Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2001-11-08
2004-08-03
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044230, C369S275400
Reexamination Certificate
active
06771584
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical disk and an optical disk apparatus wherein light is condensed at a very small spot to perform recording or playback of information, and also to a method of correcting an aberration appearing on a very small spot upon recording or playback of information using an optical disk of the type just mentioned.
2. Description of the Related Art
An optical disk is known as an information recording medium which allows high density recording without contacting with the same, and a compact disk (CD), a digital versatile or video disk (DVD) and so forth have been put into practical use. In high density recording, wherein an optical disk of the type described is used, in order to achieve an increase in the capacity and increase in the density, the numerical aperture of a condenser of an optical system to be used for formation of a very small spot is increasing. For example, where the numerical aperture for a CD in the past was 0.45, the numerical aperture for a DVD which allows higher density recording is 0.6. It is possible that a condenser of a further high numerical aperture of 0.8 or more will be used in the future.
Recording or playback of information onto or from an optical disk is performed usually by forming a very fine spot on an information recording layer of the optical disk.
FIG. 15
schematically illustrates a manner in which a very small spot is formed on an information recording layer of a conventional optical disk.
Referring to
FIG. 15
, an optical disk
100
is of the type generally placed in practical use and includes a protective transparent substrate
101
which covers a surface of an information recording layer
102
. A condenser
103
is a component of an optical system of a recording-playback section of a known optical disk apparatus, and a laser beam for information playback or for information recording is condensed by the condenser
103
so that a very small spot is formed on the surface of the information recording layer
102
of the optical disk
100
. The optical disk
100
is structured such that the information recording layer
102
having a very small structure of less than micrometer order is protected by the transparent substrate
101
so that reliability in recording or playback of information can be assured.
In a configuration where a laser beam condensed by the condenser
103
passes through the transparent substrate
101
and forms a very small spot on the surface of the information recording layer
102
as described above, if the numerical aperture of the condenser
103
is increased, then the influence of an aberration which appears with the spot when the thickness of the transparent substrate
101
is displaced from a designed value increases, and it becomes difficult to form a very small spot. If the numerical aperture becomes, for example, 0.85, then the tolerance of the displacement of the thickness of the transparent substrate becomes less than several &mgr;m.
An influence of a spherical aberration upon a very small spot when the displacement of the thickness of the transparent substrate is within its tolerance and when the displacement is outside its tolerance is described.
FIG. 16
illustrates a variation of the spot intensity distribution with respect to a variation of the focus displacement where the displacement of the thickness of the transparent substrate is within its tolerance (the thickness of the transparent substrate is substantially within a designed range (prescribed range)). In this instance, the beam diameter exhibits a substantially symmetrical variation before and after a focused point. Here, the focused point signifies a position in which the optical system including the condenser is focused on the object side (optical disk side) and corresponds to a focused point of a camera.
However, if the substrate thickness exceeds the tolerance, then a spherical aberration which provides concentric waveform variations appears.
FIG. 17
illustrates a variation of the spot intensity variation with respect to the variation of the focus displacement where the displacement of the thickness of the transparent substrate exceeds the tolerance. In this instance, the spot intensity distribution with respect to the variation of the focus displacement varies asymmetrically before and after the focused point and exhibits an irregular variation such that it has an increased side lobe or has an increased beam diameter at a certain portion thereof. In such a condition, it is difficult to obtain a good playback signal.
In such a case that the displacement of the thickness of the transparent substrate exceeds its tolerance, it is demanded to detect an aberration amount caused by a displacement in thickness of a transparent substrate by some method and correct an influence of the aberration.
Such an aberration detection apparatus as disclosed in Japanese Patent Laid-Open No. 2000-20999 has been proposed.
FIG. 18
is a schematic view showing a general configuration of the aberration detection apparatus disclosed in the document just mentioned.
Referring to
FIG. 18
, the aberration detection apparatus incorporates an interference optical system in an optical system for detecting reflected light from an optical disk and detects an aberration amount from a variation of the intensity distribution caused by interference. The aberration detection apparatus includes an interference plate
200
, a convergent lens
201
, and a split light detector
202
. Reflected light from an optical disk, or light condensed by a condenser (not shown) and reflected from the optical disk, passes through the condenser again, and part of the reflected light is split and enters the interference plate
200
, In the interference plate
200
, the incoming reflected light is split, and the split light components are overlapped with each other in a state wherein they are spatially displaced from each other a little. The overlapped light is introduced into the split light detector
202
by the convergent lens
201
. If an aberration occurs, then the interference condition changes and the aberration can be observed as a variation of the light intensity distribution by the split-light detector
202
.
When the numerical aperture of a condenser for forming a very small spot is increased in order to achieve further increase of the capacity and the density of an optical disk, if the displacement of the thickness of the transparent substrate for protecting the information recording layer of the optical disk exceeds its tolerance, then it is necessary to detect the aberration amount caused by the displacement of the thickness of the transparent substrate by some method and correct the influence of the aberration.
Although the aberration detection apparatus disclosed in Japanese Patent Laid-Open No. 2000-20999 mentioned above can solve the subject described above, since it need include a new optical system for detecting an aberration in an optical system of an optical disk apparatus, this gives rise to a new problem that it increases the scale of the apparatus and increases the cost.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical disk, an aberration correction method and an optical disk apparatus wherein a spherical aberration caused by a displacement of the thickness of a transparent substrate of the optical disk can be corrected without giving rise to an increase in scale of the apparatus or an increase of the cost.
In order to attain the object described above, according to an aspect of the present invention, there is provided an optical disk, comprising an information recording layer having a recording face on which a very small spot for recording or playing back information is formed, and a transparent substrate for covering the recording face of the information recording layer, the recording face of the information recording layer having a particular region in which a particular pattern from which a spherical aberration of the very small spot which appea
Hayes & Soloway P.C.
Huber Paul W.
NEC Corporation
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