Dynamic information storage or retrieval – Systems having plural physically distinct independent tracks... – Having layered storage medium
Reexamination Certificate
1997-04-15
2001-11-27
Klimowicz, William (Department: 2652)
Dynamic information storage or retrieval
Systems having plural physically distinct independent tracks...
Having layered storage medium
C369S286000, C369S272100, C369S288000, C428S064100, C428S064400
Reexamination Certificate
active
06324148
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an optical disk comprising a plurality of information recording layers and an optical information recording/reproducing apparatus for recording/reproducing information on/from the optical disk.
Conventionally, optical disks have been used as information recording media with a large capacity and a random access capability. And, in recent years, optical disks have been strongly demanded to have larger capacities, since information amount handled in information-associated devices is increasing more and more, and necessity of recording larger capacities of information such as digital dynamic picture images has arised as the information has been presented in multimedia.
To meet such demands, a technology has been proposed lately which enabled information to be read out from each of a plurality of information recording layers provided on one side of an optical disk by changing the focal position of a light for producing information.
As well known, optical disks are classified into three types according to functions; read-only-memory type, write-once type, and erasable type. Each type has its own characteristics and is used differently from others. The erasable type optical disk is excellent in versatility and can be used for writing and reading out dynamic image information, so it is highly expected to be provided with a large capacity. Consequently, one proposal to realize a large capacity erasable optical disk is to provide a plurality of erasable information recording layers on one side of the optical disk. Such a configuration of the disk, however, had a difficulty in being practically used because of a problem to be explained below.
FIG. 1
is a cross sectional view of an erasable optical disk provided with two information recording layers on one side, which is a typical design example of such an optical disk. The optical disk
100
in such a design example is provided with the first information recording part L
1
, which is partially reflecting; a spacer layer
105
; the second information recording part L
2
; and a reflective film
109
formed sequentially on one side of the substrate
101
. The first information recording part L
1
comprises a transparent dielectric film
102
made of SiN; an information recording layer
103
made of TbFeCo, which is a typical rare earth-transition metal magnetic material; and a transparent dielectric film
104
made of SiN, which are formed sequentially from the substrate
101
. In the same manner, the second information recording part L
2
also comprises a transparent dielectric film
106
made of SiN; an information recording layer
107
made of TbFeCo; and a transparent dielectric film
108
, made of SiN formed sequentially from the spacer layer
105
. The substrate
101
is made of, for example, polycarbonate and the spacer layer
105
is made of, for example, ultraviolet-curing resin. The reflective film
109
is made of, for example, Al.
In the design example shown in
FIG. 1
, let the refractive index n of SiN be 2.0, the complex index of refraction n−ik of TbFeCo be 3.6−i·4.1, the wave length of the information reproducing light be 690 nm, and the intensity reflectivity of the second information recording part L
2
be 30%. In this case, the respective relationships between the thickness d (nm) of the information recording layer
103
and each of the intensity reflectivity R
1
of the first information recording part L
1
, the final intensity reflectivity R
2
of the second information recording part L
2
, which means the ratio of the intensity of the returned light from the second information recording part L
2
to the intensity of the incident light to the first information recording part L
1
, and the light absorptivity A of the first information recording part L
1
is as shown in FIG.
2
.
In order to stabilize the information reproducing operation in an apparatus for reproducing information from an optical disk having two information recording layers as shown in
FIG. 1
, both the intensity reflectivity R
1
of the first information recording part L
1
and the final intensity reflectance R
2
of the second information recording part L
2
must become almost equal. And, R
1
and R
2
become almost equal under the above condition when the thickness d of the information recording layer
103
of the first information recording part L
1
is about 6 nm. At this time, both R
1
and R
2
become about 8%. However, it is very difficult to carry out a mass production of such an information recording layer
103
while it is kept accurately to a uniform thickness of about 6 nm. Because the light absorptivity in the information recording layer
103
is too large, the thickness d of the information recording layer
103
must be reduced in such way.
Because of such a problem, it was actually impossible to realize a multi-layer erasable optical disk enabling information to be overwritten at least on the information recording layer provided on the light incident side.
SUMMARY OF THE INVENTION
Under such the circumstances, it is an object of this invention to provide an optical disk which comprises a plurality of information recording layers on one side so that at least the first information recording layer on the light incident side is erasable, and is enabled to be massproduced, as well as to provide an optical recording/reproducing apparatus that can record/reproduce information on/from the optical disk.
The optical disk of this invention comprises a plurality of information recording layers on one side so that the first recording layer on the light incident side is erasable and formed with a material whose extinction coefficient, which is a coefficient of an imaginary part of a complex index of refraction, is 1 or under.
The optical information recording/reproducing apparatus of this invention is for recording/reproducing information on/from an optical disk.
The optical disk used for the apparatus comprises a plurality of information recording layers on one side of the disk and at least a first layer of the information recording layers from a light incident side is of an erasable type formed with a material whose extinction coefficient, a coefficient of an imaginary part of a complex index of refraction, is not more than 1.0, and the apparatus comprises an information recording unit and an information reproducing unit.
The information recording unit records information on the optical disk by projecting and focusing a light for recording the information on an information recording layer being of an erasable type of a plurality of the information recording layers of the optical disk. The information reproducing unit reproduces information from each of the information recording layers of the optical disk by projecting a light having a wavelength suitable for making the extinction coefficient of the first layer be not more than 1.0 with the light being selectively focused on each of the information recording layers for reproducing the information and detecting the light returned from each of the information recording layers.
In the optical disk of this invention, since the first information recording layer on the light incident side is formed with a material whose extinction coefficient, which is a coefficient of the imaginary part of a complex index of refraction to decide the absorption characteristic, is 1 or under, the light absorptivity per unit thickness of the first information recording layer becomes small satisfactorily and the thickness of the first information recording layer and its tolerance can be set large enough to cope with mass production of the optical disk.
In the optical information recording/reproducing apparatus of this invention, information is recorded on the optical disk using the information recording unit in which a light for recording information is projected and focused on the erasable information recording layer of a plurality of the information recording layers and the information is reproduced from the optical disk using the information reproducing u
Aratani Katsuhisa
Kaneko Masahiko
Nakaoki Ariyoshi
Klimowicz William
NEC Corporation
Sonnenschein Nath & Rosenthal
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