U.S. patent application number 10/650132 was filed with the patent office on 2004-05-13 for optical disc and optical disc apparatus.
This patent application is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Iwata, Katsuo, Maruyama, Sumitaka, Watabe, Kazuo.
Application Number | 20040090902 10/650132 |
Document ID | / |
Family ID | 31499126 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040090902 |
Kind Code |
A1 |
Maruyama, Sumitaka ; et
al. |
May 13, 2004 |
Optical disc and optical disc apparatus
Abstract
The refractive index of a light transmission layer of an optical
disk is set within the range of 1.45 to 1.75, the numerical
aperture of a lens emitting laser light which is incident onto the
light transmission layer is set to 0.65, and the wavelength range
of the laser light is set within the range of 395 to 415 nm.
Further, in order that aberrations fall within the range of certain
acceptable values, the thickness t of the light transmission layer
is set within the range of f(n)-t1.ltoreq.t.ltoreq.f(n)+t2,
employing constants t1, t2 determined based on an acceptable value
of aberration and function f(n) of the refractive index n.
Inventors: |
Maruyama, Sumitaka;
(Yokohama-shi, JP) ; Iwata, Katsuo; (Tokyo,
JP) ; Watabe, Kazuo; (Yokohama-shi, JP) |
Correspondence
Address: |
PILLSBURY WINTHROP, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
Kabushiki Kaisha Toshiba
Tokyo
JP
|
Family ID: |
31499126 |
Appl. No.: |
10/650132 |
Filed: |
August 28, 2003 |
Current U.S.
Class: |
369/121 ;
369/283; G9B/7.166 |
Current CPC
Class: |
G11B 7/24056
20130101 |
Class at
Publication: |
369/121 ;
369/283 |
International
Class: |
G11B 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 28, 2002 |
JP |
2002-249202 |
Mar 31, 2003 |
JP |
2003-096300 |
Jun 5, 2003 |
JP |
2003-161032 |
Claims
What is claimed is:
1. An optical disk which is constructed in such a manner that an
information recording layer formed on a substrate is covered with a
light transmission layer and in which the range of the thickness
and the refractive index of the light transmission layer is set so
that aberration due to a deviation of the thickness and the
refractive index of the light transmission layer from each standard
value falls within the range of certain acceptable values, wherein
the thickness t of the light transmission layer is set within the
range of f(n)-t1.ltoreq.t.ltoreq.f(n- )+t2, employing function f(n)
of the refractive index n of the light transmission layer and
constants t1, t2 determined based on an acceptable value of
aberration in the light transmission layer, the refractive index of
the light transmission layer is set within the range of 1.45 to
1.75, the numerical aperture of a lens emitting laser light which
is incident onto the light transmission layer is set to 0.65, and
the function f(n) is shown by 3 f ( n ) = A 1 .times. n 3 n 2 - 1
.times. n 2 + A 2 n 2 + A 3 ( m ) employing constants A.sub.1,
A.sub.2, A.sub.3.
2. The optical disk according to claim 1, wherein the refractive
index of the light transmission layer is set within the range of
1.5 to 1.7.
3. The optical disk according to claim 1, wherein the wavelength of
the laser light which is incident onto the light transmission layer
is set within the range of 395 to 415 nm.
4. The optical disk according to claim 1, wherein the constant
A.sub.1 is 0.26200, constant A.sub.2 is -0.32400, and constant
A.sub.3 is 0.00595.
5. The optical disk according to claim 1, wherein minimum values of
the constants t1, t2 are substantially set to 10 .mu.m.
6. The optical disk according to claim 1, wherein the constants t1,
t2 are substantially set to 13 .mu.m.
7. The optical disk according to claim 1, wherein predetermined
positions on curved lines that f(n)-t1 and f(n)+t2 show are
sampled, and an area encircled by connecting each sample point by
means of straight lines is set as the range of the thickness t of
the light transmission layer.
8. An optical disk which is constructed in such a manner that a
plurality of information recording layers are laminated by
sandwiching a space layer having a light transmission property
therebetween on a substrate and are covered with a light
transmission layer, wherein the thickness t of the light
transmission layer is set to f(n)-t1 or more, employing function
f(n) of the refractive index n of the light transmission layer and
constants t1, t2 determined based on an acceptable value of
aberration in the layer comprising the light transmission layer,
the information recording layers, and the space layer, the sum of
thicknesses of the light transmission layer, the space layer, and
the information recording layer excluding the information recording
layer which is closest to the substrate is set to f(n)+t2 or less,
the refractive index of the light transmission layer is set within
the range of 1.45 to 1.75, the refractive index of the space layer
is set within the range of +0.0 to -0.15 of the refractive index of
the light transmission layer, the numerical aperture of a lens
emitting laser light which is incident onto the light transmission
layer is set to 0.65, and the function f(n) is shown by 4 f ( n ) =
A 1 .times. n 3 n 2 - 1 .times. n 2 + A 2 n 2 + A 3 ( m ) employing
constants A.sub.1, A.sub.2, A.sub.3.
9. The optical disk according to claim 8, wherein the refractive
index of the light transmission layer is set within the range of
1.5 to 1.7.
10. The optical disk according to claim 8, wherein the wavelength
of the laser light which is incident onto the light transmission
layer is set within the range of 395 to 415 nm.
11. The optical disk according to claim 8, wherein the constant
A.sub.1 is 0.26200, constant A.sub.2 is -0.32400, and constant
A.sub.3 is 0.00595.
12. The optical disk according to claim 8, wherein minimum values
of the constants t1, t2 are substantially set to 10 .mu.m.
13. The optical disk according to claim 8, wherein the constants
t1, t2 are substantially set to 22 .mu.m.
14. The optical disk according to claim 8, wherein predetermined
positions on a curved line that f(n)-t1 shows are sampled so that
the thickness that a straight line connecting each sample point
shows is set to a minimum value of the thickness t of the light
transmission layer in a corresponding refractive index, and
predetermined positions on a curved line that f(n)+t2 shows are
sampled so that the thickness that a straight line connecting each
sample point shows is set to a maximum value of the thickness of
the sum of the light transmission layer in a corresponding
refractive index, the space layer, and the information recording
layer excluding the information recording layer which is closest to
the substrate.
15. An optical disk apparatus comprising: a semiconductor laser
element emitting laser light whose wavelength is 395 to 415 nm; and
a processing unit allowing the laser light from the semiconductor
laser element to be emitted to the optical disk to perform
recording processing and reproducing processing, for an optical
disk which is constructed in such a manner that an information
recording layer formed on a substrate is covered with a light
transmission layer and in which the range of the thickness and the
refractive index of the light transmission layer is set so that
aberration due to a deviation of the thickness and the refractive
index of the light transmission layer from each standard value
falls within the range of certain acceptable values, wherein the
thickness t of the light transmission layer is set within the range
of f(n)-t1.ltoreq.t.ltoreq.f(n)+t2, employing function f(n) of the
refractive index n of the light transmission layer and constants
t1, t2 determined based on an acceptable value of aberration in the
light transmission layer, the refractive index of the light
transmission layer is set within the range of 1.45 to 1.75, the
numerical aperture of a lens emitting laser light which is incident
onto the light transmission layer is set to 0.65, and the function
f(n) is shown by 5 f ( n ) = A 1 .times. n 3 n 2 - 1 .times. n 2 +
A 2 n 2 + A 3 ( m ) employing constants A.sub.1, A.sub.2,
A.sub.3.
16. The optical disk apparatus according to claim 15, wherein the
refractive index of the light transmission layer is set within the
range of 1.5 to 1.7.
17. An optical disk apparatus comprising: a semiconductor laser
element emitting laser light whose wavelength is 395 to 415 nm; and
a processing unit allowing the laser light from the semiconductor
laser element to be emitted to the optical disk to perform
recording processing and reproducing processing, for an optical
disk which is constructed in such a manner that a plurality of
information recording layers are laminated by sandwiching a space
layer having a light transmission property therebetween on a
substrate and are covered with a light transmission layer, wherein
the thickness t of the light transmission layer is set to f(n)-t1
or more, employing function f(n) of the refractive index n of the
light transmission layer and constants t1, t2 determined based on
an acceptable value of aberration in the layer comprising the light
transmission layer, the information recording layers, and the space
layer, the sum of thicknesses of the light transmission layer, the
space layer, and the information recording layer excluding the
information recording layer which is closest to the substrate is
set to f(n)+t2 or less, the refractive index of the light
transmission layer is set within the range of 1.45 to 1.75, the
refractive index of the space layer is set within the range of +0.1
of the refractive index of the light transmission layer, the
numerical aperture of a lens emitting laser light which is incident
onto the light transmission layer is set to 0.65, and the function
f(n) is shown by 6 f ( n ) = A 1 .times. n 3 n 2 - 1 .times. n 2 +
A 2 n 2 + A 3 ( m ) employing constants A.sub.1, A.sub.2,
A.sub.3.
18. The optical disk apparatus according to claim 17, wherein the
refractive index of the light transmission layer is set within the
range of 1.5 to 1.7.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Applications No.
2002-249202, filed Aug. 28, 2002; No. 2003-096300, filed Mar. 31,
2003; and No. 2003-161032, filed Jun. 5, 2003, the entire contents
of all of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical disk capable of
high density recording.
[0004] 2. Description of the Related Art
[0005] As well known, in recent years, as an optical disk capable
of high density recording of information, a DVD having a single
layer per side capacity of 4.7 GB has been put into practical use.
There exist DVD types such as a DVD-ROM for the exclusive use of
reproduction, a rewritable DVD-RAM, and the like. A DVD is
constructed in such a way that an information recording layer is
formed on a transparent substrate (hereafter, referred to as a
light transmission layer) having a thickness of 0.6 mm, and laser
light is allowed to pass through the light transmission layer to
converge on the information recording surface to write or read
information. The numerical aperture (NA) of an objective lens for
converging a beam of this time is 0.6 as a reference. The
refractive index n of the light transmission layer is specified to
be the range, n=1.45 to 1.65, with respect to the wavelength of 650
nm, and a light transmission layer material suitable for this
condition is selected. As such light transmission layer material,
polycarbonate is generally employed, and the refractive index in
this case is n=1.58.
[0006] Although the reference of the thickness of the light
transmission layer of a DVD is 0.6 mm as described above, it is
unavoidable that a thickness dispersion occurs from the viewpoint
of manufacturing of disks. In an optical system to record and
reproduce a DVD, in the case where the light transmission layer is
designed in such a way that the standard value of the thickness
thereof is 0.6 mm, if the thickness of a substrate is manufactured
departing from 0.6 mm, aberration occurs. Since such aberration of
an optical system increases a beam spot diameter and adversely
affects reproduction of a signal, it is necessary to restrain the
aberration to a predetermined value or less from the viewpoint of
the system.
[0007] The aberration of the optical system due to a thickness
error of the light transmission layer is determined by both a
deviation from a standard value of the light transmission layer and
a deviation from a standard value of the refractive index of the
light transmission layer. Accordingly, in the case of a DVD, in
order to restrain the aberration of the optical system caused by a
thickness error of the light transmission layer to a constant value
or less, the range of the light transmission layer thickness is
specified as a two-dimensional range of the light transmission
layer thickness and the refractive index thereof. This range is
disclosed for example in Jpn. Pat. Appln. KOKAI Publication No.
8-273199. That is, with respect to the range of the refractive
index, n=1.45 to 1.65, in the case where the error of the light
transmission layer thickness with respect to the standard value is
.+-.0.03 mm, when a horizontal axis represents the refractive index
and a vertical axis represents the light transmission layer
thickness, if the refractive index n becomes smaller than a lens
load specification (standard value), a range which is shifted in a
direction in which the light transmission layer thickness is
increased is specified, and if it becomes larger, a range by which
the light transmission layer thickness is not changed is
specified.
[0008] However, the specifications of the above-described publicly
known example are not appropriate in view of the following.
[0009] Presently, technology development to make a DVD further high
density has proceeded in various companies. The spot size of
focused light emitted on the information recording surface of an
optical disk is in proportion to the wavelength and is in inverse
proportion to the NA showing the iris angle of an objective lens
for focusing light. Accordingly, in order to contract the spot size
of focused light, aiming at improving the recording density, it is
necessary to shorten the light source wavelength and increase the
NA of the objective lens.
[0010] At this time since the refractive index of the light
transmission layer is dependent upon the light source wavelength,
it is necessary to newly specify the range of the light
transmission layer thickness as a two-dimensional range with its
refractive index. As an example of the light source wavelength, the
NA, and the light transmission layer thickness of a next generation
optical disk, it can be shown that the wavelength .lambda.=405 nm,
NA=0.65, and the light transmission layer thickness=0.6 mm.
BRIEF SUMMARY OF THE INVENTION
[0011] According to one aspect of the present invention, there is
provided an optical disk which is constructed in such a manner that
an information recording layer formed on a substrate is covered
with a light transmission layer and in which the range of the
thickness and the refractive index of the light transmission layer
is set so that aberration due to a deviation of the thickness and
the refractive index of the light transmission layer from each
standard value falls within the range of certain acceptable values,
wherein the thickness t of the light transmission layer is set
within the range of f(n)-t1.ltoreq.t.ltoreq.f(n- )+t2, employing
function f(n) of the refractive index n of the light transmission
layer and constants t1, t2 determined based on an acceptable value
of aberration in the light transmission layer, the refractive index
n of the light transmission layer is set within the range of 1.45
to 1.75, the numerical aperture of a lens emitting laser light
which is incident onto the light transmission layer is set to 0.65,
and the function f(n) is shown by 1 f ( n ) = A 1 .times. n 3 n 2 -
1 .times. n 2 + A 2 n 2 + A 3 ( m )
[0012] employing constants A.sub.1, A.sub.2, A.sub.3.
[0013] According to one aspect of the present invention, the
wavelength of the laser light which is incident onto the light
transmission layer is set within the range of 395 to 415 nm and the
constant A.sub.1 is 0.26200, constant A.sub.2 is -0.32400, and
constant A.sub.3 is 0.00595.
[0014] By choosing such numerical values, it is possible to
restrict aberration of an optical system to a constant value or
less and obtain operational stability.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0015] FIG. 1 is a cross-sectional view showing the structure of a
first optical disk to which the present invention is applied;
[0016] FIG. 2 is a block diagram showing an optical disk apparatus
which performs recording and reproducing for an optical disk of the
present invention;
[0017] FIG. 3 is a view showing a manner in which a light beam is
focused on a recording layer to generate a beam spot;
[0018] FIG. 4 is a view showing the relationship between the
refractive index and the thickness of the light transmission layer
of an optical disk, indicating a wave front aberration as a
parameter;
[0019] FIG. 5 is a view which shows the range of the refractive
index and the thickness of a light transmission layer according to
one embodiment of the present invention and which shows a range set
in the case where the acceptable aberration is 0.03
.lambda.rms;
[0020] FIG. 6 is a view which shows the range of the refractive
index and the thickness of a light transmission layer according to
one embodiment of the present invention and which shows a range set
in the case where the acceptable aberration is 0.05
.lambda.rms;
[0021] FIG. 7 is a view which shows the range of the refractive
index and the thickness of a light transmission layer according to
one embodiment of the present invention and which shows a straight
line approximate range that is set in the case where the acceptable
aberration is near 0.03 .lambda.rms;
[0022] FIG. 8 is a cross-sectional view showing the structure of a
second optical disk to which the present invention is applied;
[0023] FIG. 9 is a view which shows the range of the refractive
index and the thickness of a light transmission layer according to
one embodiment of the present invention and which shows a range
that is set for the second optical disk in the case where the
acceptable aberration is 0.05 .lambda.rms;
[0024] FIG. 10 is a view which shows the range of the refractive
index and the thickness of a light transmission layer according to
one embodiment of the present invention and which shows a straight
line approximate range that is set in the case where the acceptable
aberration is near 0.05 .lambda.rms;
[0025] FIG. 11 is a view which shows the range of the refractive
index and the thickness of a light transmission layer according to
one embodiment of the present invention and which shows another
straight line approximate range that is set in the case where the
acceptable aberration is near 0.03 .lambda.rms; and
[0026] FIG. 12 is a view which shows the range of the refractive
index and the thickness of a light transmission layer according to
one embodiment of the present invention and which shows another
straight line approximate range that is set in the case where the
acceptable aberration is near 0.05 .lambda.rms.
DETAILED DESCRIPTION OF THE INVENTION
[0027] Embodiments of the present invention are explained below
referring to drawings.
[0028] FIG. 1 shows one example of a cross-sectional view of an
optical disk 1 of the present invention. An information recording
layer 3 including for example a phase change recording film is
formed on (under in the figure) a substrate 2 made of
polycarbonate. In the case where the optical disk 1 is a disk for
the exclusive use of reproduction, an information recording layer 3
made of a metal reflective film instead of the phase change
recording film is formed. Then a light transmitting layer 4 having
a thickness of t is formed on (under in the figure) the information
recording layer 3. The light transmitting layer 4 is for example
polycarbonate.
[0029] Next, FIG. 2 shows an example of the formation of an optical
disk apparatus for recording and reproducing data on the present
disk. A semiconductor laser 20 of a short wavelength is employed as
a light source. The wavelength of the emitted light is that of
violet wavelength band generally of the range of 395 nm to 415 nm
(405.+-.10 nm). The light 100 emitted from the semiconductor laser
light source 20 becomes parallel light by means of a collimator
lens 21 and passes through a polarizing beamsplitter 22 and a
.lambda./4 plate 23 to enter an objective lens 25. The range of the
NA of the objective lens is for example 0.6 to 0.7. Thereafter, the
emitted light 100 passes through the light transmission layer 4 of
the optical disk 1 and is focused on the information recording
layer 3.
[0030] Reflected light 101 by the information recording layer 3 of
the optical disk 1 passes through the light transmitting layer 4 of
the optical disk 1 again to pass through an objective lens 25 and
the .lambda./4 plate 23 and is reflected by means of the polarizing
beamsplitter 22 to pass through a photo detection optical system 26
to enter a photo detector 27. A light receiving portion of the
photo detector 27 is generally divided into plural portions, and
each light receiving portion outputs current in response to light
intensity. The output current, after being converted into a voltage
by an unillustrated I/V amplifier, is arithmetically processed into
a RF signal, a focus error signal, and a track error signal by a
data processing unit 29.
[0031] Based on these error signals, a servo driver 28 drives a
lens drive coil 12 to move the lens 25 in a focus direction (lens
optical axis direction) and a tracking direction (disk radius
direction). As a result, a beam spot is generated on a target track
of the information recording layer 3.
[0032] Here, when the thickness of the light transmission layer 4
falls into a standard value (for example, 0.6 mm), the apparatus
has been designed in such a manner that the light enters the
objective lens 25 as approximately parallel light. However, in the
case where the thickness of the light transmission layer 4 departs
from the standard value, a spherical aberration caused by a
thickness error of the light transmission layer 4 occurs. At this
time since convergent spot shape on the information recording layer
3 of the optical disk 1 is distorted, stable, correct recording and
reproducing becomes difficult.
[0033] In a next generation optical disk apparatus, since the
wavelength is further shortened and the NA is further increased
compared to those of a conventional optical disk apparatus,
applying conventional specified values of a DVD or a CD to
manufacturing of a next generation optical disk as they are
produces erroneous results. Thus, the optical disk 1 of the present
invention is characterized by having the range of the thickness
error and the refractive index of the light transmission layer 4
which is obtained by considering the trend of shortening the
wavelength and increasing the NA in an optical disk apparatus.
[0034] As specifications of a next generation optical disk system,
the case where for example a light source wavelength is 405 nm and
NA of the objective lens 25 is 0.65 is considered. FIG. 3 shows a
manner in which the light beam 100 is focused on the recording
layer 3 employing an ideal objective lens to which aberration is
completely compensated with respect to the lens load in which in
the light transmission layer of an optical disk the refractive
index is 1.60 and the thickness is 0.6 mm so that a beam spot is
generated. At this time wave front is in order, and wave front
aberration, that is, curvature of wave front does not occur.
However, when wave front aberration occurs due to residual
spherical aberration of the lens system and the like, an ideal beam
spot does not occur as shown by dotted curved lines 6 in the
drawing.
[0035] FIG. 4 shows results obtained by calculation of rms (root
mean square) values of wave front aberrations which occur in the
case where optical disks 1 having various light transmission layer
refractive indexes n and light transmission layer thicknesses t are
employed instead of the above-described ideal objective lens. In
FIG. 4, the horizontal axis represents the light transmission layer
refractive index n and the vertical axis represents the light
transmission layer thickness t, rms values of wave front
aberrations at each point on the coordinates plane are displayed by
contour lines. The interval of the contour lines represents
{fraction (1/100)} of light source wavelength (.lambda.=405
[nm]).
[0036] Through these results, when disks having various light
transmission layer refractive indexes and thicknesses are employed,
in order to make residual aberration amount a constant value, it
can be seen that it is better to a bit more increase the light
transmission layer thickness than the standard value if the
refractive index is shifted in the direction in which the
refractive index becomes larger or smaller than the lens load
specification value. Accordingly, with respect to light
transmission layer specifications of an optical disk of a next
generation DVD, it is necessary to specify an error acceptable
range of light transmission layer thicknesses in a way that the
error acceptable range is changed in response to the absolute value
of a deviation from the standard value, 1.60, of the light
transmission layer refractive index.
[0037] The range of the refractive indexes and thicknesses of the
light transmission layers of optical disks according to one
embodiment of the present invention is the range shown in FIG. 5.
This shows the following area.
Refractive index n:1.45.ltoreq.n.ltoreq.1.75 (1)
[0038] Light Transmission Layer Thickness
t:f(n)-t1.ltoreq.tf(n)+t2(.mu.m) (2) 2 f ( n ) = A 1 .times. n 3 n
2 - 1 .times. n 2 + A 2 n 2 + A 3 ( m ) ( 3 )
[0039] A.sub.1=0.26200
[0040] A.sub.2=-0.32400
[0041] A.sub.3=0.00595
[0042] t1, t2=13 (.mu.m)
[0043] The contour lines of the wave front aberration amounts of
FIG. 4 line up approximately parallel to the vertical axis
direction, and their curved lines can be shown as curved lines
obtained by imparting a constant offset to equation (3) given
above. Accordingly, when acceptable values of aberrations are
determined, the range of the light transmission layer thicknesses
and the refractive indexes can be determined through the equations
(1) to (3) by making the acceptable values correspond to offset t1,
t2.
[0044] In the case of the present embodiment, the range shown in
FIG. 5 substantially corresponds to the range in which the
aberration in FIG. 4 is 0.03 .lambda.rms or less. That is, the
range of the thickness error .+-.13 .mu.m (t1, t2=13 .mu.m)
corresponds to the range in which the aberration is 0.03
.lambda.rms or less. Accordingly, by specifying an optical disk of
the range shown in FIG. 5, it is possible to maintain a condition
that the aberrations due to deviations from the standard values (as
one example, t=0.6 mm, n=1.60) of the light transmission layer
thicknesses and the refractive indexes are approximately 0.03
.lambda.rms or less.
[0045] The acceptable values of aberrations are values determined
according to performance or acceptable aberrations of an optical
disk apparatus which performs recording or reproducing on an
optical disk. At this time by changing t1, t2 in response to
aberration acceptable values, the range of the light transmission
layer thickness may be adjusted. For example, in the case where an
acceptable aberration is 0.05 .lambda., by setting t1, t2=22 .mu.m
in the equation given above, an appropriate range can be specified
(refer to FIG. 6). From the view point of present optical disk
manufacturing, it is difficult to set t1, t2 to 10 .mu.m or less.
Therefore, minimum values of t1, t2 are about 10 .mu.m.
[0046] Meanwhile, the range of the refractive index is determined
by the material of the light transmission layer 2 and the
wavelength of the light source, and a range in which an effective
material as the material of the light transmission layer of an
optical disk is contained is specified. In this case, by setting it
to about 1.45 to 1.75, the refractive index in violet wavelength
band of an effective material, such as polycarbonate, as the light
transmission layer of an optical disk can be covered.
[0047] Further, the range of the thickness and the refractive index
of the light transmission layer of an optical disk according to
another embodiment of the present invention has a range shown in
FIG. 7. This range, although being approximately similar to the
range of the optical disk of the embodiment shown in FIG. 5, is an
area which is not encircled by curved lines but is encircled by
straight lines. Its effect is similar to that of the optical disk
of the embodiment described above.
[0048] Furthermore, the range of the thickness and the refractive
index of the light transmission layer of an optical disk according
to another embodiment of the present invention is shown in FIG. 11.
In FIG. 11, the range of the thickness and the refractive index of
the light transmission layer, although being defined by values
which partly differ from those of FIG. 7, is not largely different
therefrom. That is, the range is defined at the points of the
refractive indexes 1.5 and 1.7. This embodiment has a similar
effect that the refractive index can be covered in violet
wavelength band of a material, such as polycarbonate and the like,
which is effective as the light transmission layer of an optical
disk.
[0049] Next, FIG. 8 shows an example of a cross-sectional view of
an optical disk 51 according to another embodiment of the present
invention. An information recording layer 53 including for example
a phase change recording film on (under in the drawing) a substrate
52 made of polycarbonate is formed. A space layer 54 having
transparency is formed thereon, and another information recording
layer 55 is formed further thereon. The information recording
layers 53 and 55 may be layers for the exclusive use of
reproduction, both of which are made of metallic reflection films,
and both may be recordable and reproducible layers, or only one of
which may be a reproduction-only layer and another of which may be
recordable and reproducible layer. A light transmission layer 56 is
formed on the information recording layer 55. The light
transmission layer 56 is for example made of polycarbonate. As a
manufacture process, a substrate 52 for example on which the
information recording layer 53 is formed and the light transmission
layer 56 on which the information recording layer 54 is formed are
glued via a pressure sensitive adhesive such as a ultraviolet ray
setting resin (to be the space layer 54).
[0050] The role of the space layer 54 is to optically shut out
information leak (cross talk) from another information recording
layer when one side information recording layer is reproduced. In
that sense, the interval of the two information recording layers is
better to be as large as possible, and the thickness of the space
layer 54 is better to be thick. However, in that case, a load is
put on a recording and reproducing optical system. That is, in the
case where the thickness from the surface of the light transmission
layer to the center of the space layer is specified as the load of
the objective lens, even if either one of information recording
layers is recorded and reproduced, aberration due to the thickness
error of half the thickness of the space layer occurs. Accordingly,
in view of the aberration of the recording and reproducing optical
system, the thickness of the space layer is better to be thin. That
is, the thickness of the space layer is set to a compromise point
of trade off relationship in the cross talk of the information
recording layer and the aberration of the recording/reproducing
optical system.
[0051] As specifications of a next generation optical disk system,
in the case where for example light source wavelength of 405 nm and
NA of an objective lens 25=0.65 are employed, it is appropriate
that the thickness of the space layer is about 20 .mu.m to 30
.mu.m, considering the trade off. It is better to represent a
thickness specification of the light transmission layer of a two
layer disk by a minimum value of the thickness of the light
transmission layer 56 and a maximum value of the sum of thicknesses
of the light transmission layer 56, the information recording layer
55 in contact with this light transmission layer, and the space
layer 54. The range of the thickness and the refractive index of
the light transmission layer of the optical disk at this time
becomes the range shown in FIG. 9. Similarly to the embodiments
above, it is assumed that the lens load in which the refractive
index of the light transmission layer of the optical disk is 1.60
and the thickness is 0.6 mm and that acceptable aberration of the
system is 0.05 .lambda.. The specified area is the area shown
below:
[0052] Refractive index n: 1.45.ltoreq.n.ltoreq.1.75
[0053] Light transmission layer thickness: f(n)-t1 or more
[0054] Thicknesses of the light transmission layer+the information
recording layer 55+the space layer 54: f(n)+t2 or less
[0055] t1, t2=22 .mu.m
[0056] f(n) is according to equation (3).
[0057] Like this, since the acceptable aberration is set to 0.05
.lambda., considering the thickness of the space layer, the range
of the thickness direction is widened compared to the case of
single layer. The refractive index of the space layer is set within
the range of +0.0 to -0.15 of the refractive index of the light
transmission layer 56.
[0058] Moreover, the range of the thickness and the refractive
index of the light transmission layer of an optical disk of another
embodiment of the present invention has the range shown in FIG. 10.
This range, although being substantially similar to the range of
the optical disk of the embodiment shown in FIG. 9, is an area
which is not encircled by curved lines but is encircled by straight
lines. Its effect is similar to that of the optical disk of the
embodiment described above.
[0059] Furthermore, the range of the thickness and the refractive
index of the light transmission layer of an optical disk according
to another embodiment of the present invention is shown in FIG. 12.
In FIG. 12, although the range of the thickness and the refractive
index of the light transmission layer is defined by values which
are partly different from those in FIG. 10, the range is not
largely different therefrom. That is, the range is defined at the
points of the refractive indexes 1.5 and 1.7. This embodiment has a
similar effect that the refractive index can be covered in violet
wavelength band of a material, such as polycarbonate and the like,
which is effective as the light transmission layer of an optical
disk.
[0060] Although the three embodiments show the cases where the
information recording layer are two layers, it is needless to say
that the embodiments can be applied to an optical disk having two
or more information recording layers.
[0061] As explained above, according to the present invention, it
is possible to specify the range of the light transmission layer
thickness and the light transmission layer refractive index which
is effective in a next generation disk and to provide an optical
disk which is suitable for a high density recording.
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