Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SPECIFICATION
Optical Disc Recording Medium
BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to an optical disc recording
medium, and more particularly to a multilayer film type optical
disc recording medium having two or more recording layers, such
as a DVD (digital video disc).
2. Description of the Related Art:
As the need for processing large amounts of data such as
found in digital moving pictures on personal computers has
increased in recent years, the capacity of storage devices must
be enlarged.
When an optical disc is used as a medium of storage for
digital video, it is superior to a magnetic tape medium in
recording density and access speed but is inferior to magnetic
tape in storage capacity, and thus is inferior to magnetic tape
in the amount of digital video it is capable of storing.
In the field of optical laser discs there exists a
double-sided disc produced by laminating two one-sided discs back
to back to increase storage capacity or reproduction time. This
double-sided disc is not a desirable solution when using a single
optical head for reading information, because the disc must be
taken out and flipped over to be again inserted into the optical
disc readout device after playback of the information on one side
is completed.
Therefore, as a method for increasing the recording capacity
when reproducing (reading) information on only one side of the
disc, multiple recording layers on the disc can be employed.
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Although conventional multilayer discs are exclusively used for
reading data, a system by which two recording layers are used to
double the reproduction time has been considered for digital
video discs (DVD) which have been proposed as the next generation
of optical discs.
Fig. 1 shows an example of a cross-sectional structure of
such a double-layer medium for reading information on one side
thereof. Information is recorded by means of pits on the surface
of a substrate 51 consisting of a transparent material such as
plastic, formed by an injection molding. A transparent spacer
layer 52 is provided on the substrate, the spacer layer 53 having
a refractive index larger than that of the substrate so that a
second recording layer is disposed in accordance with either a
photo-polymer method (G. Bouwhuis et al., ~Principles of Optical
Disk Systems", Adam Hilger Ltd., Bristol and Boston, P. 204-206)
or a thin-sheet laminating method. Then, a reflective layer 53
having a high reflectance, e.g., aluminum, is formed thereon by
a sputtering method or the like.
A laser beam for reading information is incident through the
above-mentioned substrate 51 and focused on a first recording
layer K1 or a second recording layer K2. The information is
recorded as a series of pits of variable length on the recording
layers K1 and K2. When the pits are exposed to the laser beam,
the amount of the reflected light is decreased by scattering or
diffraction of the light. Then, the reflected light is received
by a photodetector on the optical head to be converted into
electric signals of variable amplitudes.
In case of an optical disc which is exclusively used for
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reproducing (reading) data, such as a compact disc or a laser
disc, there is no problem in applications in which the user only
receives information, as in e.g., music or movies. However, when
such an optical disc is used as a medium of storage device for
a personal computer, it is not well suited to highly-interactive
applications which require writing/reading operations, unless
information can be recorded on that medium.
For example, when the score of a computer game still in
progress is saved, a hard disc or a semiconductor memory in
which information can be recorded must be used instead of the
optical disc, and the operation of the medium becomes
complicated.
For the above-mentioned application, a recording medium in
which a read-only area is provided can be used, as using a medium
such as a partial ROM magneto-optical disc enables a user to
record any desired information (for example, the score of a
computer game still in progress) on the same disc during
reproduction of the previously-recorded information.
However, even if the information is recorded on the same
disc, since the read only area is separate from the recordable
area on the disc such as is the case with partial ROM discs, a
track jump or a seek of an optical head must be performed to
switch from one area to the other (e.g., changing from the
recordable area to the read-only area or changing from the read-
only area to the recordable area), which disadvantageously takes
a few tens or hundreds of milliseconds.
In order to eliminate these problems in the prior art, an
object of the present invention is to provide an optical disc
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recording medium having overlying recordable and read-only layers
and being capable of selectively recording predetermined
information on the same optical disc containing the read-only
information.
SUMM~Y OF THE INVENTION
To achieve this aim, the invention is an optical disc
comprising a substrate of transparent member such as plastic
material, the substrate comprising a read-only information
plane having pits on a surface thereof; a transparent spacer
layer formed on the substrate, the transparent spacer layer
having a refractive index larger than that of the substrate; a
tracking guide groove formed on the upper surface of the
transparent spacer layer; a writing information recording
layer formed above the tracking guide groove, and a protective
layer formed on the writing information recording layer.
When reproducing the information which is previously
recorded on the read-only plane of the optical disc having the
above-mentioned structure, the light is first focused on the
read-only information plane of the first layer. Irradiation
of a laser beam onto the pits provided on the read-only
information plane decreases an amount of light reflected by
scattering or diffracting it, and when the laser beam is
irradiated on a part where no pit is formed, an amount of
light equal to the product of the incident light and the
reflectance ratio of the first layer and second layer is
returned to the optical head side.
When this reflected light is converted into electric
signals, information concerning a length of the pits is
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derived from the amplitude of the electric signals. If
information must be recorded during a playback session in
which information is reproduced from the read-only information
plane, the laser beam spot is first moved to another plane.
This is achieved by temporarily turning off a focus servo and
operating a focusing actuator to forcibly move a focusing
position of the laser beam in a focusing direction. When the
focusing position has reached the writing information
recording layer, a focus jump is carried out by again turning
on the focus servo. As a result, a shift from a read-only
area to a recording area or from a recording area to a read-
only area can be attained in a short period of time.
According to another aspect of the invention, a threshold
value of the recording level in the above-mentioned writing
information recording layer is set to a sufficiently-high
value as compared to a value of a laser power needed to
reproduce information stored in the read-only information
plane.
This configuration effectively prevents the information
from being deteriorated or erased by the application of heat
to the writing information recording layer which is apt to be
caused when the focusing point of the laser beam is moved from
the read-only information plane.
According to another aspect of the invention, an
interference layer and a reflection layer are provided between
the above-described writing information recording layer and
the protective layer.
This structure improves the quality of
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recording/reproduction by the Faraday effect for transmission.
- BRIEF DESCRIPTION OF THE DRAWINGS
The novel features believed characteristic of the
invention are set forth in the appended claims. The invention
itself, however, as well as other features and advantages
thereof, will be best understood by reference to the detailed
description which follows, read in conjunction with the
accompanying drawings, wherein:
Fig. 1 is an explanatory view showing a cross section of
a conventional two-layer read-only disc and a reflective path
for a laser beam;
Fig. 2 is a partial cross-sectional view showing an
embodiment according to the present invention; and
Fig. 3 is a partial cross-sectional view showing a
modification of Fig. 1.
DETAILED DESCRIPTION OF THE PREFERRED EM~ODIMENTS
A first embodiment according to the present invention
will now be described with reference to Fig. 2.
In Fig. 2, reference numeral 11 denotes a substrate made
of transparent plastic or the like. This substrate 11 is
formed by a technique such as injection molding and is
provided with an read-only information plane llA having pits
on the upper side.
The pits formed on the substrate material reduce the
reflection of an emitted laser beam by scattering or
diffraction of light. The amount of the reflected light thus
increases or decreases depending on existence/ahsence of the
pits at the portion of the substrate material on which the
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laser beam is focused. Therefore, when the reflected light is
received by a photodetector to be converted into electric
signals, information concerning the length of the pits can be
derived from the amplitude of the electric signals.
A transparent spacer layer 12 having a refractive index
higher than that of the substrate 11 is provided on the read-
only information plane llA. Further, a tracking guide groove
12A (a pregroove) is formed on the upper surface of the spacer
layer 12; a writing information recording layer 13 consisting
of phase-change material or magneto-optical material is
superimposed thereon; and a protective layer 14 is further
superimposed on the phase-change of magneto-optical material.
Here, the transparent spacer layer 12 having a higher
refractive index than that of the substrate 11 enables the
laser beam to be reflected on the read-only information plane.
Also, the tracking guide groove 12A formed on the upper
surface of the transparent spacer layer 12 is used for
tracking the laser beam when recording to or reading
information from the writing information recording layer 13.
The writing information recording layer 13 has a
recording level threshold value which is set to be a
sufficiently-high value as compared with the reproduction
laser power of the above read-only information plane llA. In
such a case, the threshold value of the recording level in the
writing information recording layer 13 can be set to a value
which is sufficiently high as compared with the reproduction
laser power of the above-mentioned read-only information plane
llA by setting a refractive index of the transparent spacer
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layer 12, a composition ratio of the recording layer, film
thickness and other variables to appropriate values. As a
result, the information can be prevented from being erased,
even if the writing information recording layer 13 is heated
when a focus point of the laser beam is moved away from the
read-only information plane.
More specifically, the read-only information plane llA
having phase pits is formed on the substrate 11 made of
plastic or the like by a technique such as injection molding.
The transparent spacer layer 12 having a higher refractive
index than that of the substrate 11 is formed on the read-only
information plane llA and the tracking guide groove 12A is
further formed on the upper surface of the transparent spacer
layer 12 by the photo-polymer method or the like.
In general, light is reflected at the interface between
materials having different refractive indices. Assuming that
the refractive index of the material on the light incident
side is n1 and the refractive index of the other material is
n2, a reflectance ratio R of the reflected light can be
expressed as follows:
R = (n2 - n1) / (n2 + n1) ............. (1)
Since the refractive index of polycarbonate which is
often used as substrate material is approximately 1.58, use of
a transparent material (i.e., a material having an extinction
coefficient which is substantially zero), which has a
refractive index of approximately 3.0, as the transparent
spacer layer 12 can yield a reflectance of approximately 30
at the interface and form a practical reflective plane,
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although the material of the transparent space layer 12 is not
restricted to a particular type.
The tracking guide groove (pregroove) 12A formed on the
upper surface of the transparent spacer layer 12 is required
for tracking the laser beam when recording to or reading
information from the writing information recording layer 13.
The tracking guide groove 12A is formed by, e.g., the photo-
polymer manufacturing method. The writing information
recording layer 13, which is made of a phase-change recording
material or magneto-optical recording material and which is
formed on the upper surface of the transparent spacer layer
12, has a recording threshold value that is sufficiently high
as compared with the laser power used for reading information
from the read-only information plane llA by adequately
determining the refractive index of the transparent spacer
layer 12, the recording layer composition ratio, a film
thickness and other variables. This consequently prevents the
information from being deteriorated or erased when, e.g., the
writing information recording layer 13 is heated when the
focusing point of the laser beam is moved from the read-only
information plane llA to the writing information recording
layer 13.
In Fig. 2, although the material used for the writing
information recording layer 13 is not restricted to a
particular type in the present invention, it is effective to
use phase-change recording material (e.g. GeSbTe system),
magneto-optical recording material (e.g. TbFeCo system), dye
type write once material (e.g. cyanine dye) or other
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materials. The film thickness for these is 200-1000 Angstrom.
As for the material used for the transparent spacer layer
12, it is effective to use poly-crystal SiO2 or amorphous SiN
(film thickness is 500-1500 Angstrom). As for the material
used for the protective layer 14, it is effective to use the
same material as the material used for the transparent space
layer 12. However, the film thickness for the protective
layer 14 is preferably more than 500 Angstrom.
Here, when the phase-change recording material or the
magneto-optical recording material is used, it may be possible
to incorporate the structure shown in Fig. 3 in order to
enhance signals or adjust the sensitivity. The structure
shown in Fig. 3 is obtained by further providing an
interference layer 17 and a reflective layer 18 between the
writing information recording layer 13 and the protective
layer 14 shown in Fig. 1. In such a case, the film thickness
of the writing information recording layer 13 is set to be
relatively thin so that the incident light can pass through
the interference layer 17 and can be reflected by the
reflection layer 18 to return to the incident side.
The structure using an interference layer 17 and a
reflective layer 18 is described in S. Tanaka et al., ~Design
Concept of Magneto-Optical Disk", Japanese Journal of Applied
Physics, Vol. 28, 1989 September 28-3, pp. 67-70.
In case of the magneto-optical disc, since the angle of
polar rotation generally becomes larger by the Faraday effect
for transmission rather than the Kerr effect for reflection,
the embodiment shown in Fig. 3 is more advantageous.
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Moreover, the protective layer 14, at the topmost position,
effectively functions as an element for mechanically or
chemically protecting each of the above-mentioned layers.
Although the material used for each layer in Fig. 3 is
not restricted to a particular type in this invention, it is
effective to use, as the transparent spacer layer 12, poly-
crystal SiO2 or amorphous SiN (film thickness is 500-1500
Angstrom).
Also, it is effective to use, as writing information
recording layer 13, magneto-optical material such as TbFeCo
system or phase-change material such as GeSbTe system (film
thickness is less than 600 Angstroms).
Moreover, it is effective to use as the interference
layer 17, poly-crystal SiO2 or amorphous SiN (film thickness is
less than 1000 Angstrom), to use as the reflective layer 18,
aluminum (film thickness is more than 300 Angstrom), and to
use W resin as the protective layer 14.
The functioning of the optical disc during reproduction
in the above embodiment will now be described.
When reproducing (reading) information previously
recorded on the read-only information plane llA of the optical
disc, the light is focused on the read-only information plane
llA of the first layer. The information is recorded on the
read-only information plane llA as a series of variable length
pits (irregularities on the recording plane). When the pits
are exposed to the laser beam, the amount of the light
reflected by scattering or diffraction of the light is
decreased. On the other hand, when the laser beam is
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irradiated on a part of the substrate where no pit is formed,
the amount of the light reflected and returned to the optical
head side is equal to the product of the incident light and
the reflectance ratio of the first layer 11 and second layer
12.
When the reflected light is received by a photodetector
to be converted into electric signals, information concerning
a length of the pits can be derived from the amplitude of the
electric signals. If information must be recorded during a
playback session during which information is reproduced from
the read-only information plane llA, it is first required to
move the laser beam spot to another plane.
In order to move the laser beam spot to another plane, a
focus servo is temporarily turned off, and a control current
flows to a focusing actuator to forcibly move a focusing
position of the laser beam in a focusing direction. When the
focusing position has reached the writing information
recording layer 13, the focus servo is again turned on,
thereby performing the focus jump. The focus jump takes a few
milliseconds and switches from a reproduction area to a
recording area (or from a recording area to a reproduction
area) within a shorter time compared to the time taken for
the regional movement on the disc plane.
By the above-described structure, predetermined
information can be immediately recorded onto the same optical
disc being used for reproduction without adopting any other
recording means. Further, when the present invention is
incorporated into a personal computer or a game machine, it is
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possible to provide an excellent novel optical disc medium
which is suitable for highly interactive applications (for
example, computerized game applications).
While this invention has been described with reference to
an illustrative embodiment, this description is not intended
to be construed in a limiting sense. Various modifications of
the illustrative embodiment, as well as other embodiments of
the invention, will be apparent to persons skilled in the art
upon reference to this description. It is, therefore,
contemplated that the appended claims will cover any such
modifications or embodiments as fall within the true scope of
the invention.
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