Description
Optical Recording Medium, Recording Device, and Recording
Method
Technical Field
The present invention relates to an optical
recording medium, and particularly to a recording device
and 'a recording method for a bulk type optical recording
medium.
Background Art
So-called optical disks such for example as a CD
(Compact Disc), a DVD (Digital Versatile Disc), and a BD
(Blu-ray Disc: registered trademark) have spread as
optical recording media on which signals are
recorded/reproduced by application of light. Systems for
these optical disks read minute changes in reflectance
that are formed on one side of a disk on a noncontact
basis by application of laser light via an objective lens.
As is well known, the size of a light spot on a disk is
given by A/NA (A: the wavelength of laser light, NA: a
numerical aperture), and resolution is proportional to
this value. Details of a Blu-ray Disc corresponding to
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about 25 GB as a disk having a diameter of 12 cm, for
example, are disclosed in the above Non-Patent Document 1.
In addition, Non-Patent Documents 2 and 3 propose
multilayer recording type optical disks. When recording
and reproduction is performed in multiple layers, a
recording capacity of one disk is multiplied by the
number of the layers.
Prior Art Documents
Non-Patent Documents
Non-Patent Document 1: Yutaka KASAMI, Yuji KURODA,
Katsuhiro SEO, Osamu KAWAKUBO, Shigeki TAKAGAWA, Masumi
ONO and Masahiro YAMADA "Large Capacity and High-Data-
Rate Phase-Change Disks" Jpn. J. Appl. Phys.,
Vol.39(2000), pp756
Non-Patent Document 2: Isao Ichimura, Kimihiro
Saito, Takeshi Yamasaki, and Kiyoshi Osato "Proposal for
a multilayer read-only-memory optical disk structure"
Applied Optics, Vol. 45, NO.8 2006, pp1794
Non-Patent Document 3: Noriyoshi SHIDA, Takanobu
HIGUCHI, Yasuo HOSODA, Hiroko MIYOSHI, Akio NAKANO and
Katsunori TSUCHIYA "Multilayer Optical Read-Only-Memory
Disk Applicable to Blu-ray Disc Standard Using a
Photopolymer Sheet with a Recording Capacity of 100 GB"
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Jpn. J. Appl. Phys., Vol. 43(2004), pp4983
Summary of Invention
Technical Problem
FIG. 17 shows an example of multilayer recording
type optical disks shown in Non-Patent Document 2. FIG.
17(a) represents an example of a four-layer disk. The
four-layer disk has a structure formed by laminating a
recording layer LO, a spacer layer SP1, a recording layer
L1, a spacer layer SP2, a recording layer L2, a spacer
layer SP3, a recording layer L3, and a cover layer 101 of
70 pm in order on a substrate 102 of 1.1 mm. In this
example, the spacer layer SP1 is 11.5 pm, the spacer
layer SP2 is 15.0 pm, and the spacer layer SP3 is 9.5
That is, layer intervals between the recording layers L0,
Ll, L2, and L3 are set so as to be different from each
other. In a case of a reproduction-only type disk (ROM
disk), the recording layers LO, L1, L2, and L3 have data
recorded therein by an embossed pit string, for example.
In addition, a total reflection film is formed in the
deepest recording layer L0, and a semitransparent
reflective film is formed in the other recording layers
Ll, L2, and L3.
In addition, FIG. 17(b) represents an example of an
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eight-layer disk. Recording layers LO to L7 are formed
between a substrate 102 of 1.1 mm and a cover layer of 41
pm with spacer layers SP1 to SP7 interposed between the
recording layers LO to L7. Also in this case, the
thicknesses of the spacer layers SP1 to SP7, that is,
layer intervals are not uniform.
The layer intervals are not uniform thicknesses as
in these for the following reasons. FIG. 18(a) represents
a case of a four-layer disk in which spacer layers SP1 to
SP3 have a same thickness. Suppose in this case that a
recording layer L1 is irradiated with laser light LZ. As
shown in FIG. 18(b), when the laser light is focused on
the recording layer Ll, return light modulated by the
embossed pit string of the recording layer Ll is obtained.
Information recorded in the recording layer L1 can be
read by detecting the return light with a photodetector.
However, in a case of a multilayer disk, a stray
light component modulated by another recording layer
occurs in the return light guided to the photodetector.
As indicated by a broken line in FIG. 18(b), a component
as a part of the laser light LZ focused on the recording
layer L1 is reflected by the recording layer L2, and
becomes stray light. This stray light is further
reflected by each of the recording layers L3 and L2, and
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becomes a part of the return light. This is a light
component modulated by the recording layer L3. When the
layer intervals are uniform at this time, the focus point
FP of the stray light component indicated by the broken
line is on the recording layer L3. That is, the stray
light component is applied to the embossed pit string of
the recording layer L3 in a focused state, and thereby
becomes a modulated component of a relatively high
intensity. Such a stray light component is mixed into the
return light to the photodetector, thereby causing a socalled
interlayer crosstalk and thus decreasing the
quality of a reproduced signal.
When the layer intervals are made different from
each other in this case, the effect of the interlayer
crosstalk due to such stray light can be reduced or
eliminated. FIG. 18(c) represents an example in which
layer intervals between recording layers LO to L3 are
made different from each other by setting spacer layers
SP1 to SP3 to respective different thicknesses. Also in
this case, when laser light LZ is focused on the
recording layer L1, a component as a part of the laser
light LZ is reflected by the recording layer L2, and
becomes stray light, as indicated by a broken line in FIG.
18(d), for example. This stray light is reflected by each
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of the recording layers L3 and L2, and becomes a part of
return light. However, in this case, because of the
different thicknesses of the spacer layers SP2 and SP3,
the focus point FP of the stray light component is not on
the recording layer L3, as in the figure. The recording
layer L3 is thus irradiated with the stray light
component in a so-called out-of-focus state. This stray
light component is subjected to modulation by the pit
string of the recording layer L3.' However, the stray
light component is not focused, and thus modulation
intensity becomes very weak. Therefore, the stray light
component hardly affects a reproduced signal even when
mixed in as a part of return light to a photodetector.
That is, when layer intervals are made different
from each other in a multilayer disk, the modulation
intensity of a stray light component modulated by another
recording layer than a target recording layer can be
lowered. As a result, an interlayer crosstalk can be
reduced to such a level as not to affect a reproduced
signal.
Thus making layer intervals different from each
other in a multilayer disk can improve the quality of a
reproduced signal. However, in that case, there is a
disadvantage in that many kinds of settings of spacer
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layer thickness need to be provided in the manufactured
constitution of a disk. Increases in the number of layers
to 8 layers, 16 layers, and more, in particular, have
recently been promoted. However, providing a large number
of kinds of film thickness settings for spacer layers SP
and changing a setting in each of stages of formation of
the spacer layers in a case of forming the spacer layers
SP by sputtering, for example, is disadvantageous in
terms of process efficiency and manufacturing cost.
Accordingly, the present invention proposes a
structure of an optical recording medium such as can
eliminate the effect of an interlayer crosstalk with a
minimum number of kinds of layer intervals.
Technical Solution
An optical recording medium according to the
present invention includes: N recording layers (where N >>
4); a number of kinds of layer intervals between the
recording layers adjacent to each other being an integer
M equal to or less than a minimum integer equal to or
more than log2(N); and one or more blocks including four
recording layers with a first layer interval, a second
layer interval, and the first layer interval formed in
order in the one or more blocks. For example, a four-
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layer recording medium has a minimum integer M = 2 equal
to or more than log2(4), and has one block including four
recording layers with a first layer interval, a second
layer interval, and the first layer interval formed in
order in the one block. In addition, an eight-layer
recording medium, for example, has a minimum integer M =
3 equal to or more than log2(8) , and has two blocks
including four recording layers with a first layer
interval, a second layer interval; and the first layer
interval formed in order in the two blocks. Then, a layer
interval between the blocks is a third layer interval.
In addition, a number of the recording layers is
such that the M as the number of kinds of layer intervals
is M >> 3, and the optical recording medium has each of
parts of respective layer intervals from a third layer
interval to an Mth layer interval as a layer interval
between a recording layer forming the block and a
recording layer adjacent to the recording layer, the
adjacent recording layer being included in other than the
block including the recording layer. In addition, in this
case, the first layer interval < the second layer
interval < the third layer interval < ... < the Mth layer
interval. As an example where M ? 3, when the optical
recording medium has five to eight recording layers, and
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the above M = 3, the optical recording medium has a part
of a third layer interval as a layer interval between a
recording layer forming the block and a recording layer
adjacent to the recording layer, the adjacent recording
layer being included in other than the block including
the recording layer. In addition, when the optical
recording medium has 9 to 16 recording layers, and the
above M = 4, the optical recording medium has each of a
part of a third layer interval and a part of a fourth
layer interval as a layer interval between a recording
layer forming the block and a recording layer adjacent to
the recording layer, the adjacent recording layer being
included in other than the block including the recording
layer. In addition, when the optical recording medium has
17 to 32 recording layers, and the above M = 5, the
optical recording medium has each of a part of a third
layer interval, a part of a fourth layer interval, and a
part of a fifth layer interval as a layer interval
between a recording layer forming the block and a
recording layer adjacent to the recording layer, the
adjacent recording layer being included in other than the
block including the recording layer.
When the number of the recording layers is N = 2Y
(y is an integer of three or more), the Mth layer
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interval is set between an (N/2)th recording layer and an
{(N/2) + l}th recording layer, a first recording layer to
the (N/2)th recording layer and the {(N/2) + 1}th
recording layer to an Nth recording layer each have at
least one or more blocks, and settings of respective
layer intervals from the first recording layer to the
(N/2)th recording layer are identical to settings of
respective layer intervals from the {(N/2) + 11th
recording layer to the Nth recording layer. This
corresponds to cases where the number of recording layers
is 8, 16, 32, .... To illustrate by a case of 16 layers
(M = 4), for example, a fourth layer interval is set
between an eighth recording layer and a ninth recording
layer, a first recording layer to the eighth recording
layer and the ninth recording layer to a sixteenth
recording layer each have two blocks, and settings of
respective layer intervals from the first recording layer
to the eighth recording layer are identical to settings
of respective layer intervals from the ninth recording
layer to the sixteenth recording layer. That is, the
layer intervals are set symmetrically with respect to a
center between the eighth recording layer and the ninth
recording layer.
In addition, the following is for cases in which
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the number N of recording layers is 2y-1 < N < 2y (y is an
integer of three or more) Suppose that N' = 2y, that the
Mth layer interval is set between an (N'/2)th recording
layer and an {(N'/2) + 1}th recording layer, that a first
recording layer to the (N'/2)th recording layer and the
{(N'/2) + 11th recording layer to an N'th recording layer
each have at least one or more blocks, and that settings
of respective layer intervals from the first recording
layer to the (N'/2)th recording layer are identical to
settings of respective layer intervals from the {(N'/2) +
11th recording layer to the N'th recording layer. The N
recording layers are formed with layer interval settings
in a case of extracting N consecutive recording layers
from the first recording layer to the N'th recording
layer at this time. This corresponds to cases where the
number of recording layers is 5 to 7, 9 to 15, 17 to
31, .... An illustration will be given in a case of N =
12 (M = 4), for example. First, suppose that N' = 16
(incidentally; M = 4 also in the case of 16 layers). In
this case, a fourth layer interval is set between an
eighth recording layer and a ninth recording layer, a
first recording layer to the eighth recording layer and
the ninth recording layer to a sixteenth recording layer
each have two blocks, and settings of respective layer
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intervals from the first recording layer to the eighth
recording layer are identical to settings of respective
layer intervals from the ninth recording layer to the
sixteenth recording layer. That is, the layer intervals
are set symmetrically with respect to a center between
the eighth recording layer and the ninth recording layer.
The first to twelfth recording layers are formed with
layer interval settings in a case of extracting N
consecutive recording layers (12 recording layers in this
case) from the 16 layers set as described above.
A recording device according to the present
invention includes: an optical pickup for irradiating,
with laser light, an optical recording medium having a
bulk layer in which to record optical recording
information by irradiation with laser light and form a
plurality of recording layers in which the optical
recording information is recorded; a focus control
section configured to control a focal position of the
laser light within the bulk layer; and a control section
configured to give an instruction on the focal position
to the focus control section so that each recording layer
is formed by recording operation with the laser light
such that N recording layers (where N > 4) are formed
within the bulk layer, a number of kinds of layer
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intervals between the recording layers adjacent to each
other is an integer M equal to or less than a minimum
integer equal to or more than 1og2(N), and the bulk layer
has one or more blocks including four recording layers
with a first layer interval, a second layer interval, and
the first layer interval formed in order in the one or
more blocks.
A recording method according to the present
invention as a recording method of a recording device,
the recording device including an optical pickup for
irradiating, with laser light, an optical recording
medium having a bulk layer in which to record optical
recording information by irradiation with laser light and
form a plurality of recording layers in which the optical
recording information is recorded, and a focus control
section configured to control a focal position of the
laser light within the bulk layer, performs recording
operation with the laser light after the focus control
section controls the focal position according to each
recording layer, so as to form each recording layer such
that N recording layers (where N ? 4) are formed within
the bulk layer, a number of kinds of layer intervals
between the recording layers adjacent to each other is an
integer M equal to or less than a minimum integer equal
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to or more than log2(N), and the bulk layer has one or
more blocks including four recording layers with a first
layer interval, a second layer interval, and the first
layer interval formed in order in the one or more blocks.
The layer intervals between the recording layers in
the optical recording medium according to the present
invention or the layer intervals between the recording
layers in the bulk type optical recording medium which
recording layers are formed by recording performed by the
recording device or the recording method according to the
present invention have layer interval settings that can
eliminate the effect of an interlayer crosstalk with a
minimum number of kinds of layer intervals,
Advantageous Effect
The optical recording medium according to the
present invention is, as a multilayer optical recording
medium having four layers or more, an optical recording
medium that can eliminate the effect of an interlayer
crosstalk at a time of reproduction with a minimum number
of kinds of layer intervals. It is therefore possible to
minimize kinds of film thickness settings when forming
spacer layers between recording layers by sputtering, for
example, and thus improve manufacturing efficiency.
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In addition, according to the recording device and
the recording method according to the present invention,
it is possible to realize an optical recording medium
that can eliminate the effect of an interlayer crosstalk
at a time of reproduction with a minimum number of kinds
of layer intervals when four or more recording layers are
formed by recording operation on a bulk type multilayer
optical recording medium. Therefore, simplification and
higher efficiency of setting of a-focal position by focus
control at a time of recording can be achieved.
Brief Description of Drawings
FIG. 1 is a diagram of assistance in explaining a
four-layer recording medium according to a first
embodiment of the present invention.
FIG. 2 is a diagram of assistance in explaining
stray light in an optical recording medium according to
the first embodiment.
FIG. 3 is a diagram of assistance in explaining an
eight-layer recording medium according to the first
embodiment.
FIG. 4 is a diagram of assistance in explaining
stray light in the eight-layer recording medium according
to the first embodiment.
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FIG. 5 is a diagram of assistance in explaining a
16-layer recording medium according to the first
embodiment.
FIG. 6 is a diagram of assistance in explaining a
32-layer recording medium according to the first
embodiment.
FIG. 7 is a diagram of assistance in explaining a
20-layer recording medium according to the first
embodiment.
FIG. 8 is a diagram of assistance in explaining a
20-layer recording medium according to the first
embodiment.
FIG. 9 is a diagram of assistance in explaining a
20-layer recording medium according to the first
embodiment.
FIG. 10 is a diagram of assistance in explaining a
5-layer recording medium, a 6-layer recording medium, a
9-layer recording medium, and a 12-layer recording medium
according to the first embodiment.
FIG. 11 is a diagram of assistance in explaining
microhologram recording.
FIG. 12 is a diagram of assistance in explaining
microhologram recording.
FIG. 13 is a diagram of assistance in explaining a
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bulk recording medium according to a second embodiment of
the present invention.
FIG. 14 is a diagram of assistance in explaining
servo control at a time of recording according to the
second embodiment.
FIG. 15 is a diagram of assistance in explaining a
recording and reproducing optical system according to the
second embodiment.
FIG. 16 is a flowchart of a recording process
according to the second embodiment.
FIG. 17 is a diagram of assistance in explaining a
conventional four-layer disk and a conventional eightlayer
disk.
FIG. 18 is a diagram of assistance in explaining
layer intervals set so as to be different from each other.
Mode for Carrying Out the Invention
Embodiments of the present invention will be
described in the following order.
<1. First Embodiment>
[1-1: N-layer (N = 2Y) Recording Medium]
[1-2: N-layer (2y-1 < N < 2Y) Recording Medium]
<2. Second Embodiment: Recording Device for Bulk Type
Optical Recording Medium>
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[2-1: Structure of Optical Recording Medium]
[2-2: Servo Control]
[2-3: Recording and Reproducing Optical System]
[2-4: Recording Process]
<1. First Embodiment>
[1-1: N-layer (N = 2Y) Recording Medium]
An optical. recording medium as a first embodiment
will be described. This optical recording medium can be
realized as a reproduction-only optical disk, a writeonce
optical disk, or a rewritable optical disk in Bluray
Discs, for example.
For example, a reproduction-only Blu-ray Disc (BDROM)
has a diameter of 120 mm and a disk thickness of 1.2
mm as a disk size. That is, in these respects, a BD-ROM
is similar, in external appearance, to a disk of a CD
system or a disk of a DVD system.
Then, a so-called blue laser is used as a laser for
recording/reproduction, and an optical system has a high
NA (for example NA = 0.85). In a BD-ROM, for example, an
embossed pit string in a spiral shape is formed on a
substrate (disk substrate) formed by a resin such as
polycarbonate having a thickness of about 1.1 mm, and a
surface having depressions and projections formed by the
pit string is coated with a reflective film to form a
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recording layer. In addition, in the BD-ROM, not only
one-layer disks having one recording layer but also
multiple-layer disks having two recording layers, three
recording layers, ..., and n recording layers have been
developed. Each recording layer is formed on the disk
substrate with a spacer layer interposed between the
recording layers. Of course, a recording capacity can be
greatly increased by providing a large number of
recording layers. Then, a cover layer of a predetermined
thickness is further formed, whereby a disk having a
thickness of 1.2 mm is formed.
In addition,.a BD-R (Blu-ray Disc Recordable) and a
BD-RE (Blu-ray Disc Rewritable) are known as recordable
disks. A BD-R is a write-once disk in the Blu-ray Disc
standards. A BD-RE is a rewritable disk in the Blu-ray
Disc standards, BD-R and BD-RE disks have only a groove
formed in advance on a disk substrate, and have no pit
string. Of these recordable disks, disks having multiple
recording layers have also been developed.
Incidentally, a recording medium according to the
present embodiment having a structure to be described in
the following is not limited to optical disks of the Bluray
Disc system, but has a structure realizable as that
of a Blu-ray Disc or a next-generation disk. In addition,
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the recording medium according to the present embodiment
is not necessarily limited to disk type recording media,
but is also applicable to other kinds of recording media
such for example as an optical recording medium having
the shape of a card. That is, the recording medium
according to the present embodiment is widely applicable
as multilayer optical recording medium having four layers
or more.
First, the optical recording medium according to
the present embodiment is based on the following premises.
FIG. 2(a) schematically shows recording layers LO to L8
in an eight-layer recording medium, for example. In this
case, when the recording layer Ll is to be reproduced,
laser light LZ is focused on the recording layer Ll as
indicated by a solid line in the figure. At this time, as
indicated by a broken line, a stray light component is
focused on another recording layer L5 (focus point FP),
thereby modulated by the recording layer L5, and mixed
into return light as a modulated component of a
relatively high intensity. This constitutes an interlayer
crosstalk, as described above. In addition, as indicated
by a broken line in FIG. 2(b), there is a component
reflected twice and mixed into the return light. In the
present embodiment, cases of being reflected by a certain
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recording layer only once and focused on another
recording layer as in FIG. 2(a) are avoided, and cases of
being focused as a result of multiple reflection as shown
in FIG. 2(b) are ignored. This is because in cases of
multiple reflection, the number of times of reflection is
increased by two or more, so that an amount of light is
small and produces little effect.
First, a four-layer recording medium will be
described as the present embodiment with reference to
FIG. 1(b). Incidentally, FIG. 1(a) shows an example of a
structure of a two-layer recording medium for reference.
As in FIG. l(a), in the case of the two-layer recording
medium, recording layers LO and L1 are formed between a
substrate 12 and a cover layer 11. A spacer layer SP1 is
formed between the recording layers LO and Ll. Such a
two-layer structure is extended to develop recording
media having more layers such as three layers, four
layers, ....
As shown in FIG, 1(b), the four-layer recording
medium according to the present embodiment has recording
layers LO, L1, L2, and L3 formed between a substrate 12
and a cover layer 11. Spacer layers SP1, SP2, and SP3 are
formed between the recording layers LO, Ll, L2, and L3.
In the case of the above-described Blu-ray Disc, for
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example, the substrate 12 has a thickness of about 1.1 mm,
and a thickness from the recording layer LO to the
surface of the cover layer 11 is about 100 pm, so that
the disk having a thickness of 12 mm is formed. Each of
the recording layers LO to L3 has an embossed pit string
and a wobbling groove formed therein.
The four-layer recording medium in the present
example has two kinds as kinds of layer intervals, that
is, a first layer interval A and a second layer interval
B as shown in the figure. Specifically,
L0-Ll layer interval (thickness of the spacer layer
SP1) ... A
Ll-L2 layer interval (thickness of the spacer layer
SP2) ... B
L2-L3 layer interval (thickness of the spacer layer
SP3) ... A
In this example, A < B. For example, A = 9 pm, and B = 10
According to such a layer structure, stray light
being focused on another recording layer as shown in
FIG. 2(a) can be avoided. Description will be made with
reference to FIGS. 2(c) and 2(d). Consideration will be
given to a case where the recording layer LO is set as a
reproduction object as shown in FIG. 2(c), for example.
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When laser light LZ is focused on the recording layer LO,
as indicated by a broken line, a component as a part of
the laser light LZ is reflected by the recording layer L1
on a side nearer to a laser incidence surface, and
becomes stray light. This stray light is further
reflected by each of the recording layers L1 and L2, and
becomes a part of return light. However, in this case,
the L0-Li layer interval A is a different layer interval
from the L1-L2 layer interval B. 'Therefore, the focus
point FP of the stray light component is not on the
recording layer L2 as shown in the figure. The recording
layer L2 is thus irradiated with the stray light
component in a so-called out-of-focus state. This stray
light component is subjected to modulation by the pit
string of the recording layer L3. However, the stray
light component is not focused, and thus modulation
intensity becomes very weak. Therefore, the stray light
component hardly affects a reproduced signal even when
mixed in as a part of the return light to a photodetector.
Though not shown in the figure, the laser light LZ
focused on the recording layer LO also includes a stray
light component reflected by the recording layer L2.
However, the stray light component does not have a focal
position on the recording layer L3, either.
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The same is also true for a case where the
recording layer Ll is set as a reproduction object as
shown in FIG. 2(d). When laser light LZ is focused on the
recording layer Ll, as indicated by a broken line, a
component as a part of the laser light LZ is reflected by
the recording layer L2, and becomes stray light. However,
in this case, the L1-L2 layer interval B is a different
layer interval from the L2-Ll layer interval A. Therefore
the focus point FP of the stray light component is not on
the recording layer L3 as shown in the figure. Thus, the
modulation intensity of the stray light component in the
return light is very weak, and hardly affects a
reproduced signal.
An ability itself to reduce the: effect of an
interlayer crosstalk with such different layer intervals
is similar to that described with reference to FIG. 17
and FIG. 18. However, the present embodiment can achieve
a similar effect with a small number of kinds of layer
intervals. Specifically, the four-layer recording medium
according to the present embodiment has four recording
layers LO to L3, and the number of kinds of layer
intervals between the recording layers adjacent to each
other is M = 2, where M is a minimum integer equal to or
more than log2(4). That is, the four-layer recording
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medium according to the present embodiment has two kinds
of layer intervals A and B. Then, the four-layer
recording medium according to the present embodiment has
one block of four recording layers (an "AB block" shown
in FIG. l(b)) in which block the first layer interval A,
the second layer interval B, and the first layer interval
A are formed in order. Whereas for example the four-layer
disk described in FIG. 17(a) has three kinds of layer
intervals, it suffices to have two kinds of layer
intervals in the present example.
Further, recording media having N layers (N = 2Y)
according to embodiments, that is, recording media having
8 layers, 16 layers, 32 layers, ... can be formed by
extending this AB block, that is, the constitution
including four recording layers with the layer intervals
A-B-A.
An example of an eight-layer recording medium is
shown in FIG. 3. In the case of the eight-layer recording
medium, two AB blocks as described above are used, and a
third layer interval C is set between the AB blocks. As
shown in FIG. 3, recording layers LO to L7 are formed
between a substrate 12 and a cover layer 11. Spacer
layers SPl to SP7 are formed between the recording layers
L0, Ll, L2, ..., and L7.
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Then, three kinds of layer intervals, that is, a
first layer interval A, a second layer interval B, and a
third layer interval C are provided as kinds of layer
intervals. Specifically,
LO-L1 layer interval (thickness of the spacer layer
SP1) ... A
Ll-L2 layer interval (thickness of the spacer layer
SP2) ... B
L2-L3 layer interval (thickness of the spacer layer
SP3) ... A
L3-L4 layer interval (thickness of the spacer layer
SP4) ... C
L4-L5 layer interval (thickness of the spacer layer
SP5) ... A
L5-L6 layer interval (thickness of the spacer layer
SP6) ... B
L6-L7 layer interval (thickness of the spacer layer
SP7) A
In this example, A < B < C. For example, A = 9 pm, B = 10
pm, and C = 11 pm.
Even with such a layer structure, stray light being
focused on another recording layer as shown in FIG. 2(a)
can be avoided. Cases where the recording layers L4 and
L5 are set as a reproduction object can be considered to
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be similar to the cases of FIGS. 2(c) and 2(d) described
for the above four-layer recording medium. FIG. 4(a)
represents a case where the recording layer L3 is set as
a reproduction object. Specifically, when laser light LZ
is focused on the recording layer L3, as indicated by
broken lines, a component as a part of the laser light LZ
is reflected by recording layers on a side nearer to a
laser incidence surface, such as the recording layer L4
and the recording layer L3, and becomes stray light.
However, none of the focus point F21 of the stray light
reflected by the recording layer L4, the focus point FP2
of the stray light reflected by the recording layer L5,
and the focus point FP3 of the stray light reflected by
the recording layer L6 are on recording layers. This is
because the L3-L4 layer interval is C. Thus, modulation
intensity in the stray light components is very we,k, and
hardly affects a reproduced signal even when the stray
light components are mixed in as a part of return light
to a photodetector.
The same is also true for a case where the
recording layer L2 is set as a reproduction object as
shown in FIG. 4(b). When laser light LZ is focused on the
recording layer L2, as indicated by broken lines, a
component as a part of the laser light LZ is reflected by
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recording layers on a side nearer to a laser incidence
surface, and becomes stray light. However, none of the
focus point FPl of the stray light reflected by the
recording layer L3, the focus point FP2 of the stray
light reflected by the recording layer L4, the focus
point FP3 of the stray light reflected by the recording
layer L5, and the focus pointFP4 of the stray light
reflected by the recording layer L6 are on recording
layers. This is because the L3-L4 layer interval is C and
the L2-L3 layer interval is A. Thus, modulation intensity
in the stray light components is very weak, and hardly
affects a reproduced signal even when the stray light
components are mixed in as a part of return light to a
photodetector.
The same is also true for the recording layers Ll
and LO. As for a case where the recording layer LO is set
as a reproduction object, for example, stray light
reflected by the recording layers Ll and L2 within the
same AB block as the recording layer LO is not focused on
the recording layers L2 and L3. Because of the A-B-A
layer interval setting, consideration can be given in a
similar manner to FIGS. 2(c) and 2(d). Stray light
reflected by the recording layers L3 to L6 can be
considered to be similar to that of FIG. 4(b) described
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above, and none of the stray light is focused on
recording layers. Thus, the stray light hardly affects a
reproduced signal.
Thus, the eight-layer recording medium can reduce
an interlayer crosstalk with only three kinds of layer
intervals. Specifically, the eight-layer recording medium
in the present example has eight recording layers LO to
L7, and the number of kinds of layer intervals between
the recording layers adjacent to each other is M = 3,
where M is a minimum integer equal to or more than log2(8).
That is, the eight-layer recording medium according to
the present embodiment has three kinds of layer intervals
A, B, and C. Then, the eight-layer recording medium
according to the present embodiment has two "AB blocks"
of four recording layers in which blocks the first layer
interval A, the second layer interval B, and the first
layer interval A are formed in order. Further, a layer
interval between a recording layer forming an AB block
and a recording layer adjacent to the recording layer,
the adjacent recording layer being included in other than
the AB block including the recording layer, that is, a
layer interval between the recording layers L3 and L4 as
a layer interval between the two AB blocks in the present
example is the third layer interval C. This constitution
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makes it possible to take measures against an interlayer
crosstalk with three kinds of layer intervals even in the
case of an eight-layer recording medium.
It suffices to consider a 16-layer recording medium
and a 32-layer recording medium by extending the above
constitution. FIG. 5 shows the layer intervals of a 16-
layer recording medium. In this case, two "ABC blocks"
corresponding to the constitution of the above eightlayer
recording medium are used, and a fourth layer
interval D is set between the ABC blocks. In this example,
A < B < C < D. For example, A = 9 pm, B = 10 pm, C = 11
pm, and D 13 pm.
The 16-layer recording medium in the present
example has 16 recording layers LO to:L15, and the number
of kinds of layer intervals between the recording layers
adjacent to each other is M = 4, where M is a minimum
integer equal to or more than log2(16). That is, the 16-
layer recording medium in the present example has four
kinds of layer intervals A, B, C, and D. Then, the 16-
layer recording medium in the present example has four
"AB blocks" of four recording layers in which blocks the
first layer interval A, the second layer interval B, and
the first layer interval A are formed in order. The 16-
layer recording medium in the present example further has
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each of parts of the third layer interval C and a part of
the fourth layer interval D as a layer interval between a
recording layer forming an AB block and a recording layer
adjacent to the recording layer, the adjacent recording
layer being included in other than the AB block including
the recording layer. In the present example, an L3-L4
layer interval between two AB blocks and an L11-L12 layer
interval between two AB blocks are the third layer
interval C, and an L7-L8 layer interval is the fourth
layer interval D. This constitution makes it possible to
reduce an interlayer crosstalk with only four kinds of
layer intervals even in the case of the 16-layer
recording medium.
FIG. 6 shows the layer intervals of a 32-layer
recording medium. In this case, two "ABCD blocks"
corresponding to the constitution of the above 16-layer
recording medium are used, and a fifth layer interval E
is set between the ABCD blocks. In this example, A < B <
C < D < E. For example, A = 9 pm, B = 10 pm, C = 11 pm, D
= 13 pm, and E = 15 pm.
The 32-layer recording medium in the present
example has 32 recording layers LO to L31, and the number
of kinds of layer intervals between the recording layers
adjacent to each other is M = 5, where M is a minimum
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integer equal to or more than log2(32). That is, the 32-
layer recording medium in the present example has five
kinds of layer intervals A, B, C, D, and E. Then, the 32-
layer recording medium in the present example has eight
"AB blocks" of four recording layers in which blocks the
first layer interval A; the second layer interval B, and
the first layer interval A are formed in order. The 32-
layer recording medium in the present example further has
each of parts of the third layer interval C, parts of the
fourth layer interval D, and a part of the fifth layer
interval as a layer interval between a recording layer
forming an AB block and a recording layer adjacent to the
recording layer, the adjacent recording layer being
included in other than the AB block including the
recording layer. In the present example, an L3-L4 layer
interval between two AB blocks, an Lll-L12 layer interval
between two AB blocks, an L19-L20 layer interval between
two AB blocks, and an L27-L28 layer interval between two
AB blocks are the third layer interval C. In addition, an
L7-L8 layer interval and an L23-L24 layer interval are
the fourth layer interval D, and an L15-L16 layer
interval is the fifth layer interval E. This constitution
makes it possible to reduce an interlayer crosstalk with
only five kinds of layer intervals even in the case of
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the 32-layer recording medium.
Though not shown in the figures, recording media
having more layers such as a 64-layer recording medium
and a 128-layer recording medium can be considered in a
similar manner.
Recording media having a number N of recording
layers where N 2y, such as 4, layers, 8 layers, 16 layers,
and 32 layers, have been described above. These optical
recording media according to the present embodiment are
optical recording media capable of eliminating the effect
of an interlayer crosstalk at a time of reproduction with
a minimum number of kinds of layer intervals as
multilayer optical recording media having four layers or
more. It is therefore possible to minimize kinds of film
thickness settings when forming spacer layers between
recording layers by sputtering, for example, and thus
improve manufacturing efficiency. In addition, when each
spacer layer is formed by a sheet material, kinds of
thicknesses of sheet material can be reduced. The optical
recording media according to the present embodiment are
thus suitable for improving manufacturing efficiency and
reducing cost. Further, a method of laminating sheets
fabricated in units of AB blocks to each other can be
adopted.
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In addition, when the number of recording layers is
N = 2Y, and y is 3 or more, that is, in the cases of 8
layers, 16 layers, 32 layers, ..., in particular, the
following features are obtained. The Nth layer interval
is present between the (N/2)th recording layer and the
{(N/2) + l}th recording layer. The first recording layer
(LO) to the (N/2)th recording layer and the {(N/2) + 1}th
recording layer to the Nth recording layer each have at
least one AB block or more. Further, the settings of the
respective layer intervals from the first recording layer
(LO) to the (N/2)th recording layer are identical to the
settings of the respective layer intervals from the
{(N/2) + 1}th recording layer to the Nth recording layer.
For example, in the case of 16;layers (M = 4, M
being the number of kinds of layer intervals), the fourth
layer interval D is present between the 8th recordi.ng
layer (L7) and the 9th recording layer (L8) The first to
eighth recording layers (LO) to (L7) and the ninth to
sixteenth recording layers (L8) to (L15) each have two AB
blocks. Further, the settings of the respective layer
intervals from the first recording layer (LO) to the
eighth recording layer (L7) are identical to the settings
of the respective layer intervals from the ninth
recording layer (L8) to the sixteenth recording layer
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(Ll5). That is, the layer intervals are set symmetrically
with respect to a center between the eighth recording
layer (L7) and the ninth recording layer (L8).
The layer interval configuration symmetric with
respect to the center between such central recording
layers not only can reduce the number of kinds of layer
intervals as described above but also facilitate the
settings of the layer intervals, which is advantageous
for improving efficiency of a manufacturing process. In
addition, regularity of the settings of the layer
intervals facilitates focus control onto each recording
layer in the optical recording media and the operation of
a focus jump (interlayer movement) on the side of a
recording and reproducing device.
While the layer intervals are set such that A < B <
C < D ... in each of the above examples, the layer
intervals are not limited to this. It suffices at least
to set A # B ^ C # D .... However, making at least the
first layer interval A smallest can minimize a total
layer thickness. This is because there are a largest
number of spacer layers having the layer interval A.
In addition, as for concrete layer intervals, it
suffices to set the smallest first layer interval A to
12.4'X/NA/tan(asin(NA/n))
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using the wavelength A of reproduction laser light, an NA,
and an index n-of refraction between layers (the index of
refraction is assumed to be the same throughout). This
value is described in the document "K. Saito and S.
Kobayashi; Proc. SPIE 6282 (2006)." For example, when A =
0.405 um, NA = 0.85, and n = 1.55, Layer Interval A = 9
pm as in the above examples.
In addition, the layer intervals B, C, D, ... are
each separated from the shallower' layer interval by a
depth of focus (nA/NA2). For example, it suffices to set
Layer Interval B = 10 pm, Layer Interval C = 11 pm, and
Layer Interval D = 13 pm. Incidentally, it is appropriate
to set B + C # A + D.
[1-2: N-layer (2y-1 < N < 2Y) Recording, Medium]
The above-described examples are cases where the
number of recording layers is a power of two. In the
following, description will be made of recording media in
which the number N of recording layers is 2y-1 < N < 2Y.
To be short, in cases of 2Y-1 < N < 2Y, it suffices to
extract a structure for the number of layers from the 2Y
constitution.
The following is for cases where the number N of
recording layers is 2y-1 < N < 2Y (y is an integer of three
or more) . First, a constitution in which the above-
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SP248080
described number of recording layers is 2Y is assumed
provisionally. That is, a setting is made such that N' _
2Y, and an Mth layer interval is set between an (N'/2)th
recording layer and an {(N'/2) + 11th recording layer for
the number M of kinds of layer, intervals. A first
recording layer to the (N'/2)th recording layer and the
{(N'/2) + 11th recording layer, to an N'th recording layer
each have at least one AB block or more. Further, the
settings of respective layer intervals from the first
recording layer to the (N'/2)th recording layer are
identical to the settings of respective layer intervals
from the {(N'/2) + 11th recording layer to the N'th
recording layer. This corresponds to the constitutions of
8 layers, 16 layers, 32 layers, and the like described
above. At this time, a number N of recording layers are
formed with layer interval settings in a case where N
consecutive recording layers are extracted from the first
to N'th recording layers in the assumption in question.
To be short with a concrete example, when a 20-
layer recording medium is formed, for example, it
suffices to extract 20 layers from the constitution of
the 32-layer recording medium in FIG. 6.
FIG. 7, FIG. 8, and FIG. 9 each show an example of
an interlayer structure of a 20-layer recording medium.
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FIG. 7 represents an example of a 20-layer recording
medium in which the recording layers L12 to L31 in the
32-layer recording medium of FIG. 6 are set as recording
layers LO to L19. FIG. 8 represents an example of a 20-
layer recording medium in which the recording layers LO
to L19 in the 32-layer recording medium of FIG. 6 are set
as recording layers LO to L19., FIG. 7 represents an
example of a 20-layer recording medium in which the
recording layers L8 to L27 in the'32-layer recording
medium of FIG. 6 are set as recording layers LO to L19.
As described above, in the 32-layer recording
medium, for example, layer intervals are set such that at
a time of reproduction of each recording layer, stray
light components reflected by recording layers on a side
nearer to a laser incidence surface than the recording
layer are not focused on certain recording layers.
Therefore, even when any arbitrary set of 20 layers is
extracted from the constitution of the 32-layer recording
medium in which each layer interval is set as described
above, stray light components at a time of reproduction
are not focused on certain recording layers. Hence, in a
case of forming a 20-layer recording medium, for example,
when each layer interval is set as a constitution
extracted from the example of the 32-layer constitution
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as in the example of FIG. 7, FIG. 8, or FIG. 9, an
optical recording medium reducing the effect of crosstalk
can be realized, and the number of kinds of layer
intervals can be minimized.
FIG. 10 shows other examples. A case of a fivelayer
recording medium is as shown in FIG. 10(a).
Specifically, the recording layers L3 to L7 in the eightlayer
recording medium of FIG. 3 are set as recording
layers LO to L4. In a case of a s'ix-layer recording
medium, as shown in FIG. 10(b), for example, the
recording layers L2 to L7 in the eight-layer recording
medium of FIG. 3 are set as recording layers LO to L5. In
a case of a nine-layer recording medium, as shown in FIG.
10(c), the recording layers L7 to L15:in the 16-layer
recording medium of FIG. 5 are set as recording layers LO
to L8. In a case of a 12-layer recording medium, is in
FIG. 10(d), for example, the recording layers L4 to Ll5
in the 16-layer recording medium of FIG. 5 are set as
recording layers LO to Lll.
Similar settings are made in cases of other numbers
of layers. There are of course constitutions other than
the above illustrations. For example, in a case of a
five-layer recording medium, the recording layers LO to
L4 in the eight-layer recording medium of FIG. 3 may be
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set as recording layers LO to L4.
With such constitutions, even when the number N of
recording layers is 2y-1 < N < 2Y (y is an integer of three
or more), recording media effective in reducing an
interlayer crosstalk with a minimum of kinds of layer
intervals can be realized as in the above-described cases
where the number of recording layers is 2Y. Effects
including an improvement in efficiency of a manufacturing
process and the like are thereby obtained.
<2. Second Embodiment: Recording Device for Bulk Type
Optical Recording Medium>
[2-1: Structure of Optical Recording Medium]
A recording device for a bulk type optical
recording medium will next be described as a second
embodiment. A bulk type optical recording medium does not
have recording layers formed therein in advance, bit
recording layers are formed in the bulk type optical
recording medium for the first time by performing
information recording by a recording device. Hence, the
position of a recording layer is determined by setting a
focus position at a time of recording in the recording
device. Thus, when an interlayer crosstalk at a time of
reproduction is taken into consideration, the recording
device needs to set the position of the recording layer
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(layer intervals) properly.
A bulk recording type optical recording medium will
first be described. Bulk recording is a technique for
achieving a high recording capacity by irradiating an
optical recording medium having at least a cover layer 2
and a bulk layer 5 as shown in FIG. 11, for example, with
laser light while changing a focal position sequentially,
and thereby performing multilayer recording within the
bulk layer 5.
In relation to such bulk recording, a recording
technique referred to as a so-called microholographic
system is known. The microholographic system uses a socalled
holographic recording material as a recording
material for the bulk layer 5. A photopolymerizable
photopolymer, for example, is widely known as a
holographic recording material.
As shown in FIG. 12, the microholographic system is
a method of forming minute interference fringes
(hologram) by condensing two beams opposed to each other
(a beam A and a beam B) at a same position and setting
this as a recorded mark.
Further, the present applicant has proposed a void
recording (hole recording) system as another method for
bulk recording. The void recording system is a method of
41
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irradiating a bulk layer 5 formed of a recording material
such for example as a photopolymerizable photopolymer
with laser light at a relatively high power, and thereby
recording holes (voids) within the bulk layer 5.
The thus formed void parts have a different index
of refraction from other parts within the bulk layer 5,
and the reflectance of light is increased at boundary
parts between these parts. Hence, the above void parts
function as recorded marks. Thereby information recording
by the formation of void marks is realized.
Such a void recording system does not form a
hologram, and thus needs only light irradiation from one
side in recording. Specifically, a need to form recorded
marks by condensing two beams at a same position as in
the case of the positive type microholographic system can
be eliminated, and a need for high position control.
accuracy for condensing two beams at a same position can
be obviated.
In the following, an optical recording medium used
for the void recording system that performs light
irradiation from one side at a time of recording will be
taken as an example. However, the present embodiment to
be described in the following is not necessarily
applicable only to the void recording system, but is
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applicable to systems that perform information recording
in a bulk layer.
FIG. 13 is a sectional structural view of a bulk
recording medium 1 in which recording is performed by a
recording device according to the second embodiment. The
bulk recording medium 1 shown in FIG. 13 is an optical
recording medium having the shape of a disk. Mark
recording (information recording) is performed by
irradiating the bulk recording medium 1 being rotationdriven
with laser light. In addition, recorded
information is reproduced by irradiating the bulk
recording medium 1 being rotation-driven with laser light.
In FIG. 13, the bulk recording medium 1 has a cover
layer 2, a reference plane 3, and a bulk layer 5 formed
in order in a direction of thickness from an upper layer
side (side of a laser incidence surface) as shown i.n the
figure. Incidentally, words such as the "direction of
thickness" and a "direction of depth" are used in the
description of the present example. The "direction of
thickness" and the "direction of depth" refer to the
direction of thickness of the bulk recording medium 1 as
a direction parallel with a direction of incidence of
laser light.
In the bulk recording medium 1, the cover layer 2
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is formed of a resin such as polycarbonate or acrylic and
the reference plane 3 is formed on the lower surface side
of the cover layer 2 as shown in the figure. The
reference plane 3 is given a sectional shape of
depressions and projections attendant on the formation of
a guiding groove for guiding a recording/reproduction
position. As viewed in a direction of a flat surface of
the disk, the guiding groove is formed in a spiral shape.
A continuous groove (groove) or a pit string is formed as
the above guiding groove. When the guiding groove is a
groove, for example, the groove is formed in a state of
being periodically meandered (wobbling), so that address
information can be recorded by the periodic information
of the meander. Description in the following will be made
supposing that the reference plane 3 is formed as a
continuous groove (wobbling groove) having address
information recorded therein.
The cover layer 2 is formed by injection molding or
the like using a stamper on which a shape of depressions
and projections as such a meandering guiding groove
(wobbling groove) is formed. Thereby the shape of
depressions and projections is transferred to the lower
surface side of the cover layer 2. The reference plane 3
is formed by forming a selective reflecting film on the
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surface in the depression and projection shape of the
cover layer 2.
In this case, the recording system for the bulk
recording medium 1 applies servo light (hereinafter
second laser light) for obtaining a tracking error signal
and a focus error signal on the basis of the abovedescribed
reference plane 3 separately from recording
light (hereinafter first laser light) for mark recording
in the bulk layer 5. At this time, if the second laser
light reaches the bulk layer 5, the mark recording within
the bulk layer 5 may be adversely affected. Therefore, a
reflective film having selectivity to reflect the second
laser light and transmit the first laser light is
necessary. In the present example, the first laser light
and the second laser light have respective different
wavelengths, such for example as a wavelength of 405 nm
of the first laser light and a wavelength of 660 nm of
the second laser light. In order to make provision for
this, a selective reflecting film having wavelength
selectivity to reflect light in the same wavelength range
as the second laser light and transmit light of other
wavelengths is used as the above selective reflecting
film.
The bulk layer 5 is formed on the lower layer side
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of the reference plane 3 (deeper side as viewed from the
side of the laser incidence surface). It suffices to use
an appropriate material as a material for forming the
bulk layer 5 (recording material) according to the
recording system such as the microholographic system or
the void recording system. For example, in the case of
the void recording system, a plastic material is used.
In the bulk layer 5, information recording by mark
formation is performed by sequentially focusing laser
light on each position determined in advance in a
direction of depth of the bulk layer S. Hence, the bulk
recording medium 1 that has been recorded has a plurality
of recording layers L formed within the bulk layer 5. In
the example of the figure, as shown as recording layers
LO to L7, eight recording layers are formed. The
thickness size and the like of the bulk layer 5 are not
determined. However, when a case of applying blue laser
light (wavelength of 405 nm) with an optical system
having an NA of 0.85, for example, it is appropriate to
form recording layers at positions of 50 pm to 300 pm
from the surface of the disk (surface of the cover layer
2) in the direction of depth. This range is given with
spherical aberration correction taken into account.
In addition, marks are recorded in each recording
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layer L in a state of tracking servo control being
attained by using the groove formed in the reference
plane 3. Hence, a mark string formed in the recording
layer L is formed in a spiral shape as viewed in the
direction of a flat surface of the disk.
In this case, in the example of FIG. 13, depth
positions at which the recording layers LO to L7 are
formed are indicated by broken lines. As described above,
information recording by mark formation is performed by
sequentially focusing laser light on each position
determined in advance in the direction of depth of the
bulk layer 5. As a result, a recording layer is formed.
"Each position determined in advance" in this case is
each position determined according to a layer interval
configuration as described in the foregoing first
embodiment. Thereby recording layer positions determined
with crosstalk taken into consideration are realized.
In the case of FIG. 13, for example, eight layers
are taken as an example, which has three kinds of layer
intervals, that is, a first layer interval A, a second
layer interval B, and a third layer interval C. Then, a
layer interval configuration is as follows:
L0-Ll Layer Interval ... A
Ll-L2 Layer Interval ... B
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L2-L3 Layer Interval ... A
L3-L4 Layer Interval ... C
L4-L5 Layer Interval ... A
L5-L6 Layer Interval ... B
L6-L7 Layer Interval ... A
That is, a layer interval configuration similar to that
of the eight-layer recording medium of FIG. 3 is formed.
Incidentally, an interval from the reference plane 3 to
the recording layer LO is set as R.
[2-2: Servo Control]
Servo control at a time of recording/reproduction
on the bulk recording medium 1 will be described with
reference to FIG. 14. As described above, the bulk
recording medium 1 is irradiated with first laser light
for forming recorded marks and reproducing information
from the recorded marks and second laser light of a
different wavelength from that of the first laser light.
As will be described later with reference to FIG.
15, the bulk recording medium 1 is irradiated with the
first laser light and the second laser light via a common
objective lens (an objective lens 21 in FIG. 15).
In this case, as shown in FIG. 13, the bulk layer 5
in the bulk recording medium 1 does not include a
reflecting surface having a guiding groove formed by pits,
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S2248080
a groove, or the like in each layer position as an object
of recording, unlike multilayer disks in relation to
current optical disks such for example as DVDs and Bluray
Discs. Therefore, at a time of recording when no
marks are formed yet, a focus servo and a tracking servo
for the first laser light cannot be performed using the
reflected light of the first laser light itself. Thus, at
a time of recording on the bulk recording medium 1, the
tracking servo and the focus servo for the first laser
light are both performed using the reflected light of the
second laser light as servo light.
Specifically, first, a focus mechanism for the
first laser light which mechanism can change only the infocus
position of the first laser light independently (an
expander formed by lenses 17 and 18 and a lens driving
part 19 in FIG. 15) is provided for the focus servo for
the first laser light at a time of recording. Then, the
focus servo for the.first laser light is performed by
controlling the focus mechanism (expander) for the first
laser light on the basis of an offset of as shown in FIG.
2 with the reference plane 3 as a reference.
In this case, as described above, the first laser
light and the second laser light are applied to the
recording medium 1 via the common objective lens. Then,
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the focus servo for the second laser light is performed
by controlling the objective lens using the reflected
light (return light) of the second laser light from the
reference plane 3. When the first laser light and the
second laser light are thus applied via the common
objective lens, and the focus servo for the second laser
light is performed by controlling the objective lens on
the basis of the reflected light of the second laser
light from the reference plane 3,'the in-focus position
of the first laser light basically follows on the
reference plane 3. That is, in other words, such a focus
servo for the objective lens on the basis of the
reflected light of the second laser light from the
reference plane 3 provides a function of following
surface variations of the bulk recording medium 1 for the
in-focus position of the first laser light. Then, the
focus mechanism for the first laser light as described
above offsets the in-focus position of the first laser
light by the value of the above offset of. The in-focus
position of the first laser light can be thereby made to
follow a required depth position within the bulk layer 5.
FIG. 14 shows an example of each offset of
corresponding to a case where information recording
layers LO to L7 are set in the bulk layer 5. Specifically,
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a case of setting an offset of-LO corresponding to the
layer position of the recording layer LO, an offset of-L1
corresponding to the layer position of the recording
layer L1, ..., and an offset of-L7 corresponding to the
layer position of the recording layer L7 is shown. When
the focus mechanism for the first laser is driven using
the values of these offsets of, a position of formation
of marks (recording position) in the direction of depth
can be selected properly among the layer positions of the
recording layers LO to L7.
In addition, a tracking servo for the first laser
light at a time of recording is realized by performing a
tracking servo for the objective lens using the reflected
light of the second laser light from the reference plane
3 utilizing the fact that the first laser light and the
second laser light are applied via the common objective
lens as described above. Further, address information is
obtained. at a time of recording from the reflected light
information of the second laser light from the reference
plane 3 utilizing the fact that a wobbling groove having
address information recorded therein is formed in the
reference plane 3.
On the other hand, at a time of reproduction, the
recording layers L are formed in the bulk layer 5 as
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shown in FIG. 13, and therefore the reflected light of
the first laser light from such recording layers L can be
obtained. Thus, the focus servo for the first laser light
at a time of reproduction is performed using the
reflected light of the first laser light itself.
Specifically, the focus servo for the first laser light
at a time of reproduction is performed by controlling the
above-described focus mechanism for the first laser light
on the basis of the reflected light of the first laser
light. Incidentally, the tracking servo for the first
laser light at a time of reproduction is also realized by
performing the tracking servo for the objective lens on
the basis of the reflected light of the second laser
light.
In this case, also at a time of reproduction, a
focus servo and a tracking servo for the second laser
light targeted at the reference plane 3 are performed to
read the address information recorded in the reference
plane 3. That is, also at a time of reproduction, as at a
time of recording, the position of the objective lens is
controlled by realizing the focus servo and the tracking
servo for the second laser light targeted at the
reference plane 3 on the basis of the reflected light of
the second laser light.
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Summarizing the above, servo control is performed
as follows.
First Laser Light Side
At a time of recording, the focus servo is
performed by driving the common objective lens using the
reflected light of the second laser light and giving an
offset using the focus mechanism (expander) for the first
laser light. The tracking servo is performed
automatically because the objective lens is driven using
the reflected light of the second laser light. At a time
of reproduction, the focus servo is performed by driving
the focus mechanism (expander) for the first laser light
using the reflected light of the first laser light. The
tracking servo for the first laser light at a time of
reproduction is also performed automatically because the
objective lens is driven using the reflected light of the
second laser light.
Second Laser Light Side
Both at a time of recording and at a time of
reproduction, the focus servo and the tracking servo are
performed by driving the objective lens using the
reflected light of the second laser light.
Incidentally, at a time of reproduction, it is
possible not to use the second laser light after
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completing access to a predetermined address and actually
starting reproducing a recording layer L. That is, after
a recorded mark string is formed, the focus servo and the
tracking servo for the objective lens can be performed on
the'basis of reflected light of the first laser light
from the recording layer L, and addresses included in the
data recorded in the recorded mark string can be read.
[2-3: Recording and Reproducing Optical System]
FIG. 15 shows a configuration of a recording and
reproducing device 10 for performing recording and
reproduction on the bulk recording medium 1 of FIG. 13.
The bulk recording medium 1 loaded in the recording and
reproducing device 10 is rotation-driven by a spindle
motor 39 in the figure. Then, the recording and
reproducing device 10 has an optical pickup OP for
irradiating the thus rotation-driven recording medium 1
with first laser light and second laser light.
The optical pickup OP includes a first laser diode
11 as a light source of the first laser light for
recording information by forming recorded marks and
reproducing the information recorded by the recorded
marks and a second laser diode 12 as a light source of
the second laser light as servo light. In this case, the
first laser light and the second laser light have
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respective different wavelengths, as described above. In
the present example, the first laser light has a
wavelength of about 405 nm (so-called blue-violet laser
light), and the second laser light has a wavelength of
about 660 nm (red laser light).
The optical pickup OP also includes an objective
lens 21 as a terminal for the output of the first laser
light and the second laser light to the recording medium
1. The objective lens 21 has an NA of 0.85. Further, a
first photodetector 25 for receiving the reflected light
of the first laser light from the bulk recording medium 1
and a second photodetector 34 for receiving the reflected
light of the second laser light from the bulk recording
medium 1 are provided.
An optical system for guiding the first laser light
emitted from the first laser diode 11 to the objective
lens 21 and guiding the reflected light of the first
laser light from the bulk recording medium 1 which
reflected light enters the objective lens 21 to the first
photodetector 25 is formed within the optical pickup OP.
The path of the first laser light is indicated by a solid
line. The first laser light emitted from the first laser
diode 11 is first converted into collimated light via a
collimator 13, and then enters a polarization beam
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splitter 14. The polarization beam splitter 14 is formed
so as to transmit the first laser light made incident via
the collimator 13.
The first laser light transmitted by the
polarization beam splitter 14 passes through a liquid
crystal element 15 and a quarter-wave plate 16. The
liquid crystal element 15 is provided to correct socalled
off-axis aberration such for example as comatic
aberration and astigmatism.
The first laser light that has passed through the
quarter-wave plate 16 enters an expander composed of
lenses 17 and 18 and a lens driving part 19. This
expander has a fixed lens as the lens 17 and has a
movable lens as the lens 18. The lens driving part 19
drives the lens 18 in a direction parallel with the
optical axis of the first laser light. Thereby
independent focus control is performed on the first laser
light. The expander (lens driving part 19) offsets the
in-focus position of the first laser light on the basis
of an instruction from a controller 50 at a time of
recording, and performs focus control on the first laser
light on the basis of an output signal from a focus servo
circuit 36 for the first laser at a time of reproduction.
The first laser light passed through the expander
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enters a dichroic mirror 20. The dichroic mirror 20 is
formed so as to transmit light in the same wavelength
range as the first laser light and reflect light of other
wavelengths. Hence, the first laser light made incident
through the expander passes through the dichroic mirror
20.
The first laser light that has passed through the
dichroic mirror 20 is applied to the recording medium 1
via the objective lens 21. The objective lens 21 is
provided with an actuator 22 for retaining the objective
lens 21 so as to be able to displace the objective lens
21 in a focus direction (direction of approaching and
separating from the bulk recording medium 1) and a
tracking direction (direction orthogonal to the focus
direction: the direction of the radius of the bulk
recording medium 1) . The actuator 22 has a focus coil and
a tracking coil supplied with driving currents from a
focus servo circuit 42 for the second laser and a
tracking servo circuit 43, respectively. The actuator 22
thereby displaces the objective lens 21 in each of the
focus direction and the tracking direction.
At a time of reproduction, according to the
irradiation of the bulk recording medium 1 with the first
laser light as described above, the reflected light of
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the first laser light is obtained from the bulk recording
medium 1 (particularly a recording layer L as an object
of reproduction within the bulk layer 5) The thus
obtained reflected light of the first laser light is
guided to the dichroic mirror 20 via the objective lens
21, and transmitted by the dichroic mirror 20. The
reflected light of the first laser light which reflected
light has passed through the dichroic mirror 20 is passed
through the lens 17 and the lens 18 forming the expander,
and thereafter enters the polarization beam splitter 14
via the quarter-wave plate 16 and the liquid crystal
element 15.
In this case, due to the action of the quarter-wave
plate 16 and the action of the reflection by the bulk
recording medium 1, the reflected light (return light) of
the first laser light which reflected light thus enters
the polarization beam splitter 14 is different in
polarization direction by 90 degrees from the first laser
light (outward light) that entered the polarization beam
splitter 14 from the side of the first laser diode 11. As
a result, the reflected light of the first laser light
which reflected light entered as described above is
reflected by the polarization beam splitter 14.
The reflected light of the first laser light which
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reflected light is reflected by the polarization beam
splitter 14 is guided to the side of a condensing lens 23
in the figure. Then, the reflected light is condensed on
the detecting surface of the first photodetector 25 via
the condensing lens 23 and a cylindrical lens 24.
In addition, an optical system for guiding the
second laser light emitted from the second laser diode 12
to the objective lens 21 and guiding the reflected light
of the second laser light from the bulls recording medium
1 which reflected light enters the objective lens 21 to
the second photodetector 34 is formed within the optical
pickup OP. The path of the second laser light is
indicated by a broken line. As shown in the figure, the
second laser light emitted from the second laser diode 12
is converted into collimated light by a collimator 26,
and then enters a polarization beam splitter 27. The
polarization beam splitter 27 is formed so as to transmit
the second laser light (outward light) made incident via
the collimator 26.
The second laser light transmitted by the
polarization beam splitter 27 enters the dichroic mirror
20 via a quarter-wave plate 28 and lenses 29 and 30. As
described above, the dichroic mirror 20 is formed so as
to transmit light in the same wavelength range as the
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first laser light and reflect light of other wavelengths.
Hence, the second laser light is reflected by the
dichroic mirror 20, and applied to the bulk recording
medium 1 via the objective lens 21 as shown in the figure.
In addition, the reflected light of the second
laser light (reflected light from the reference plane 3)
which reflected light is obtained according to such
irradiation of the bulk recording medium 1 with the
second laser light passes through!the objective lens 21,
is reflected by the dichroic mirror 20, passes through
the lenses 30 and 29 and the quarter-wave plate 28, and
thereafter enters the polarization beam splitter 27. As
in the case of the above first laser light, due to the
action of the quarter-wave plate 28 and the action of the
reflection by the bulk recording medium 1, the reflected
light (return light) of the second laser light which
reflected light enters from the side of the bulk
recording medium 1 is different in polarization direction
by 90 degrees from the outward light. Hence, the
reflected light of the second laser light as the return
light is reflected by the polarization beam splitter 27.
Then, the reflected light of the second laser light which
reflected light is reflected is reflected by a mirror 31,
and condensed on the detecting surface of the second
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photodetector 34 via a condensing lens 29 and a
cylindrical lens 33.
Though not shown, the recording and reproducing
device 10 has a slide driving section for slide-driving
the whole of the optical pickup OP in the tracking
direction. By driving the optical pickup OP, the slide
driving section can displace a laser light irradiation
position over a wide range.
The recording and reproducing device 10 also
includes a recording processing section 40, a matrix
circuit 35 for the first laser, a focus servo circuit 36
for the first laser, a reproduction processing section 38,
a matrix circuit 41 for the second laser, a focus servo
circuit 42 for the second laser, a tracking servo circuit
43, an address decoder 44, and a controller 50.
First, data to be recorded on the bulk recording
medium 1 (recording data) is input to the recording
processing section 40. The recording processing section
40 adds an error correcting code to the input recording
data and applies predetermined recording modulation
coding to the input recording data. The recording
processing section 40 thereby obtains a recording
modulated data string as a binary data string of "Os" and
"ls" to be actually recorded on the bulk recording medium
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1. Further, a write strategy is performed on the basis of
the recording modulated data string, and a laser driving
signal is generated. Then, the laser driving signal is
supplied to the first laser diode 11 to drive the first
laser diode 11 for light emission . The recording
processing section 40 performs such processing according
to an instruction (control signal CNT) from the
controller 50.
The matrix circuit 35 for the first laser includes
a current-to-voltage converting circuit, a matrix
operation/amplifying circuit, and the like so as to
correspond to output currents from a plurality of light
receiving elements as the first photodetector 25. The
matrix circuit 35 for the first laser generates a
necessary signal by matrix operation processing.
Specifically, a high-frequency signal (reproduced signal
RF) corresponding to a reproduced signal, a focus error
signal FE for focus servo control, and the like are
generated. In the present example, there are two kinds of
focus error signals FE based on the reflected light of
the first laser light and based on the reflected light of
the second laser light. In order to distinguish the two
kinds of focus error signals FE from each other, the
focus error signal FE generated in the matrix circuit 32
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for the first laser will hereinafter be referred to as a
focus error signal FE-l.
The reproduced signal RF generated in the matrix
circuit 35 for the first laser is supplied to the
reproduction processing section 38. In addition, the
focus error signal FE-1 is supplied to the focus servo
circuit 37 for the first laser.
The reproduction processing section 38 subjects the
reproduced signal RF generated in'the matrix circuit 35
for the first laser to reproduction processing for
reconstructing the recorded data described above, such as
binarization processing, decoding for recording
modulation code, and error correction processing, and
obtains reproduced data as a result of reproducing the
recorded data. In addition, at a time of reproduction,
address information is extracted from the reproduced data.
The address information is supplied to the controller 50.
In addition, the focus servo circuit 36 for the
first laser generates a focus servo signal based on the
focus error signal FE-l, and driving-controls the lens
driving part 19 on the basis of the focus servo signal.
The focus servo circuit 36 for the first laser thereby
performs focus servo control for the first laser light.
At a time of recording, the focus servo circuit 36 for
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the first laser drives the lens driving part 19 on the
basis of a predetermined offset of (see FIG. 14)
according to an instruction (control signal CNT) given
from the controller 50. In addition, at a time of
reproduction, the focus servo circuit 36 for the first
laser performs focus servo control for the first laser
light by driving the lens driving part 19 on the basis of
the reflected light of the first laser light. Further,
according to an instruction (control signal CNT) given
from the controller. 50 at a time of reproduction, the
focus servo circuit 36 for the first laser drivingcontrols
the lens driving part 19 such that interlayer
jump operation between recording layers L formed in the
bulk recording medium 1 and a focus servo pull-in to a
required information recording surface L are performed.
Meanwhile, as for the side of the second laser
light, the matrix circuit 41 for the second laser
includes a current-to-voltage converting circuit, a
matrix operation/amplifying circuit, and the like so as
to correspond to output currents from a plurality of
light receiving elements as the second photodetector 34.
The matrix circuit 41 for the second laser generates a
necessary signal by matrix operation processing.
Specifically, the matrix circuit 41 for the second laser
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generates a focus error signal FE-2 and a tracking error
signal TE for servo control and a push-pull signal P/P
for address extraction. The focus error signal FE-2 is
supplied to the focus servo circuit 42 for the second
laser. In addition, the tracking error signal TE is
supplied to the tracking servo circuit 43. The push-pull
signal P/P is supplied to the address decoder 44.
The focus servo circuit 42 for the second laser
generates a focus servo signal based on the focus error
signal FE-2, and drives the focus coil of the actuator 22
on the basis of the focus servo signal. The focus servo
circuit 42 for the second laser thereby performs focus
servo control on the objective lens 21. As described
earlier, the focus servo control on the objective lens 21
is performed on the basis of the reflected light of the
second laser light both at a time of recording and at a
time of reproduction. The focus servo circuit 42 for the
second laser drives the focus coil according to a control
signal CNT from the controller 50 such that a focus servo
pull-in to the reference plane 3 formed in the bulk
recording medium 1 is performed. Incidentally, an
astigmatism method, an SSD (Spot Size Detection) method,
a differential astigmatism method, or the like can be
used for the focus servo control.
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The tracking servo circuit 43 generates a tracking
servo signal based on the tracking error signal TE from
the matrix circuit 41 for the second laser, and drives
the tracking coil of the actuator 22 on the basis of the
tracking servo signal. As described above, tracking servo
control on the objective lens 21 is performed on the
basis of the reflected light of the second laser light
both at a time of recording and at a time of reproduction.
Incidentally, a PP (Push-Pull) method, a DPP
(Differential Push-Pull) method, or the like can be used
for the tracking servo control.
The address decoder 44 decodes address information
from the input push-pull signal P/P. Because the second
laser light is focused on the reference plane 3, the
reflected light of the second laser light has the
information of the wobbling groove formed in the
reference plane 3. The matrix circuit 41 for the second
laser supplies the address decoder 44 with the
information of the wobbling groove as the push-pull
signal P/P. The address decoder 44 decodes the address
information from the push-pull signal P/P, and supplies
the address information to the controller 50.
The controller 50 is formed by a microcomputer
including a CPU (Central Processing Unit), for example. A
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memory section 51 includes a RAM, a ROM, a flash memory,
and the like. The memory section 51 represents areas for
storing information used for various kinds of processing
by the controller 50. The controller 50 performs
processing according to a program stored in the memory
section 51, for example. Then, control signals CNT are
supplied to various required parts to perform overall
control of the recording and reproducing device 10. At a
time of recording, the controller'50 controls the infocus
position of the first laser light (selects a
recording position in the direction of depth) on the
basis of the value of the offset of set so as to
correspond to each layer position as described with
reference to FIG. 14. That is, the controller 50 selects
a recording position in the direction of depth by
instructing the focus servo circuit 36 for the first
laser to drive the lens driving part 19 on the basis of
the value of an offset of set so as to. correspond to a
layer position as an object of recording. The value of
the offset of is stored in the ROM, the flash memory, or
the like within the memory section 51. The positions of
the respective recording layers LO to L(n) in the bulk
recording medium 1 are set by setting the values of the
offsets of-LO to of-L(n).
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As described above, tracking servo control at a
time of recording is performed on the basis of the
reflected light of the second laser light. Therefore the
controller 50 at the time of recording instructs the
tracking servo circuit 43 to perform tracking servo
control based on the tracking error signal TE. In
addition, the controller 50 at the time of recording
instructs the focus servo circuit 42 for the second laser
to perform focus servo control based on the focus error
signal FE-2 (focus servo control on the objective lens
21).
On the other hand, at a time of reproduction, the
controller 50 instructs the focus servo circuit 36 for
the first laser to focus the first laser light on a
recording layer L in which data to be reproduced is
recorded. That is, focus servo control targeted at the
recording layer L is made to be performed for the first
laser light. In addition, the controller 50 at the time
of reproduction also makes the tracking servo circuit 43
perform tracking servo control based on the tracking
error signal TE. In addition, the controller 50 at the
time of reproduction makes the focus servo circuit 42 for
the second laser perform focus servo control based on the
focus error signal FE-2 (focus servo control on the
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objective lens 21).
[2-4: Recording Process]
FIG. 16 shows a process at a time of recording by
such a recording and reproducing device 10. In step F101,
the controller 50 determines a recording layer as an
object of recording. In the case of eight layers as in
FIG. 13, for example, one recording layer among the
recording layers LO to L7 is selected.
Next, in step F102, the controller 50 determines an
offset value of-L(x) corresponding to the recording layer
L(x) in which to perform recording. When the values of
the offsets of-LO to of-L(7) are stored in the memory
section 51 as described above, for example, an offset
value corresponding to the target recording layer is read.
In step F103, the controller 50 makes focus control
and tracking control performed for the second laser light
on the reference plane 3. Specifically, the focus servo
circuit 42 for the second laser is instructed to drive
the actuator 22 to position the objective lens 21 such
that the in-focus position of the second laser light is
on the reference plane 3. Further, after the focus
control, the tracking servo by the tracking servo circuit
43 is turned on so that tracking control is performed.
After performing the focus and tracking control by
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the second laser light, the controller 50 in next step
F104 focus-controls the first laser light onto the
recording layer as an object of recording. Specifically,
the offset value of-L(x) determined in step F102 is given
to the focus servo circuit 36 for the first laser to
drive the expander (lens driving part 19) . Thereby, the
first laser light is focused at a depth position at which
to form the recording layer L(x) set as an object of
recording this time. In this state, recording operation
is started in step F105. Specifically, a laser driving
signal based on recording data is output from the
recording processing section 40, and the first laser
light modulated by the recording data as recording power
is output from the first laser diode. Thereby recording
on the recording layer L(x) (and the formation of the
recording layer) is performed.
In this case, the above-described offset values are
set as follows. Incidentally, "R" is a distance from the
reference plane 3 to the recording layer LO as shown in
FIG. 13.
offset value of-LO ... R
offset value of-L1 ... R + A
offset value of-L2 ... R + A + B
offset value of-L3 ... R + A + B + A
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offset value of-L4 ... R + A + B + A + C
offset value of-L5 ... R + A + B + A + C + A
offset value of-L6 ... R + A + B + A + C + A + B
offset value of-L7 ... R + A + B + A + C + A + B +
A
With such settings, the recording layers LO to L7
are formed at layer intervals as in FIG. 13 as an eightlayer
recording medium, for example. That is, the bulk
recording medium of a similar layer interval
configuration to that of FIG. 3 described in the first
embodiment is formed by the recording operation. Then, it
is needless to say that stray light is not focused on a
certain recording layer at a time of reproduction on such
a bulk recording medium. Thus the effect of an interlayer
crosstalk can be reduced or eliminated. Further, the
kinds of layer intervals can be three kinds (A, B, and C)
in the case of eight layers. Therefore, offset values can
be set easily.
Without being limited to eight layers, of course,
similar layer interval configurations to those of the
foregoing first embodiment can be adopted when four or
more recording layers are formed. Even when the number of
layers is greatly increased as in cases of 20 layers, 32
layers, or the like in particular, offset values are set
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easily because the kinds of layer intervals are a minimum.
Further, even when the recording and reproducing
device 10 does not store offset values, a process of
determining an offset value according to a recording
layer as an object of recording or reproduction by a
simple calculation is also made possible because of a
regular layer interval configuration based on an AB block
as in the 32-layer recording medium described above.
In addition, when a fact that no recording layers
are formed in the bulk recording medium 1 in a state of a
so-called virgin disk is taken into consideration, a mode
of use in which the number of layers is determined at a
time of recording is also assumed. In such a case, when a
regular layer interval configuration with a minimum of
kinds of layer intervals as described in the first
embodiment is adopted, the recording and reproducing
device 10 can easily set appropriate offset values.
Explanation of Reference Symbols
1 Bulk recording medium
2, 11 Cover layer
3 Reference plane
5 Bulk layer
10 Recording and reproducing device
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12 Substrate
21 Objective lens
22 Actuator
37 Tilt servo circuit
LO, Ll, L2 Recording layer
SP1, SP2, SP3 Spacer layer
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Claims
1. An optical recording medium comprising:
N recording layers (where N ? 4);
a number of ]rinds of layer intervals between the
recording layers adjacent to each other being an integer
M equal to or less than a minimum integer equal to or
more than 1092 (N),; and
one or more blocks including four recording layers
with a first layer interval, a second layer interval, and
said first layer interval formed in order in the one or
more blocks.
2. The optical recording medium according to
claim 1, wherein
a number of the recording layers is such that said
M as the number of kinds of layer intervals is M ? 3, and
the optical recording medium has each of parts of
respective layer intervals from a third layer interval to
an Mth layer interval as a layer interval between a
recording layer forming said block and a recording layer
adjacent to said recording layer, the adjacent recording
layer being included in other than the block including
said recording layer.
3. The optical recording medium according to
claim 2, wherein
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said first layer interval < said second layer
interval < the third layer interval < ... < the Mth layer
interval.
4. The optical recording medium according to
claim 2, wherein
the optical recording medium has five to eight
recording layers,, and said M = 3, and
the optical recording medium has a part of a third
layer interval as a layer interval between a recording
layer forming said block and a recording layer adjacent
to said recording layer, the adjacent recording layer
being included in other than the block including said
recording layer.
5. The optical recording medium according to
claim 2, wherein
the optical recording medium has 9 to 16 recording
layers, and said M = 4, and
the optical recording medium has each of a part of
a third layer interval and a part of a fourth layer
interval as a layer interval between a recording layer
forming said block and a recording layer adjacent to said
recording layer, the adjacent recording layer being
included in other than the block including said recording
layer.
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6. The optical recording medium according to
claim 2, wherein
the optical recording medium has 17 to 32 recording
layers, and said M = 5, and
the optical recording medium has each of a part of
a third layer interval, a part of a fourth layer interval,
and a part of a fifth layer interval as a layer interval
between a recording layer forming said block and a
recording layer adjacent to said recording layer, the
adjacent recording layer being included in other than the
block including said recording layer.
7. The optical recording medium according to
claim 2, wherein
when the number of the recording layers is N = 2Y
(y is an integer of three or more),
said Mth layer interval is set between an (N/2)th
recording layer and an {(N/2) + 11th recording layer,
a first recording layer to the (N/2)th recording
layer and the {(N/2) + 1}th recording layer to an Nth
recording layer each have at least one or more said
blocks, and
settings of respective layer intervals from the
first recording layer to the (N/2)th recording layer are
identical to settings of respective layer intervals from
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the {(N/2) + l}th recording layer to the Nth recording
layer.
8. The optical recording medium according to
claim 2, wherein
when the number N of recording layers is 2y-1 < N <
2Y (y is an integer of three or more),
the N recording layers are formed with layer
interval settings in a case of extracting N consecutive
recording layers from a first recording layer to an N'th
recording layer when assuming that N' = 2Y, that said Mth
layer interval is set between an (N'/2)th recording layer
and an {(N'/2) + l}th recording layer for the number M of
kinds of layer intervals, that the first recording layer
to the (N'/2)th recording layer and the {(N'/2) + 11th
recording layer to the N'th recording layer each have at
least one or more said blocks, and that settings of
respective layer intervals from the first recording layer
to the (N'/2)th recording layer are identical to settings
.of respective layer intervals from the {(N'/2) + l}th
recording layer to the N'th recording layer.
9. The optical recording medium according to
claim 1, wherein
a minimum layer interval among the first layer
interval to an Mth layer interval is
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12.4•A/NA/tan(asin(NA/n))
where h is a wavelength of reproduction light on
the optical recording medium, an NA is a numerical
aperture of a reproducing optical system, and n is an
index of refraction between recording layers.
10. A recording device comprising:
an optical pickup for irradiating, with laser light,
an optical recording medium having a bulk layer in which
to record optical recording information by irradiation
with laser light and form a plurality of recording layers
in which the optical recording information is recorded;
a focus control section configured to control a
focal position of the laser light within said bulk layer;
and
a control section configured to give an instruction
on the focal position to said focus control section so
that each recording layer is formed by recording
operation with said laser light such that N recording
layers (where N ? 4) are formed within said bulk layer, a
number of kinds of layer intervals between the recording
layers adjacent to each other is an integer M equal to or
less than a minimum integer equal to or more than log2(N),
and the bulk layer has one or more blocks including four
recording layers with a first layer interval, a second
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layer interval, and said first layer interval formed in
order in the one or more blocks.
11. A recording method of a recording device, the
recording device including an optical pickup for
irradiating, with laser light, an optical recording
medium having a bulk layer in which to record optical
recording information by irradiation with laser light and
form a plurality of recording layers in which the optical
recording information is recorded,^and a focus control
section configured to control a focal position of the
laser light within said bulk layer, the recording method
comprising:
performing recording operation with said laser
light after said focus control section controls the focal
position according to each recording layer, so as to form
each recording layer such that N recording layers (where
N ? 4) are formed within said bulk layer, a number of
kinds of layer intervals between the recording layers
adjacent to each other is-an integer M equal to or less
than -a minimum integer equal to or more than log2(N), and
the bulk layer has one or more blocks including four
recording layers with a first layer interval, a second
layer interval, and said first layer interval formed in
order in the one or more blocks.