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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates a read-only optical disk and more
particularly to an optical disk having at least two information recording
layers.
2. Description of the Related Art
Optical disks such as a digital audio disk (a so-called compact disk), a
video disk or the like are known as an information recording medium. When
information is recorded on an optical disk, the information is recorded as
phase pits formed along a circumference-direction track or the like. The
typical phase pits are a large number of minute convex or concave portions
formed discontinuously along the track. Therefore, the phase pits and
portions where no phase pit is formed are alternately formed along the
track.
When information is reproduced from the optical disk, rays of laser light
are irradiated on the optical disk which is being rotated. When the rays
of laser light are irradiated on the land, the rays of laser light are
reflected by the land as they are. On the other hand, when the rays of
laser light are irradiated on the phase pit, diffraction occurs and
diffracted light is dispersed to both sides. The diffracted light from the
phase pit and the reflected light from the land are detected by a
photodetector.
Intensity of the diffracted light detected by the photodetector is
sufficiently small as compared with that of the reflected light from the
land. Therefore, the photodetector can discriminate the diffracted light
from the phase pit and the reflected light from the land. The
photodetector converts an amount of detected received light into a current
signal. Thus, the phase pit and the land are encoded.
To increase a recording density of the optical disk, a length t of the
phase pit and a pitch p thereof may be set smaller. However, resolution of
information reproduction depends upon a radiation-spot diameter D. To
discriminate the phase pit and the land without intersymbolic
interference, the radiation-spot diameter D must be sufficiently small as
compared with the pitch p of the phase pit. Therefore, when the length t
of the phase pit and the pitch p thereof are set smaller to increase the
recording density of the optical disk, the radiation-spot diameter D must
be set smaller.
The radiation-spot diameter D is generally in proportion to a ratio
.lambda./NA of a wavelength .lambda. of light from a light source to a
numerical aperture NA of an objective lens. Therefore, although it is
preferable that, to reduce the radiation-spot diameter D, the wavelength
.lambda. of the light from the light source is set shorter or the
numerical aperture NA of the objective lens is set larger, it is difficult
because of various conditions.
A super-high-density information recording optical disk having a phase pit
pitch p which is sufficiently small as compared with a radiation-spot
diameter D is manufactured to reproduce information therefrom. This
reproduction is called a super high resolution or super resolution (SR)
information reproduction. Examples thereof are disclosed in Japanese
patent applications Nos. 94452/1990, 291773/1990 and 249511/1991 which
were filed by the same assignee as that of this application. Summary of
these applications will hereinafter be described.
FIGS. 1A and 1B show an example of an arrangement of a super high density
information recording optical disk. FIGS. 1A and 1B are cross-sectional
views of the optical disks each of which is cut along a perpendicular
plane in the diameter direction. An optical disk shown in FIG. 1A has a
transparent substrate 11 having phase pits 11P formed thereon and also has
a transmittance varying layer (reflectivity varying layer) 13 disposed on
the phase pit 11P. The transmittance varying layer 13 is made of a
material whose light transmittance (reflectivity) is remarkably changed
reversibly as its temperature is increased by irradiation of laser light
thereon.
An optical disk shown in FIG. 1B has dielectric protective layers 12, 14
provided on the upper and lower sides of the transmittance varying layer
(reflectivity varying layer) 13, and further has a reflective layer 15
formed on the dielectric protective layer 14. Therefore, the optical disk
has the transparent substrate 11, the first dielectric protective layer
12, the transmittance varying layer 13, the second dielectric protective
layer 14 and the reflective layer 15.
A principle of the super high resolving power or super resolution
information reproduction will be described with reference to FIGS. 2A to
2D. FIG. 2A is a partially enlarged, plan view of an information recording
surface of the optical disk. A large number of minute phase pits 103 are
formed along a track 101 on the information recording surface. As shown in
FIG. 2A, the length of the phase pit 103 and the pitch between the two
adjacent phase pits 103 are marked with reference symbols D, p,
respectively. It is assumed that a radiation spot 201 formed by the laser
light scans along the track 101 in the direction shown by an arrow A in
FIG. 2A. Although the phase pit 103 is practically moved relative to the
radiation spot 201 since the optical disk is rotated, the principle will
be described on the above assumption to simplify the description.
Since the rays of the laser light are irradiated on the optical disk, a
portion thereof where the radiation spot 201 is formed by irradiation of
the laser light is heated. However, since the radiation spot 201 scans in
the direction shown by the arrow A, a portion of the optical disk where
the radiation spot 201 has just passed has the most high temperature
rather than the portion where the radiation spot 201 is now formed. In
FIG. 2A, a high-temperature portion where a temperature is higher than a
predetermined threshold temperature, e.g., a melting point MP is an
ellipse portion.
FIG. 2B two-dimensionally shows a light intensity and a temperature
distribution in the track direction. In FIG. 2B, a curve 301 represents
distribution of a light intensity obtained on the radiation spot 201
formed on a surface of the optical disk, and a curve 302 represents a
temperature distribution of the transmittance varying layer 13.
FIG. 2C shows change of the reflectivity of the material used for making
the transmittance varying layer 13. The reflectivity of the material is
changed depending upon the change of the temperature. The reflectivity
becomes remarkably high at the high temperature portion 202 and becomes
remarkably low at an area other than the high temperature portion 202,
i.e., at a low temperature area. Therefore, the reflected light is
detected from the high temperature portion 202 of the radiation spot 201,
i.e., from a common portion (cross-hatched portion) 203 of the area of the
radiation spot 201 and the area of the high temperature portion 202. Such
common portion (cross-hatched portion) 203 is a substantial radiation spot
and referred to as a substantial radiation spot.
FIG. 2D shows change of the reflectivity of another material used for
making the transmittance varying layer 13. The reflectivity of this
material becomes remarkably low at the high temperature portion 202 and
becomes remarkably high at an area other than the high temperature portion
202, i.e., at a low temperature area. For example, this material has high
reflectivity at an ordinary or room temperature. Therefore, the reflected
light is detected from a portion 204 which is an area of the radiated spot
201 except the area of the high temperature portion 202. Such portion 204
is a substantial radiation spot and referred to as a substantial radiation
spot.
It is not the track-direction diameter of the actual radiation spot 201 but
the track-direction diameters of the substantial radiation spots 203, 204
which affect the resolving power with respect to the phase pit of the
optical disk. In FIG. 2A, since the diameters of the substantial radiation
spots 203, 204 are substantially half of the diameter of the radiation
spot 201, even if the pitch p of the phase pit 103 is half as long as the
ordinary pitch, then the phase pits 103 can be resolved. Specifically, the
resolving power becomes double.
In general, the diameters of the substantial radiation spots 203, 204 are
determined depending upon relationship between the radiation spot 201 and
the area of the high temperature portion 202. Therefore, by properly
selecting the material of the transmittance varying layer 13 and a
condition of thermal conductivity at a peripheral portion of the
transmittance varying layer 13, it is possible to set the diameters of the
substantial radiation spots 203, 204 to desired values. This can be
realized by selecting thicknesses of the dielectric protective layers 12,
14 to be desired values.
A material whose reflectivity (transmittance) is remarkably changed
depending upon its temperature includes a material whose reflectivity
(transmittance) is remarkably changed as a phase is changed. Phase change
includes phase change among a crystalline state, an amorphous state, a
molten state and so on. Typically, the reflectivity (transmittance) is
changed at a melting point MP as a threshold value. There are two kinds of
materials: one material whose reflectivity is low in its crystalline state
(solid state) but is high in its molten state (liquid state) as shown by a
curve of FIG. 2C; and a material whose reflectivity is high in its
crystalline state (solid state) but is low in its molten state (liquid
state) as shown by a curve of FIG. 2D. The former is called a rear
aperture detection (RAD) type material and the latter is called a front
aperture detection (FAD) type material.
Other than the phase change material, various organic dyes, an interference
filter and so on are employed as a material of the transmittance varying
layer (reflectivity varying layer). For example, the reflectivity
(transmittance) of the organic dye material is changed depending upon an
intensity of radiated light. In this case, the diameter of the substantial
radiation spot is one obtained when the light intensity represented by the
light intensity curve 301 of FIG. 2B is larger than a predetermined
threshold value, and the diameter is smaller than an actual diameter of
the radiation spot 201. Details are described in the above applications.
For increasing the recording density of the optical disk, it is proposed to
provide a plurality of information recording surfaces other than to reduce
the length t of the phase pit and the pitch p thereof.
A structure of an optical disk having a plurality of information recording
surfaces will be described with reference to FIG. 3. The optical disk has
a transparent substrate 11, a first information recording layer 10
deposited thereon, a transparent layer 19 deposited thereon, and a second
information recording layer 20 deposited thereon.
Phase pits 11P, 19P are respectively formed on the transparent substrate 11
and the transparent layer 19. The first information recording layer 10 is
formed of a semitransparent reflective film, and the second information
recording layer 20 is formed of a reflective layer.
An example of an apparatus for recording and reproducing the optical disk
having a plurality of information recording surfaces will be described
with reference to FIG. 4. The recording and reproducing apparatus has a
light source 31 formed of a semiconductor laser, a collimator lens 33, a
beam splitter 35, a quarter-wave plate 37, an objective lens 39, a
condenser lens 41, a cylindrical lens 43, and a photosensor 45 formed of a
photodiode.
Laser light from the light source 31 is converted by the collimator lens 33
into parallel rays of laser light. The parallel rays of laser light pass
through the beam splitter 35 and travel to the quarter-wave plate 37. The
quarter-wave plate 37 subjects the laser light to linear polarization and
then the objective lens 39 focuses the linearly polarized laser light.
Rays of laser light reflected by the information recording layers 10, 20 of
an optical disk 1 pass through the quarter-wave plate 37 and travel to the
beam splitter 35. The beam splitter 35 deflects an optical path of the
reflected light, thereby the reflected light passing through the condenser
lens 41 and the cylindrical lens 43. The reflected light is then detected
by the photodetector 45.
The optical disk 1 has a structure shown in FIG. 3, including the first
information recording layer 10 and the second information recording layer
20. An optical system of this recording and reproducing apparatus includes
a known auto-focussing mechanism and a known auto-tracking mechanism. The
auto-focusing mechanism shifts the objective lens 39 in the direction
shown by an arrow B of FIG. 4, thereby the radiation spot 201 being formed
on the first information recording layer 10 or the second information
recording layer 20.
The optical disk having a plurality of information recording layers 10, 20
advantageously has a large recording capacity as compared with an optical
disk having a single information recording layer.
However, since it is necessary to discriminate from which information
recording layer information is being reproduced during the reproduction of
the information, identification informations therefor are recorded on
tracks of the respective information recording layers 10, 20. Therefore,
user data of the optical disk having the two information recording layers
10, 20 are substantially small as compared with data twice of user data of
the optical disk having a single information recording layer.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical disk having
a plurality of information recording layers on which more information can
be recorded.
An optical disk having a plurality of information recording layers
according to the present invention includes a transparent substrate, a
transparent layer and at least two information recording layers formed on
the transparent substrate. The information recording layers are provided
so as to sandwich the transparent layer. At least one of the information
recording layers has a transmittance varying layer.
According to the present invention, the transmittance varying layer is made
of a material whose transmittance is changed at a predetermined
temperature as a threshold value. The transmittance varying layer is made
of a phase change material. The threshold value is within the temperature
range from 700.degree. to 500.degree.. Crystallization speed of the
transmittance varying layer is equal to or slower than 500 ns.
According to the present invention, the transmittance varying layer is made
of a material whose transmittance is changed at a predetermined luminous
intensity as a threshold value. The transmittance varying layer is made of
an organic dye material.
According to the present invention, since the read-only optical disk has a
plurality of information recording layers at least one of which includes
the transmittance varying layer and hence the optical disk is a multilayer
optical disk allowing super high resolution reproduction, it is possible
to secure a larger user-data recording area as compared with other
multilayer optical disks.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are diagrams each showing an example of an arrangement of a
super high resolution information recording optical disk;
FIGS. 2A to 2D are a diagram and graphs used to explain a principle of a
super high resolution information reproduction;
FIG. 3 is a diagram showing an example of an arrangement of an optical disk
having two information recording layers;
FIG. 4 is a diagram showing an example of an arrangement of an apparatus
for reproducing an optical disk having two information recording layer;
and
FIG. 5 is a diagram showing an example of an arrangement of an optical disk
according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
An optical disk according to an embodiment of the present invention will
hereinafter be described with reference to FIG. 5. The optical disk
according to the present invention has a transparent substrate 11, a first
information recording layer 10, a transparent layer 19 and a second
information recording layer 20. The first information recording layer 10
is formed of a semitransparent reflective layer.
The second information recording layer 20 has a first dielectric protective
layer 12, a transmittance varying layer (reflectivity varying layer) 13
formed thereon, a second dielectric protective layer 14 formed thereon, a
reflective layer 15 formed thereon, and a third dielectric protective
layer 16 formed thereon.
The optical disk according to this embodiment is different from the optical
disk shown in FIG. 3 in that the second information recording layer 20 has
the transmittance varying layer 13, and may be arranged similarly thereto
in other points. Any of the two dielectric protective layers 12, 14 formed
on both sides of the transmittance varying layer 13, the reflective layer
15 and the third dielectric protective layer 16 may be omitted properly.
When recorded information is reproduced from the optical disk according to
this embodiment, the recording and reproducing apparatus shown in FIG. 4
may be used.
The dielectric protective layers are provided to protect the transmittance
varying layer 13 and the reflective layer 15 and also to adjust
characteristics of the transmittance varying layer 13 and the reflective
layer 15. For example, by properly selecting thicknesses and materials of
the dielectric protective layers, their thermal conductive characteristics
are changed. This change leads to change of temperature characteristics of
the transmittance varying layer 13, thereby the transmittance
(reflectivity) characteristics being adjusted. The dielectric protective
layers help heat to be satisfactorily radiated from the reflective layer
15.
For example, a phase change material or an organic dye material is employed
as a material of the transmittance varying layer 13 of the optical disk
according to this embodiment. The phase change material is reversibly
changed from its crystalline state to its molten state when its
temperature becomes higher than a predetermined threshold temperature.
When being cooled, the phase change material is changed from its molten
state to its crystalline state. At this time, it is preferable that the
phase change material is free from composition change such as phase
separation or the like and segregation. It is also preferable that the
phase change material has less kinds of crystals and composition of the
crystal is approximate to composition of a compound composition.
If the melting point of the phase change material is too high, the heating
of the phase change material brings thermal load to the protective layer
or the transparent layer around the phase change material. Therefore, the
melting point is preferably smaller than 700.degree., e.g., within the
range from 700.degree. to 500.degree..
Time required for crystallization will be considered. For example, if a
linear velocity of the optical disk is 2 to 20 m/sec. and a diameter of
the beam spot on the information recording layer of the optical disk is 1
.mu.m, then time of beam radiation is within the range from 50 to 500
nsec. Therefore, crystallization speed must be 500 nsec. or shorter.
An alloy made of three elements, e.g., Ge.sub.2 Sb.sub.2 Te.sub.5 is
employed as the phase change material. The phase change material may be an
alloy containing at least one kinds of the following elements; Au, Al, Ag,
Bi, Cu, Cr, Co, Cd, Ce, Cs, Dy, Fe, Ge, Gd, Ga, Hf, In, K, La, Li, Mn, Mo,
Ni, Nb, Nd, Na, Os, Pd, Pr, Pb, Ru, Rh, Rb, Sn, Sb, Si, Sm, Sc, Se, Te,
Ti, Tb, Ta, Tl, V, W, Y, Zn and Zr.
Further, such phase change material may be made of SbSe system, SbSeSi
system, InSe system, InSeSi system, AgInTeSb system, AsTeGe system, TeGeSn
system, TeGeSnO system, TeSe system, SnTeSe system, TeGeSnAu system,
SbTeSe system, InSeTl system, InSb system, InSbSe system, AgZn alloy,
CuAlNi alloy, InSeTlCo system, SiTeSn system, suboxide such as TeOx
(0<x<2) or the like, or the like.
The organic dye material employed in the transmittance varying layer 13
includes a thermochromic material whose transmittance is changed when its
temperature becomes higher than a predetermined threshold temperature, and
a photochromic material whose transmittance is changed when luminous
intensity of received light becomes higher than a predetermined threshold
luminous intensity. The thermochromic material includes polyacene class,
phthalocyanine class, spiropyran dye, lactone dye, fluoran dye and so on.
The photochromic material includes xanthene dye, azo dye, cyanin dye and
so on.
For example, polycarbonate is employed as a material of the transparent
substrate 11. However, acrylic resin such as poly(methyl methacrylate)
(PMMA) or the like and glass photopolymer (2P) may be employed as a
material of the transparent substrate 11. A ZnS--SiO.sub.2 mixture may be
employed as the dielectric protective layers 12, 14, 16. However, a
material employed as the dielectric protective layers 12, 14, 16 may be a
material having a low absorptance in a wavelength range of the laser light
to be used, e.g., may be nitride, oxide, sulfide or the like of metal such
as Al, Si, or the like or of a semiconductor element.
The reflective film 15 of the second information recording layer 20 is
formed of a metal reflective film. It is preferable to employ dysprosium
(Dy) as such metal. Such metal may be Al, Au, Ag or BiSe.sub.4 or alloy of
these metals and Ti, Cr or the like.
While the optical disk shown in FIG. 5 includes the two information
recording layers 10, 20, the optical disk can generally be arranged so as
to have the information recording layer having multilayer structure. The
optical disk having the information recording layer formed of N layers
(where N is an integer that is equal to or greater than 2) is arranged
such that the first to (N-1)th information recording layers include
semitransparent layers and the Nth information recording layer includes a
reflective layer. Since the metal reflective film cannot be provided in
any of the first to (N-1)th information recording layers, heat is easily
accumulated in the information recording layers. Therefore, it is
preferable to provide a dielectric protective layer made of a material
having a comparatively high thermal conductivity in each of the first to
(N-1)th information recording layers. Such dielectric protective layer may
be formed of Al.sub.3 N.sub.4, SiC or the like, for example.
An inventive example of the present invention will be described. The
optical disk having the two information recording layers 10, 20 as shown
in FIG. 5 was manufactured experimentally. Initially, the phase pits 11P
were formed by a stamper on a main surface of the polycarbonate
transparent substrate 11 to transfer first information thereto. The
semitransparent layer 18 made of silicon nitride was formed on the
transparent substrate 11. The transparent photopolymer layer 19 was formed
on the semitransparent layer 18 by photopolymerization. The phase pits 19P
were formed by a stamper on the photopolymer layer 19 to transfer second
information thereto.
The dielectric protective layer 12, the transmittance varying layer 13, the
dielectric protective layer 14, the reflective film 15 and the dielectric
protective layer 16 were successively formed on the photopolymer layer 19
having the phase pits 19P.
An arrangement of the manufactured optical disk, materials thereof and
thicknesses thereof were as shown on the following Table 1.
TABLE 1
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portion material thickness
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transparent substrate 11
polycarbonate
first information recording
silicon nitride
60 nm
layer 10
second transparent layer 19
photopolymer (2P)
40 .mu.m
second information recording
layer 20
first dielectric
ZnS-SiO.sub.2 mixture
105 nm
protective layer 12
transmittance varying
Ge.sub.2 Sb.sub.2 Te.sub.5 alloy
19 nm
layer 13
second dielectric
ZnS--SiO.sub.2 mixture
180 nm
protective layer 14
reflective film 15
dysprosium (Dy)
150 nm
third dielectric
ZnS--SiO.sub.2 mixture
400 nm
protective layer 16
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Rays of laser light having a wavelength of 680 nm were irradiated on the
optical disk having above structure. Reflectivities of the first and
second information recording layers 10 and 20 were as shown on the
following Table 2.
TABLE 2
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layer reflectivity
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first information recording layer 10
30%
second information recording layer 20
when transmittance varying layer 13
20%
is in its crystalline state
when transmittance varying layer 13
1.6%
is in its molten state
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Informations recorded on the first and second information recording layers
10 and 20 were satisfactorily reproduced.
According to the present invention, it is possible to perfectly and
precisely reproduce information from the read-only optical disk having a
plurality of information recording layers. Moreover, this advantage leads
to substantial enlargement of the user area.
Having described a preferred embodiment of the present invention with
reference to the accompanying drawings, it is to be understood that the
present invention is not limited to the above-mentioned embodiment and
that various changes and modifications can be effected therein by one
skilled in the art without departing from the spirit or scope of the
present invention as defined in the appended claims.
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