Title of Invention

MULTILAYER OPTICAL INFORMATION RECORDING MEDIUM,OPTICAL HEAD AND OPTICAL DRIVE

Abstract A disclosed optical recording medium includes multiple recording layer units in each of which one or more recording layers (3) and one or more middle layers (2) are stacked alternately; and one or more spacer layers. In the disclosed optical recording medium, the recording layer units and the spacer layers (4) are stacked alternately in a depth direction of the optical recording medium.
Full Text

DESCRIPTION
TECHNICAL FIELD
The present invention generally relates to a
multilayer optical information recording medium, an optical
head, and an optical drive, and more particularly relates
to a multilayer optical information recording medium with
multiple recording layers, an optical head for the
multilayer optical information recording medium, and an
optical drive capable of recording, reproducing, and/or
deleting information on the multilayer optical information
recording medium.
BACKGROUND ART
Patent document 1 discloses a method of optically
writing, reading, and/or deleting information on a
conventional multilayer optical information recording
medium having at least two recording layers and two guide
layers; and an apparatus for writing, reading, and/or
deleting information on the conventional multilayer optical
information recording medium. FIG. 14 shows the structure
of an exemplary recording medium described in patent
document 1. The exemplary recording medium includes

multiple recording layers 3 and one tracking layer (control
layer) 5 for the multiple recording layers 3.
Patent document 2 discloses a recording medium
made by stacking control layers used for tracking and
layers made of a photosensitive material. Patent document 3
discloses an optical memory device in which a recording
layer is provided next to a core layer made of resin or a
clad layer made of resin and a barrier layer is provided
between the recording layer and the core layer or the clad
layer to prevent them from blending.
Also, patent document 4 discloses a multilayer
optical recording medium made by stacking recording layers
and non-recording layers alternately using adhesive sheets
each made up of an optical recording layer containing a
photosensitive material and an adhesive layer.
[Patent document 1] Japanese Patent No. 3110532
[Patent document 2] Japanese Patent Application
Publication No. 2003-36537
[Patent document 3] Japanese Patent Application
Publication No. 2003-141739
[Patent document 4] Japanese Patent Application
Publication No. 2005-259192
However, in a multilayer optical information
recording medium having a structure as described above, as
the number of layers increases, fluctuation of reflectance

caused by differences in the wavelength or incidence angle
of light becomes greater and wavelength dependence becomes
greater. In other words, the amount of reflected light in
recording or reading a signal fluctuates depending on the
wavelength or incidence angle of the light. This
fluctuation causes an increase in noise and results in a
decrease in the S/N ratio.
Also, in a multilayer optical recording medium
with a control layer used for tracking as shown in FIG. 14,
as the number of recording layers increases, the distance
between each recording layer and the control layer
increases. This makes it difficult to position a laser beam
accurately.
Further, since the recording layers are formed
just above the control layer, methods that can be used to
process the control layer are limited and therefore
flexibility in designing the control layer is reduced.
Meanwhile, in recent years, with the development
of digital technologies and the improvement in data
compression techniques, optical disks such as a digital
versatile disk (DVD) have gotten a lot of attention as
media for recording information such as music, movies,
photographs, and computer programs (hereafter, may also be
called "contents"). Also, as the prices of optical disks

become lower, optical drives for recording and/or
reproducing information on optical disks have become
widespread.
As the data sizes of contents increase year by
year, there is an increasing demand for an optical disk
with higher storage capacity. One way to increase the
storage capacity of an optical disk is to provide multiple
recording layers. Currently, development of optical disks
having multiple recording layers (hereafter, may also be
called "multilayer disks" or "multilayer optical disks")
and optical drives for recording/reproducing information on
such multilayer disks are very active.
However, if the number of recording layers in a
conventional optical disk is increased, the amount of light
reflected from a recording layer decreases as the distance
between the recording layer and the incidence plane
increases, because light is absorbed by other recording
layers. As a result, the amount of light reflected from a
distant recording layer decreases to such a level that it
is difficult to detect a signal. Also, a conventional laser
diode may not be powerful enough to record information on
such a multilayer optical disk. These problems have been
limiting the number of recording layers in an optical disk.
To solve the above problems and thereby to
increase the number of recording layers, multilayer disks

using two-photon absorption materials have been proposed
(see, for example, patent documents 5 and 6). The
refractive index of a two-photon absorption material
changes when it absorbs two photons simultaneously. The
proposed multilayer disks utilize this characteristic of
two-photon absorption materials. On a proposed multilayer
disk, information is recorded by changing the refractive
index of target areas. These refractive index changed areas
are called pits. More specifically, information is
represented by the lengths and combination of refractive
index changed areas and refractive index unchanged areas.
The probability of occurrence of two-photon
absorption is proportional to the square of an applied
optical-electric field (intensity of an incident light).
Therefore, two-photon absorption occurs only in an area
where the energy of an incident light is concentrated. When
an incident light is focused by a lens, two-photon
absorption occurs only around the focal point and does not
occur in other areas where the incident light is not
focused. In other words, the refractive indices of
recording layers other than that on which incident light is
focused do not change and those recording layers transmit
the incident light without changing its intensity.
Therefore, in this case, increasing the number of recording

layers does not make it difficult to detect a signal or
cause recording power shortage problems.
Thus, using two-photon absorption materials makes
it possible to increase the number of recording layers and
thereby to greatly increase the storage capacity of an
optical disk. However, as in the case of conventional
multilayer disks, forming guide tracks on each of the
recording layers results in increased costs.
To obviate this problem, multilayer disks having
guide tracks on a layer other than recording layers have
been proposed (see, for example, patent documents 7 and 8).
Patent document 7 discloses a recording medium
having a servo layer. With the disclosed recording medium,
servo control is performed by detecting reflected light
from the servo layer. However, if the recording medium is
tilted in the radial direction in relation to the incidence
angle of light, a tracking error may occur on a data layer
that is distant from the servo layer. For example, on a
data layer that is 1 mm distant from the servo layer, when
the recording medium is tilted 1 degree in relation to the
incidence angle of the light, the focal point of the light
is shifted as much as 17.4 µm. On a Blu-ray disk with a
track pitch of 0.32 µm, 17.4 µm is equivalent to about 50
tracks. For this reason, the recording medium disclosed in
patent document 7 requires a tilt control that is different

from that for a recording medium with a few recording
layers. Also, although a small light spot can be formed on
a data layer where a two-photon absorption material is used,
a light spot becomes large on the servo layer where no two-
photon absorption material is used. This problem makes it
difficult to increase the track density of a recording
medium and thereby makes it difficult to increase the
storage capacity per data layer.
Patent document 8 discloses an optical information
recording medium including a first layer having alternate
convexities and concavities and a second layer having
alternate convexities and concavities. In this case,
however, it is very difficult to accurately align the
convexities and concavities on the first and second layers.
[Patent document 5] Japanese Patent Application
Publication No. 6-28672
[Patent document 6] Japanese Patent Application
Publication No. 2004-79121
[Patent document 7] Japanese Patent Application
Publication No. 2002-312958
[Patent document 8] Japanese Patent Application
Publication No. 2005-18852
DISCLOSURE OF THE INVENTION
The present invention provides a multilayer

optical information recording medium, an optical head, and
an optical drive that substantially obviate one or more
problems caused by the limitations and disadvantages of the
related art.
Embodiments of the present invention provide a
multilayer optical information recording medium that makes
it possible to reduce the fluctuation in the amount of
reflected light in recording or reading a signal even when
the wavelength or incidence angle of the light varies and
thereby to prevent the S/N ratio from decreasing; to
accurately position a laser beam even when the number of
recording layers is large; and to use various methods to
process a control layer and thereby to design the control
layer flexibly.
According to an embodiment of the present
invention, an optical recording medium includes multiple
recording layer units in each of which one or more
recording layers and one or more middle layers are stacked
alternately; and one or more spacer layers; wherein the
recording layer units and the spacer layers are stacked
alternately in a depth direction of the optical recording
medium.
According to an embodiment of the present
invention, an optical recording medium includes multiple
recording layers in each of which recording marks each

having a refractive index different from that of a
surrounding area are arranged so as to form multiple layers
of the recording marks, wherein the recording marks in each
of the layers of the recording marks are horizontally
arranged at intervals and the layers of the recording marks
are vertically arranged at intervals; and one or more
spacer layers; wherein the recording layers and the spacer
layers are stacked alternately in a depth direction of the
optical recording medium.
An embodiment of the present invention provides a
high capacity multilayer optical information recording
medium with a tilt tolerance that is substantially equal to
that of a recording medium having only a few recording
layers.
Another embodiment of the present invention
provides an optical head that can accurately detect a
signal from a multilayer optical information recording
medium according to an embodiment of the present invention.
Still another embodiment of the present invention
provides an optical drive that can accurately record,
reproduce, and/or delete information on a multilayer
optical information recording medium according to an
embodiment of the present invention.
According to an embodiment of the present

invention, an optical recording medium includes multiple
multilayer units each including a guide layer corresponding
to light with a first wavelength and multiple recording
layers corresponding to light with a second wavelength that
is different from the first wavelength; wherein the
multilayer units are stacked in a depth direction of the
optical recording medium.
According to an embodiment of the present
invention, an optical recording medium includes multiple
guide layers corresponding to light with a first
wavelength; and multiple recording layers corresponding to
light with a second wavelength that is different from the
first wavelength.
According to an embodiment of the present
invention, an optical recording medium includes multiple
multilayer units each including multiple guide layers
corresponding to light with a first wavelength, and
multiple recording layers corresponding to light with a
second wavelength that is different from the first
wavelength; wherein the multilayer units are stacked in a
depth direction of the optical recording medium.
An optical head for recording or reproducing
information on an optical recording medium according to an
embodiment of the present invention includes a first light
source configured to emit a light beam with the first

wavelength; a second light source configured to emit a
light beam with the second wavelength; an objective lens
configured to focus the light beam with the first
wavelength on the guide layer and to focus the light beam
with the second wavelength on one of the recording layers;
an optical system configured to guide the light beam with
the first wavelength and the light beam with the second
wavelength to the objective lens and to separate a light
beam reflected from the guide layer and a light beam
reflected from the one of the recording layers; a first
photodetector configured to detect the light beam reflected
from the guide layer; and a second photodetector configured
to detect the light beam reflected from the one of the
recording layers.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
FIG. 1 is a drawing illustrating the configuration
of a first exemplary multilayer optical information
recording medium according to a first embodiment of the
present invention;
FIG. 2 is a graph showing the relationship between
the number of layers in the first exemplary multilayer
optical information recording medium shown in FIG. 1 and
its reflectance;

FIG. 3 is a graph showing the relationship between
the thickness of each middle layer and the reflectance of
the first exemplary multilayer optical information
recording medium;
FIG. 4 is a drawing illustrating the configuration
of a second exemplary multilayer optical information
recording medium according to a second embodiment of the
present invention;
FIG. 5 is a drawing illustrating the configuration
of a third exemplary multilayer optical information
recording medium according to a third embodiment of the
present invention;
FIG. 6 is a drawing illustrating the configuration
of a fourth exemplary multilayer optical information
recording medium according to a fourth embodiment of the
present invention;
FIG. 7 is a drawing illustrating the configuration
of a fifth exemplary multilayer optical information
recording medium according to a fifth embodiment of the
present invention;
FIG. 8 is a drawing illustrating the configuration
of a sixth exemplary multilayer optical information
recording medium according to a sixth embodiment of the
present invention;

FIG. 9 is a drawing illustrating the configuration
of a seventh exemplary multilayer optical information
recording medium according to a seventh embodiment of the
present invention;
FIG. 10 is a drawing illustrating the
configuration of an eighth exemplary multilayer optical
information recording medium according to an eighth
embodiment of the present invention;
FIG. 11 is a drawing illustrating the
configuration of a ninth exemplary multilayer optical
information recording medium according to a ninth
embodiment of the present invention;
FIG. 12 is a drawing illustrating the
configuration of a tenth exemplary multilayer optical
information recording medium according to a tenth
embodiment of the present invention;
FIG. 13 is a drawing illustrating the
configuration of an exemplary signal recording/reproducing
apparatus according to an eleventh embodiment of the
present invention for recording and reproducing a signal on
a multilayer optical information recording medium according
to an embodiment of the present invention;
FIG. 14 is a drawing illustrating a conventional
multilayer optical information recording medium;

FIG. 15 is a drawing illustrating an exemplary-
structure of an optical disk 100 that is a multilayer
optical information recording medium according to an
embodiment of the present invention;
FIG. 16 is a drawing illustrating an information
layer M in the optical disk 100 shown in FIG.15
FIG. 17 is a graph showing sizes of pits formed by
two-photon absorption and sizes of pits formed by a
conventional method;
FIG. 18 is a drawing illustrating the
configuration of an exemplary optical pickup that is an
optical head according to an embodiment of the present
invention;
FIG. 19 is a drawing used to describe the
exemplary optical pickup shown in FIG. 18;
FIG. 20 is another drawing used to describe the
exemplary optical pickup shown in FIG. 18;
FIG. 21 is another drawing used to describe the
exemplary optical pickup shown in FIG. 18;
FIG. 22 is a block diagram illustrating the
configuration of an exemplary optical disk apparatus that
is an optical drive according to an embodiment of the
present invention;
FIG. 23 is a flowchart showing an exemplary
recording process performed by the exemplary optical disk

apparatus shown in FIG. 22 when a recording request is
received from an upstream apparatus;
FIG. 24 is a flowchart showing an exemplary
reproduction process performed by the exemplary optical
disk apparatus shown in FIG. 22 when a reproduction request
is received from an upstream apparatus;
FIG. 25 is a drawing illustrating an exemplary
structure of an optical disk 100a that is a multilayer
optical information recording medium according to an
embodiment of the present invention;
FIG. 26 is a drawing illustrating exemplary focal
points of a servo beam and a recording/reproducing beam in
the optical disk 100a shown in FIG. 25;
FIG. 27 is another drawing illustrating exemplary
focal points of a servo beam and recording/reproducing
beams in the optical disk 100a shown in FIG. 25;
FIG. 28 is another drawing illustrating exemplary
focal points of a servo beam and recording/reproducing
beams in the optical disk 100a shown in FIG. 25;
FIG. 29 is a drawing illustrating an exemplary
structure of an optical disk 100b that is a multilayer
optical information recording medium according to an
embodiment of the present invention; and
FIG. 30 is a drawing illustrating an exemplary
structure of an optical disk 100c that is a multilayer

optical information recording medium according to an
embodiment of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
Preferred embodiments of the present invention are
described below with reference to the accompanying drawings.
FIG. 1 is a drawing illustrating the configuration
of a first exemplary multilayer optical information
recording medium according to a first embodiment of the
present invention. The first embodiment of the present
invention is described below with reference to FIG. 1. As
shown in FIG. 1, in the first exemplary multilayer optical
information recording medium, a first middle layer 2, a
first recording layer 3, a second middle layer 2, and a
second recording layer 3 are stacked on a substrate 1. In a
similar manner, for example, five recording layers 3 and
six middle layers 2 are stacked to form a recording layer
unit. Further, a separating layer (spacer layer) 4 is
stacked on the recording layer unit. Thus, the first
exemplary multilayer optical information recording medium
is produced by stacking multiple recording layer units and
one or more spacer layers 4 alternately on the substrate 1.
To form a recording mark on the recording layer 3
of the first exemplary multilayer optical information
recording medium, light with a light source wavelength is

focused (a light spot is formed) on the recording layer 3
using an objective lens.
In this embodiment, the recording layer 3 may be
made of a material the refractive index of which is
increased by a light spot or a material the refractive
index of which is decreased by a light spot. The thickness
of the recording layer 3 is preferably smaller than the
depth of a light spot. On the other hand, the thickness of
the middle layer 2 is preferably equal to or larger than
the depth of a light spot.
The refractive index in an area where a recording
mark is formed and the refractive index in an area where no
recording mark is formed are different. Therefore, the
reflectance at the surface boundary between the middle
layer 2 and the recording layer 3 differs depending on
whether a recording mark is formed. The difference in
reflectance results in the difference in intensity of
reflected light. Information on the first exemplary
multilayer optical information recording medium is read as
the difference in intensity of reflected light.
For the substrate 1, materials such as glass,
crystalline oxide, polycarbonate, and polyolefin that are
transparent to a light source wavelength may be used. For
the middle layer 2, a material that is transparent to a
light source wavelength may be used. For the recording

layer 3, a material that absorbs a part of light with a
light source wavelength may be used. The middle layer 2 is,
for example, formed by applying a resin such as polyvinyl
alcohol or ethylene vinyl alcohol. Also, the middle layer 2
may also be formed by fusion-bonding, extruding, laminating,
or vapor-depositing a transparent resin, such as
polycarbonate, polystyrene, polyamide, epoxy, or urethane
resin, or its film.
Further, the material for the middle layer 2 is
not limited to organic materials. The middle layer 2 may be
formed by vapor-depositing or sputtering a material such as
glass or an oxide. The recording layer 3 may be formed by
applying or vapor-depositing a mixture of a resin, such as
polymethyl methacrylate or polystyrene, and a photochromic
dye, such as spiropyran, diarylethene, or fulgide, the
refractive index or absorption wavelength of which changes
by absorption of light with a specific light source
wavelength. On a recording layer containing a photochromic
dye, information can be recorded by causing multiphoton
absorption, for example, two-photon absorption, using a
short pulse, high-power laser. The separating layer (spacer
layer) 4 may be formed using substantially the same method
and materials as those used for forming the middle layer 2.
Also, the spacer layer 4 may be an adhesive layer or a
pressure-sensitive adhesive layer.

Also, a barrier layer (not shown) may be provided
between the recording layer 3 and the middle layer 2. The
barrier layer prevents a solvent used in one layer from
dissolving the other layer and thereby makes it possible to
form each layer by applying a solution. In other words, the
barrier layer makes it easer to form the recording layer 3
and the middle layer 2. The barrier layer may also make it
easier to reduce the thickness of the recording layer 3 and
the middle layer 2 to increase light transmission.
The characteristics of the first exemplary
multilayer optical information recording medium are
described below with reference to FIGs. 2 and 3. FIG. 2 is
a graph showing the relationship between the number of
layers in the first exemplary multilayer optical
information recording medium shown in FIG. 1 and its
reflectance. More specifically, the graph shows changes in
the reflectance, measured from the side opposite to the
substrate 1, when the total number of the middle layers 2
and the recording layers 3 is changed between 5 and 50. The
changes in the reflectance are calculated by just changing
the total number of the middle layers 2 and the recording
layers 3 without taking into account the effects of the
spacer layer 4 and the substrate 1.
In the calculation, refractive index n2 of the
recording layer 3 is set to 1.6, refractive index n3 of the

middle layer 2 is set to 1.5, and light source wavelength λ
is set to 0.66 µm. In FIG. 2, the solid line shows the
reflectances when the thickness of the recording layer 3 is
λ/4n2 and the thickness of the middle layer 2 is an odd
multiple of λ/4n3; and the dotted line shows the
reflectances when the thicknesses of the recording layer 3
and the middle layer 2 are slightly different from λ/4n2
and an odd multiple of λ/4n3.
As the graph shows, when the thickness of the
middle layer 2 is an odd multiple of λ/4n3, the reflectance
of the first exemplary multilayer optical information
recording medium increases sharply as the total number of
layers increases. This means that light cannot reach a
layer that is distant from the side opposite to the
substrate 1 and is therefore not preferable.
FIG. 3 is a graph showing the relationship between
the thickness of the middle layer 2 and the reflectance of
the first exemplary multilayer optical information
recording medium. In this calculation, the thickness of the
recording layer 3 is fixed at λ /4n2 and the thickness of
the middle layer 2 is varied. The reflectance at five
layers is indicated by a dotted arrow and the reflectance
at 50 layers is indicated by a solid arrow. The exemplary
multilayer optical information recording medium used in
this calculation includes 10 recording layer units each

composed of five layers. As the graph shows, with a large
number of layers, the reflectance becomes high when the
middle layer 2 has one of certain thicknesses that exist at
regular intervals.
Accordingly, it is preferable to avoid a
combination of a thickness and a refractive index (optical
path length: the product of a thickness and a refractive
index) of the middle layer 2 which combination results in
increased reflectance as shown in FIG. 3.
Also, when producing a multilayer optical
information recording medium by stacking multiple recording
layer units each composed of at least one recording layer 3
and one middle layer 2, the thicknesses and the refractive
indices of the middle layers 2 among the recording layer
units are not necessarily the same and are preferably
varied. Further, the optical path length (the product of a
thickness and a refractive index) of the spacer layer 4 is
preferably an even multiple of λ/2.
When a recording medium is produced by alternately
stacking recording layer units and the spacer layers 4, the
spacer layer 4 with the optical path length as described
above contributes to preventing the decrease in light
transmission of the recording medium even when each of the
recording layer units have a high reflectance.

FIG. 4 is a drawing illustrating the configuration
of a second exemplary multilayer optical information
recording medium according to a second embodiment of the
present invention. The second embodiment of the present
invention is described below with reference to FIG. 4. As
shown in FIG. 4, in the second exemplary multilayer optical
information recording medium, a first recording layer 3 is
formed on a substrate 1. In the first recording layer 3,
recording marks 3a, which are recording spots each having a
refractive index different from the surrounding area, are
horizontally arranged at intervals; and layers of the
horizontally arranged recording marks 3a are vertically
(perpendicularly to the substrate 1) arranged at regular
intervals.
In this embodiment, the recording layer 3 in which
the recording marks 3a are horizontally and vertically
arranged at intervals is treated as a recording layer unit.
A spacer layer 4 is stacked on the recording layer 3. On
the spacer layer 4, a second recording layer 3 is stacked.
As in the first recording layer 3, the recording marks 3a
are horizontally and vertically arranged at intervals in
the second recording layer 3. Another spacer layer 4 is
stacked on the second recording layer 3. Thus, the second
exemplary multilayer optical information recording medium

is produced by alternately stacking the recording layers 3
and the spacer layers 4.
The recording mark 3a is formed by focusing light
with a light source wavelength (by forming a light spot) on
the recording layer 3 using an objective lens. In this
embodiment, the recording layer 3 may be made of a material
the refractive index of which is increased by a light spot
or a material the refractive index of which is decreased by
a light spot. The refractive index in the recording mark 3a
and the refractive index in the surrounding area are
different. Therefore, the reflectance in an area differs
depending on whether a recording mark is formed. The
difference in reflectance results in the difference in
intensity of reflected light. Information on the second
exemplary multilayer optical information recording medium
is represented by the difference in intensity of reflected
light.
For the substrate 1, the same materials described
in the first embodiment may be used. The recording layer 3
may be formed by applying or vapor-depositing a mixture of
a resin, such as polymethyl methacrylate or polystyrene,
and a photochromic dye, such as spiropyran, diarylethene,
or fulgide, the refractive index or absorption wavelength
of which changes by absorption of light with a specific
light source wavelength. On a recording layer containing a

photochromic dye, information can be recorded by causing
multiphoton absorption, for example, two-photon absorption,
using a short pulse, high-power laser. The separating layer
(spacer layer) 4 may be formed using substantially the same
method and materials as those used for forming the middle
layer 2 in the first embodiment. Also, the middle layer 2-
in the first embodiment may be used instead of the spacer
layer 4.
The second exemplary multilayer optical
information recording medium has characteristics similar to
those of the first exemplary multilayer optical information
recording medium shown in FIGs. 2 and 3. The interval
(distance) between the layers of the recording marks 3a in
the second embodiment corresponds to the thickness of the
middle layer 2 in the first embodiment. When the interval
between the layers of the recording marks 3a is an odd
multiple of λ/4n2 (λ is the light source wavelength and n2
is the refractive index of the recording layer 3), the
reflectance of the second exemplary multilayer optical
information recording medium increases sharply as the
number of the layers of the recording marks 3a increases.
This means that light cannot reach a layer that is distant
from the side opposite to the substrate 1 and is therefore
not preferable.

Also, with a large number of layers, the
reflectance becomes high when the interval between the
layers of the recording marks 3a takes certain values that
exist at regular intervals. Accordingly, it is preferable
to avoid a combination of an interval between the layers of
the recording marks 3a and a refractive index of the
recording layer 3 (optical path length: the product of an
interval and a refractive index) which combination results
in increased reflectance.
Also, when producing a multilayer optical
information recording medium by stacking multiple recording
layer units each composed of one recording layer 3, the
intervals between the layers of the recording marks 3a and
the refractive indices of the recording layers 3 among the
recording layer units are not necessarily the same and are
preferably varied. Further, the optical path length (the
product of a thickness and a refractive index) of the
spacer layer 4 is preferably an even multiple of 1/2.
When a recording medium is produced by alternately
stacking multiple recording layer units, each of which is
composed of one recording layer 3 in which the recording
marks 3a are horizontally and vertically arranged at
intervals, and the spacer layers 4, the spacer layer 4 with
the optical path length as described above contributes to
preventing the decrease in light transmission of the

recording medium even when each of the recording layer
units have a high reflectance.
FIG. 5 is a drawing illustrating the configuration
of a third exemplary multilayer optical information
recording medium according to a third embodiment of the
present invention. The third embodiment of the present
invention is described below with reference to FIG. 5. As
shown in FIG. 5, the third exemplary multilayer optical
information recording medium has a structure similar to
that of the first exemplary multilayer optical information
recording medium shown in FIG. 1, except that a first
adhesive layer 6, a control layer 5, and a second adhesive
layer 6 are formed between the substrate 1 and the middle
layer 2. Grooves used for tracking are formed on the
control layer 5 and the refractive index of the control
layer 5 is different from that of the adhesive layer 6. The
refractive index of the control layer 5 may be higher or
lower than that of the adhesive layer 6.
On the second adhesive layer 6, the middle layers
2 and the recording layers 3 are stacked alternately as in
the first exemplary multilayer optical information
recording medium. Also, the spacer layer 4 is stacked on
top of them. The third exemplary multilayer optical
information recording medium is produced by stacking sets
of the above mentioned layers.

In the third embodiment, a recording layer unit is
made up of five recording layers 3 and six middle layers 2,
and one control layer 5 is provided for each recording
layer unit. However, the present invention is not limited
to this configuration. As a value obtained by a formula
[(thickness of recording layer 3) x number of layers +
(thickness of middle layer 2) x number of layers] increases,
the distance between the control layer 5 and the most
distant recording layer 3 increases. In a tracking method
that uses light beams from two light sources, a large
distance between the control layer 5 and the recording
layer 3 results in a low accuracy in positioning a light
spot of a recording light beam. For this reason, the number
of layers in a recording layer unit is preferably no more
than 100.
In this embodiment, as shown in FIG. 5, grooves
are formed on a surface of the control layer 5, which
surface is closer to the substrate 1, to detect a tracking
error using a push-pull method. However, grooves may be
formed on the opposite surface of the control layer 5. Also,
tracking guides formed by refractive index modulation may
be used instead of grooves. Further, the pattern of the
grooves is not limited to a specific pattern. For example,
the grooves may be formed concentrically or spirally.

Other configurations and characteristics of the
third exemplary multilayer optical information recording
medium are substantially the same as those of the first
exemplary multilayer optical information recording medium.
The control layer 5 may be formed by applying a UV curing
resin and by pressing a transparent stump with a patterned
indented surface onto the applied UV curing resin. For the
adhesive layer 6, an adhesive or a pressure-sensitive
adhesive made of resin may be used.
FIG. 6 is a drawing illustrating the configuration
of a fourth exemplary multilayer optical information
recording medium according to a fourth embodiment of the
present invention. The fourth embodiment of the present
invention is described below with reference to FIG. 6. The
fourth exemplary multilayer optical information recording
medium is produced by adding the control layers 5 and the
adhesive layers 6 according to the third embodiment shown
in FIG. 5 to the structure of the second exemplary
multilayer optical information recording medium shown in
FIG. 4. As shown in FIG. 6, the first adhesive layer 6, the
control layer 5, and the second adhesive layer 6 are formed
between the substrate 1 and the first recording layer 3,
and between the spacer layer 4 and the second recording
layer 3. Other configurations and characteristics of the
fourth exemplary multilayer optical information recording

medium are substantially the same as those of the second
and third exemplary multilayer optical information
recording media.
FIG. 7 is a drawing illustrating the configuration
of a fifth exemplary multilayer optical information
recording medium according to a fifth embodiment of the
present invention. The fifth embodiment of the present
invention is described below with reference to FIG. 7. The
fifth exemplary multilayer optical information recording
medium has a structure similar to that of the first
exemplary multilayer optical information recording medium
shown in FIG. 1, except that spacer layers 4-1 and 4-2 have
different thicknesses. The optical path lengths of the
spacer layers 4 may be varied by changing their refractive
indices instead of changing their thicknesses. Other
configurations and characteristics of the fifth exemplary
multilayer optical information recording medium are
substantially the same as those of the first exemplary
multilayer optical information recording medium. The
difference in thickness between the spacer layers 4 is
preferably, but not limited to, about 0 to λ/2 or more in
terms of optical path length.
FIG. 8 is a drawing illustrating the configuration
of a sixth exemplary multilayer optical information
recording medium according to a sixth embodiment of the

present invention. The sixth embodiment of the present
invention is described below with reference to FIG. 8. The
sixth exemplary multilayer optical information recording
medium has a structure similar to that of the second
exemplary multilayer optical information recording medium
shown in FIG. 4, except that spacer layers 4-1 and 4-2 have
different thicknesses. The optical path lengths of the
spacer layers 4 may be varied by changing their refractive
indices instead of changing their thicknesses. Other
configurations and characteristics of the sixth exemplary
multilayer optical information recording medium are
substantially the same as those of the second exemplary
multilayer optical information recording medium. The
difference in thickness between the spacer layers 4 is
preferably, but not limited to, about 0 to λ/2 or more in
terms of optical path length.
FIG. 9 is a drawing illustrating the configuration
of a seventh exemplary multilayer optical information
recording medium according to a seventh embodiment of the
present invention. The seventh embodiment of the present
invention is described below with reference to FIG. 9. The
seventh exemplary multilayer optical information recording
medium has a structure similar to that of the first
exemplary multilayer optical information recording medium
shown in FIG. 1, except that middle layers 2-1, 2-2, and 2-

3 have different thicknesses and/or recording layers 3-1,
3-2, and 3-3 have different thicknesses. The optical path
lengths of the middle layers 2 and/or the recording layers
3 may be varied by changing their refractive indices
instead of changing their thicknesses. Other configurations
and characteristics of the seventh exemplary multilayer
optical information recording medium are substantially the
same as those of the first exemplary multilayer optical
information recording medium. The difference in thickness
between the middle layers 2 or between the recording layers
3 is preferably, but not limited to, about 0 to λ/2 or more
in terms of optical path length. Also, the difference in
thickness is preferably varied irregularly.
FIG. 10 is a drawing illustrating the
configuration of an eighth exemplary multilayer optical
information recording medium according to an eighth
embodiment of the present invention. The eighth embodiment
of the present invention is described below with reference
to FIG. 10. The eighth exemplary multilayer optical
information recording medium has a structure similar to
that of the second exemplary multilayer optical information
recording medium shown in FIG. 4 except that the layers of
the recording marks 3a in recording layers 3-1, 3-2, and 3-
3 are arranged at different intervals 3b-l, 3b-2, and 3b-3
(distances between the layers of the recording marks 3a are

different among the recording layers 3) and, as a result,
the recording layers 3-1, 3-2, and 3-3 have different
thicknesses. The optical path lengths of the recording
layers 3 may be varied by changing their refractive indices
instead of changing the intervals between the layers of the
recording marks 3a. Other configurations and
characteristics of the eighth exemplary multilayer optical
information recording medium are substantially the same as
those of the second exemplary multilayer optical
information recording medium. The difference in thickness
between the recording layers 3 is preferably, but not
limited to, about 0 to λ/2 in terms of optical path length.
Also, the difference in thickness is preferably varied
irregularly.
FIG. 11 is a drawing illustrating the
configuration of a ninth exemplary multilayer optical
information recording medium according to a ninth
embodiment of the present invention. The ninth embodiment
of the present invention is described below with reference
to FIG. 11. The ninth exemplary multilayer optical
information recording medium has a structure similar to
that of the third exemplary multilayer optical information
recording medium shown in FIG. 5, except that the first
adhesive layer 6 and the control layer 5 are formed in the
middle of each recording layer unit composed of the middle

layers 2 and the recording layers 3 rather than between the
substrate 1 and the middle layer 2. As shown in FIG. 11, in
the ninth exemplary multilayer optical information
recording medium, the second adhesive layer 6 shown in FIG.
5 is omitted and the middle layer 2 is formed directly on
the control layer 5. Other configurations and
characteristics of the ninth exemplary multilayer optical
information recording medium are substantially the same as
those of the second exemplary multilayer optical
information recording medium.
FIG. 12 is a drawing illustrating the
configuration of a tenth exemplary multilayer optical
information recording medium according to a tenth
embodiment of the present invention. The tenth embodiment
of the present invention is described below with reference
to FIG. 12. As shown in FIG. 12, the tenth exemplary
multilayer optical information recording medium has a
structure similar to that of the fourth exemplary
multilayer optical information recording medium shown in
FIG. 6, except that the first adhesive layer 6 and the
control layer 5 are formed between the recording layers 3
rather than between the spacer layer 4 and the recording
layer 3. In the tenth exemplary multilayer optical
information recording medium, the second adhesive layer 6
is omitted and the recording layer 3 is formed directly on

the control layer 5. Other configurations and
characteristics of the tenth exemplary multilayer optical
information recording medium are substantially the same as
those of the fourth exemplary multilayer optical
information recording medium.
FIG. 13 is a drawing illustrating the
configuration of an exemplary signal recording/reproducing
apparatus according to an eleventh embodiment of the
present invention for recording and reproducing a signal on
a multilayer optical information recording medium according
to an embodiment of the present invention. In this
embodiment, the third exemplary multilayer optical
information recording medium shown in FIG. 5 is used as an
exemplary optical recording medium to describe exemplary
signal recording and reproduction processes by the
exemplary signal recording/reproducing apparatus.
The eleventh embodiment of the present invention
is described below with reference to FIG. 13. In the
exemplary signal recording/reproducing apparatus shown in
FIG. 13, a light beam emitted from a light source 11 (first
light source) passes through a lens 12, a polarization beam
splitter 13, a dichroic prism 14, a 1/4 wavelength plate 15,
and an objective lens 16; and is thereby focused on a track
on the control layer 5 in the exemplary optical recording
medium. The light beam reflected from the track on the

control layer 5 passes through the objective lens 16, the
1/4 wavelength plate 15, and the dichroic prism 14; is
reflected by the polarization beam splitter 13; passes
through a condenser lens 17, a pinhole 18 (first pinhole),
condenser lenses 19 and 20, and a cylindrical lens 21; and
is thereby focused on a quadrant detector 22.
Focusing a light beam with the condenser lens 20
and the cylindrical lens 21 causes astigmatism and thereby
causes the focused light beam to have two focal points. The
quadrant detector 22 is positioned between the two focal
points. The diameter of the pinhole 18 placed between the
condenser lenses 17 and 19 is, for example, slightly larger
than that of the light spot formed by the condenser lens 17.
This allows a light beam to pass through the pinhole 18
even when the light beam is not accurately focused on the
control layer 5.
Another light beam emitted from another light
source 23 (second light source) passes through a lens 24;
is reflected by a polarization beam splitter 25; passes
through condenser lenses 26 and 27; is reflected by the
dichroic prism 14; passes through the 1/4 wavelength plate
15 and the objective lens 16; and is thereby focused on the
recording layer 3 in the exemplary optical recording medium.
A positioning mechanism such as a coil is provided for the

objective lens 16 to adjust the position of the objective
lens 16.
The light beam reflected from the recording layer
3 passes through the objective lens 16 and the 1/4
wavelength plate 15; is reflected by the dichroic prism 14;
passes through the condenser lenses 27 and 26, the
polarization beam splitter 25, a condenser lens 28, and a
pinhole 29 (second pinhole); and is thereby focused on a
photodetector 30.
The light source 23 and the pinhole 29 are placed
in confocal positions. The condenser lenses 26 and 27 form
a beam expander and function as a positioning mechanism.
The focal point of a light beam in the exemplary optical
recording medium can be changed by changing the positions
of the condenser lenses 26 and 27 along the optical axis.
In other words, the focal point of a light beam from the
light source 23 can be adjusted along the optical axis by
changing the distance between the condenser lenses 26 and
27. The diameter of the pinhole 29 is preferably equal to
or around that of a light spot formed by the condenser lens
28.
In the exemplary signal recording/reproducing
apparatus as described above, a light beam emitted from the
light source 11 is focused on a track on the control layer
5 and the distance between the condenser lenses 26 and 27

is adjusted so that a light beam emitted from the light
source 23 is focused on a point that is a specific distance
away along the optical axis from the focal point of the
light beam emitted from the light source 11. This mechanism
makes it possible to focus the light beam emitted from the
light source 23 on a specific recording layer 3. Also, as
described above, the light beam emitted from the light
source 11 is reflected by the track and enters the quadrant
detector 22, and the quadrant detector 22 generates a
signal. Based on the signal generated by the quadrant
detector 22, a focus error signal is obtained by an
astigmatism method and a track error signal is obtained by
a push-pull method. The obtained signals are used to
control the position of the objective lens 16.
The exemplary signal recording/reproducing
apparatus may also be configured to include multiple sets
of the light source 23 and the photodetector 30 and thereby
to record and reproduce information on multiple recording
layers 3 using multiple light beams. Further, the exemplary
signal recording/reproducing apparatus may be configured to
include multiple sets of the light source 11 and the
quadrant detector 22 and thereby to simultaneously perform
focus servo control for plural recording layers 3. In this
case, a dynamic focusing unit such as a liquid crystal

focusing element is necessary in the exemplary signal
recording/reproducing apparatus.
A twelfth embodiment of the present invention is
described below. According to the twelfth embodiment of the
present invention, the control layer 5 in the third or
ninth exemplary multilayer optical information recording
medium is configured to store information on the
arrangement of the recording layers 3 and the middle layers
2 in a corresponding recording .layer unit and information
on the locations of the recording layers 3 and the middle
layers 2 in the medium. Also, the control layer 5 in the
fourth or tenth exemplary multilayer optical information
recording medium is configured to store information on the
horizontal and vertical arrangements of the recording marks
3a in a corresponding recording layer 3 and information on
the locations of the recording marks 3a in the medium.
The above information can be recorded by
physically forming lands and pits on the control layer 5 at
the same time when grooves used for tracking are formed on
the control layer 5. Other configurations and
characteristics of the exemplary multilayer optical
information recording media according to the twelfth
embodiment are substantially the same as those of the
exemplary multilayer optical information recording media
according to other embodiments.

In a multilayer optical information recording
medium according to an embodiment of the present invention,
the total number of the recording layers 3 is preferably
from several tens to several hundreds, and the number of
layers in a recording layer unit is preferably from several
to 100. When materials and specifications of a product are
taken into account, the thickness of the recording layer 3
and the middle layer 2 is preferably from 0.1 µm to several
tens of µm, and the thickness of the spacer layer 4 is
preferably between 1 and about 100 µm. Also, the thickness
of the middle layer 2 is preferably equal to or larger than
that of the recording layer 3.
An embodiment of the present invention provides a
multilayer optical information recording medium that makes
it possible to reduce the fluctuation in the amount of
reflected light in recording or reading a signal even when
the wavelength or incidence angle of the light varies and
thereby to prevent the S/N ratio from decreasing; and to
accurately position a laser beam even when the number of
recording layers is large. Such a multilayer optical
information recording medium is suitable, for example, to
be used with an optical disk filing system or an optical
information recording/reproducing apparatus for recording
information such as video data.

According to an embodiment of the present
invention, a multilayer optical information recording
medium includes multiple recording layer units in each of
which one or more recording layers and one or more middle
layers are stacked alternately; and one or more spacer
layers; wherein the recording layer units and the spacer
layers are stacked alternately in a depth direction of the
optical recording medium. The spacer layers make it
possible to control and optimize the optical phase change
between the multiple recording layer units and thereby to
reduce the fluctuation in reflectance or in the amount of
reflected light even when the wavelength or incidence angle
of the light varies.
According to an embodiment of the present
invention, a multilayer optical information recording
medium includes multiple recording layers treated as
recording layer units in each of which recording marks each
having a refractive index different from that of a
surrounding area are arranged so as to form multiple layers
of the recording marks, wherein the recording marks in each
of the layers of the recording marks are horizontally
arranged at intervals and the layers of the recording marks
are vertically arranged at intervals; and one or more
spacer layers; wherein the recording layers and the spacer
layers are stacked alternately in a depth direction of the

optical recording medium. In such a multilayer optical
information recording medium, varying the thicknesses or
refractive indices of the spacer layers or varying the
vertical distances between the layers of the recording
marks among the recording layers makes it possible to
change relative phase of light when the light is
transmitted through the layers. This, in turn, makes it
possible to reduce the fluctuation in reflectance or in the
amount of reflected light by mutual interference even when
the wavelength or incidence angle of the light varies.
A multilayer optical information recording medium
according to embodiments of the present invention may also
include a control layer used for tracking for each of the
recording layer units and/or an adhesive or pressure-
sensitive adhesive layer. Further, the control layer may be
configure to store information on the arrangements of
layers or recording marks in a corresponding one of the
recording layer units and information on locations of the
layers or the recording marks in the optical recording
medium. Such configurations make it possible to perform
tracking accurately for each of the recording layer units
and to process the control layer with various methods.
An embodiment of the present invention is descried
below with reference to FIGs. 15 through 24. FIG. 15 is a

drawing illustrating an exemplary structure of an optical
disk 100 that is a multilayer optical information recording
medium according to an embodiment of the present invention.
As shown in FIG. 15, the optical disk 100 includes
a cover layer C and three multilayer units (U1, U2, and U3)
stacked on the cover layer C. In FIG. 15, the Z direction
indicates a direction along the thickness of the optical
disk 100 (upward direction in FIG. 15). A laser beam LB is
emitted from a light source positioned upstream of the
optical disk 100 in the Z direction.
The cover layer C is the lowest layer of the
optical disk 100. Therefore, the laser beam LB is incident
on the lower surface of the cover layer C (the lower
surface of the cover layer C is the incidence plane). The
multilayer unit Ul is stacked on the upper surface of the
cover layer C, the multilayer unit U2 is stacked on the
upper surface of the multilayer unit Ul, and the multilayer
unit U3 is stacked on the upper surface of the multilayer
unit U2.
Each of the multilayer units Ul through U3
includes a guide track layer S and an information layer M.
The guide track layer S corresponds to light with
a wavelength between 390 and 420 nm. Guide grooves (or
tracks) are formed spirally or concentrically on the guide

track layer S. Also, the guide tracks are formed so as to
wobble at intervals.
As shown in FIG. 16, the information layer M is
positioned downstream of the guide track layer S in the Z
direction and is composed of five recording layers D and
five resin layers G stacked alternately. In other words,
one guide track layer S is provided for five recording
layers D. Also, in each of the multilayer units, the guide
track layer S is positioned closer to the incidence plane
than the recording layers D.
Each of the recording layers D is made of a two-
photon absorption material that is suitable for light with
a wavelength between 650 and 680 nm. Information is
recorded on the recording layer D in a photon mode.
Examples of two-photon absorption materials include
photorefractive crystal, photopolymer, and photochromic
materials.
In a photon-mode recording, the refractive index
of an area exposed to a light spot changes in proportion to
the light intensity distribution of the light spot.
Therefore, the diameter of a spot formed by photon-mode
recording is about 0.71 (= 1/2) times as large as that of
a spot formed by normal recording. For example, as shown in
FIG. 17, a pit recorded by two-photon absorption (length:
Dz2, width: Dr2) is smaller than a pit recorded by normal

one-photon absorption (length: Dz1, width: Dr1). Therefore,
two-photon absorption recording makes it possible to record
information at a recording density higher than in one-
photon absorption recording even when light with a same
light source wavelength is used (see "Two-photon absorption
recording on photochromic material using laser diode",
Teruhiro Shiono, OPTRONICS, July 2005, No. 28, p 174,
published by the Optronics Co., Ltd.). In other words, two-
photon absorption recording makes it possible to increase
the storage capacity of an optical disk. Each value on the
horizontal scale of the graph in FIG. 17 indicates a
distance from the center of a pit.
As described above, one way to increase the
storage capacity of an optical disk is to increase the
number of recording layers in the optical disk. At the same
time, it is important to increase the storage capacity of
each recording layer. To increase the storage capacity of
each recording layer, it is preferable to record
information at a high density using light with a wavelength
as short as possible. However, at the current state of
technology, it is difficult to find a two-photon absorption
material suitable for blue light. Therefore, it is
preferable to use a two-photon absorption material suitable
for green light or red light. Since green-light emitting
laser diodes are not being mass-produced currently, in this

embodiment, a two-photon absorption material suitable for
red light is used for the recording layer D. Even with red
light, a spot with a diameter about 0.71 (= 1/√2) times as
large as that of a spot formed by one-photon absorption
recording can be formed on the recording layer D and
therefore information can be recorded at a density as high
as that possible with blue light.
The guide track layer S contains no two-photon
absorption material. Since two-photon absorption materials
degrade when they are exposed to light in the spectrum from
blue to ultraviolet, it is preferable not to use two-photon
absorption material for a layer that is, for example,
irradiated with an ultraviolet ray in a production process
(for example, 2P process) for curing an ultraviolet curing
resin or an adhesive.
As described above, the guide track layer S
contains no two-photon absorption material. Therefore, if
red light for the recording layer D is also used for the
guide track layer S, the diameter of a spot cannot be
reduced to a satisfactory level and it becomes difficult to
increase the storage capacity per recording layer.
Generally, the diameter of a spot is proportional to a
value obtained by the following formula:
wavelength/numerical aperture (NA) of lens. Therefore, even
on a layer with no two-photon absorption material, the

diameter of a spot can be reduced by using light with a
short wavelength. In this embodiment, blue light, which has
a wavelength shorter than that of light used to irradiate
the recording layer D, is used to irradiate the guide track
layer S. Using blue light makes it possible to narrow the
pitch between tracks (track pitch) on the guide track layer
S and thereby makes it possible to increase the storage
capacity per recording layer. And the increased storage
capacity per recording layer, in turn, makes it possible to
increase the storage capacity of an optical disk. Also, a
narrower track pitch makes it possible to accurately
perform servo control on a high-density recording medium.
Meanwhile, forming a guide track layer S and
multiple recording layers D as a multilayer unit improves
the tilt tolerance of an optical disk. Take, for example,
an information layer M including five recording layers each
having a thickness of 3 µm (the total thickness of the
information layer M is 3 µm x 5 = 15 µm) . In this case,
even when the optical disk 100 is tilted 1 degree in
relation to the incidence angle of light, the focal point
of the light is shifted only 0.26 µm. With an optical disk
having the above structure, even when the track pitch is as
narrow as 0.32 µm like a Blu-ray disk, the focal point of
the light may not be shifted out of a target track and
therefore information can be recorded/reproduced reliably

by performing conventional tilt control. In other words,
the optical disk 100 satisfies the condition expressed by-
formula (1) shown below. In formula (1), n indicates the
number of recording layers in each multilayer unit, d
indicates the thickness of each recording layer, and p
indicates a track pitch.

In summary, an optical disk with a structure where
the guide track layers S and the information layers M are
stacked alternately provides, even when the optical disk
includes a large number of recording layers, a tilt
tolerance that is substantially equal to that of an optical
disk having only a few recording layers and thereby makes
it possible to stably and reliably record/reproduce
information.
An exemplary configuration of an optical pickup
123 that is an optical head according to an embodiment of
the present invention is described below with reference to
FIGs. 18 through 21.
As shown in FIG. 18, the optical pickup 123
includes a light source LD1, a polarization beam splitter
151, a collimator lens 152, an aberration correction
optical element 153, a dichroic prism 154, a 1/4 wavelength
plate 155, an objective lens 160, a light source LD2, a
detection lens 156, a photodetector PD1, a half mirror 159,

a diffractive-optical element 158, a collimator lens 157,
photodetector PD2, and a driving mechanism (not shown) for
driving the objective lens 160.
The light source LD1 includes a laser diode that
emits light with a wavelength of about 405 nm. The light
source LD1 emits light at its maximum intensity in the Z
direction. The light emitted from the light source LD1 is,
for example, p-polarized. Hereafter, the light emitted from
the light source LD1 is also called a "servo beam".
The polarization beam splitter 151 is positioned
downstream of the light source LD1 in the Z direction. The
reflectance of the polarization beam splitter 151 differs
depending on the polarization state of an incoming light.
In this embodiment, for example, the reflectance of the
polarization beam splitter 151 is low for a p-polarized
light and high for an s-polarized light. Therefore, most of
the servo beam emitted from the light source LD1 can pass
through the polarization beam splitter 151.
The collimator lens 152 is positioned downstream
of the polarization beam splitter 151 in the Z direction
and substantially collimates the servo beam from the
polarization beam splitter 151.
The aberration correction optical element 153 is
positioned downstream of the collimator lens 152 in the Z

direction and corrects the aberration of an incoming light
beam.
The light source LD2 includes a laser diode array
having at least five light-emitting parts each of which
emits light with a wavelength of about 660 nm. The light
source LD2 emits five light beams in the -Y direction. The
five light beams emitted from the light source LD2 are, for
example, p-polarized. Hereafter, the light beams emitted
from the light source LD2 are also called
"recording/reproducing beams".
The half mirror 159 is positioned downstream of
the light source LD2 in the -Y direction and bends the
light path of a part of an incoming light beam at right
angles.
The diffractive-optical element 158 is positioned
downstream of the half mirror 159 in the -Y direction and
diffracts an incoming light. The light paths of the five
recording/reproducing beams from the half mirror 159 are
changed by the diffractive-optical element 158 so that
their light axes join and their divergence angles differ
from each other.
The collimator lens 157 is positioned downstream
of the diffractive-optical element 158 in the -Y direction
and substantially collimates the five recording/reproducing
beams from the diffractive-optical element 158. However,

since the five recording/reproducing beams from the
diffractive-optical element 158 have different divergence
angles, each of the beams from the collimator lens 157
becomes parallel, slightly divergent, or slightly
convergent.
The dichroic prism 154 is positioned downstream of
the aberration correction optical element 153 in the Z
direction and downstream of the collimator lens 157 in the
-Y direction. The dichroic prism 154 bends the light path
of light with a wavelength of about 660 nm (a
recording/reproducing beam) at right angles.
The 1/4 wavelength plate 155 is positioned
downstream of the dichroic prism 154 in the Z direction and
gives an optical phase difference of a 1/4 wavelength to an
incoming light.
The objective lens 160 is positioned downstream of
the 1/4 wavelength plate 155 in the Z direction and focuses
light from the 1/4 wavelength plate 155. As shown in FIGs.
19 through 21, each of the five recording/reproducing beams
LB2 is focused on a different one of the recording layers D
in a multilayer unit, and the servo beam LB1 is focused on
the guide track layer S in the multilayer unit.
The detection lens 156 is positioned downstream of
the polarization beam splitter 151 in the Y direction and
gives astigmatism to light that is returned from the guide

track layer S and reflected by the polarization beam
splitter 151 in the Y direction.
The photodetector PD1 is positioned downstream of
the detection lens 156 in the Y direction and receives
light from the detection lens 156.
The photodetector PD2 is positioned downstream of
the half mirror 159 in the -Z direction and receives light
returned from the information layer M and reflected by the
half mirror 159 in the -Z direction.
The driving mechanism includes a focusing actuator
for finely adjusting the position of the objective lens 160
in the focus direction that is along the light axis of the
objective lens 160; and a tracking actuator for finely
adjusting the position of the objective lens 160 in the
tracking direction that is orthogonal to a tangent to a
track.
The working of the optical pickup 123 configured
as mentioned above is described below. In the descriptions
below, it is assumed that the five recording layers D in
the multilayer unit U2 of the optical disk 100 are target
recording layers.
The linearly polarized (p-polarized) servo beam
LB1 emitted from the light source LD1 enters the
polarization beam splitter 151. Most of the servo beam LB1
passes through the polarization beam splitter 151; is

substantially collimated by the collimator lens 152; is
aberration-corrected by the aberration correction optical
element 153; and enters the dichroic prism 154. The servo
beam LB1 passes through the dichroic prism 154; is
circularly-polarized by the 1/4 wavelength plate 155; and
is focused on the guide track layer S in the multilayer
unit U2 by the objective lens 160.
The light beam reflected from the guide track
layer S in the multilayer unit U2 (returned light beam) is
circularly-polarized in a direction opposite to that of the
circular polarization of the incoming servo beam LB1. The
returned light beam enters the 1/4 wavelength plate 155 via
the objective lens 160 and is linear-polarized (s-
polarized) in a direction that is orthogonal to that of the
linear polarization of the incoming servo beam LB1. Then,
the returned light beam passes through the dichroic prism
154, the aberration correction optical element 153, and the
collimator lens 152, and enters the polarization beam
splitter 151.
The returned light beam is reflected by the
polarization beam splitter 151 in the Y direction and, via
the detection lens 156, received by the photodetector PD1.
As in a conventional optical disk apparatus, the
photodetector PD1 includes multiple light-receiving
elements (or multiple light-receiving areas) each outputs a

signal (control signal) containing information such as
wobble signal information and servo information (focus
error information, tracking error information, and so on).
Each of the light-receiving elements (or light-receiving
areas) generates a signal in proportion to the amount of
received light by photoelectric conversion.
On the other hand, the linearly polarized (p-
polarized) five recording/reproducing beams LB2 emitted
from the light source LD2 enter the half mirror 159. The
five recording/reproducing beams LB2 from the half mirror
159 pass through the diffractive-optical element 158 and
the collimator lens 157; and enter the dichroic prism 154.
The light paths of the five recording/reproducing beams LB2
are bended by the dichroic prism 154 in the Z direction.
Then, the five recording/reproducing beams LB2 are
circularly polarized by the 1/4 wavelength plate 155; and
focused on the five recording layers D in the multilayer
unit U2 by the objective lens 160.
The five light beams reflected from the five
recording layers D in the multilayer unit U2 (returned
light beams) are circularly-polarized in a direction
opposite to that of the circular polarization of the
incoming recording/reproducing beams LB2. The returned
light beams enter the 1/4 wavelength plate 155 via the
objective lens 160 and are linear-polarized (s-polarized)

in a direction that is orthogonal to that of the linear
polarization of the incoming recording/reproducing beams
LB2. Then, the returned light beams are reflected by the
dichroic prism 154 in the Y direction; pass through the
collimator lens 157 and the diffractive-optical element
158; and enter the half mirror 159. The returned light
beams are reflected by the half mirror 159 and received by
the photodetector PD2. The photodetector PD2 includes five
light-receiving elements (or five light-receiving areas)
where each element (or area) receives a different one of
the five returned light beams and outputs a signal
containing information such as reproduction information.
Each of the light-receiving elements (or light-receiving
areas) generates a signal in proportion to the amount of
received light by photoelectric conversion. In other words,
the photodetector PD2 can read signals from the five
recording layers D at the same time.
As described above, since the photodetector PD1
for receiving a light beam representing servo information
and the photodetector PD2 for receiving light beams
representing reproduction information are provided
separately, each of the photodetectors can be configured to
best suit its purpose. For example, a low-speed
photodetector may be used as the photodetector PD1 and a
high-speed photodetector may be used as the photodetector

PD2. Especially, it is preferable to design an optical
drive using a low-speed photodetector as the photodetector
PD1 for receiving a short-wavelength light that is
comparatively difficult to detect. Also, since a servo beam
with a fixed intensity can be used for both recording and
reproduction, no gain switch is necessary for the
photodetector PDl. This makes it possible to simplify the
circuit configuration of the photodetector PDl.
Further, a Si-PIN photodiode with a wide dynamic
range may be used for the photodetector PDl and an
avalanche photodiode (APD) with a high multiplication
factor may be used for the photodetector PD2. A Si-PIN
photodiode is suitable for accurate detection of a light
beam representing servo information which detection
requires a wide dynamic range and linearity to the amount
of light. On the other hand, an avalanche photodiode can
amplify a weak light beam representing reproduction
information which light beam is reflected from the
recording layer D having a low reflectance.
An exemplary configuration of an optical disk
apparatus 120 that is an optical drive according to an
embodiment of the present invention is described below with
reference to FIG. 22.
As shown in FIG. 22, the optical disk apparatus
120 includes a spindle motor 122 for rotating an optical

disk, the optical pickup 123, a seek motor 121 for driving
the optical pickup 123, a laser control circuit 124, an
encoder 125, a drive control circuit 126, a reproduction
signal processing circuit 128, a buffer RAM 134, a buffer
manager 137, an interface 138, a flash memory 139, a CPU
140, and a RAM 141. Arrows in FIG. 22 indicate flow of
signals and information and do not represent all
connections between the blocks. The optical disk apparatus
120 is usable for recording/reproducing information on the
optical disk 100.
The reproduction signal processing circuit 128
obtains, for example, address information, a
synchronization signal, and servo signals such as a focus
error signal and a tracking error signal based on the
output signals (photoelectric conversion signals) from the
photodetector PD1. Also, the reproduction signal processing
circuit 128 obtains RF signals from the recording layers D
based on the output signals (five photoelectric conversion
signals) from the photodetector PD2.
The servo signals are output to the drive control
circuit 126, the address information is output to the CPU
140, and the synchronization signal is output to the
encoder 125 and the drive control circuit 126. Further, the
reproduction signal processing circuit 128 performs
decoding and error detection on each of the RF signals and

then stores the RF signal as reproduced data via the buffer
manager 137 in the buffer RAM 134. When an error is
detected in an RF signal, the reproduction signal
processing circuit 128 performs error correction before
storing the RF signal in the buffer RAM 134. The address
information contained in the reproduced data is output to
the CPU 140.
The drive control circuit 126 generates driving
signals for the driving mechanism of the optical pickup 123
based on servo signals from the reproduction signal
processing circuit 128 and outputs the driving signals to
the optical pickup 123. The optical pickup 123 performs
tracking control and focus control according to the driving
signals. The drive control circuit 126 generate a driving
signal for driving the seek motor 121 and a driving signal
for driving the spindle motor 122 according to an
instruction from the CPU 140. The driving signals are
output to the seek motor 121 and the spindle motor 122.
The buffer RAM 134 temporarily stores, for example,
data to be recorded on the optical disk 100 (recording
data) and data reproduced from the optical disk 100
(reproduced data). Data input/output to or from the buffer
RAM 134 is controlled by the buffer manager 137.
The encoder 125, according to an instruction from
the CPU 140, retrieves recording data in the buffer RAM 134

via the buffer manager 137; modulates the recording data;
attaches an error correcting code to the recording data;
and generates recording signals to be written on the
information layer M of the optical disk 100. For example,
to record information on the five recording layers D, five
recording signals are generated. The generated recording
signals are output to the laser control circuit 124.
The laser control circuit 124 controls the light
emission power of each of the light sources of the optical
pickup 123.
When recording information, a driving signal for
the light source LD2 is generated based on the recording
signal, recording conditions, light emission
characteristics of the laser diode array of the light
source LD2, and so on. For example, to simultaneously
record information on the five recording layers D, five
driving signals are generated for the five light-emitting
parts of the laser diode array.
The interface 138 enables two-way communication
between the optical disk apparatus 120 and an upstream
apparatus 190 (for example, a personal computer). The
interface 138 is a standard interface such as an AT
attachment packet interface (ATAPI), a small computer
system interface (SCSI), or a universal serial bus (USB).

The flash memory 139 stores, for example, programs
written in code that the CPU 140 can understand, light
emission characteristics of the laser diode of the light
source LD1, light emission characteristics of the laser
diode array of the light source LD2, and recording
conditions including recording power and recording strategy
information.
The CPU 140 controls the operations of other units
in the optical disk apparatus 120 according to the programs
stored in the flash memory 139 and stores, for example,
control data in the RAM 141 and the buffer RAM 134.

An exemplary recording process in the optical disk
apparatus 120, which recording process is performed when
recording of user data is requested from the upstream
apparatus 190, is described below with reference to FIG. 23.
The flowchart shown in FIG. 23 corresponds to a set of
processing algorithms executed by the CPU 140. In the
exemplary recording process, it is assumed that user data
are recorded on multiple recording layers.
When a recording command is received from the
upstream apparatus 190, the initial address of a program
corresponding to the flowchart shown in FIG. 23 is set in
the program counter of the CPU 140 and a recording process
is started.

In step 401, the CPU 140 instructs the drive
control circuit 126 to cause the spindle motor 122 to
rotate the optical disk 100 at a specified linear velocity
(or angular velocity) and reports the reception of the
recording command from the upstream apparatus 190 to the
reproduction signal processing circuit 128.
In step 403, the CPU 140 analyzes the recording
command and determines target recording layers and a target
multilayer unit based on the addresses specified in the
recording command. Then, the CPU 140 reports the determined
information to the reproduction signal processing circuit
128, the drive control circuit 126, the encoder 125, and
the laser control circuit 124. Based on the determined
information, the drive control circuit 126 controls the
objective lens 160 so that the servo beam LB1 is focused on
the guide track layer S in the target multilayer unit. Also,
the CPU 140 determines light-emitting parts of the light
source LD2 that are to be driven and light-receiving
elements (or light-receiving areas) of the photodetector
PD2 that are to generate signals.
In step 405, the CPU 140 refers to address
information obtained based on an output signal from the
photodetector PD1 and instructs the drive control circuit
126 to cause the seek motor 121 to seek the optical pickup
123 so that a light spot is formed around a target position

corresponding to the specified address. If seek operation
is not necessary, this step is skipped.
In step 407, the CPU 140 permits recording of data.
With the permission, the encoder 125 and the laser control
circuit 124 causes the optical pickup 123 to record data on
the target recording layers substantially at the same time.
During the recording, tracking control and focus control
described above are performed at specified timings.
In step 409, the CPU 140 determines whether the
recording of data is completed. If the recording of data is
not completed, the recording is continued and the CPU 140
performs this step again after a specified period of time.
If the recording of data is completed, the recording
process is terminated. In the exemplary recording process,
data are recorded on multiple recording layers
substantially at the same time. Therefore, the exemplary
recording process makes it possible to reduce the time for
recording.

An exemplary reproduction process in the optical
disk apparatus 120, which reproduction process is performed
when reproduction of data is requested from the upstream
apparatus 190, is described below with reference to FIG. 24.
The flowchart shown in FIG. 24 corresponds to a set of
processing algorithms executed by the CPU 140. In the

exemplary reproduction process, it is assumed that data on
multiple recording layers are reproduced.
When a reproduction command is received from the
upstream apparatus 190, the initial address of a program
corresponding to the flowchart shown in FIG. 24 is set in
the program counter of the CPU 140 and a reproduction
process is started.
In step 501, the CPU 140 instructs the drive
control circuit 126 to cause the spindle motor 122 to
rotate the optical disk 100 at a specified linear velocity
(or angular velocity) and reports the reception of the
reproduction command from the upstream apparatus 190 to the
reproduction signal processing circuit 128.
In step 503, the CPU 140 analyses the reproduction
command and determines target recording layers and a target
multilayer unit based on the addresses specified in the
reproduction command. Then, the CPU 140 reports the
determined information to the reproduction signal
processing circuit 128, the drive control circuit 126, and
the laser control circuit 124. Based on the determined
information, the drive control circuit 126 controls the
objective lens 160 so that the servo beam LB1 is focused on
the guide track layer S in the target multilayer unit. Also,
the CPU 140 determines light-emitting parts of the light
source LD2 that are to be driven and light-receiving

elements (or light-receiving areas) of the photodetector
PD2 that are to generate signals.
In step 505, the CPU 140 instructs the drive
control circuit 126 to cause the seek motor 121 to drive
the optical pickup 123 so that a light spot is formed
around a target position corresponding to the specified
address. If seek operation is not necessary, this step is
skipped.
In step 507, the CPU 140 permits reproduction of
data. With the permission, the optical pickup 123 and the
reproduction signal processing circuit 128 reproduce data
on the target recording layers substantially at the same
time. The reproduced data are stored in the buffer RAM 134.
When the reproduced data reach a specified amount, the
reproduced data are transferred to the upstream apparatus
190.
In step 509, the CPU 140 determines whether the
reproduction of data is completed. If the reproduction of
data is not completed, the reproduction is continued and
the CPU 140 performs this step again after a specified
period of time. If the reproduction of data is completed,
the reproduction process is terminated. In the exemplary
reproduction process, data on multiple recording layers are
reproduced substantially at the same time. Therefore, the

exemplary reproduction process makes it possible to reduce
the time for reproduction.
As described above, in the optical disk 100
according to an embodiment of the present invention, the
guide track layer S functions as a guide layer.
In the optical pickup 123 according to an
embodiment of the present invention, the light source D1
emits a light beam with a first wavelength, the light
source LD2 emits light beams with a second wavelength, the
photodetector PD1 receives a light beam reflected from a
guide layer, and the photodetector PD2 separately receives
multiple light beams reflected from multiple recording
layers.
In the optical disk apparatus 120 according to an
embodiment of the present invention, the reproduction
signal processing circuit 128, the CPU 140, and programs
executed by the CPU 140 constitute a processing unit. A
part or the whole of the processing implemented by the
programs executed by the CPU 14 0 may be implemented by
hardware.
As described above, the optical disk 100 according
to an embodiment of the present invention includes multiple
multilayer units each including the guide track layer S
(guide layer) corresponding to light with a wavelength
between 390 and 420 nm (a first wavelength) and multiple

recording layers D made of a two-photon absorption material
and corresponding to light with a wavelength between 650
and 680 nm (a second wavelength). This structure provides a
high capacity multilayer optical information recording
medium with a tilt tolerance that is substantially equal to
that of a recording medium having only a few recording
layers.
With the optical disk 100 according to an
embodiment of the present invention, a laser beam with a
wavelength of 660 nm may be used as a recording/reproducing
beam; and a laser beam with a wavelength of 405 nm may be
used as a servo beam. This eliminates the need to use an
expensive laser such as a femtosecond laser and thereby
makes it possible to produce an optical pickup and an
optical disk apparatus for recording/reproducing
information on the optical disk 100 at low costs.
In the optical disk 100 according to an embodiment
of the present invention, one guide track layer S is
provided for multiple recording layers D. This structure
eliminates the need to form guide grooves on each recording
layer and thereby makes it possible to simplify a part of
the production process.
In the optical disk 100 according to an embodiment
of the present invention, the guide track layer S and the
recording layers D are provided separately. With this

structure, a servo beam does not form a small light spot
near recording layers and, therefore, even when blue light
is used for the servo beam, degradation of recording layers
made of a two-photon absorption material, which is
sensitive to blue through ultraviolet light, can be
prevented.
In the optical disk 100 according to an embodiment
of the present invention, the guide track layer S is
positioned closer to the incidence plane than the
information layer M. With this structure, the substrate
thickness that a servo beam passes through becomes small.
Therefore, even when blue light is used for the servo beam
and even when the optical disk 100 is tilted 1 degree in
relation to the objective lens 160, highly accurate servo
information can be obtained. In other words, the optical
disk 100 has a high tilt tolerance.
The optical pickup 123 according to an embodiment
of the present invention uses a blue-light emitting laser
diode as the light source of a servo beam and therefore is
able to accurately obtain signals for servo control from
the optical disk 100.
The optical pickup 123 according to an embodiment
of the present invention uses a blue-light emitting laser
diode as the light source of a servo beam and red-light
emitting laser diodes for the light source of

recording/reproducing beams. This configuration makes it
possible to reduce the size and costs of an optical pickup.
In the optical pickup 123 according to an
embodiment of the present invention, the focal point of
light with a wavelength of about 405 nm is closer to the
objective lens 160 than that of light with a wavelength of
about 660 nm. This configuration prevents degradation of
the recording layers D in the optical disk 100. Also, since
the refractive index of glass generally becomes higher as
the wavelength of light becomes shorter, it is rather easy
to design an objective lens with a short focal length for
short wavelength light. More specifically, an inexpensive
lens designed to handle both short wavelength light and
long wavelength light implemented by using chromatic
aberration of a lens material may be used as an objective
lens.
In the optical pickup 123 according to an
embodiment of the present invention, a photodetector for
receiving a reflected servo beam and a photodetector for
receiving reflected recording/reproducing light beams are
provided separately. This configuration makes it possible
to optimize the response speed, gain, sensitivity,
modulation characteristic, and so on of each photodetector
so that each photodetector can generate an appropriate
signal.

The optical disk apparatus 120 according to an
embodiment of the present invention includes the optical
pickup 123 described above and is therefore able to
accurately record, reproduce, and/or delete information on
the optical disk 100.
In the embodiments described above, the optical
disk 100 includes three multilayer units. However, the
number of multilayer units in the optical disk 100 is not
limited to three.
In the embodiments described above, each
multilayer unit includes five recording layers. However,
the number of recording layers in a multilayer unit is not
limited to five. When the number of recording layers in
each multilayer unit is less/more than five, the number of
light-emitting parts of the light source LD2 may be changed
according to the number of recording layers.
Also, a multilayer optical information recording
medium according to an embodiment of the present invention
may have a structure like that of an optical disk 100a
shown in FIG. 25. In the optical disk 100a, multiple guide
track layers (S1, S2, and S3) are stacked on the upper
surface of the cover layer C; and multiple information
layers (Ml, M2, and M3) are stacked on top of the guide
track layers (S1, S2, and S3). In the optical disk 100a,
the distance between the guide track layer S1 and the

information layer M1, the distance between the guide track
layer S2 and the information layer M2, and the distance
between the guide track layer S3 and the information layer
M3 are the same distance t.
In the optical disk 100a, as shown FIGs. 26
through 28, the guide track layer S1 is used for servo
control when recording/reproducing information on the
information layer Ml, the guide track layer S2 is used for
servo control when recording/reproducing information on the
information layer M2, and the guide track layer S3 is used
for servo control when recording/reproducing information on
the information layer M3. Therefore, in this case, the
objective lens 160 is configured so that the distance
between the focal point of the servo beam LB1 and the focal
point of the closest one of the recording/reproducing beams
LB2 equals the distance t.
Also, in the optical disk 100a, the guide track
layers S1 through S3 are positioned closer to the incidence
plane than the information layers M1 through M3. Therefore,
even when a servo beam moves from one guide track layer to
another, the servo beam does not pass through a recording
layer. This prevents reflection (flare) of the servo beam
from layers other than the target layer and thereby makes
it possible to obtain a stable servo signal and to perform
servo control at high speed without interruption.

Further, in the optical disk 100a, recording
layers and guide track layers are separated. This structure
makes it possible to simplify the production process.
According to another embodiment of the present
invention, a multilayer optical information recording
medium may have a structure like that of an optical disk
100b shown in FIG. 29. In the optical disk 100b, a filter
layer F is provided between a set of guide track layers and
a set of information layers. The filter layer F reflects
light with a wavelength between 390 and 420 nm and
transmits light with a wavelength between 650 and 680 nm.
In other words, the filter layer F reflects the servo beam
LB1 and thereby prevents the servo beam LB1 from reaching
the recording layers D made of a two-photon absorption
material. This, in turn, eliminates one of the causes that
degrade the recording layers D and makes it possible to
reliably record/reproduce information.
According to still another embodiment of the
present invention, a multilayer optical information
recording medium may have a structure like that of an
optical disk 100c shown in FIG. 30. The optical disk 100c
includes multiple multilayer units (UN1, UN2, ...) each
including multiple guide track layers and multiple
information layers.

The optical pickup 123 may be easily adapted for
the optical disk 100a by changing the effective diameters
of the servo beam LB1 and the recording/reproducing beams
LB2 according to the distance t in the optical disk 100a
and by changing the numerical aperture of the objective
lens 160.
According to embodiments of the present invention,
the wavelength of light emitted from the light source LD1
is preferably between 390 and 420 nm and the wavelength of
light emitted from the light source LD2 is preferably
between 650 and 680 nm.
In the above embodiments, the guide track layer S
corresponds to light with a wavelength between 390 and 420
nm and the recording layer D corresponds to light with a
wavelength between 650 and 680 nm. However, the wavelength
range of the light for the guide track layer S and the
wavelength range of the light for the recording layer D are
not limited to the above ranges as long as the wavelength
ranges do not overlap. Even when the wavelength ranges are
different from those described above, the light source LD1
emits light for the guide track layer S and the light
source LD2 emits light for the recording layer D.
The optical disk apparatus 120 according to an
embodiment of the present invention is configured to record
and reproduce information on an optical disk. However, the

optical disk apparatus 120 may be configured to only
reproduce information on an optical disk.
The optical disk apparatus 120 and the optical
pickup 123 according to an embodiment of the present
invention are configured to record/reproduce information on
multiple recording layers at substantially the same time.
However, the optical disk apparatus 120 and the optical
pickup 123 may be configured to record/reproduce
information on one recording layer at a time. In this case,
the light source LD2 may be configured to include only one
light-emitting part. Also, the photodetector PD2 may be
configured to include only one light-receiving element (or
light-receiving area).
In the above embodiments, guide grooves are formed
on the guide track layer S. However, guide pits (prepits)
may be formed on the guide track layer S instead of the
guide grooves. Also, both grooves and prepits may be formed
on the guide track layer S.
The guide track layer S may be designed to be
recordable. A recordable guide track layer S makes it
possible to further increase the storage capacity of an
optical disk. In this case, like a hybrid disk, the guide
track layer S may be used, for example, to store read-only
programs or security data. Also, recording such data
according to the Blu-ray standard makes it possible to use

a signal processing system of an optical disk apparatus
conforming to the Blu-ray standard and thereby makes it
possible to reduce the production costs of an optical disk
apparatus.
Further, the guide track layer S may contain pre-
recorded information such as a unit number to identify its
location in an optical disk. Such identification
information helps reduce the time to access the guide track
layer S or to jump between multiple guide track layers S.
As described above, an embodiment of the present
invention provides a high capacity multilayer optical
information recording medium with a tilt tolerance that is
substantially equal to that of a recording medium having
only a few recording layers. Another embodiment of the
present invention provides an optical head that can
accurately receive a signal from a multilayer optical
information recording medium according to an embodiment of
the present invention. Still another embodiment of the
present invention provides an optical drive that can
accurately record, reproduce, and/or delete information on
a multilayer optical information recording medium according
to an embodiment of the present invention.
The present invention is not limited to the
specifically disclosed embodiments, and variations and
modifications may be made without departing from the scope

of the present invention.
The present application is based on Japanese
Priority Application No. 2005-349202 filed on December 2,
2005 and Japanese Priority Application No. 2006-016382
filed on January 25, 2006, the entire contents of which are
hereby incorporated herein by reference.

WE CLAIM :
1. An optical recording medium (100; 100c), comprising:
multiple multilayer units (U1, U2, U3, UN1, UN2) stacked in a depth direction of the
optical recording medium (100),
characterised in that
each multilayer unit (U1, U2, U3, UN1, UN2) comprises:
a guide layer (S) corresponding to light with a first wavelength, and
multiple recording layers (D) corresponding to light with a second wavelength that is
longer than the first wavelength.
2. The optical recording medium (100, 100c) as claimed in claim 1, wherein the recording
layers (D) are stacked on an upper side or a lower side of the guide layer (S).
3. The optical recording medium (100, 100c) as claimed in claim 2, wherein both the light
with the first wavelength and the light with the second wavelength enter the optical recording
medium (100, 100c) through a same incidence plane; and
the guide layer is positioned closer to the incidence plane than the recording layers (D)
in each of the multilayer units.
4. The optical recording medium (100, 100c) as claimed in claim 3, wherein tracks are
formed spirally or concentrically on the guide layer (S); and
when n indicates a number of the recording layers (D) in each of the multilayer units
(U1, U2, U3, UN1, UN2), d indicates a thickness of each of the recording layers (D), and p
indicates a pitch between the tracks, n x d x sin(1°) 5. The optical recording medium (100c) as claimed in claim 1, wherein each multilayer
units (UN1, UN2) comprises:
multiple guide layers (S) corresponding to light with the first wavelength, and
multiple recording layers (D) corresponding to light with the second wavelength.


6. The optical recording medium (100c) as claimed in claim 5, wherein the guide layers
(S) are positioned closer to the incidence plane than the recording layers (D) in each of the
multilayer units (UN1, UN2).
7. The optical recording medium (100, 100c) as claimed in claim 1, wherein information
can be recorded on the guide layer (S),
8. The optical recording medium (100, 100c) as claimed in claim 1, wherein information
is prerecorded on the guide layer (S).
9. The optical recording medium (100, 100c) as claimed in claim 8, wherein the
information prerecorded on the guide layer (S) comprises information to identify a location of
the guide layer (S) in the optical recording medium (100).
10. The optical recording medium (100, 100c) as claimed in claim 1, wherein the first
wavelength is between 390 and 420 nm and the second wavelength is between 650 and 680
nm.
11. The optical recording medium (100, 100c) as claimed in claim 1, wherein at least guide
grooves or guide pits are formed on the guide layer (S).
12. An optical head (123) for recording or reproducing information on the optical
recording medium (100, 100c) as claimed in claim 1, comprising:
a first light source (LD1) configured to emit a light beam with the first wavelength;
a second light source (LD2) configured to emit a light beam with the second
wavelength;

an objective lens (160) configured to focus the light beam with the first wavelength on
the guide layer (S) and to focus the light beam with the second wavelength on one of the
recording layers (D);
an optical system configured to guide the light beam with the first wavelength and the
light beam with the second wavelength to the objective lens (160) and to separate a light beam
reflected from the guide layer (S) and a light beam reflected from the one of the recording
layers (D);
a first photodetector (PD1) configured to detect the light beam reflected from the guide
layer (S); and
a second photodetector (PD2) configured to detect the light beam reflected from the
one of the recording layers (D).
13. The optical head (123) as claimed in claim 12, wherein the second light source (LD2)
comprises multiple light-emitting parts and is configured to emit multiple light beams with the
second wavelength from the light-emitting parts;
the second photodetector (PD2) comprises multiple light-receiving parts and is
configured to detect the light beams reflected from the recording layers (D) separately with the
light-receiving parts.
14. The optical head (123) as claimed in claim 13, wherein the objective lens (160) is
configured so that a focal point of the light beam with the first wavelength becomes closer to
the objective lens (160) than a focal point of the light beam with the second wavelength.
15. An optical drive (120) for recording, reproducing, or deleting information on the
optical recording medium (100, 100c) as claimed in claim 1, comprising:
the optical head (123) as claimed in any one of claims 12 to 14; and

a processing unit (128) configured to reproduce the information on the optical
recording medium (100, 100c) based on an output signal from the second photodetector (PD2)
of the optical head (123).



ABSTRACT


MULTILAYER OPTICAL INFORMATION RECORDING MEDIUM,OPTICAL HEAD
AND OPTICAL DRIVE


A disclosed optical recording medium includes multiple recording layer units in each
of which one or more recording layers (3) and one or more middle layers (2) are stacked
alternately; and one or more spacer layers. In the disclosed optical recording medium, the
recording layer units and the spacer layers (4) are stacked alternately in a depth direction of
the optical recording medium.

Documents:

02832-kolnp-2007-abstract.pdf

02832-kolnp-2007-claims 1.0.pdf

02832-kolnp-2007-claims 1.1.pdf

02832-kolnp-2007-correspondence others 1.1.pdf

02832-kolnp-2007-correspondence others.pdf

02832-kolnp-2007-description complete.pdf

02832-kolnp-2007-drawings.pdf

02832-kolnp-2007-form 1.pdf

02832-kolnp-2007-form 18.pdf

02832-kolnp-2007-form 3.pdf

02832-kolnp-2007-form 5.pdf

02832-kolnp-2007-international publication.pdf

02832-kolnp-2007-international search report.pdf

02832-kolnp-2007-pct request form.pdf

02832-kolnp-2007-priority document.pdf

2832-KOLNP-2007-(06-07-2012)-ABSTRACT.pdf

2832-KOLNP-2007-(06-07-2012)-AMANDED CLAIMS.pdf

2832-KOLNP-2007-(06-07-2012)-AMANDED PAGES OF SPECIFICATION.pdf

2832-KOLNP-2007-(06-07-2012)-DESCRIPTION (COMPLETE).pdf

2832-KOLNP-2007-(06-07-2012)-DRAWINGS.pdf

2832-KOLNP-2007-(06-07-2012)-EXAMINATION REPORT REPLY RECEIVED.pdf

2832-KOLNP-2007-(06-07-2012)-FORM-1.pdf

2832-KOLNP-2007-(06-07-2012)-FORM-13.pdf

2832-KOLNP-2007-(06-07-2012)-FORM-2.pdf

2832-KOLNP-2007-(06-07-2012)-FORM-3.pdf

2832-KOLNP-2007-(06-07-2012)-OTHERS.pdf

2832-KOLNP-2007-(06-07-2012)-PETITION UNDER RULE 137.pdf

2832-KOLNP-2007-(09-10-2012)-CORRESPONDENCE.pdf

2832-KOLNP-2007-(15-02-2013)-CORRESPONDENCE.pdf

2832-KOLNP-2007-(18-03-2013)-CORRESPONDENCE.pdf

2832-KOLNP-2007-(18-03-2013)-FORM 3.pdf

2832-KOLNP-2007-(25-04-2012)-CORRESPONDENCE.pdf

2832-KOLNP-2007-ASSIGNMENT.pdf

2832-KOLNP-2007-CANCELLED PAGES.pdf

2832-KOLNP-2007-CORRESPONDENCE OTHERS 1.2.pdf

2832-KOLNP-2007-CORRESPONDENCE OTHERS 1.3.pdf

2832-KOLNP-2007-CORRESPONDENCE-1.4.pdf

2832-KOLNP-2007-CORRESPONDENCE.pdf

2832-KOLNP-2007-EXAMINATION REPORT.pdf

2832-KOLNP-2007-FORM 13.pdf

2832-KOLNP-2007-FORM 18.pdf

2832-KOLNP-2007-FORM 3-1.1.pdf

2832-KOLNP-2007-GRANTED-ABSTRACT.pdf

2832-KOLNP-2007-GRANTED-CLAIMS.pdf

2832-KOLNP-2007-GRANTED-DESCRIPTION (COMPLETE).pdf

2832-KOLNP-2007-GRANTED-DRAWINGS.pdf

2832-KOLNP-2007-GRANTED-FORM 1.pdf

2832-KOLNP-2007-GRANTED-FORM 2.pdf

2832-KOLNP-2007-GRANTED-FORM 3.pdf

2832-KOLNP-2007-GRANTED-FORM 5.pdf

2832-KOLNP-2007-GRANTED-SPECIFICATION-COMPLETE.pdf

2832-KOLNP-2007-INTERNATIONAL PUBLICATION.pdf

2832-KOLNP-2007-INTERNATIONAL SEARCH REPORT & OTHERS.pdf

2832-KOLNP-2007-OTHERS.pdf

2832-KOLNP-2007-PA.pdf

2832-KOLNP-2007-PETITION UNDER RULE 137.pdf

2832-KOLNP-2007-REPLY TO EXAMINATION REPORT.pdf

2832-KOLNP-2007-TRANSLATED COPY OF PRIORITY DOCUMENT.pdf

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Patent Number 256596
Indian Patent Application Number 2832/KOLNP/2007
PG Journal Number 28/2013
Publication Date 12-Jul-2013
Grant Date 05-Jul-2013
Date of Filing 02-Aug-2007
Name of Patentee RICOH COMPANY, LTD.
Applicant Address 3-6, NAKAMAGOME 1-CHOME, OHTA-KU, TOKYO
Inventors:
# Inventor's Name Inventor's Address
1 MISAWA SHIGEYOSHI 5-8, DENENCHOFU 2-CHOME, OHTA-KU, TOKYO 145-0071
2 OOUCHIDA SHIGERU 20-5 NIHONBASHININGYOCHO 2-CHOME, CHUO-KU, TOKYO 103-0013
PCT International Classification Number G11B 7/24, G11B 7/09
PCT International Application Number PCT/JP06/324406
PCT International Filing date 2006-11-30
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 2005-349202 2005-12-02 Japan
2 2006-016382 2006-01-25 Japan