Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
1998-03-16
2002-06-04
Neyzari, Ali (Department: 2653)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
Reexamination Certificate
active
06400677
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical disc and in particular, to an optical disc having a land portion and groove portion, on both of which recording is carried out.
2. Description of the Prior Art
In the field of information recording, various groups are studying on the optical information recording method. This optical information recording method have a variety of merits, enabling to record and reproduce in a non-contact state and to obtain a high recording density than in a magnetic recording method by ten times or more, coping with various memory forms. The optical information recording method has been widely spread in practice for industrial use and civil use as a method enabling to realize a low-cost large-capacity file.
As a recording medium of reproduction-dedicated type, there are widely spread a digital audio disc on which a music information is recorded and an optical video disc on which a video information is recorded. Moreover, as a recording medium of writable type, there are spread a magneto-optical disc and an optical disc of phase change type.
There have been suggested various methods to increase the density of these optical discs. As one of these methods, there is an approach to record a data bn each of a land portion and a groove portion of a disc so as to realize a high density.
Normally, on an optical disc substrate is formed a continuous groove in a spiral form or coaxial form and a land formed between grooves. Conventionally, a data signal is recorded on either of the land portion or the groove portion as a recording track of the optical disc. This is because if recording is carried out on both of the land portion and the groove portion, a cross talk is caused, i.e., a signal is read from an adjacent track when reproducing an information.
However, from the viewpoint of recording density increase, it is by far more advantageous to utilize both of the land portion and the groove portion for recording a data signal. Consequently, Japanese Patent Application 5-282705 discloses an optical disc in which both of a land and a groove are used as tracks for recording by setting a groove depth Gd in the vicinity of &lgr;/6n (wherein &lgr; represents a laser wavelength used for data reproduction and n represents a refraction index of the substrate). Hereinafter, this method will be referred to as a land/groove method. In an optical disc having a groove depth Gd set to satisfy the aforementioned condition, when reproducing the groove, for example, even if a laser beam is radiated to an adjacent land, cross talk from the land can be suppressed to a extremely small value. The same applies to a case when the relationship between the land and the groove is reversed.
In the optical disc of the aforementioned land/groove method, a groove and land are formed on a substrate so as to have an almost identical width and a recording layer is formed on the groove and land thus formed.
However, this optical disc of the land/groove method has a problem that optical characteristics and other conditions differ between the land and the groove, disabling to carry out a preferable land/groove recording.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical disc of the land/groove method in which optical characteristics and other conditions are equivalent in the land and the groove, which can be preferably used to carry out a preferable land/groove recording.
In order to achieve the aforementioned object, the inventor of the present invention studied on an optical disc appropriate for land/groove recording and found the cause of the aforementioned problem in the conventional land/groove type optical disc, i.e., different optical characteristics between the land and the groove formed on the recording layer. By removing the cause, the inventor has reached the present invention.
The land/groove type optical disc is prepared by forming a land and a groove so as to have an identical width on a transparent substrate made from polycarbonate or the like on which a recording layer is formed. A signal recording and reproducing is carried out by applying a laser beam from the surface of the disc not having the recording layer, and the laser beam is scattered and diffracted depending on the configurations of the land and the groove formed on the recording layer.
As shown in
FIG. 1
, in a conventional land/groove type optical disc, the recording layer formed on the slanting portion of the groove has a slightly thinner thickness and the groove formed on the substrate is half-filled with the recording layer, resulting that the groove formed on the recording layer has a smaller width than that of the land. Thus, even if the land and the groove of an identical width are formed on the substrate, the groove and the land formed on the recording layer have different width values and different optical characteristics.
In order to obtain identical optical characteristics between the land and the groove in the land/groove type optical disc, it is necessary that the land and the groove formed on the recording layer have approximately identical width.
Consequently, the present invention provides an optical disc having a substrate on which a groove and a land are formed so that a data signal is recorded on both of the land and the groove portions, the optical disc being characterized in that the groove formed on the substrate has a greater width than the land formed on the substrate and at least a recording layer is formed on the substrate where a data signal is recorded.
Furthermore, the inventor of the present invention used a typical schematic figure showing correspondence between the configurations of the substrate and the reflection film surface, so as to create an approximate expression which determines the optimal substrate configuration, and checked various optical discs to verify this approximate expression.
Firstly, as the typical schematic figure, the inventor used an optical disc as shown in
FIG. 1
having a substrate on which a recording layer is formed.
Here, Wg represents an average width of a groove formed on the substrate; W1, an average width of a land formed on the substrate; Tg, a film thickness of a recording layer formed on the slanting portion of the groove formed on the substrate; To, a film thickness of the recording layer formed on the flat portion of the groove formed on the substrate; &thgr;, an angle determined by the slanting portion and the flat portion of the groove formed on the substrate; and Tp, a sum of the average width Wg of the groove and average width W1 of the land formed on the substrate. As shown in
FIG. 1
, a groove formed on the recording layer after film formation has an average width smaller by 2&Dgr; than the average value Wg of the groove formed on the substrate and as a result, a land formed on the recording layer after film formation has an average width greater by this value.
In the land/groove method, it is necessary that the groove formed on the recording layer after film formation have almost identical value as the average width of the land. Consequently, the following relationship should be satisfied.
Wg
−2&Dgr;=
W
1+2&Dgr; (2)
If the track pitch is assumed to be Tp, then Tp=Wg+W1. This can be introduced into the Expression (2) to obtain the following expression.
Wg=
0.5
Tp
+2&Dgr; (3)
If the &thgr; is the angle determined by the slanting portion and the flat portion of the groove formed on the substrate, the &Dgr; can be expressed as follows.
&Dgr;=(
Tg−To
)/tan &thgr; (4)
Thus, if the &thgr; and the Tg are known, the Wg value as the design target can be determined.
The value of theta can be defined by the following expression, assuming Ge as the width of the groove formed on the slanting portion and Gd as the depth of the groove formed on the substrate.
&thgr;=tan
−1
(
Gd/Ge
) (5)
The values of Ge and Gd can be easily obtained from an A
Kagawa Masaki
Nakano Jun
Neyzari Ali
Sonnenschein Nath & Rosenthal
Sony Corporation
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