Information recording medium

Details Information recording medium Information recording medium

2002-02-12

2004-08-31

Turner, Archene (Department: 1775)

Stock material or miscellaneous articles

Circular sheet or circular blank

Recording medium or carrier

C428S064200, C428S064800, C428S065100, C428S065100, C428S690000

Reexamination Certificate

active

06783832

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical information recording medium for recording and/or reading information optically; and, more particularly, the invention relates to a multi-layered optical disk having an improved recoding/reading accuracy.
One way to achieve an optical disc having a high capacity is by use of a multilayered medium having plural laminated information recording layers. As a read-only disk, a dual-layered DVD-ROM has been proposed. As for a rewritable dual-layered medium, the Japanese Journal of Applied Physics, Vol. 38, pp. 1679 to 1686 (1999), introduces such a technique therein, for example. In these techniques, recording layers are built up with an interval of several 10 &mgr;m, and recording/reading of information is effected by focusing optical spots onto each layer. Information on a side opposite that which receives the incident light is recorded/read as the light passes through the layer on the side of the incident light. When reading information from an inner layer, the light passes through the layers on the side of the incident light twice.
These techniques can increase the recording capacity of a medium of the same size to about twice.
JP-A-21720-1993 discloses a triple-layered recordable medium using a high transmittance organic recording film. In accordance with this method, the transmittances of the three layers are 70%, 80% and 90%, respectively, and the transmittance of a recording mark thereof is 100%. By detecting the amount of transmitting light, data recorded on the three layers are read at the same time. The method is capable to increasing the recording density and data transfer rate by about three times.
As for an optical disk using a nonlinear optical layer, a method of photon super-resolution has been proposed. Several methods using this technique have been proposed, and the Japanese Journal of Applied Physics, Vol. 32, p. 5210, discloses one of them, for example. The described method is characterized in that, by providing a mask to block a part of the optical spot so as to transmit light only through an unmasked portion thereof, the effective spot diameter is reduced, thus increasing the density and capacity of the optical disk. Specifically, in the process of photon super-resolution, when the light focuses on a film, the light transmittance of the light of higher intensity is increased, whereas the reflectance of the non-focused portion is high. In known techniques based on the use of photon super-resolution, a medium has reflective films, and the transmittance of the medium itself is always substantially 0%.
The abovementioned techniques, however, have some inherent problems. In recordable and rewritable optical disks, in particular, it is difficult to form a dual-layered medium that secures a satisfactory process margin in consideration of mass productivity and the products or margins for recording/reading conditions. This is because it is difficult to achieve an optimum optical design that allows for high signal modulation in both layers. To increase the signal quality obtained from the light-incident side, the transmittance of the layer should be lowered, and it is better to increase the reflectance and create larger differences in reflectance between a marked portion and a spaced portion. On the other hand, however, for the layer of the farthest side, the higher the transmittance of the layer on the light incident side is, the higher will be the signals that can be received. As such, as for setting the transmittance of the light incident side layer, the signal needs to be shared by two layers, because the optimum transmittance is in conflict for both layers. More details thereof will be described hereinbelow.
Hereinbelow, a dual-layered phase change medium will be described in which L
0
denotes a layer on the side facing the incident light, L
1
denotes a layer on the other side side thereof, Rc denotes the disk reflectance of a crystal state, and Ra denotes the amorphous state thereof. Rc and Ra of layers L
0
and L
1
are indicated respectively as Rc
0
, Ra
0
, Rc
1
, and Ra
1
. The amount of reflected light/incident light, i.e., the drive reflectance, is indicated as Rcd and Rad, and the transmittance of layer L
0
is indicated as T
0
.
Suppose T
0
=60% and (Rcd, Rad)=(15%, 2%). The reflectance takes on values close to the reflectance of a phase change disk that is currently produced. It is desirable to obtain the same amount of signal from layers L
0
and L
1
. Calculating reflectance, while taking the above into consideration, a setting value for the reflectance of the layer L
1
is (Rc
1
,Ra
1
)=(41.7%, 5.6%). However, it is difficult to design a disk for a phase change medium that is capable of overwriting that has a reflectance of 40% or higher. If T
0
is set higher than 60%, the reflectance and light absorption of the layer L
0
is lowered significantly, and it becomes impossible to obtain a desirable property at the layer L
0
. Moreover, it is necessary that the transmittance is substantially the same in the crystalline state and the amorphous state for the following reason: when the layer L
0
has a marked portion and an unmarked portion, and if the light spot passes on a border of two areas of the layer L
0
as the light reads the layer L
1
, the direct current element and amplitude of the signal in reading the layer L
1
fluctuate, thereby causing an increase in jitter or the error rate. Therefore, any accidental error of the transmittance for those two states should be suppressed to less than 5 to 10%. However, maintaining a translucent transmittance with the range is difficult when considering the processing margin.
Moreover, while a dual-layered medium generates problems, such as the above, it is almost impossible to achieve a recordable/rewritable optical disk having three or more layers. The triple-layered recordable disk technique described above detects transmittance. In this method, however, optical systems need to be located above and below the disk. Such a structure makes it difficult to adjust the optical systems, thereby lowering the production margin of the drive. Moreover, the method is not applicable to a rewritable disk.
In the super-resolution technique, the effective spot diameter can be made smaller, thus allowing for a higher density. However, the technique has drawbacks, as follows: A. When considering the processing margin for mass productivity, it is difficult to make the size of the light transmitting portion constant over the entire surface of the disk. B. In an optical disk, the signal to noise ratio S/N becomes an issue; and, in this regard, the area of an effective spot as a part of the spot diameter determines the signal level, while the spot diameter irradiating the disk determines the noise, whereby the signal is increased for a short mark, but the overall S/N, including the one for a long mark, is lowered.
SUMMARY OF THE INVENTION
In view of the above, the transmittance of the layer L
0
should be high at least while reading the layer L
1
in consideration of the layer L
1
. When reading the layer L
1
, a signal for reading the layer L
1
is determined by a square of the transmittance of layer L
0
, that is, T
0
2
. The value of the obtained signal should be no lower than a half of the signal obtained from a single layer L
1
, thus a desirable value is expressed as:
 
T
0
2
≧50%
∴
T
0
≧71%  EXPRESSION 1
In the case of a triple-layered medium, the signals are determined by a square of the transmittance of layer L
0
when reading the layer L
1
, i.e., T
0
2
, and a product of the squared transmittance of layers L
0
and L
1
, i.e., T
0
2
T
1
2
, when reading the layer L
2
. In this case, each signal for reading layers L
0
and L
1
is desirably expressed by:
T
0
2
≧⅔=67%
∴
T
0
={square root over ({fraction (2/3)})}≈82
%  EXPRESSION 2
T
0
2
T
1
2
≧⅓=33%
∴
T
1
={square root over ({fraction (1/2)})}=71
%  EXPRESSION

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