Title: Readout method and apparatus for optical information medium
Abstract: In an optical information medium comprising an information recording layer having a mark train of marks and spaces, the mark train is read out by scanning it with a laser beam and detecting a light intensity change pattern of reflected laser beam. Provided that the reflected laser beam includes polarized light components which define an angle θ with the mark train, a polarized light component giving θ=0 is x0 component, and a polarized light component giving θ=90° is y0 component, the mark train is read out utilizing at least a light intensity change of x0 component. When pits or recorded marks having a size approximate to or below the resolution limit are read out, the present invention allows high read outputs to be obtained and prevents omission of readout signals.
Patent Number: 7,016,290 Issued on 03/21/2006 to Nakano, et al.
| Inventors:
|
Nakano; Takashi (Ibaraki, JP);
Fukuda; Hisako (Ibaraki, JP);
Tominaga; Junji (Ibaraki, JP);
Atoda; Nobufumi (Ibaraki, JP);
Kikukawa; Takashi (Tokyo, JP)
|
| Assignee:
|
TDK Corporation (Tokyo, JP);
National Institute of Advanced Industrial Science and Technology (Tokyo, JP)
|
| Appl. No.:
|
270047 |
| Filed:
|
October 15, 2002 |
Foreign Application Priority Data
| Oct 15, 2001[JP] | 2001-317507 |
| Current U.S. Class: |
369/110.04; 369/112.16 |
| Current Intern'l Class: |
G11B 7/00 (20060101) |
| Field of Search: |
369/11001,110.02,110.04,112.16,110.03,109.2
|
References Cited [Referenced By]
U.S. Patent Documents
| 5528575 | Jun., 1996 | Saito.
| |
| Foreign Patent Documents |
| 5-205314 | Aug., 1993 | JP.
| |
| 8-96412 | Apr., 1996 | JP.
| |
| 2844824 | Oct., 1998 | JP.
| |
| 10-340482 | Dec., 1998 | JP.
| |
| 11-86342 | Mar., 1999 | JP.
| |
| 2001/-250274 | Sep., 2001 | JP.
| |
Other References
Takashi Kikukawa, et al., "High-Density Read-Only Memory Disc With Super Resolution
Reflective Layer", Jpn. J. Appl. Phys., vol. 40, Part 1, No. 3B, Mar. 2001, pp.
1624-1628, no day.
|
Primary Examiner: Hindi; Nabil
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
What is claimed is:
1. An information readout method for an optical information medium comprising
an information recording layer having a mark train of marks and spaces, said method
comprising the steps of scanning the mark train with a laser beam, and reading
out the mark train on the basis of a light intensity change pattern of reflected
laser beam, wherein
provided that the laser beam reflected by the mark train includes polarized light
components which define an angle θ with the mark train, a polarized light
component giving θ=0° is designated x
0 component, and a polarized
light component giving θ=90° is designated y
0 component,
the mark train is read out utilizing at least a light intensity change of x
0 component.
2. The information readout method of claim 1 wherein linearly polarized light
having an angle θ of from 0° to less than 90° is taken out of the
laser beam reflected by the mark train, and the mark train is read out utilizing
at least a light intensity change of said linearly polarized light.
3. The information readout method of claim 1 wherein linearly polarized light
having an angle θ of from 0° to less than 45° is taken out of the
laser beam reflected by the mark train, and the mark train is read out utilizing
at least a light intensity change of said linearly polarized light.
4. An information readout method for an optical information medium comprising
an information recording layer having a mark train of marks and spaces, said method
comprising the steps of scanning the mark train with a laser beam, and reading
out the mark train on the basis of a light intensity change pattern of a reflected
laser beam, wherein
provided that the laser beam reflected by the mark train includes polarized light
components which define an angle θ with the mark train, a polarized light
component giving θ=0° is designated x
0 component, and a polarized
light component giving θ=90° is designated y
0 component, and
linearly polarized light having an angle θ of from 0° to 5° is
taken out of the laser beam reflected by the mark train, and the mark train is
read out utilizing at least a light intensity change of said linearly polarized light.
5. The information readout method of claim 4 wherein the mark train includes
plural types of marks having different lengths and plural types of spaces having
different lengths, and
the mark train is read out on the basis of a difference between the light intensity
change pattern of x
0 component and the light intensity change pattern
of y
0 component.
6. The information readout method of claim 5 wherein two types of linearly polarized
light having different θ are taken out of the laser beam reflected by the
mark train,
of the two types of linearly polarized light, one having smaller θ is designated
x polarized light and the other having larger θ is designated y polarized light,
the mark train is read out on the basis of a change of the value obtained by
subtracting a multiple of the intensity of y polarized light from the intensity
of x polarized light, whereby those marks and/or spaces which cannot be read out
solely with x polarized light are read out.
7. An information readout method for an optical information medium comprising
an information recording layer having a mark train of marks and spaces, said method
comprising the steps of scanning the mark train with a laser beam, and reading
out the mark train on the basis of a light intensity change pattern of a reflected
laser beam, wherein
provided that the laser beam reflected by the mark train includes polarized light
components which define an angle θ with the mark train, a polarized light
component giving θ=0° is designated x
0 component, and a polarized
light component giving θ=90° is designated y
0 component,
the mark train includes plural types of marks having different lengths and plural
types of spaces having different lengths, two types of linearly polarized light
having different θ are taken out of the laser beam reflected by the mark train,
of the two types of linearly polarized light, one having a smaller θ than
the other linearly polarized light is designated x polarized light and the other
having a larger θ than the one linearly polarized light is designated y polarized light,
the intensity of the x polarized light is X, the intensity of the y polarized
light is Y, and X/Y corresponding to the longest mark is α
LM, and
the mark train is read out on the basis of a change of (X-α
LMY).
8. An information readout method for an optical information medium comprising
an information recording layer having a mark train of marks and spaces, said method
comprising the steps of scanning the mark train with a laser beam, and reading
out the mark train on the basis of a light intensity change pattern of a reflected
laser beam, wherein
provided that the laser beam reflected by the mark train includes polarized light
components which define an angle θ with the mark train, a polarized light
component giving θ=0° is designated x
0 component, and a polarized
light component giving θ=90° is designated y
0 component,
the mark train includes plural types of marks having different lengths and plural
types of spaces having different lengths, two types of linearly polarized light
having different θ are taken out of the laser beam reflected by the mark train,
of the two types of linearly polarized light, one having a smaller θ than
the other linearly polarized light is designated x polarized light and the other
having a larger θ than the one linearly polarized light is designated y polarized
light, and
the angle θ for the x polarized light is from 0° to less than 45°,
and the angle θ for the y polarized light is from more than 45° to 90°.
9. An information readout method for an optical information medium comprising
an information recording layer having a mark train of marks and spaces, said method
comprising the steps of scanning the mark train with a laser beam, and reading
out the mark train on the basis of a light intensity change pattern of a reflected
laser beam, wherein
provided that the laser beam reflected by the mark train includes polarized light
components which define an angle θ with the mark train, a polarized light
component giving θ=0° is designated x
0 component, and a polarized
light component giving θ=90° is designated y
0 component,
the mark train includes plural types of marks haying different lengths and plural
types of spaces having different lengths, two types of linearly polarized light
having different θ are taken out of the laser beam reflected by the mark train,
of the two types of linearly polarized light, one having a smaller θ than
the other linearly polarized light is designated x polarized light and the other
having a larger θ than the one linearly polarized light is designated y polarized
light, and
the angle θ for the x polarized light is from 0° to 5° , and
the angle θ for the y polarized light is from 85° to 90°.
10. An information readout method for an optical information medium comprising
an information recording layer having a mark train of marks and spaces, said method
comprising the steps of scanning the mark train with a laser beam, and reading
out the mark train on the basis of a light intensity change pattern of a reflected
laser beam, wherein
provided that the laser beam reflected by the mark train includes polarized light
components which define an angle θ with the mark train, a polarized light
component giving θ=0° is designated x
0 component, and a polarized
light component giving θ=90° is designated y
0 component, and
the laser beam having a wavelength λ is irradiated to the mark train through
an objective lens having a numerical aperture NA for reading out the mark train,
and the minimum length M
L of the marks is up to 0.36λ/NA.
11. An information readout method for an optical information medium comprising
an information recording layer having a mark train of marks and spaces, said method
comprising the steps of scanning the mark train with a laser beam, and reading
out the mark train on the basis of a light intensity change pattern of a reflected
laser beam, wherein
provided that the laser beam reflected by the mark train includes polarized light
components which define an angle θ with the mark train, a polarized light
component giving θ=0° is designated x
0 component, and a polarized
light component giving θ=90° is designated y
0 component, and
the laser beam having a wavelength λ is irradiated to the mark train through
an objective lens having a numerical aperture NA for reading out the mark train,
and the minimum length M
L of the marks is less than 0.25λ/NA.
12. The information readout method of claim 1 wherein the marks are formed by
changes in shape and/or property of the information recording layer.
13. A readout apparatus for use in the information readout method of claim 1,
comprising at least means for detecting linearly polarized light including x
0 component.
14. The readout apparatus of claim 13 wherein said means is capable of independently
detecting two types of linearly polarized light having different θ.
15. An information readout method for an optical information medium which is
one of a read-only optical disk, a phase change optical disk and a write-once optical
recording disk, the optical information medium comprising an information recording
layer having a mark train of marks and spaces, said method comprising the steps
of scanning the mark train with a laser beam, and reading out the mark train on
the basis of a light intensity change pattern of a reflected laser beam, wherein
provided that the laser beam reflected by the mark train includes polarized light
components which define an angle θ with the mark train, a polarized light
component giving θ=0° is designated x
0 component, and a polarized
light component giving θ=90° is designated y
0 component, and
the mark train is read out utilizing at least a light intensity change of the
x
0 component.
16. The information readout method of claim 7, wherein the marks are formed by
changes in shape andior property of the information recording layer.
17. A readout apparatus for use in the information readout method of claim 7,
comprising at least means for detecting linearly polarized light including the
x
0 component.
18. The readout apparatus of claim 17, wherein said means for detecting linearly
polarized light is capable of independently detecting two types of linearly polarized
light having different θ.
Description
This invention relates to a method and apparatus for reading out information
in an optical information medium.
BACKGROUND OF THE INVENTION
Optical information media include read-only optical disks such as compact
disks, rewritable optical recording disks such as magneto-optical recording disks
and phase change optical recording disks, and write-once optical recording disks
using organic dyes as the recording material.
In general, optical information media can have an increased information density.
Nowadays, optical information media are required to further increase their information
density in order to process a vast quantity of information data as in images. The
information density per unit area can be increased either by narrowing the track
pitch or by reducing the interval between recorded marks or pits to increase a
linear density. However, if the linear density is too high relative to the beam
spot of reading light, the carrier-to-noise (C/N) ratio lowers, eventually to a
level where signals are unreadable. The resolution upon signal readout is determined
by the diameter of a beam spot. More illustratively, provided that the reading
light has a wavelength λ and the optical system of the readout apparatus
has a numerical aperture NA, the spatial frequency 2NA/λ generally becomes
a resolution limit. Accordingly, reducing the wavelength of reading light and increasing
the NA are effective means for improving the C/N and resolution upon readout. A
number of technical studies that have been made thus far reveal that many technical
problems must be solved before such effective means can be introduced.
Under the circumstances, several methods have been proposed for going over
the resolution limit (or diffraction limit) determined by light diffraction. They
are generally known as super-resolution readout methods.
The most common super-resolution readout method is to form a mask layer over
a recording layer. Based on the fact that a laser beam defines a spot having an
intensity distribution approximate to the Gaussian distribution, an optical aperture
smaller than the beam spot is formed in the mask layer whereby the beam spot is
reduced below the diffraction limit. This method is generally divided into a heat
mode and a photon mode, depending on the optical aperture-forming mechanism.
The heat mode is such that upon irradiation to a beam spot, the mask layer changes
its optical properties in a region whose temperature is raised above a certain
value. The heat mode is utilized, for example, in the optical disk disclosed in
JP-A 5-205314. This optical disk has on a transparent substrate in which optically
readable recorded pits are formed in accordance with information signals, a layer
of a material whose reflectance changes with temperature. That is, the material
layer serves as a mask layer. The elements described in JP-A 5-205314 as the material
of which the mask layer is constructed are lanthanoids, with Tb being used in Examples.
In the optical disk of JP-A 5-205314, when reading light is irradiated, the reflectance
of the material layer changes due to temperature distribution within the scanned
spot of the reading light. After reading operation, the reflectance resumes the
initial state as the temperature lowers. It never happens that the material layer
be melted during reading. Another known example of the heat mode is a medium capable
of super-resolution readout, as disclosed in Japanese Patent No. 2,844,824, the
medium having a mask layer of an amorphous-crystalline phase transition material
in which a high-temperature region created within a beam spot is transformed into
crystal for increasing the reflectance. This medium, however, is impractical in
that after reading, the mask layer must be transformed back to amorphous.
The heat mode media require that the power of reading light be strictly controlled
in consideration of various conditions including the linear velocity of the medium
since the size of the optical aperture depends solely on the temperature distribution
in the mask layer. This, in turn, requires a complex control system and hence,
an expensive medium drive. The heat mode media also suffer from the problem that
reading characteristics degrade with the repetition of reading operation because
the mask layer is prone to degradation by repetitive heating.
On the other hand, the photon mode is such that upon exposure to a beam spot,
the mask layer changes its optical properties in a region whose photon quantity
is increased above a certain value. The photon mode is utilized, for example, in
the information recording medium of JP-A 8-96412, the optical recording medium
of JP-A 11-86342, and the optical information recording medium of JP-A 10-340482.
More illustratively, JP-A 8-96412 discloses a mask layer formed of phthalocyanine
or a derivative thereof dispersed in a resin or inorganic dielectric, and a mask
layer formed of a chalcogenide. JP-A 11-86342 uses as the mask layer a super-resolution
readout film containing a semiconductor material having a forbidden band which
upon exposure to reading light, is subject to electron excitation to the energy
level of excitons to change light absorption characteristics. One illustrative
mask layer is CdSe microparticulates dispersed in a SiO
2 matrix. JP-A
10-340482 uses as the mask layer a glass layer in which the intensity distribution
of transmitted light varies non-linearly with the intensity distribution of irradiated light.
Unlike the super-resolution readout media of the heat mode, the super-resolution
readout media of the photon mode are relatively resistant to degradation by repetitive reading.
In the photon mode, the region whose optical characteristics change is determined
by the number of incident photons which in turn, depends on the linear velocity
of the medium relative to the beam spot. Also in the photon mode, the size of an
optical aperture depends on the power of reading light, indicating that supply
of an excessive power makes so large an optical aperture that super-resolution
readout may become impossible. Therefore, the photon mode also requires to strictly
control the power of reading light in accordance with the linear velocity and the
size of pits or recorded marks (objects to be read out). Additionally, the photon
mode requires to select the mask layer-forming material in accordance with the
wavelength of reading light. That is, the photon mode media are rather incompatible
with multi-wavelength reading.
Under the circumstances, JP-A 2001-250274 proposes a medium comprising a layer
(functional layer) made of a specific material such as Si and having a specific
thickness corresponding to the specific material. This medium enables to read out
pits or recorded marks of a size which is below the resolution limit determined
by light diffraction.
The medium of JP-A 2001-250274 is expected to find practical use because a carrier-to-noise
ratio (CNR) of about 40 dB is available in reading out pits of a size which is
below the resolution limit. Regrettably, the patent lacks the disclosure of an
optimum method for enhancing read outputs.
It is reported in Jpn. J. Appl. Phys., Vol. 40 (2001), pp. 1624-1628 that in
the
readout operation on the medium of JP-A 2001-250274, some readout signals are omitted
from a certain arrangement pattern of pits and spaces (which are regions between
two adjacent pits). The pit train of alternately arranged pits and spaces usually
includes pits and spaces of differing lengths depending on a particular modulation
system used, and these pits and spaces are arranged in a pattern compliant with
the modulation system and the information to be recorded. If the arrangement pattern
which causes some readout signals to be omitted is identified, then the modulation
system can be devised such that the signal-omitting arrangement pattern may not
develop. However, the resulting modulation system has increased redundancy and
is detrimental to the efforts of increasing the capacity of media. All the patent
and literature references cited above are incorporated herein by reference.
SUMMARY OF THE INVENTION
An object of the invention is to provide a readout method for an optical information
medium having pits or recorded marks of a size approximate to or below the resolution
limit determined by the diffraction theory, which method produces high and accurate
read outputs; and a readout apparatus for use in the readout method.
The above and other objects are attained by the present invention which is defined below.
(1) An information readout method for an optical information medium comprising
an information recording layer having a mark train of marks and spaces, said method
comprising the steps of scanning the mark train with a laser beam, and reading
out the mark train on the basis of a light intensity change pattern of reflected
laser beam, wherein
provided that the laser beam reflected by the mark train includes polarized
light components which define an angle θ with the mark train, a polarized
light component giving θ=0° is designated x
0 component, and
a polarized light component giving θ=90° is designated y
0 component,
the mark train is read out utilizing at least a light intensity change of x
0 component.
(2) The information readout method of (1) wherein linearly polarized light having
an angle θ of from 0° to less than 90° is taken out of the laser
beam reflected by the mark train, and the mark train is read out utilizing at least
a light intensity change of said linearly polarized light.
(3) The information readout method of (1) wherein linearly polarized light having
an angle θ of from 0° to less than 45° is taken out of the laser
beam reflected by the mark train, and the mark train is read out utilizing at least
a light intensity change of said linearly polarized light.
(4) The information readout method of (1) wherein linearly polarized light having
an angle θ of from 0° to 5° is taken out of the laser beam reflected
by the mark train, and the mark train is read out utilizing at least a light intensity
change of said linearly polarized light.
(5) The information readout method of any one of (1) to (4) wherein the mark
train includes plural types of marks having different lengths and plural types
of spaces having different lengths, and
the mark train is read out on the basis of a difference between the light intensity
change pattern of x
0 component and the light intensity change pattern
of y
0 component.
(6) The information readout method of (5) wherein two types of linearly polarized
light having different θ are taken out of the laser beam reflected by the
mark train,
of the two types of linearly polarized light, one having smaller θ is designated
x polarized light and the other having larger θ is designated y polarized light,
the mark train is read out on the basis of a change of the value obtained by
subtracting a multiple of the intensity of y polarized light from the intensity
of x polarized light, whereby those marks and/or spaces which cannot be read out
solely with x polarized light are read out.
(7) The information readout method of (6) wherein provided that the intensity
of x polarized light is X, the intensity of y polarized light is Y, and X/Y corresponding
to the longest mark is α
LM, the mark train is read out on the
basis of a change of (X-α
LMY).
(8) The information readout method of (6) or (7) wherein the angle θ for
x polarized light is from 0° to less than 45°, and the angle θ
for y polarized light is from more than 45° to 90°.
(9) The information readout method of (6) or (7) wherein the angle θ for
x polarized light is from 0° to 5°, and the angle θ for y polarized
light is from 85° to 90°.
(10) The information readout method of any one of (1) to (9) wherein the laser
beam having a wavelength λ is irradiated to the mark train through an objective
lens having a numerical aperture NA for reading out the mark train, and the minimum
length M
L of the marks is up to 0.36λ/NA.
(11) The information readout method of any one of (1) to (9) wherein the laser
beam having a wavelength λ is irradiated to the mark train through an objective
lens having a numerical aperture NA for reading out the mark train, and the minimum
length M
L of the marks is less than 0.25λ/NA.
(12) The information readout method of any one of (1) to (11) wherein the marks
are formed by changes in shape and/or property of the information recording layer.
(13) A readout apparatus for use in the information readout method of any one
of (1) to (12), comprising at least means for detecting linearly polarized light
including x
0 component.
(14) The readout apparatus of (13) wherein the means is capable of independently
detecting two types of linearly polarized light having different θ.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a cross-sectional view of the pattern of one mark train used for
simulation, FIG. 1B is a graph showing the light intensity Y
0 of y
0
component, FIG. 1C is a graph showing the light intensity X
0 of
x
0 component, and FIG. 1D is a graph of (X
0-αY
0).
FIG. 2A is a cross-sectional view of the pattern of another mark train used
for simulation, FIG. 2B is a graph showing the light intensity Y
0 of
y
0 component, FIG. 2C is a graph showing the light intensity X
0
of x
0 component, and FIG. 2D is a graph of (X
0-αY
0).
FIG. 3 illustrates a readout apparatus according to one embodiment of the invention.
FIG. 4 illustrates a readout apparatus according to another embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As used herein, the mark train is present within an information recording layer
and consists of a series of marks and spaces (which are regions between two adjacent
marks). As the mark train is scanned with a laser beam, the light intensity of
reflected laser beam changes in accordance with the arrangement of marks and spaces
to provide a pattern. The marks that bring about such changes of reflected light
intensity can be formed by changes in shape and/or property of the information
recording layer. The marks formed by changes in shape of the information recording
layer are, for example, pits in read-only optical disks. The marks formed by changes
in property of the information recording layer are, for example, amorphous recorded
marks in phase change optical recording disks. The marks formed by changes in shape
and property of the information recording layer are, for example, pits in write-once
optical recording disks having an organic dye-containing information recording
layer where the pits are formed.
Reference is now made to the resolution limit determined by the diffraction
theory. An optical system delivers a laser beam through an objective lens to the
mark train in the information recording layer of the medium. The resolution limit
is determined by the wavelength of the laser beam and the numerical aperture of
the objective lens. Assume that λ is the wavelength of the reading laser
beam and NA is the numerical aperture of the objective lens. Since the cutoff spatial
frequency is 2NA/λ, a mark train in which marks and spaces have an equal
length are readable as long as the spatial frequency is equal to or below 2NA/λ
(line pairs/nm). The mark length (=space length) corresponding to the readable
spatial frequency is given as
λ/4
NA=0.25
λ/NA.
It is then concluded that super-resolution readout is possible if a mark train
whose minimum mark length M
L is less than 0.25λ/NA can be read out.
Making studies on the super-resolution readout mechanism of reading out a
mark train of minute marks and spaces of a size below the resolution limit in the
medium described in the above-referred JP-A 2001-250274 and Jpn. J. Appl. Phys.,
Vol. 40 (2001), pp. 1624-1628, the present inventors have discovered that super-resolution
readout is closely correlated to the angle between the mark train and the oscillation
or polarization direction of an electric field vector of a reading laser beam.
Herein, the angle defined between the mark train and the polarization direction
is represented by θ. Also herein, θ is an acute angle, that is, in
the range of 0° to 90°.
The medium of interest is one including a functional layer capable of super-resolution
readout having a mark train of minute marks and spaces of a size below the resolution
limit. The inventors have empirically found that the mark train can be read out
when retrieval is carried out with a laser beam whose polarization direction is
parallel to the mark train (i.e., θ=0°). By contrast, the mark train
cannot be read out when retrieval is carried out on the same medium using a laser
beam whose polarization direction is perpendicular to the mark train (i.e., θ=90°).
Now a simulation model is used to describe that the readout of a mark train consisting
of minute marks and spaces is dependent on the polarization direction of a reading
laser beam; and to describe a signal processing method necessary to carry out super-resolution
readout to produce high, accurate outputs, utilizing the dependency.
For the simulation, a finite difference time domain method was used. The medium
used had a substrate bearing a mark train in its surface and a Si layer of 20 nm
thick formed thereon. The mark train included short marks and short spaces both
having a length of 200 nm and long marks and long spaces both having a length of
800 nm. Each mark is a pit of 60 nm deep and 200 nm wide. The pit has end walls
which are semi-circular and side walls which extend perpendicular to the substrate
surface. The Si layer extends to the side walls of the pits while keeping its thickness
of 20 nm. The substrate has a refractive index of 1.56; Si has a complex refractive
index of 3.88+0.02i; and air has a refractive index of 1. The laser beam used for
reading has a wavelength of 650 nm. The objective lens of the laser beam delivery
optical system has a numerical aperture of 0.60. The length of long marks and spaces
is greater than the resolution limit (0.25λ/NA=271 nm), and the length of
short marks and spaces is less than the resolution limit.
In carrying out simulation, the medium was divided into sections having planar
dimensions of 20 nm×20 nm and a depth of 3 nm. For each section, the distribution
of an electromagnetic field created within the medium by irradiating a laser beam
was determined. Based on this distribution, the total intensity of reflected light
from all the sections falling within the beam spot was determined. With this simulation,
the sum of the reflected light intensity dependent on the electromagnetic field
distribution and the reflected light intensity dependent on optical diffraction
and reflection is obtainable, but the reflected light intensity dependent on optical
interference by medium surface irregularities is not obtainable.
In the mark train, marks and spaces are arranged in a pattern as shown in FIG.
1A. In FIG. 1A, SM, SS, LM and LS stand for short marks, short spaces, long marks
and long spaces, respectively.
In this simulation, the reflected light from the mark train contains polarized
light components. Of the polarized light components, a polarized light component
having θ=90° is designated y
0 component having a light intensity
Y
0, and a polarized light component having θ=0° is designated
x
0 component having a light intensity X
0.
FIGS. 1B and 1C show changes of the intensity of Y
0 and X
0,
respectively. In FIGS. 1B and 1C, the ordinate represents the intensity of Y
0
and X
0, and the abscissa represents positions corresponding to
the mark train shown in FIG. 1A.
Attention is first paid to five, in total, short marks and short spaces
alternately arranged near the center of the substrate in FIG. 1A (referred to as
"short mark-and-space train," hereinafter). Long marks are located at opposite
ends of the short mark-and-space train. In FIG. 1B showing changes of the intensity
of Y
0, any intensity change corresponding to short marks in the short
mark-and-space train is not ascertained, but a broad intensity drop is ascertained
as if spaces substantially longer than the short spaces were present. On the other
hand, in FIG. 1C showing changes of the intensity of X
0, intensity drops
corresponding to two short marks in the short mark-and-space train is ascertained.
As understood from these results, at least x
0 component must be used
in order to read out marks of a size smaller than the resolution limit.
It is noted that although five peaks indicative of intensity changes corresponding
to the short mark-and-space train must appear by nature, only three peaks are ascertained
in FIG. 1C at positions corresponding to the short mark-and-space train. In the
event of a mark train including a short mark which is separated from a long mark
by a short space (which is referred to as "long mark-adjoining short mark," hereinafter)
as in this short mark-and-space train, an apparent signal omission occurs. More
specifically, it becomes impossible to independently detect a light intensity change
associated with a short space located between a short mark and a long mark. Then,
in reading out such a mark train which can induce signal omission, signal processing
must be properly modified.
Next, it is described how to recover the signals omitted in proximity to the
long mark-adjoining short mark.
In FIG. 1C showing the intensity change of X
0, the X
0 intensity
change pattern corresponding to short marks and short spaces in the short mark-and-space
train is reverse to the pattern corresponding to long marks and long spaces. That
is, in the train of long marks and long spaces, the intensity increases at marks
and decreases at spaces, whereas in the short mark-and-space train, the intensity
decreases at marks and increases at spaces. It is thus presumed that the intensity
increase of X
0 by a short space located at each end of the short mark-and-space
train is buried or hidden in the intensity increase of X
0 by a long mark.
On the other hand, in FIG. 1B showing the intensity change of Y
0,
the
intensity change pattern is similar to that of FIG. 1C except that a intensity
change corresponding to the short mark-and-space train is not ascertained. It is
notable that the maximum light intensity of Y
0, is about 5 times the
maximum light intensity of X
0. It is presumed from these results that
a intensity change corresponding to the short mark-and-space train can be extracted
if appropriate processing is made such that the light intensity change pattern
associated with long marks and long spaces, that is, the change pattern common
to X
0 and Y
0, is eliminated from the light intensity change
pattern of X
0 shown in FIG. 1C. To this end, subtraction operation of
subtracting a multiple of Y
0 from X
0 may be carried out.
Provided that the multiple of Y
0 used in the subtraction operation is
αY
0, α may be determined as appropriate in accordance with
the construction of the medium and the construction of the mark train such that
a light intensity change associated with a short space located between a short
mark and a long mark may develop as a result of the subtraction operation.
Although an appropriate value of α can be experimentally determined,
it is recommended that the value of X
0/Y
0 at the long mark
be employed as α when X
0 and Y
0 change as shown in
FIGS. 1B and 1C, respectively. In this event, for the long mark, X
0-αY
0=X
0-X
0=0.
Provided that X
0/Y
0 at the long mark is α
LM,
(X
0-α
LMY
0) is shown in FIG. 1D. In FIG.
1D, the abscissa represents positions corresponding to the mark train shown in
FIG. 1A, like FIGS. 1B and 1C, and α
LM is 0.178.
(X
0-α
LMY
0)
is a value obtained by extracting the difference between the intensity change pattern
of X
0 and the intensity change pattern of Y
0, using the reflected
light intensity from the long mark as a reference. Then, the curve of (X
0-α
LMY
0)
shown in FIG. 1D definitely reveals the changes of a minute amplitude which have
been buried in the changes of a large amplitude in FIG. 1C, that is, intensity
changes of all marks and spaces in the short mark-and-space train.
Also, since (X
0-α
LMY
0)>0 stands
at the long spaces, the pattern of intensity change is reversed for both the long
marks and the long spaces, as compared with the X
0 signal shown in FIG.
1C. That is, in the curve of (X
0-α
LMY
0),
the intensity lowers at long marks, but increases at long spaces. On the other
hand, the intensity change pattern associated with the short mark-and-space train
is such that the intensity lowers at marks, but increases at spaces as in FIG.
1C. As a result, in the curve of (X
0-α
LMY
0),
all the long marks, long spaces and short mark-and-space train show a light intensity
change pattern similar to that of phase pits in prior art optical information media.
It is noted that the subtraction operation of computing (X
0-α
LMY
0)
is preferably applied in the situation where (X
0-α
LMY
0)>0
stands at long spaces.
While α
LM is one example of α as described above, α
may take a value other than α
LM even when X
0 and Y
0
change as shown in FIGS. 1B and 1C, respectively. Note that α must
be determined while taking care such that the intensity change pattern that the
intensity lowers at marks and increases at spaces in the short mark-and-space train
is not reversed.
Next, simulation was carried out as above on a mark train having the arrangement
pattern shown in FIG. 2A. The results are shown in FIGS. 2B,
2C and
2D.
In FIGS. 2B,
2C and
2D, the ordinate represents the intensity of
Y
0, X
0 and (X
0-α
LMY
0),
and the abscissa represents positions corresponding to the mark train shown in
FIG. 2A. In this case, α
LM is 0.237.
In the mark train shown in FIG. 2A, attention is paid to a short mark-and-space
train consisting of two short marks and two short spaces alternately arranged near
the center of the substrate. Although four peaks indicative of intensity changes
must appear from this short mark-and-space train by nature, only two peaks are
ascertained in FIG. 2C at positions corresponding to the short mark-and-space train.
Specifically, as in FIG. 1C, a light intensity change corresponding to the short
space located at one end of the short mark-and-space train is omitted, and a light
intensity change corresponding to the short mark located at the other end is omitted
as well. It is seen from this result that an apparent signal omission also occurs
in a mark train including a short space which is separated from a long space by
a short mark (which is referred to as "long space-adjoining short space," hereinafter).
More specifically, it becomes impossible to independently detect a light intensity
change associated with a short mark located between a short space and a long space.
Such signal omission occurs presumably because a intensity increase of X
0
by a short mark located between a short space and a long space is buried in a intensity
decrease of X
0 by a long space. This is evident from the fact that all
four peaks corresponding to the short mark-and-space train definitely appear in
the curve of (X
0-α
LMY
0) shown in FIG. 2D.
It is appreciated from the results of both the aforementioned simulation models
that those marks and/or spaces, which have not been read out only with x
0
polarized light, can be read out utilizing (X
0-α
LMY
0).
From the results of these simulation models, the super-resolution readout mechanism
on the medium having the aforementioned functional layer is presumed as below.
In a medium of the type wherein reflectance changes at marks are utilized for
readout, the recorded information is reproduced by scanning a mark train with a
laser beam, the intensity of reflected light changing in accordance with the arrangement
pattern of marks and spaces, and detecting changes of reflected light intensity
for thereby reading out the mark train. However, with respect to a mark train in
which arrangement intervals are below the resolution limit, reflectance changes
corresponding to the arrangement pattern of marks and spaces cannot be detected,
and reproduction is impossible.
It is believed that when the mark train is scanned with a laser beam, interaction
occurs between the electric field of the laser beam and the medium, depending on
many parameters including the shape of marks (including a three-dimensional shape),
the dimensions of marks and spaces, and the refractive index of the material of
which marks and spaces are made. Presumably, the localization of the electric field
is enhanced particularly in proximity to the outer periphery of a mark. It is then
presumed that on scanning a mark train with a laser beam, the aforementioned interaction
changes in accordance with the arrangement pattern of marks and spaces and that
change is reflected by a change pattern of reflected light intensity.
The aforementioned interaction is enhanced particularly when a functional layer
for enabling super-resolution readout, such as the Si layer used in the aforementioned
simulation models is provided. In contrast, no or insignificant interaction occurs
with a layer incapable of super-resolution readout, such as an Ag layer. Also no
or insignificant interaction occurs when the oscillation or polarization direction
of the electric field vector of a laser beam is perpendicular to the mark train,
but strong interaction occurs when the polarization direction is parallel to the
mark train.
A comparison of FIG. 1B with FIG. 1C indicates the presence of another effect
different
from the aforementioned interaction. The other effect is to increase the intensity
of reflected light at long marks of a size equal to or above the resolution limit
and reduce the intensity of reflected light at long spaces of a size equal to or
above the resolution limit. For convenience sake, the aforementioned interaction
is designated as first effect and the other effect is designated as second effect.
It is believed that the second effect is not substantially exerted with minute
marks and spaces of a size below the resolution limit.
A comparison of FIG. 1B with FIG. 1C reveals that the reflected light intensity
arising from the second effect is less dependent on the polarization direction
when reflected from long spaces and more dependent on the polarization direction
when reflected from long marks. It is noted that the light intensity change pattern
based on the second effect is not dependent on the polarization direction. That
is, the reflected light intensity increases at long marks and decreases at long
spaces, regardless of the polarization direction.
As compared with the first effect, the second effect has substantial influence
on the reflected light intensity, and the influence of the second effect on the
(decreasing or increasing) direction of reflected light intensity change is reverse
to the influence of the first effect. Then, as shown in FIG. 1C, the light intensity
change by the first effect at a short space is hidden by the light intensity change
by the second effect at the long mark located adjacent to the short space. Also,
as shown in FIG. 2C, the light intensity change by the first effect at a short
mark is hidden by the light intensity change by the second effect at the long space
located adjacent to the short mark. Accordingly, from a short mark-and-space train
consisting of a total number "n" of short spaces and short marks and located adjacent
to long marks or long spaces, only (n-2) peaks indicative of light intensity changes
appear, and omission of signals occurs.
The present invention aims to recover the signals that are omitted in this way.
Now that the first effect is substantially exerted only on short marks and short
spaces, the reflected light intensity change pattern based on the first effect
has polarization dependency, the second effect is substantially exerted only on
long marks and long spaces, and the reflected light intensity change pattern based
on the second effect has no polarization dependency; the present invention carries
out signal processing operation of extracting the light intensity change brought
by the first effect which has been buried in the light intensity change pattern
of x
0 component, by eliminating the light intensity change pattern of
y
0 component from the light intensity change pattern of x
0 component.
This prevents any signals from being omitted from the short mark-and-space train
as shown in FIGS. 1D and 2D.
The results of the aforementioned simulation do not include the results of light
interference. Long marks function as phase pits in the above simulation, and the
reflected light intensity at long marks decreases due to light interference. On
the other hand, short marks cannot be read out utilizing light interference. In
actually retrieving information from the medium, the light intensity changes shown
in FIGS. 1B,
1C and
1D, respectively, are added to the reflected
light intensity change by such light interference.
Provided that the actual light intensity of x
0 component is X,
this light intensity X is the reflected light intensity change by interference
on which the light intensity change pattern shown in FIG. 1C is superposed. Accordingly,
in the change pattern of light intensity X, there develop both a light intensity
change brought by light interference corresponding to a long mark and a light intensity
change corresponding to a short mark as shown in FIG. 1C. Namely, both a long mark
and a short mark can be read out.
Provided that the actual light intensity of y
0 component is Y,
the actual light intensity (X-α
LMY) is the reflected light intensity
change by interference (this intensity is the original intensity multiplied by
(1-α
LM)) on which the light intensity change pattern shown in
FIG. 1D is superposed. Accordingly, in the change pattern of light intensity (X-α
LMY),
there develop both a light intensity change corresponding to a long mark and light
intensity changes corresponding to all marks and spaces included in the short mark-and-space
train. Then, all marks can be read out.
Although a mark train consisting of pits has been verified in the aforementioned
simulation, the present invention is effectively applicable to the reading out
of a mark train consisting of marks other than pits, for example, a mark train
consisting of amorphous recorded marks formed in a crystalline recording layer
of a phase change optical recording disk. It is presumed that since the complex
index of refraction differs between amorphous and crystalline states, the irradiation
of a laser beam to the recording layer results in an electric field which is strongly
localized near the boundary between an amorphous recorded mark and the surrounding
crystalline material. This localization of the electric field is dependent on the
polarization direction like the localization of the electric field associated with
pits. Therefore, a reflected light intensity change by the first effect also occurs
with the short mark-and-space train in the phase change recording layer. On the
other hand, long marks invite a lowering of reflected light intensity due to the
difference of reflectance from the surrounding crystalline material. Accordingly,
the present invention is applicable to phase change media as well.
In the aforementioned simulation, x
0 component having a polarization
direction parallel to the mark train and y
0 component having a polarization
direction perpendicular to the mark train are used to exaggerate the polarized
light dependence. However, any linearly polarized light taken out arbitrarily from
the laser beam reflected by the mark train is regarded as the resultant of x
0
and y
0 components. Accordingly, the benefits of the present invention
are attainable, as will be described below, using two types of linearly polarized
light taken out arbitrarily from the laser beam reflected by the mark train and
having different angles between the polarization direction and the mark train.
It is now assumed that, of the two types of linearly polarized light, one having
smaller θ is designated x polarized light and the other having larger θ
is designated y polarized light, the intensity of x polarized light is X, and the
intensity of y polarized light is Y. The x polarized light includes at least x
0
component, and the y polarized light includes at least y
0 component.
In this case, a change of (X-αY) reflects the light intensity change pattern
which is extracted from the change pattern of x
0 component and given
by those marks and/or spaces which have not been read out only with x
0 polarized
light. Accordingly, using (X-αY), the mark train can be read out on the basis
of a difference between the light intensity change pattern of x
0 component
and the light intensity change pattern of y
0 component. Then, those
marks and/or spaces which have not been read out only with x polarized light can
be read out.
To increase the read output of minute marks, better results are obtained as the
angle θ of x polarized light becomes closer to 0° and the angle θ
of y polarized light becomes closer to 90°. Specifically, the angle θ
of x polarized light is preferably from 0° to less than 45°, and then,
the angle θ of y polarized light is preferably from more than 45° to
90°. It is most preferred that the angle θ of x polarized light be 0°
and the angle θ of y polarized light be 90°, that is, to use x polarized
light consisting solely of x
0 component and y polarized light consisting
solely of y
0 component. However, sufficiently high read outputs of minute
marks are obtainable as long as the angle θ of x polarized light is from
0° to 5° and the angle θ of y polarized light is from 85°
to 90°.
In taking two types of linearly polarized light having different θ out
of
the light reflected by the mark train, a polarizing beam splitter is generally
used. The two types of polarized light separated by the polarizing beam splitter
have orthogonal polarization directions.
On the other hand, in the case of readout operation utilizing only the characteristic
light intensity change of x
0 component shown in FIG. 1C, the mark train
may be read out by taking linearly polarized light having θ of from 0°
to less than 90° out of the laser beam reflected by the mark train and utilizing
the light intensity change of this linearly polarized light. Since this linearly
polarized light includes x
0 component, the light intensity change pattern
of this linearly polarized light reflects the pattern shown in FIG. 1C. Accordingly,
even when only this linearly polarized light is used, readout of a minute pattern
is possible like the case where only x
0 polarized light is used. To
increase the read output of minute marks, better results are obtained as the angle
θ of the linearly polarized light becomes closer to 0°, and specifically,
when θ is preferably from 0° to less than 45°, more preferably
from 0° to 5°, and most preferably 0°.
The present invention is effective for reading out a mark train including at
least marks of a size below the resolution limit, that is, a mark train wherein
the shortest mark length M
L is less than 0.25λ/NA. Understandably,
the first effect dependent on the polarization direction of a laser beam is exerted
relatively strongly on marks having a size equal to or above the resolution limit,
as long as the mark length is approximate to the resolution limit. For marks having
a size of such order, conventional readout methods are difficult to increase read
outputs. Accordingly, the present invention is also effective for reading out a
mark train wherein the shortest mark length M
L is equal to or above
the resolution limit (0.25λ/NA). However, the first effect becomes weak if
the shortest mark length M
L is too long. Then the shortest mark length
M
L is given an upper limit of 0.36λ/NA which is slightly greater
than the resolution limit, and is preferably up to 0.31λ/NA.
To utilize the readout method of the invention, a readout apparatus comprising
at least means for detecting linearly polarized light including x
0 component
is used. When a mark train is read out using two types of linearly polarized light
having different θ, a readout apparatus comprising means for independently
detecting two types of linearly polarized light having different θ is used.
FIG. 3 illustrates the arrangement of a readout apparatus used in the practice
of the readout method of the invention. The readout apparatus includes a laser
diode LD for emitting linearly polarized light which passes through a polarizing
beam splitter PBS and a quarter waveplate ¼WP for conversion to circularly
polarized light. The circularly polarized light enters a beam splitter BS. A light
component that has passed straight ahead through the beam splitter BS is focused
at the surface of a disk through a lens. The beam splitter BS is not a polarizing
beam splitter, but has a function of dividing light into two directions like a
half mirror. Of the circularly polarized light entering the beam splitter BS, a
light component reflected upward as viewed in the figure is absorbed by a beam
stop so that no scattering light prevails within the apparatus.
Of the circularly polarized light reflected by the medium surface, a light component
turned downward as viewed in the figure by the beam splitter BS enters a second
polarizing beam splitter PBS where it is divided into two types of linearly polarized
light having orthogonal polarization directions which enter photodiodes PD
1
and PD
2, respectively. By processing the signals from the photodiodes PD
1
and PD
2, a readout operation based on (X-αY) is enabled.
On the other hand, of the circularly polarized light reflected by the medium
surface,
a light component that has passed straight ahead through the beam splitter BS enters
the quarter waveplate ¼WP again for conversion to linearly polarized light.
The linearly polarized light enters the polarizing beam splitter PBS where it is
reflected downward as viewed in the figure and enters a photodiode PD
3.
Therefore, the laser beam is not fed back to the laser diode LD. It is noted that
although the provision of the photodiode PD
3 is not essential, the photodiode
PD
3 may be utilized for the purpose of tracking servo or focusing servo.
FIG. 4 illustrates the arrangement of another readout apparatus used in the
practice of the readout method of the invention. In this readout apparatus, linearly
polarized light emitting from a laser diode LD has a polarization direction which
is inclined at an angle of 45° with respect to the plane of paper. Of this
linearly polarized light, a light component that has passed straight ahead through
a beam splitter BS is focused at the surface of a disk through a lens.
Of the linearly polarized light reflected by the medium surface, a light component
turned downward as viewed in the figure by the beam splitter BS enters a polarizing
beam splitter PBS where it is divided into two types of linearly polarized light
having orthogonal polarization directions which enter photodiodes PD
1 and
PD
2, respectively. By processing the signals from the photodiodes PD
1
and PD
2, a readout operation based on (X-αY) is enabled.
Described below is one exemplary construction of the medium to which the
readout method of the invention is applied.
The medium to which the invention is applie
