
9
in the amorphous state, for example, a heat source such as a laser beam source. When a laser beam source is employed, the focus of a laser beam is formed by a condensing lens on the recording layer
9
made of the phase-changing material so that the focused area is subject to a large energy density.
FIG. 11
is a schematic view for explaining a process in which the recording layer
9
is changed from the amorphous state into the crystalline state by irradiating a laser beam, which is emitted from a laser beam source and focused through a condensing lens, to the recording layer
9
, in the amorphous state, of an optical recording medium to be initialized which is a pre-stage member of the phase-changing optical recording medium.
As shown in
FIG. 11
, a spot of the laser beam emitted from the laser beam source and focused through the condensing lens is formed on the recording layer
9
, in the amorphous state, of the optical recording medium to be initialized which rotates at a predetermined rotational speed. The temperature of an area, in which the beam spot is formed, is raised beyond the crystallizing temperature of the phase-changing material, and is then moved away from the beam spot with the rotation of the initialized optical recording medium for crystallization under slow cooling. Through such a process, the phase-changing optical recording medium is completed in which an area of the medium taking part in recording and reproduction of an information signal is entirely held in the crystalline state.
For bringing the entire area of the medium taking part in recording and reproduction of an information signal into the crystalline state, the energy density of the laser beam, which is emitted from the laser beam source toward the recording layer
9
made of the phase-changing material, must be held constant. Usually, the optical recording medium to be initialized involves an inherent slight warp attributable to flatness of the transparent base plate
7
. Therefore, when the optical recording medium to be initialized is, e.g., a disk-shaped medium, there occurs a plane runout on the order of ±500 &mgr;m during the rotation. Assuming that the numerical aperture of the condensing lens is NA and the wavelength of the laser beam emitted from the laser beam source is &lgr;, the focal depth d of the beam spot focused by the condensing lens is expressed by d=±&lgr;/2NA
2
. Given &lgr;=680 nm and NA=0.45, for example, d=±1.68 &mgr;m is resulted. This value is much smaller than the plane runout of ±500 &mgr;m that occurs during the rotation of the disk-shaped medium.
For that reason, a focus servo means utilizing the astigmatism process, the Foucault process or the critical angle process, for example, is indispensable in the initializing apparatus for the optical recording medium to be initialized, for the purpose of controlling the beam spot focused through the condensing lens to be always held within the focal depth with respect to the recording layer
9
of the initialized optical recording medium.
However, the initializing apparatus including the focus serve means has two problems as follows.
The first problem is concerned with the condensing lens.
The transparent base plate
7
or the transparent cover layer
13
has a different thickness depending on the type of the optical recording medium to be initialized. To form a normal beam spot without suffering the effect of spherical aberration, therefore, the initializing apparatus must i
Ito Kazumine
Kawakubo Osamu
Kawase Hiroshi
Nakamura Manami
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