Optical recording medium with multiple recording layers and...

Details Optical recording medium with multiple recording layers and... Optical recording medium with multiple recording layers and...

1999-04-27

2002-04-30

Evans, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064400, C428S064500, C430S270130

Reexamination Certificate

active

06379767

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical recording medium in which an information is optically recorded, and more particularly to a multi-layer optical recording medium having a number of recording layers.
2. Description of the Prior Art
Generally, an optical recording medium has been fabricated in a disc type having a fast random access ability. The disc-type optical recording medium is classified into a read-only type and a recordable type. Also, the recordable disc is subdivided into a write-once-read-only(WROM) type which is recordable once and a rewritable type which is recordable repeatedly. The read-only type disc includes a compact disc-read only memory(CD-ROM) and a digital versatile disc-read only memory(DVD-ROM), etc. Furthermore, the WORM type disc includes a recordable compact disc(CD-R) and a recordable digital versatile disc(DVD-R), etc. and the rewritable disc includes a rewritable compact disc(CD-RW), a rewritable digital versatile disc(DVD-RW or DVD-RAM) and a magneto-optical disc MOD, etc.
Such an optical recording medium requires a large recording capacity enough to record a large information such as a moving picture. In order to satisfy the requirement for a large recording capacity, there has been developed a double-face optical recording medium wherein a track pitch is narrow and a recording face exists in each of the upper and lower surfaces thereof. For example, U.S. Pat. No. 4,450,553 discloses a multi-layer optical recording medium having at least two recording layers alternately disposed with light transmission layers used as a substrate. The multi-layer optical recording medium is noticed in that it can assure a larger recording capacity than a double-face optical recording medium. According to the U.S. Patent, the recording layer used for the multi-layer optical recording medium has a light reflection characteristic as well as a light transmission characteristic. This is caused by a fact that, when a certain recording layer is accessed, the upper recording layers thereof must transmit a light while the certain recording layer must reflect a received light. Accordingly, the recording layers of the multi-layer optical recording medium is formed of a semi-permeable material having the two optical characteristics. The semi-permeable material has an intrinsic light reflection coefficient Rc expressed as a certain constant value to reflect a constant ratio of incident light amount irrespective of an intensity of the incident light as shown in FIG.
1
. The intrinsic light reflection coefficient Rc of the semi-permeable material becomes different depending upon a type of semi-permeable material. Due to the recording layers of the semi-permeable material, a light loss is generated at the multi-layer optical recording medium. For instance, when a second recording layer
8
included in a two-layer optical disc as shown in
FIG. 2
is accessed, that is, when a light spot is formed on the second recording layer
8
, a light incident to the second recording layer
8
, via a first light transmission layer
2
, a first recording layer
4
and a second light transmission layer
6
, from a light source(not shown) is lost by an amount corresponding to a reflection coefficient of the first recording layer
4
. Also, a reflective light reflected by the second recording layer
8
is more reduced by an amount corresponding to a light transmissivity of the second recording layer
8
. Furthermore, a reflective light passing through the second light transmission layer
6
, the first recording layer
4
and the first light transmission layer
2
from the second recording layer
8
is again reduced by an amount corresponding to a light reflection coefficient of the first recording layer
1
. Herein, it is assumed that an absorption coefficient of each of the first and second recording layers
4
and
8
is “0”, a light reflection coefficient R
1
of the first recording layer
4
is 0.3, that is, a light transmissivity (1−R
1
) of the first recording layer
4
is 0.7, and a light reflection coefficient R
2
of the second recording layer
8
is 0.7. In this case, a light amount reflected by the second recording layer
8
and emitting from the first light transmission layer
2
becomes equal to “0.343=(1−R
1
)×R
2
×(1−R
1
)=0.7×0.7×0.7” times of the light amount incident to the first light transmission layer
2
. On the other hand, when the first recording layer
4
of the two-layer optical disc in
FIG. 2
is accessed, a reflective light amount emitting from the first light transmission layer
2
becomes equal to 0.3 times of the light amount incident to the first light transmission layer
2
. In other words, when the first recording layer
4
is accessed, a light loss amount becomes 70% of the incident light amount.
In order to increase a reflective light amount emitted from the first light transmission layer
2
when the first recording layer
4
is accessed, the reflection coefficient R
1
of the first recording layer
4
can be heightened. For instance, if the reflection coefficient of the first recording layer
4
has been set to 40%, then a reflective light amount emitted from the first light transmission layer
2
when the first recording layer
4
is accessed, increases into 40% of the light amount incident to the first light transmission layer
2
. But, a reflective light amount emitted from the first light transmission layer
2
when the second recording layer
8
is accessed, decreases dramatically due to a low light transmissivity (1−R
1
) of the first recording layer
4
. In other words, a reflective light amount emitted from the first light transmission layer
2
when the second recording layer
8
is accessed, becomes equal to “0.252=(1−R
1
)×0.7×(1−R
1
)=0.6×0.7×0.6” times of a light amount incident to the first light transmission layer
2
. As described above, a reflective light amount when the second recording layer
8
is accessed, is dramatically reduced in comparison to a reflective light amount when the first recording layer
4
is accessed. Accordingly, a signal detected from the second recording layer
8
also has a dramatically lower voltage level than a signal detected from the first recording layer
4
. In other words, the signal detected from the first recording layer
4
can have a reproducible voltage level, but the signal detected from the second recording layer
8
fails to obtain the reproducible voltage level. As a result, a data recorded on the second recording layer
8
can not be read out by an optical pickup. In view of this, it is necessary to provide a multi-layer optical recording medium with a high light efficiency so that all the multiple recording layers can be accessed.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an optical recording medium with a number of recording layers that is adaptive for enhancing a light efficiency, and a method of fabricating the same.
Further object of the present invention is to provide an optical recording medium with a number of recording layers that is adaptive for improving a responsibility in the information reproduction, and a method of fabricating the same.
Still further object of the present invention is to provide an optical recording medium that is adaptive for increasing a recording capacity, and a method of fabricating the same.
In order to achieve these and other objects of the invention, in an optical recording medium with a number of recording layers according to one aspect of the present invention, at least one of the number of recording layers is made from a non-linear material having a non-linear optical characteristic.
A method of fabricating an optical recording medium with a number of recording layers according to another aspect of the present invention includes the steps of providing a number of substrates; forming a recording layer on each of the number of substrates in such a

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