U.S. patent application number 12/182460 was filed with the patent office on 2009-03-19 for three-dimensional recording and reproducing apparatus.
Invention is credited to Takeshi Maeda, Kiyoshi Matsumoto, Harukazu Miyamoto, Tetsuya Nishida, Shigenori Okamine, Hisataka Sugiyama, Motoyasu Terao.
Application Number | 20090075013 12/182460 |
Document ID | / |
Family ID | 17391261 |
Filed Date | 2009-03-19 |
United States Patent
Application |
20090075013 |
Kind Code |
A1 |
Sugiyama; Hisataka ; et
al. |
March 19, 2009 |
THREE-DIMENSIONAL RECORDING AND REPRODUCING APPARATUS
Abstract
A recording medium including a plurality of recording layers,
including: an optional first recording layer on which a light spot
at a diffraction limit is formed; and a second recording layer on
which a mark string pattern is formed, said second recording layer
being different from said first recording layer, wherein when said
mark string pattern is formed on a light receiving plane, while
information of said first recording layer is reproduced, assuming
that an optical distance between said first and second recording
layers is dm, an optical distance d between optional two recording
layers among a plurality of said recording layers is different from
said dm.
Inventors: |
Sugiyama; Hisataka;
(Kodaira-shi, JP) ; Maeda; Takeshi;
(Kokubunji-shi, JP) ; Matsumoto; Kiyoshi;
(Kokubunji-shi, JP) ; Terao; Motoyasu; (Tokyo,
JP) ; Okamine; Shigenori; (Kodaira-shi, JP) ;
Nishida; Tetsuya; (Hachioji-shi, JP) ; Miyamoto;
Harukazu; (Kodaira-shi, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
17391261 |
Appl. No.: |
12/182460 |
Filed: |
July 30, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11781491 |
Jul 23, 2007 |
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12182460 |
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08427866 |
Apr 26, 1995 |
7286153 |
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11781491 |
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Current U.S.
Class: |
428/64.4 |
Current CPC
Class: |
G11B 7/004 20130101;
G11B 7/1372 20130101; G11B 7/14 20130101; G11B 7/24038 20130101;
G11B 7/24 20130101 |
Class at
Publication: |
428/64.4 |
International
Class: |
B32B 3/02 20060101
B32B003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 11, 1991 |
JP |
03-263561 |
Claims
1. (canceled)
2. An optical disc having a plurality of recording layers to which
information is recorded as mark pattern formed by an irradiation of
a light spot that is emitted from an optical head employing a light
source, an optical detector, an optical system for imaging
reflected light from the optical disc to a light receiving plane of
the optical detector, comprising: a substrate; a plurality of
recording layers formed on the substrate with an intermediate layer
therebetween; wherein an optical spot at a diffraction limit is
formed on a target recording layer among said plurality of
recording layers at a time of recording information; and a distance
between said target recording layer and a neighboring recording
layer is designed to not include a distance of which a mark pattern
in a recording layer different to the target recording layer is
imaged to the light receiving plane of the optical detector.
3. An optical disc having a plurality of recording layers to which
information is recorded as a mark pattern formed by an irradiation
of a light spot that is emitted from an optical head employing a
light source, an optical detector, an optical system for imaging
reflected light from the optical disc to a light receiving plane of
the optical detector, comprising: a substrate; a plurality of
recording layers formed on the substrate with an intermediate layer
therebetween; wherein an optical distance d between one of the
plurality of recording layers and its neighboring recording layer
is different to dm, where dm is an optical distance that a mark
pattern in recording layers different to said one of the recording
layers is imaged to the light receiving plane of the optical
detector.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a continuation of U.S. application Ser. No.
11/781,491, filed Jul. 23, 2007, which is a continuation of U.S.
application Ser. No. 08/427,866, filed Apr. 26, 1995, which relates
to U.S. application Ser. No. 08/464,461, filed Jun. 5, 1995 (now
U.S. Pat. No. 5,614,938) and U.S. application Ser. No. 07/959,162,
filed Oct. 8, 1992 (now U.S. Pat. No. 5,414,451). This application
relates to and claims priority from Japanese Patent Application No.
03-263561, filed on Oct. 11, 1991. The entirety of the contents and
subject matter of all of the above is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to an optical information
recording/reproducing apparatus, and more particularly to an
information recording/reproducing apparatus for achieving a high
recording density.
[0003] A method for increasing the recording density for an optical
information recording/reproducing apparatus has conventionally been
accomplished by improving a recording plane density on a
two-dimensional plane of a recording medium. However, since the
size of information recording media, such as a disc, is restricted
due to compactization of the apparatus, two-dimensional approach to
high density will reach its limit. As a method of achieving a
higher density, there has been proposed a three-dimensional
recording/reproducing method which is related with the record in
the depth direction of a recording medium.
[0004] For example, the JP-A-59-127237 discloses that information
is recorded on two recording layers by using two lights of
different wavelengths. In this event, when a light irradiates a
recording layer on the incident plane side through the other
recording layer, energy of the light is absorbed in the recording
layer on the incident plane side, whereby information is
unintentionally recorded thereon. Therefore, in JP-A-59-127237,
writing on the other recording layer is only allowed after
information has been recorded on the recording layer on the
incident side. More specifically, following three-value
informations can be recorded for the states where information is
recorded on both two recording layers; no information is recorded
on both recording layers; and information is recorded only on the
recording layer on the incident plane side.
[0005] However, the JP-A-59-127237 does not disclose a method of
independently recording binary information on each of multi-layer
recording film.
[0006] Also, the JP-A-60-202545 discloses a method of focusing a
laser beam on each layer of the multi-layer recording films.
Generally, a focusing servo circuit focuses a laser beam on a
recording film by supplying an electric offset when the beam is out
of focus. By utilizing this method, offset voltages corresponding
to respective inter-layer gaps in the multi-layer recording film
have previously been prepared. Then, one of the offset voltages
corresponding to a layer to be focused is supplied to focus the
laser beam on that layer.
[0007] However, the JP-A-60-202545 does not disclose any means for
corresponding the multi-layer recording film to the offset
voltages.
[0008] Also, the JP-A-60-202554 discloses a multi-layer recording
medium in which inter-layer gaps are formed equal or larger than an
operating range of a focus error signal by more than the same
operating range. However, the JP-A-60-202554 does not describe
specific inter-layer gaps and an access method of actually focusing
a beam on a target layer.
SUMMARY OF THE INVENTION
[0009] The foregoing conventional documents disclose performing
multiple-value recording and multiplex recording by using a
multi-layer recording film. However, in order to practically
perform multiplex recording or reproduction, optical systems
including a focus control or a track control must be investigated
on the influence of reflection or absorption of light on each layer
of the multi-layer film.
[0010] It is an object of the present invention to provide a
three-dimensional recording and reproducing apparatus including an
optical system capable of stably recording and reproducing
information by the use of a recording medium comprising a multiple
recording layer.
[0011] It is another object of the present invention to provide a
signal control method suitable for use in the recording medium
comprising a multiple recording layer, particularly, a coding
system for suppressing cross-talk between adjacent layers, and a
cross-talk canceling system.
[0012] It is a further object of the present invention to provide a
structure of a recording medium suitable for multiplex-recording, a
three-dimensional data format and a medium producing method.
[0013] The above objects are achieved by the following means.
[0014] In a disc comprising a plurality of recording film layers on
which optical properties are locally changed by irradiating locally
with a light and intermediate layers each composed of an assistant
layer for the operation of the recording layer (a layer provided
for the purpose of reflection protection, multiple reflection,
light absorption, transfer of changes in the local optical
properties of the recording layer, heat insulation, heat
absorption, heat generation or reinforcement) or a stack of
assistant layers, each local optical property of the recording
layers is individually and two-dimensionally changed by irradiating
with a light focused on each recording layer, thereby performing
recording corresponding to modulated data "1" and "0."
[0015] Further, in a three-dimensional recording and reproducing
apparatus for detecting changes of the local optical properties as
changes in a reflected light amount (or a transmitting light
amount) of a light spot irradiated to each assistant layer and
reproducing data based on the detected change, the structure of the
disc is determined as follows:
[0016] The refractivity and thickness of the optically transparent
substrate are represented by NB and d0, respectively. An
intermediate layer and a recording layer are collected as a single
layer, and first to N.sub.th layers are designated sequentially
from the top layer. A distance between the centers of adjacent
k.sub.th and (k-1).sub.th recording film layers is represented by
dk. The thicknesses of an arbitrary k.sub.th recording layer and
intermediate layer are represented by dFk and dMk, respectively,
and the real parts of the refractivities of the same are
represented by NFk and NMk, respectively. A cycle of changes of the
local optical properties on the plane of each layer is represented
by b [.mu.m]. A focusing optical system employs, for example, a
semiconductor laser emitting a light of a wavelength .lamda.
[.mu.m] as a light source. The emitted light is converted to a
parallel light by a collimator lens and incident to the focus lens
through a polarization beam splitter. Here, the numerical aperture,
effective radius and focal length of the focus lens are represented
by NAF, a [mm] and fF (.apprxeq.a/NAF), respectively. The light
reflected from the disc passes through the focus lens and is
introduced to a light receiving image lens by a beam splitter. A
change of the reflected light amount is converted to an electric
signal by a photo detector positioned near the focal point of the
image lens. The numerical aperture and focal length of the image
lens are represented by NAI and fI (.apprxeq.a/NAI), respectively.
Assuming that the diameter of a light receiving plane of the
optical detector is represented by D, a light focused on a k.sub.th
layer as a target layer reflected from the target layer is imaged
on the focal point of the image lens, and a spot diameter Uk' on
the focal plane is given by:
Uk'=.lamda./NAI=.lamda..times.(fI/a).
[0017] Next, a spot diameter U(k.+-.1)' on the focal plane from the
(k.+-.1).sub.th layer spaced from the k.sub.th target layer by the
inter-layer distance d is given by:
U ( k .+-. 1 ) ' .apprxeq. a .times. m 2 d / fI = NAI m 2 d
##EQU00001##
where m is a horizontal scaling ratio of the receiving optical
system.
[0018] From the above equation, assuming that the diameter D of the
photo detector is D=Uk'=.lamda./NAI, a detected amount In of light
reflected from other layers is given by:
1 / 10 .gtoreq. .SIGMA. Ij ( n = 1 to N , n k ) / I k = .SIGMA. [
.delta. 2 jk .times. .alpha. jk .times. ( D / U j i ) 2 ] .apprxeq.
I ( k - 1 ) / Ik = .delta. 2 ( k - 1 ) , k .times. .alpha. ( k - 1
) , k .times. ( D / U ( k - 1 ) ' ) 2 ##EQU00002##
where .delta.jk represents the transmissivity of layers between the
target k.sub.th layer and another j.sub.th layer, and .alpha.jk
represents a reflectivity ratio.
[0019] The disc structure and optical systems are designed so as to
satisfy the above equation.
[0020] Further, a minimum value bmin of the two-dimensional cycle b
is set to .lamda./NAF, and a maximum value bmax of the same is set
to be smaller than 2d.times.NAF.
[0021] In a light receiving optical system shown in FIG. 1, optical
property functions H0(S), H1(S) of a target layer plane on which
recording/reproduction is performed and an adjacent layer plane
spaced therefrom by a distance are indicated by straight lines 13
and 14, respectively, in FIG. 4. S represents a normalized spatial
frequency.
[0022] Now, as to the optical property function H1(S) for a case
where out-of-focus occurs at the inter-layer distance d, a maximum
repetition bmax of the cycle b is defined from S=2 where H1(S)=0 is
satisfied. By thus defining the relationship among the cycle b of
changes in the local optical properties on the layer plane, the
disc structure and the light receiving optical system, inter-layer
cross-talk components are made larger than the cycle b of changes
in the local optical properties.
[0023] Further, a code which defines that a total area occupied by
local optical changes (marks) included in the area defined by the
spot diameter (2d.times.NAF) on an adjacent layer is constant is
employed.
[0024] Further,
dk=dF(k-1)+dMk+dFk.apprxeq.dMk (Equation 1)
and the effective refractivity NMk of the intermediate layer is
assumed to be equal to the refractivity NB of the substrate.
[0025] In a disc structure where a thickness d up to an N.sub.th
layer of a multi-layer disc is given by the following equation:
d.apprxeq..SIGMA.dk+d0 (Equation 2)
a thickness dk of the intermediate layer of each layer and the
total number N are combined so as to satisfy a spherical aberration
amount W40 which is given by:
W40=1/(8.times.NB).times.(1/NB.sup.2-1).times.NAF.sup.4.times..DELTA.d.l-
toreq..lamda./4
0.5.times..SIGMA.dk=.DELTA.d (Equation 4)
[0026] Optical constants of the k.sub.th recording layer 1, i.e.,
the transmissivity, reflectivity and absorption ratio are
represented by Tk, Rk and Ak, respectively. Here, the relationship
Tk+Rk+Ak=1 is satisfied. The optical constants, when the local
optical properties are changed by recording, are indicated by
adding a dash "'" thereto. Generally, in thermal recording, to
cause a change in thermal structure, an energy threshold value Eth
[nJ] must exist. A light spot focused to the refractory limit on a
target recording layer is scanning on the disc at a linear velocity
V [m/s].
[0027] To locally cause a change in thermal structure corresponding
to a modulated binary signal, a light intensity P (recording power)
[mW] of the light incident to the disc should be defined. Here,
given a linear velocity V and an irradiation time t, a light
intensity density threshold value is represented by Ith
[mW/.mu.m.sup.2].
[0028] For a light intensity density Ik on a k.sub.th layer when
the focus is placed on the k.sub.th layer, a 1/e.sup.2 spot area Sk
when the focus is placed on the k.sub.th layer is given by:
Sk=.pi.(0.5.times..lamda./2NAF).sup.2
[0029] A light intensity Pk [mW] on the k.sub.th layer is given
by:
Pk=P.delta.k
.delta.k=.PI.Tn (Equation 6)
where .delta.k represents the transmissivity of layers between the
light incident plane of the disc and the k.sub.th recording layer,
and Tn represents the transmissivity of n layers.
[0030] From Equation 6, a minimum recording power Pmin required to
enable recording on the k.sub.th layer is expressed by:
Pmin.gtoreq.Ik.sub.th.times.Sk/.delta.k (Equation 7)
[0031] Also, a light intensity density Ijk [mW] on a j.sub.th layer
when the focus is placed on the k.sub.th layer for recording
thereon is:
Pjk = Pk .times. .delta. jk = P .times. .delta. j .delta. jk = .PI.
/ .PI. ( = ( transmissivity up to j th layer / transmissivity up to
k th layer ) ) ( Equation 9 ) ##EQU00003##
[0032] An upper limit Pmax of the recording power for recording on
a k.sub.th layer without destroying data recorded on a j.sub.th
layer is given by the following equation:
Pmax=Ij.sub.th.times.Sjk/.delta.j (Equation 10)
where Sjk represents the diameter of a light spot on the j.sub.th
layer when the focus is placed on the k.sub.th layer,
Sjk = .pi. [ ( .SIGMA. dn ) .times. TAN .phi. ] 2 ( when j > k )
= .pi. [ ( .SIGMA. dn ) .times. TAN .phi. ] 2 ( when j < k ) =
.pi. [ ( .SIGMA. dn ) .times. NAF ] 2 ( Equation 11 )
##EQU00004##
where dn represents a thickness of an n.sub.th layer.
TAN .phi.=a/fF.apprxeq.NAF
[0033] The focusing optics, disc structure and recording conditions
are defined so as to simultaneously satisfy Equations 6, 7, 9, 10
and 11.
[0034] As a role of each layer, a disc is provided with a ROM (Read
Only Memory) layer or a WOM (Write Once Memory) layer together with
layers for recording and reproducing user data.
[0035] The ROM or WOM layer may be used as a management layer, and
data conditions of each layer, for example, the presence or absence
of data, error management, an effective data area, the frequency of
overwrite are recorded thereon at any time.
[0036] Also, it may be used as a spare layer such that information
is recorded thereon in place of a layer from which a recording
error has been detected.
[0037] As a management format on each layer plane of the disc,
sectors and tracks are provided, and recording is performed
sequentially from the top layer, i.e.,
1.sub.st.fwdarw.k.sub.th.fwdarw.N.sub.th layers or from the
lowermost layer, i.e., N.sub.th.fwdarw.k.sub.th.fwdarw.1.sub.st
layers. Note, however, that recording proceeds to the next layer
after all user sectors and tracks have been filled with information
in each layer.
[0038] While recording proceeds to the next layer after all user
sectors and tracks have been filled with information in each layer,
the order of layers to be accessed for recording is at random.
[0039] While layers to be recorded are randomly accessed, after
data has been recorded in a sector of a layer, the same sector of
the next layer is filled. When the same sector of all layers has
been filled, data is recorded on the next sector.
[0040] On a track, random access is performed in the layer
direction. In this case, a variable length block is employed, not a
fixed block management based on the sector.
[0041] As a light spot positioning mechanism, a two-dimensional
actuator for driving a focus lens in the layer direction and the
radial direction of the disc or a combination of a one-dimensional
actuator for driving a focus lens only in the layer direction and a
galvano mirror for deflecting light flux in the radial direction of
the disc is employed, where a layer address recorded on a preformat
portion is read by a layer number detecting circuit to recognize
the number of a layer on which the focus is currently being placed.
Then, it is recognized in which of upward or downward direction (+
or -(k-j)) and how many layers (|k-j|) the spot should jump from
the j.sub.th layer on which the spot is now focused to the k.sub.th
target layer instructed by an upper level controller, and a layer
jump signal generating circuit is instructed to generate a jump
force signal which is inputted to an AF actuator driver.
[0042] The jump signal is composed of a pair of positive-polarity
and negative-polarity pulses for a one-layer jump, and replaces the
positive or negative pulse in accordance with the upward or
downward jumping direction. The first pulse is used to drive the
spot approximately by a jumping distance in a jumping direction,
and the next polarity inverted pulse is provided to stabilize the
spot so as not to excessively jump. A number of pairs of pulses
equal to the number of layers over which the spot jumps is inputted
to a driver circuit. Next, the layer number is detected, and j=k is
recognized.
[0043] A zero-cross pulse of the AF error signal and a total light
amount pulse are used as gates, and a cross layer signal detecting
circuit is provided for detecting the detection of a focused point
on each recording layer.
[0044] A saw-tooth wave is generated from an AF actuator shift
signal generating circuit so as to shift a focus position at least
from the top layer to the lower-most layer of the disc, and the AF
actuator is driven by this saw-tooth wave, wherein the focal points
on T layers are counted by the cross layer signal detecting
circuit, and the top layer (n=1) is recognized from an upper limit
of an up pulse when the lens is shifted upwardly while the
lowermost layer (n=N) is recognized from a lower limit of a down
pulse when the lens is shifted downwardly, thereby always
recognizing the focus position in the layer direction of the
disc.
[0045] When recording is to be stably performed on a target
k.sub.th recording layer, a recording power P (light intensity) is
set in consideration of the transmissivity up to the k.sub.th layer
(.SIGMA.Tn (n=1, 2, . . . , k-1)). Also, the transmissivity up to
the k.sub.th layer is set for recognition of the layer address.
[0046] The recording power is set by address recognition in
consideration of a ratio of the transmissivity up to the k.sub.th
layer (.SIGMA.Tn (n=1, 2, . . . , k-1)) upon shipment of the disc
(or designed value) to the transmissivity up to the k.sub.th layer
(.SIGMA.Tn' (n=1, 2, . . . , k-1)) immediate before recording,
i.e., a change G in transmissivity.
[0047] A management layer for layer data is provided for recording
on which layer recording is being performed. The management layer
is reproduced before recording on a target layer to recognize the
transmissivity up to the k.sub.th layer (.SIGMA.Tn' (n=1, 2, . . .
, k-1)) immediate before recording and a change G in
transmissivity.
[0048] The change G in transmissivity may be obtained by previously
reproducing an area to be recorded before recording on the target
layer.
[0049] As a method of previously reproducing an area to be
recorded, a reproduction check is done in the first rotation of the
disc in a recording mode, recording is performed in the next
rotation, and then a recording error check is done in the third
rotation. In this event, a plurality of spots are employed, and the
reproduction check is done by a preceding spot.
[0050] The reproduction check employs a reproduced signal
C'k(t-.tau.) reproduced by the preceding spot, where .tau.
represents the distance between the preceding spot and a recording
spot converted into a time. Here, the transmissivity change G may
be calculated as a square root of a ratio of a reproduced signal
Ck' in a state where the spot is focused on the target k.sub.th
recording layer to a reproduced signal Ck as a design value which
has previously been set upon shipment of the disc.
[0051] In the reproduction check, the value of the reproduced
signal Ck may be recorded on a non-recording area previously
provided as a check area in a disc format with respect to the layer
direction on a disc plane.
[0052] As a reproduction control circuit, reflected light
components from adjacent layers which particularly include a
majority of inter-layer cross-talk is detected, in addition to the
detection of reflected light components from a target layer, and
mutually included components are removed by a calculation.
[0053] Three photo detectors are positioned on imaging planes of a
target k.sub.th layer and the adjacent (k+1).sub.th and
(k-1).sub.th layers on the light receiving plane side when the
focus is placed on the k.sub.th layer. The shape of the photo
detectors are selected to be a circle, the diameter D of which is
given by D=(.lamda./NAI). Alternatively, pinholes are used to
restrict light receiving areas. Then, the following calculation is
performed for a reproduced signal by the photo detector on the
k.sub.th layer, a reproduced signal C(k-1) by the photo detector on
the (k-1).sub.th layer, and a reproduced signal by the photo
detector on the (k+1).sub.th layer.
Calculation F .ident. Ck - .gamma. .times. C ( k - 1 ) - .gamma.
.times. C ( k + 1 ) .apprxeq. CkR + .beta. .times. C ( k - 1 ) R +
.beta. .times. C ( k + 1 ) R - .gamma. .times. { C ( k - 1 ) R +
.beta. .times. CkR + .beta. .times. C ( k - 2 ) R } - .gamma.
.times. { C ( k + 1 ) R + .beta. .times. CkR + .beta. .times. C ( k
+ 2 ) R } ##EQU00005##
where .beta. represents a ratio of cross-talk components included
in each signal to necessary signal components.
[0054] Since C(k-2)R and C(k+2)R are sufficiently small and
frequency components are also low, these terms can be
neglected.
Thus,
F.apprxeq.(1-2.gamma..beta.).times.CkR+(.beta.-.gamma.).times.C(k--
1)R+(.beta.-.gamma.).times.C(k+1)R
[0055] Here, if .gamma..ident..beta.<1,
F.apprxeq.(1-.beta.).sup.2.times.CkR
[0056] By employing the calculation function given by the above
equations, signal components on the target layer alone can be
derived.
[0057] A plurality of spots are employed. A spot having the same
spot diameter as an out-of-focus spot on the adjacent layers when
the focus is placed on the k.sub.th layer are used to scan the two
adjacent layers prior to the spot focused on the k.sub.th layer, to
obtain reproduced signals from these layers, and the above
calculation is performed.
[0058] As shown in FIG. 18, a diaphragm is inserted to reduce the
effective aperture of the focus lens. Specifically, the effective
diameter a' is reduced to
[.lamda./(2d.times.NAF.sup.2).times.a].
[0059] The optical axis is considered for three separate optical
systems employing three different spots, and the numerical aperture
of the focus lenses are reduced in two optical systems with
preceding spots. Specifically, NAF'=.lamda./2d.times.NAF is
given.
[0060] A reproduced signal detected by the preceding spot is
multiplied with a weighting function 80 derived by approximating a
Gaussian distribution, which is an intensity distribution of the
spot, to a triangle distribution, and integration is performed to
this product.
[0061] In a weight setting circuit for setting each calculation
coefficient .gamma. (.ident..beta.), mark recording areas on at
least three layers including upper and lower adjacent layers are
located as a disc format such that they are not included in the
same light flux, and h(k-1)/hk and h(k+1)/h are set to .beta.(-1)
and .beta.(+1).
[0062] By employing a plurality of spots and placing the focus on
each layer, recording/reproduction is performed simultaneously on
two or more layers, i.e., parallel recording/reproduction is
achieved.
[0063] A recording medium, the transmissivity of which is increased
after recording, is employed.
[0064] Guide grooves in each layer plane of the multi-layer disc
and prepits such as address are provided in a UV cured resin layer
for each layer and formed by using a transparent frame for each
layer by a 2P method which employs the light incident from the
plane of the frame.
[0065] The intermediate layer is provided with a quarter wave plate
layer.
[0066] By applying the above structure, there can be provided a
three-dimensional recording/reproducing apparatus including an
optical system which enables stable information recording and
reproduction.
[0067] Particularly, since a photo detector in a predetermined
shape is disposed on the focal plane of the optical system, when
information recorded on a target recording layer is to be
reproduced from among a plurality of recording layers constituting
a recording medium, leak of reflected lights from other recording
layers are reduced and signal components on the target recording
layer alone can be detected.
[0068] A predetermined relationship is established between a
recording frequency of information on a recording layer subjected
to reproduction and the numerical aperture of a focus lens in the
optical system, whereby cross-talk components from adjacent layers
included when information is being reproduced from the target layer
is limited to direct current components (of a fixed value), and
signal components from the target layer alone can be extracted by
removing the direct current components.
[0069] Further, spherical aberration caused by a change in optical
distance from one layer to another is suppressed within a tolerable
value, and a light spot at the diffraction limit can be formed on
each recording layer.
[0070] Also, a recording power can be set to an incident light
which allows stable recording on a target layer without destroying
data on other recording layers during the recording process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0071] FIG. 1, comprising FIGS. 1A-1C, shows the principle of a
recording/reproducing system according to the present
invention;
[0072] FIG. 2, comprising FIGS. 2A and 2B, shows the structure of
basic optical system which is applied to the present invention;
[0073] FIG. 3 shows the principle of a recording system according
to the present invention, where FIG. 3A is a graph showing a light
intensity on each layer; FIG. 3B is a graph showing spot plane
densities on other layers when the focus is placed on a k-layer;
and FIG. 3C is a graph showing power densities on other layers when
the focus is placed on the k-layer;
[0074] FIG. 4, comprising FIGS. 4A-4C, shows the principle of a
reproducing system according to the present invention;
[0075] FIG. 5, comprising FIGS. 5A-5C, is a diagram showing a disc
format according to the present invention;
[0076] FIG. 6 is a block diagram showing the whole arrangement of a
three-dimensional recording/reproducing apparatus according to the
present invention;
[0077] FIGS. 7A, 7B are block diagrams showing a recording control
method according to the present invention;
[0078] FIG. 8 shows RBW (Read Before Write) by a preceding
beam;
[0079] FIG. 9 shows a concept of a recording control method
according to the present invention;
[0080] FIG. 10 shows an example of a three-layer film structure and
its recording characteristic, where FIG. 10A is a diagram
illustrating a three-layer film structure of a recording medium,
and FIG. 10B is a graph illustrating the recording
characteristic;
[0081] FIG. 11 is a cross-sectional view showing the structure of a
phase change type information recording medium used in an
embodiment of the present invention;
[0082] FIGS. 12A, 12B are partial cross-sectional view of a third
information recording medium used in the present invention;
[0083] FIG. 13 is a block diagram showing a reproduction control
method according to the present invention;
[0084] FIGS. 14-16 show a concept of the reproduction control
method of the present invention;
[0085] FIG. 17 shows an optical system for realizing the
reproduction control method of the present invention, where FIG.
17A illustrates the principle of the optical system; FIG. 17B an
actual optical system; and FIG. 17C the formation of a pinhole;
[0086] FIG. 18 is a diagram showing the structure of an optical
system for realizing the reproduction control method of the present
invention;
[0087] FIG. 19 shows a calculation coefficient .gamma.
(.ident..beta.) check area and a diagram of the principle;
[0088] FIG. 20 shows a disc structure for realizing a third
reproducing method according to the present invention;
[0089] FIG. 21 shows a disc structure to which a two-dimensional
recording/reproducing method is applied;
[0090] FIG. 22 shows another disc structure to which a
two-dimensional recording/reproducing method is applied;
[0091] FIG. 23 is a block diagram for explaining a layer access in
the present invention;
[0092] FIG. 24, comprising FIGS. 24A-24C, shows a concept of how
out-of-focus is detected in each recording layer;
[0093] FIG. 25 is a block diagram for explaining a layer access in
the present invention;
[0094] FIG. 26 shows detection of out-of-focus in a recorded layer,
where FIG. 26A is a graph illustrating a signal indicative of an
out focus on a recorded layer; and FIG. 26B is a block diagram
illustrating an out-of-focus detecting circuit; and
[0095] FIG. 27 is a diagram for explaining a method of reducing
interference of reflected lights between adjacent layers according
to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0096] Embodiments of the present invention will hereinafter be
described in the following order: [0097] (1) Principle of
Three-dimensional Recording/reproducing Method; [0098] (2)
Three-dimensional Disc Format and Data Management; [0099] (3)
Structure of Apparatus; [0100] (4) Access Method; [0101] (5)
Recording Control Method; [0102] (6) Reproduction Control Method;
and [0103] (7) Embodiment of Disc Structure and Disc Producing
Method.
(1) Principle of Three-Dimensional Recording/Reproducing Method
[0104] The principle of recording and reproduction performed by a
three-dimensional recording/reproducing apparatus according to the
present invention will first be explained with reference to FIG. 1.
Information is recorded on and reproduced from a disc 4 in which a
combination of a recording layer 1 and an intermediate layer film 2
is stacked on an optically transparent substrate 3 a plurality of
times. The recording layer 1 is such that its optical properties
change by a local light irradiation. The intermediate layer 2,
serving as a an assistant for the recording layer 1, consists of a
layer or a stack of layers provided for the purpose of reflection
protection, multiplex reflection, light absorption, transfer of
local optical property changes on the recording layer, heat
insulation, heat generation, reinforcement and so on. A light spot
focused on each layer is irradiated thereonto to two-dimensionally
change the local optical properties of recording layers
independently of each other. Then, recording is performed on each
layer corresponding to modulated data "1" and "0," and the light
spot is irradiated onto a recording layer whose optical properties
have been changed to detect a change in a reflected light amount
(or a transmitting light amount) to reproduce data.
[0105] In the disc 4 shown in FIG. 1B, the refractivity and the
width of the optically transparent substrate 3 are designated NB
and d0, respectively. Further, the intermediate layer and the
recording layer 1 are blocked as a combination of layers k, and
these combinations are sequentially numbered from 1 to N from the
top layer (on the light incident plane side). The distance between
adjacent layers is in principle indicated by dk which is the
distance between the centers of adjacent k.sub.th and (k+1).sub.th
recording film layers in the thickness direction. Further, a film
thickness of an arbitrary k.sub.th recording layer is designated
dFk; the real part of the refractivity of the same NFk; the film
thickness of the intermediate layer 2 dMk; and the real part of the
refractivity of the same NMk. Also, a cycle of changes in the local
optical properties on the plane of each layer is designated b
[.mu.m]. A focusing optical system shown in FIG. 1A employs as a
light source, for example, a semiconductor laser 5 which emits the
light with wavelength .lamda. [.mu.m]. The light emitted from the
semiconductor laser 5 is converted to a parallel light by a
collimator lens 6, and is incident to a focus lens 8 through a
polarization beam splitter 7. Here, the numerical aperture,
effective radius and focal length are designated NAF, a [mm] and fF
(.apprxeq.a/NAF), respectively. A light spot 11 at diffraction
limit is focused on each recording layer to be irradiated
thereon.
[0106] As light receiving optical system, an example of reflection
light receiving system is shown. The light reflected from the disc
4 is led through the lens 8 to an image lens 9 for receiving light
by the beam splitter 7. A photo detector 10 is disposed in the
vicinity of the focal point of the lens 9 such that a change in a
reflected light amount detected by the detector 10 is converted to
an electric signal. The photo detector 10 is illustrated in FIG.
1C. The numerical aperture and focal length of the image lens 9 are
designated NAI and fI (.apprxeq.a/NAI), respectively. Also, the
diameter of a light receiving plane of the photo detector 10 is
designated D (=NAI/.lamda.).
[0107] Although in this embodiment, infinite optics of FIG. 2A is
shown as an example of the optical system, limited optics shown in
FIG. 2B may also be used to produce similar effects. Also, as the
light receiving optical system, transmitting light detecting scheme
can be used to produce similar effects to this embodiment.
[0108] In the three-dimensional recording/reproduction, it is
necessary for performing recording/reproduction to focus a light
spot at the diffraction limit on each layer. With a conventional
optical disc, a light spot is focused on a recording plane through
a substrate for protecting a recording film. In this event, the
focus lens 8 should be designed so as to prevent spherical
aberration from occurring to distort the light spot, in
consideration of the refractivity of the substrate and the
thickness of the recording film.
[0109] However, in the multi-layer disc 4, the influence of layer
films other than a layer to be recorded cannot be neglected. For
example, as indicated in a known literature by Kubota et al,
entitled "Optical Code 14, Analysis of Jitter of Eye Patterns on an
Optical Disc I-V", 1985, as the number of layers other than a layer
subjected to recording increases, spherical aberration also
increases, which hinders the light from being focused to the
diffraction limit. To solve this problem, the present invention
proposes a design of a focus lens for providing a light spot having
a sufficient range for recording and reproduction, and a disc
structure. It is assumed for simplicity of a designing method that
the film thickness dFk of the recording layer 1 is thin enough
relative to the film thickness dMk of the intermediate layer 2 to
be neglected. Namely, the following equation 1 is satisfied:
dk.apprxeq.dMk (Equation 1)
[0110] Further, an intermediate layer k is assumed to have the same
refractivity NB as the substrate 3. In this case, a thickness d of
the disk from the light incident plane to the N.sub.th layer
is:
d.apprxeq..SIGMA.dk+d0 (Equation 2)
[0111] On the other hand, at Rayleigh limit, a spherical aberration
amount W40=.lamda./4 is given as a tolerable value where 80% of a
focus spot without aberration is ensured as a peak intensity.
[0112] The spherical aberration amount W40 caused by a change in
film thickness .DELTA.d from the first to N.sub.th layers is given
by the following Equation 3:
W40=1/(8.times.NB).times.(1/NB.sup.2-1).times.NAF.sup.4.times..DELTA.d
(Equation 3)
[0113] Thus, the design of a focus lens and the disc structure are
determined so as to satisfy W40.ltoreq..lamda./4. As an example,
when a glass substrate with the refractivity NB equal to 1.5 is
used as the substrate 3, a UV cured resin having a refractivity
substantially equal to that of glass is used as the intermediate
layer, and the focal length NAF of the focus lens 8 is selected to
be 0.55, .DELTA.d.ltoreq.50 .mu.m is derived from Equation 3. Here,
by combining the thickness dk of the intermediate layer of each
layer and the total number N so as to satisfy the following
Equation 4:
d0=1.2 mm-.DELTA.d=(1.15 to 1.2 mm)
0.5.times..SIGMA.dk=.DELTA.d (.ltoreq.50 .mu.m) (Equation 4)
a focus lens for a substrate thickness equal to 1.2 mm used for a
conventional optical disc can be used as it is to form an optical
spot sufficiently usable for recording on and reproducing from each
of the first to Nth layers. As a combination, with the thickness of
the intermediate layer dMk=10 .mu.m and the thickness of the
recording layer dFk=200 .ANG., do=1.15 mm, .SIGMA.dk=100.4
.mu.m.apprxeq.100 .mu.m, and the total number N=10 are
possible.
[0114] With Equation 4, spherical aberration is zero on the fifth
layer, while maximum spherical aberration within the tolerable
value occurs on the topmost and lowermost layers. Such spherical
aberration can also be corrected. The wave optics indicates that
spherical aberration can be corrected by shifting the focus
position. This may be done on condition of
W40=-W20=-0.5.times.NAF.sup.2.DELTA.z, and
.DELTA.z=-2/NAF.sup.2.times.W40, where W20 represents aberration
due to out-of-focus, and .DELTA.z the out-of-focus amount. In the
above example, spherical aberration Wk40 occurring on the k.sub.th
layer spaced from the fifth layer by an inter-layer distance
.DELTA.dk=(k-5).times.d is derived from Equation 3, and an
out-of-focus amount .DELTA.zk for correcting this aberration is
.DELTA.zk=-2/NAF.sup.2.times.Wk40.
[0115] On the lowermost layer (k=10), an out-of-focus amount equal
to 1.4 .mu.m may be given as an offset, and on the topmost layer
(k=1), -1.4 .mu.m may be given likewise.
[0116] A second problem for performing recording/reproduction lies
in a thermal recording process. Restrictive conditions for the
recording are the following two items:
<1> A sufficient and stable recording power density can be
given to a target recording layer; and <2> When recording is
performed on an arbitrary k.sub.th layer, data recorded on other
layers are not destroyed.
[0117] Factors related to these conditions are classified into
those concerning the light intensity and those concerning the
thermal conductivity. Here, the former factors will be described.
The latter factors can be attended to by providing the intermediate
layer 2 with a heat insulating effect. This method will be shown
later in the paragraph describing "Embodiment of Recording
Medium".
[0118] To satisfy these two items, the present invention primarily
optimizes the disc structure and the focusing optical system.
[0119] Referring to FIG. 1, it is assumed for simplicity of the
explanation, the substrate 2 and the intermediate layer 3 both have
the transmissivity equal to 100%, by way of example. Also, optical
constants of the k.sub.th recording layer, i.e., the
transmissivity, reflectivity and absorption ratio are represented
by Tk, Rk and Ak, respectively. Here, the relationship Tk+Rk+Ak=1
is satisfied. The optical constants, when the local optical
properties are changed by recording, are indicated by adding a dash
"'" thereto. Generally in the thermal recording, the thermal
structure on a recording film changes due to a temperature rise
caused by a heat generated by the optical film absorbing the light
and thermal diffusion occurring with thus generated heat as a heat
source. This change in thermal structure corresponds to a movement
of a recording film due to melting in a hole forming type recording
medium; crystallization and non-crystallization in a phase change
type recording medium; and inversion of vertical magnetization in a
magneto-optical recording medium. This change in thermal structure
may cause a change in the local optical properties. To cause the
change in thermal structure, an energy threshold value Eth [nj]
must exist irrespective of the kind of recording film. In a
recording process, an optical spot 11 focused on a target recording
layer at the diffraction limit is scanning on a disc at a linear
velocity V [m/s]. To locally give rise to a change in thermal
structure corresponding to modulated binary signals, the intensity
P (recording power) [mW] of the light irradiated on the disc plane
is modulated by a time t [s]. If the linear velocity V and the
irradiation time t are given, the energy threshold value E.sub.th
can be discussed with a light intensity density threshold value Ith
[mW/.mu.m.sup.2].
[0120] To satisfy the foregoing item <1>, the following
Equation 5 may stand with respect to the light intensity density Ik
on the k.sub.th layer when the light spot is focused on the
k.sub.th spot:
Ik=Pk/Sk.gtoreq.Ik.sub.th (Equation 5)
where Ikth: Light intensity density threshold value on the k.sub.th
recording layer (mW/.mu.m.sup.2); [0121] Sk: 1/e.sup.2 spot area
when the light spot is focused on the k.sub.th layer:
Sk=.pi.(0.5.times..lamda./2NAF).sup.2
[0122] Further, the diameter of the light spot focused at the
diffraction limit is represented by .lamda./NAF.
[0123] A light intensity Pk [mW] on the k.sub.th layer is:
Pk=P.delta.k, .delta.k=.PI.Tn (Equation 6)
where .delta.k represents the transmissivity of an area between the
light incident plane and the k.sub.th recording layer of the disc,
and Tn the overall transmissivity of n layers. The transmissivity
Tn is as shown in FIG. 3a. From Equations 5 and 6, a minimum
recording power Pmin required to record on the k.sub.th layer is
given by Equation 7:
Pmin.gtoreq.Ik.sub.th.times.Sk/.delta.k (Equation 7)
[0124] Generally, the lowermost layer N exhibits the lowest light
intensity. If a medium is such that the transmissivity Tn decreases
after recording has been performed on n layers (n=1 to N-1), the
transmissivity Tn is replaced by Tn' (transmissivity after
recording).
[0125] To satisfy the foregoing item (2), the light intensity
density Ljk [mW/.mu.m.sup.2] on the j.sub.th layer when the focus
is placed on the k.sub.th layer for recording on the k.sub.th layer
may satisfy Equation 8:
Ijk=Pjk/Sjk<Ij.sub.th (Equation 8)
Pjk=Pk.times..delta.jk=P.times..delta.j (Equation 9)
where .delta.jk=.PI.tn/.PI.tn (=(transmissivity of layers up to the
j.sub.th layer)/(transmissivity of layers up to the k.sub.th
layer).
[0126] When recording is performed on the k.sub.th layer, an upper
limit of the recording power to avoid destroying recording contents
on the j.sub.th layer is given by the following equation:
Pmax=Ij.sub.th.times.Sjk/.delta.j (Equation 10)
where Sjk represents a light spot dimension on the j.sub.th layer
when the focus is placed on the k.sub.th layer, and can be derived
by a geometrical optics method if the inter-layer distance d is
larger than the wavelength .lamda..
Sjk = .pi. [ ( .SIGMA. dn ) .times. TAN .phi. ] 2 ( when j > k )
= .pi. [ ( .SIGMA. dn ) .times. TAN .phi. ] 2 ( when j < k ) =
.pi. [ ( .SIGMA. dn ) .times. NAF ] 2 ( Equation 11 )
##EQU00006##
where dn: the film thickness of the n.sub.th layer; and
TAN .phi.=a/fF.apprxeq.NAF
[0127] Here, 1/Sjk [.mu.m.sup.2] represents an areal density which
is shown as in FIG. 3B. From FIGS. 3A and 3B, the light intensity
density Ijk [mW/.mu.m.sup.2] is derived, which is as shown in FIG.
3C.
[0128] By setting the focusing optical system, disc structure and
recording conditions so as to simultaneously satisfy (Equation 5)
and (Equation 8), highly reliable recording can be achieved on each
recording layer. As an example, a recordable inter-layer distance d
is calculated for a three-layer disc shown in FIG. 10A. Note that
the focusing optical system has a wavelength .lamda.=0.78 .mu.m and
NAF=0.55, while the optical constants of each recording layer are:
R1=R2=R3=0.1; T1=T2=T3=0.8; and A1=A2=A3=0.1. Also, a linear
velocity V is set to 7 m/s, an irradiation time t to 100 ns to 500
ns, and light intensity density threshold values of the recording
layers at this time are set to I1.sub.th=I2.sub.th=I3.sub.th=2.53
(mW/.mu.m.sup.2). With these prior conditions, a recordable
inter-layer distances d=d1=d2=d3 and a recording power range are
determined.
[0129] FIG. 10B shows a power of the light irradiated onto a disc
and a modulation degree of a reproduced signal generated
corresponding to the light power when the focus is placed on each
recording layer of a disc where recording has not been performed
other than on a target recording layer. The signal modulation
degree indicates a standard on the size of a mark formed by a
change in local optical properties on the surface of each recording
layer. When the mark is large enough such as the diameter of the
focus spot, the modulation degree presents a tendency of
saturation. The ordinate in FIG. 10B indicates the normalized
modulation degree with a saturation value being determined to be
one. In FIG. 10B, a threshold value power which can form the mark
is 4 mW (=I.sub.th.times.S1) on the first layer, and 5 mW
(=I.sub.th.times.S2/.delta.2) on the second layer. The power on the
third layer (k=3) determines a minimum power which is calculated
from Equations 5, 6 and 7:
.delta.3=.PI.Tn=0.64
S3=.pi.(0.5.times..lamda./NAF).sup.2=1.58 (.mu.m.sup.2)
Pmin.gtoreq.I3.sub.th.times.S3/.delta.3=6.3 mW (Equation 12)
[0130] From (Equations 8, 9 and 11:
S23=0.95.times.d.sup.2
S13=3.8.times.d.sup.2
.delta.23=1.25, .delta.13=1.5625
P23=1.25.times.P3=0.8.times.P
P13=1.5625.times.P3=P
I23=0.8.times.P/(0.95.times.d.sup.2)=0.886.times.P/d.sup.2
I12=P/(3.8.times.d.sup.2)=0.07.times.P/d.sup.2
I23<I2.sub.th=2.53
d>0.59.times. {square root over (Pmin)}=1.48 .mu.m (Equation
13)
[0131] For example, when d=2.5 .mu.m, Pmax is calculated to be 16
mW (P3max=10 mW), whereby a signal can record a sufficient mark as
shown in FIG. 10B.
[0132] By thus designing the focusing optical system, data can be
highly reliably recorded on a target layer without destroying
recorded data on other layers.
[0133] A third problem for recording/reproduction lies in a
reproduction process. Restrictive conditions for reproduction are
the following items:
<3> Noise components are reduced to be minimum. Here,
inter-layer cross-talk noise should be reduced. <4> Signal
components from a target layer is made maximum.
[0134] A first method for achieving the item <3> will be
shown.
[0135] A first method consists of optimizing the light receiving
optical system in FIG. 1. In other words, an amount of the light
reflected from layers other than the target layer is made
sufficiently small. Consequently, inter-layer cross-talk can be
reduced, with the result that reproduction can be performed with a
large S/N ratio. In FIG. 1A, a reflected light amount from a
recording layer from which data is to be reproduced is all detected
by the photo detector 10 disposed on the focal point of the image
lens 9. This operation is now explained with reference to FIG. 1C.
Unlike a reflected light from a recording layer from which data is
to be reproduced, a reflected light from an adjacent layer spreads
over a focal plane 12 of the image lens, as indicated by a broken
line. Therefore, by restricting the size of the photo detector 10,
such a reflected light from an adjacent layer can be reduced.
Hereinafter, restriction of the size of the focal plane will be
shown.
[0136] When the light spot 11 is focused on the k.sub.th recording
layer as a target layer, the diameter of the light spot 11 which
provides an intensity equal to a peak value multiplied by
1/e.sup.2, i.e., a spot diameter Uk is given by Uk=(.lamda./NAF). A
reflected light from the target layer is imaged at the focal point
of the image lens 9. A spot diameter Uk' on this focal plane 12 is
given by:
Uk ' = mUk = m .times. ( .lamda. / NA FL ) = ( NAF / NAI ) .times.
( .lamda. / NAF ) = .lamda. / NAI = .lamda. .times. ( fI / a ) (
Equation 14 ) ##EQU00007##
where m: a horizontal scaling ratio of the light receiving optical
system. Next, a spot diameter U(k.+-.1) on the focal plane from the
(k.+-.1).sub.th layer spaced from the k.sub.th target layer by the
inter-layer distance d is calculated. A distance d' between a
position at which a reflected light from the (k.+-.1).sub.th layer
is focused by the image lens 9 and the focal plane is given by:
d'=Y.times.d=m.sup.2.times.d (Equation 15)
where Y: a vertical scaling ratio.
U ( k .+-. 1 ) ' = d ' .times. tan .phi. I = d ' .times. a / ( fI +
d ' ) = m 2 d .times. a / ( fI + m 2 d ) ##EQU00008##
Here, if fI>m.sup.2d stands,
U(k.+-.1)'.apprxeq.a.times.m.sup.2d/fI=NAIm.sup.2d (Equation
16)
[0137] Assuming that the diameter D of the photo detector is given
by D=Uk'=.lamda./NAI from the above equations, an area ratio
.epsilon. is calculated by .epsilon.=(D/U(k.+-.1)').sup.2. Thus,
the reflected light amount from the adjacent layer can be reduced,
a change in reflected light amount from the target layer can be
detected with a high S/N ratio as compared with a case where the
diameter of the photo detector is not restricted.
[0138] Actually, a reflected light amount from another recording
layer is detected in consideration of the transmissivity .delta.jk
between the target k.sub.th layer and the other j.sub.th layer as
well as a reflectivity ratio .alpha.jk. Assuming that an
inter-layer cross-talk noise amount required for a reliable signal
detection is -20 db ( 1/10), the following equation may generally
be satisfied:
[0139] If a reflected light amount from n layers detected by the
photo detector 10 is represented by In,
1 / 10 .gtoreq. .SIGMA. Ij ( n = 1 to N , n k ) Ik = .SIGMA. [
.delta. 2 jk .times. .alpha. jk .times. ( D / Uj ' ) 2 ] ( Equation
17 ) ##EQU00009##
[0140] Note, however, that hereinafter the layer (k-1).sub.th
adjacent to the k.sub.th layer will alone be considered. Although
the influence exerted by other layers may be likewise considered,
the value is ignorably small.
.apprxeq.I(k-1)/Ik=.delta..sup.2(k-1), (Equation 17)
k.times..alpha.(k-1), k.times.(D/U(k-1)').sup.2 (Equation 17.5)
[0141] For example, with .lamda.=0.78 .mu.m, NAF=0.55 and fI=30 mm,
in a case where NAI=0.075, m=7.33 and m.sup.2=53.8,
D=Uk'.apprxeq.10.4 .mu.m.
[0142] Given FIG. 10 as an example, from .delta.23=1.25 and
.alpha.23=1, a suppression ratio is expressed by
.epsilon..times..delta..sup.223.times..alpha.23.
I 2 / I 3 = .delta. 2 23 .times. .alpha. 23 .times. = .delta. 2 23
.times. .alpha. 23 .times. ( D / U 2 ' ) 2 = .delta. 2 23 .times.
.alpha. 23 .times. ( .lamda. / NAI ) 2 / ( NAI .times. m 2 d ) 2
.delta. 2 23 .times. .alpha. 23 .times. ( .lamda. / NAF 2 / d ) 2 (
Equation 18 ) ##EQU00010##
[0143] If d is calculated so as to satisfy (I2/I3).ltoreq.
1/10:
d .gtoreq. ( 10 .times. .alpha. 23 ) .times. .delta. 23 .times. (
.lamda. / NAI 2 / m 2 ) = ( 10 .times. .alpha. 23 ) .times. .delta.
23 .times. ( .lamda. / NAF 2 ) .apprxeq. 12.9 m ( Equation 19 )
##EQU00011##
[0144] While the influence of cross-talk from the second layer has
been considered in this example, the influence of cross-talk from
the first layer can also be calculated, however, its value
(I1/I3)=0.024 (=-32 dB) is small enough to be neglected.
[0145] In the foregoing example, the diameter D of the photo
detector is determined to be D=Uk'=.lamda./NAI, however, there is a
certain degree of freedom in design, including a position shift of
the photo detector, such that inter-layer cross-talk may present a
certain value.
[0146] Next, a second method will be shown to achieve the item
<3>.
[0147] The second method defines the relationship between a cycle b
of changes (mark) in the local optical properties on a recording
layer plane, the disc structure and light receiving optical system,
thereby making inter-layer cross-talk components larger than the
cycle b of changes in the local optical properties. Stated another
way, frequency components of the inter-layer cross-talk are made
smaller than a signal band of data, thereby reproducing data on the
plane of a target layer with a high S/N ratio. The principle of
this method will be explained with reference to FIGS. 1 and 4.
Although the diameter of an optical detector is not restricted in
order to distinguish the second method from the first method, the
second method may be combined with the first method to provide a
higher S/N ratio.
[0148] Next, the item <4> will be examined.
[0149] Since a light spot at the diffraction limit is formed on a
target layer, if a two-dimensional cycle b is as long as a spot
diameter (.lamda./NAF) on the target layer, the light spot can
provide a sufficient resolution. In other words, if a minimum value
bmin of the two-dimensional cycle b is set to (.lamda./NAF), signal
components can be extracted with a sufficiently large proportion.
This is a condition for satisfying the item <4>. A spot
diameter on an adjacent layer, since the light spot is out of focus
on this layer, is expressed by (2d.times.NAF), where d represents
an inter-layer distance, and accordingly the optical resolution is
degraded. Therefore, by utilizing this characteristic, if a maximum
value bmax of the two-dimensional cycle b is set to be smaller than
(2d.times.NAF), leak of signal components from the adjacent layer,
i.e., frequency components of the inter-layer cross-talk becomes
smaller than a signal band (1/bmax-1/bmin), whereby the inter-layer
cross-talk can be removed by using a filter or AGC (auto gain
control).
[0150] Now, the degradation of the optical resolution, i.e., the
degradation of the signal modulation degree is calculated from the
optical theory.
[0151] In the light receiving optical system shown in FIG. 1,
optical property functions (OTF) H0(S) and H1(S) on the plane of a
target layer subjected to recording and reproduction and on the
plane of an adjacent layer spaced therefrom by an optical distance
d [.mu.m] are indicated by lines 13 and 14, respectively. The
abscissa corresponds to a repetition frequency S of an object,
while the ordinate corresponds to its modulation degree
(H(S)/H(0)), where S represents a normalized spatial frequency.
Namely, the following equation is satisfied:
S=.lamda..times.fF/(2.pi.a)=.lamda./NAF.times.b (Equation 20)
[0152] The optical property function H0(S), when no out-of-focus or
aberration is observed, is as indicated by a line 13. In this case,
the cut-off frequency at which the optical resolution is zero is
S=2. In an actual recording/reproducing apparatus, since noise
components such as laser noise and amplifier noise are included and
the optical system itself has aberration other than out-of-focus,
it is difficult to detect the cycle b corresponding to the cut-off
frequency S equal to 2. Therefore, a half of the modulation degree
(-6 dB) is determined to be a tolerable value therefor. At this
time, the cut-off frequency S is 1, and a minimum repetition bmin
is defined for the cycle b.
bmin=.lamda./NAF (Equation 21)
[0153] On the other hand, for the optical property function H1(S)
when out-of-focus, the amount of which is equal to the inter-layer
distance d, occurs, a maximum repetition bmax is defined for the
cycle b from S=2 at which H1(S)=0 stands.
[0154] As the out-of-focus amount d increases, the optical property
function H1(S) changes in the direction indicated by an arrow 15,
and bmax can also be made larger.
[0155] Thus, frequency components of cross-talk from adjacent
layers are not more than fmin (=1/bmax), so that such cross-talk
components can be absorbed by using an auto gain control circuit
which has a follow-up characteristic as shown in FIG. 4B.
[0156] Some numerical examples will be shown below.
[0157] The relationship between the out-of-focus amount d and an
amount B1 of wave front aberration is expressed by the following
equation:
B1=-d/2.times.(NAF).sup.2
As a numerical example, the cut-off frequency S for the
out-of-focus d is calculated, and further bmax is calculated from
the cut-off frequency S as follows:
[0158] When d=6.7 .mu.m, bmax=4.7 .mu.m; and [0159] B1=-.lamda.
[0160] When d=10 .mu.m, bmax=7.9 .mu.m; and [0161]
B=-1.5.lamda.
[0162] Also, bmin=(.lamda./NAF)=1.42 .mu.m.
[0163] For example, when a pit edge recording method disclosed in a
known patent document JP-A-63-53722 is employed for a disc where a
2-7 code, which is a variable length code, is used in the spot
scanning direction, and a track pitch is constantly equal to 1.5
.mu.m, a reproducible minimum bit pitch q (am) and the inter-layer
distance d are calculated. As shown in FIG. 4C, a minimum pattern
repetition cycle is calculated as follows:
3q=bmin=1.42 .mu.m
q=0.47 .mu.m
Here, a maximum pattern repetition length is 8q: 8q=3.76
.mu.m.ltoreq.bmax.
[0164] Also, for the cycle of marks formed in the radial direction
of the disc, it is necessary that the track pitch is 1.5 .mu.m
(constant) and 1.5 .mu.m.ltoreq.bmax is satisfied. Therefore,
d.gtoreq.5 .mu.m is sufficient.
[0165] Next, a third method will be shown for achieving the
foregoing item (3). Although in the second method, the frequency
components of inter-layer cross-talk noise are fmin or less,
signals from a target layer suffer from fluctuations due to
variations in local optical property change in certain modulation
methods. The 2-7 modulation code employed in the foregoing example
is also one of such cases. The power spectra characteristic of this
modulated signal is shown in FIG. 4A. It can be seen from FIG. 4A
that the signal has slight components below fmin. These components
can be suppressed by a filter circuit and an AGC circuit, as
described above. Even without these circuits, however, inter-layer
cross-talk noise can be suppressed by removing variations in local
optical property change and making direct current components
constant. The principle of the third method is based on the
employment of a code which defines that a total area occupied by
local optical changes (marks) included in the area defined by the
spot diameter (2d.times.NAF) on an adjacent layer is constant. By
employing this code, an amount of inter-layer cross-talk included
in a reproduced signal when scanning a spot presents a constant
value in direct current. The third method may be used together with
the first method.
[0166] An example will be shown. A power spectra 87 of a modulated
signal when employing an EFM (Eight to Fourteen Modulation)
modulation method described in a known literature "Digital Audio",
pp 322-324, by Toshitada Doi and Akira Iga, presents a feature that
the spectra of low range components abruptly falls as shown in FIG.
4A. Therefore, the inter-layer distance d may be set such that a
turning point 88 from which the spectra abruptly falls coincides
with the cut-off frequency at which the optical property function
H1(s) becomes zero in an adjacent layer.
[0167] For example, assuming that q=0.6 .mu.m, 2.82q=1.7
.mu.m.gtoreq.bmin=1.42 .mu.m, and 10.36q=6.2 .mu.m.ltoreq.bmax,
where the repetition cycle at the turning point 88 is 24 .mu.m, and
the inter-layer distance d is 22 .mu.m. At this time, an occupying
ratio of marks included in an area defined by the spot diameter
(2d.times.NAF=24 .mu.m) on the adjacent layer is maintained to be
approximately 50%, whereby components of a reflected light amount
from the adjacent layer included in a detected reproduced signal
always presents a constant value.
[0168] In FIGS. 21 and 22, the present invention is applied to a
case where two-dimensional recording is performed within layer
planes. As shown in FIG. 21, the two-dimensional
recording/reproducing method employs, for example, four points
arranged in a 2.times.2 lattice as one block to represent
2.sup.4=16 data by a combination of four bits which are marks on
the four lattice points, thereby achieving high density recording.
The two-dimensional recording can be implemented by the first and
second methods. Further, as shown in FIG. 22, it is required that
the same number of marks (one in FIG. 22) is included in lattice
points within each 4.times.4 lattice block. If more lattice blocks
are included in the spot area (2d.times.NAF) on an adjacent layer,
the number of marks included in the spot and accordingly the area
occupied by the marks are substantially constant, whereby the third
method can be applied thereto.
[0169] Incidentally, in an optical disc, a light spot at the
diffraction limit is formed on the plane of each recording layer.
In each optical system shown in FIG. 2, if an out-of-focus of a
certain value dm occurs, conditions of a focusing system of a
microscope are satisfied, whereby an image on a recording layer
plane may be formed on a light receiving plane. For example, when a
target layer receives light formed into a spot at the diffraction
limit, and a distance from the target layer to another layer is dm,
a mark string pattern on this target layer is formed on the light
receiving plane, whereby cross-talk noise in a signal band may be
added to information signals on the target layer. It is therefore
desirable to design the disc structure such that the inter-layer
distance does not coincide with dm.
[0170] Also, since recording layers are irradiated with the same
light, if the inter-layer distance is as short as an inteferable
distance, lights reflected from the respective recording layers
interfere with each other. As a result, cross-talk noise between
layers cannot be expressed by a ratio of a received light amount on
the target layer to a received light amount on other layers on the
light receiving planes. Stated another way, since interference
occurs, inter-layer cross-talk noise appears in the form of the
square root of the received light amount ratio in the worst case.
It is between adjacent layers when this influence actually causes
problems.
[0171] An embodiment intended to solve this problem is shown in
FIG. 27. The principle of this embodiment lies in that the
polarization direction of the light reflected from an adjacent
layer is changed to prevent interference. As a means for changing
the polarization direction, a disc shown in FIG. 27 is provided
with a wave plate layer 201 in each intermediate layer 2. A quarter
wave plate layer 201 deviates the phase difference of waves in an
electric field generated by travelling lights by an angular
distance of 90.degree. with respect to the depth direction of the
layers. Stated another way, the difference in optical thicknesses
in two directions is changed by a quarter wavelength portion. By
providing a disc with such a structure, assuming that the
polarization direction of an emitted light is E-polarization,
lights reflected from layers adjacent to each other are different
in phase by a difference produced by reciprocating the quarter wave
plate layer, i.e., a half wavelength or a 180.degree.-phase
portion, whereby the polarization direction crosses alternately
with E-polarization and H-polarization. For this reason, reflected
light components between adjacent layers do not interfere with each
other, so that cross-talk noise between these layers can be
expressed by a simple received light amount ratio on the light
receiving plane, with the result that cross-talk between layers can
be reduced. Further, a polarization beam splitter 202 is inserted
in the optical system, as shown in FIG. 27, to separately employ
detector 203 or 204 depending on the polarization direction of a
reflected light. Since this structure prevents a reflected light
from being detected from adjacent layers, a tolerable value for
variations of the size of the optical detectors can be set to a
larger value in the foregoing first reproduction method.
[0172] Next, description will be made as to an apparatus for
achieving the principle of the three-dimensional
recording/reproducing method of the present invention shown in the
foregoing section (1).
(2) Three Dimensional Disc Format and Data Management
[0173] FIG. 5A shows an exemplary format of the multi-layer disc 4.
The layers are numbered from 1 to n from the base 3 to which the
light is incident toward the progressing direction of the light.
FIG. 5B shows a dada format on a k.sub.th layer, where m represents
a sector which radially divides the disc, and l represents a track
for managing a data position in the radial direction. Data is
managed by the three addresses (l, m, k). The format on an
arbitrary track l and a sector m comprises a preformat area in
which a timing for recording/reproduction and address information
have previously been stored, and data area for
recording/reproducing user data and recording and managing a
variety of management data such as the presence or absence of data,
read-out inhibition, and so on, as shown in FIG. 5C. The disc is
also provided, in addition to the layers for recording/reproducing
user data, with a ROM (Read Only Memory) layer or a WOM (Write Once
Memory) layer which permits an OS (Operating System) of an upper
level controller or recording or reproduction conditions on each
layer, as will be later described, to be preformatted upon
producing the disc or recorded thereon at the time of shipment.
Also, as a management layer for data written on the user layers,
data conditions of each layer, e.g., the presence or absence of
data, error management, an effective data area, and the frequency
of overwrite may be recorded on the ROM layer at any time. It may
also be used as an exchange layer such that data can be recorded
thereon and reproduced therefrom in place of a layer where a
recording error is detected.
[0174] The order of data recording includes, for example, the
following combinations (a)-(e).
(a) Recording is perform sequentially from the top layer, i.e.,
1.sub.st.fwdarw.k.sub.th.fwdarw.N.sub.th layers. It should be noted
that recording proceeds to the next layer after all user sectors
and tracks have been filled with information in each layer.
[0175] When this type of data recording is performed, a recording
medium which has the characteristic of increasing the
transmissivity after recording may be used to carry out further
reliable recording/reproduction. Specifically, since the
transmissivity up to the lower-most layer increases, light with an
intensity substantially equal to that necessary to record on the
top layer can provide a lower target layer with a sufficient light
intensity required for recording thereon. Also upon reproduction,
since reflected light components from the target layer returns to
the detector substantially without being attenuated, a reproduced
signal with a high SN ratio is generated. A recording medium having
the above-mentioned characteristic is, for example, a perforation
recording medium. When recording is performed on this medium, a
reflection layer thereof is perforated, thereby decreasing the
reflectivity, i.e., increasing the transmissivity.
(b) Recording is performed sequentially from the lowermost layer,
i.e., N.sub.th.fwdarw.k.sub.th.fwdarw.1.sub.st layers. The rest of
the operation is the same as the order (a). (c) Although recording
proceeds to the next layer after information has been recorded on
all user sectors and tracks of each layer, a layer to be recorded
is accessed at random. (d) Although layers to be recorded are
accessed at random, after data has been recorded on a particular
sector in a layer, the same sector in the next layer is filled with
data. After the same sector in all the layers has been full, data
is recorded on the next sector. (e) On a particular track, random
access is performed in the layer direction. In this case, a
variable length block, which is a data management for magnetic
disc, not a fixed block management based on the sector, is applied
to correspond cylinders of a magnetic disc to the layers, whereby a
data format for the magnetic disc can be applied as it is to the
recording medium of the present invention.
[0176] In the random access, the information recording area is
managed by an upper level controller or the foregoing management
area, for example, so as to prevent a recorded area from being
erroneously accessed upon recording.
(3) Whole Arrangement of Apparatus
[0177] FIG. 6 shows the whole arrangement of a three-dimensional
recording/reproducing apparatus. When recording, user data 17 is
supplied to a modulation circuit 18 to generate modulated binary
data 19. The modulated binary data 19 is passed to a recording
condition setting circuit 20 which drives a laser driving circuit
21 so as to modulate the intensity under optimal recording
conditions at a position at which a light spot is positioned. Then,
the laser driving circuit 21 modulates the intensity of light
emitted from a semiconductor laser disposed in an optical head 22
to record user data on a disc 4.
[0178] Conversely, when previously recorded data is reproduced, a
light spot is located at a track position on a target recording
layer on the disc 4, a feeble light is irradiated thereon, and an
intensity change of a reflected light is converted by a photo
detector 10 to an electric signal to generate reproduced signals
23, 24. The reproduced signals 23, 24 are passed through a
reproduction control circuit 25 to suppress inter-layer cross-talk,
and then supplied to an AGC (auto gain control) circuit 26 to
absorb fluctuations of low frequency components which are lower
than a data band to conform the signals to an absolute level which
is processed by subsequent circuits.
[0179] Thereafter, the reproduced signals are passed to a waveform
equalizer 27 to correct distorted waveform (deterioration of
amplitude, phase shift, etc) by using a data pattern, and converted
to binary signals by a shaper 28. The shaper 28 may be one which
converts a signal to a binary code by slicing the amplitude, or one
which detects zero-cross by differentiation.
[0180] The binary signals are next passed to a phase
synchronization circuit 29 where a clock is extracted therefrom.
The phase synchronization circuit 29 is composed of a phase
comparator 30, a low pass filter (LPF) 31 and a voltage control
oscillator 32. The binary signals are passed to an identifier 33
which determines whether a data bit is "1" or "0" by using the
clock extracted by the phase synchronization circuit 29, and
converted to user data 17 by a decoder 34. In the foregoing
recording/reproducing processes, if the light spot is located on a
target layer and at a target position on the target layer by an
instruction from an upper level controller, an out-of-focus signal
and a track shift signal from the optical head 22 are detected by a
detector 35, an appropriate signal for servo control is generated
by a compensation circuit 36, and a light spot positioning
mechanism is driven by a driving circuit 37.
(4) Access Method
[0181] The optical spot positioning mechanism may be a
two-dimensional actuator which drives a focus lens in the layer
direction and the radial direction of the disc or a combination of
a one-dimensional actuator which drives the focus lens 8 only in
the layer direction and a galvano mirror for deflecting light flux
incident to the focus lens 8 to the radial direction of the
disc.
[0182] Here, for a case where random access is performed to record
and reproduce data, as described in Section (2), a method of
firstly focusing on a target layer k will be described. Since the
size of a reflected light spot from the target layer changes due to
out-of-focus, a detection of an out-of-focus signal can employ a
front-to-rear differential out-of-focus detecting method disclosed
in a known document "JP-A-63-231738 and JP-A-1-19535." FIG. 24A
shows an AF error signal 35 generated when the position of the
focus lens 8 is shifted in the layer direction Z with respect to
the disc plane. It can be seen from FIG. 24A that an out-of-focus
error signal from each recording layer and a zero-cross point 105
which represents a focused point are generated in order.
[0183] FIG. 23 shows a block diagram of the first embodiment when a
target k.sub.th layer is accessed. For the rotating disc 4, a
saw-tooth wave 106 is generated by an AF (autofocus) actuator shift
signal generating circuit 93 to drive an AF actuator driver 91,
thus shifting the focus lens 8 in the +Z direction (direction in
which the lens is approached to the disc) with respect to the disc
plane. At this time, an AF detecting circuit 89 generates the AF
error signal 35. From this signal, the zero-cross point 105 is
detected by a withdraw point determination circuit 92, thereby
informing an AF servo system controller 99 of a focused point on
the surface of a certain recording layer. The determination circuit
generates an AF pulse 37 as shown in FIG. 24B by a slice level 37
which is slightly shifted from a zero slice level, and a falling
edge of the AF pulse 37 is detected to supply the controller 99
with a timing immediately before the lens 8 passes a focused
point.
[0184] The controller 99 recognizes a focus withdraw state by an
instruction from the upper level controller, and changes over a
switch 97 at the time the timing is inputted to connect an AF servo
circuit 90 to the AF actuator driver 91 to close the servo loop. In
this state, the AF servo circuit 90 drives the AF actuator such
that the AF error signal is always zero. Thus, a spot at the
diffraction limit can be stably formed on a layer even if the disc
4 swings when rotating.
[0185] Next, a layer number detecting circuit 95 reads a layer
address recorded on the preformat area shown in FIG. 5C to
recognize the number of a layer on which the focus is placed, and
sends the number to the AF servo system controller 99. The
controller 99 recognizes in which of upward or downward direction
(+ or -(k-j)) and how many layers (|k-j|) the spot should jump from
a j.sub.th layer on which the spot is now focused to a k.sub.th
target layer instructed by the upper level controller, and has a
layer jump signal generating circuit 96 generate a jump force
signal 107 which in turn is inputted to the AF actuator driver 91.
The jump signal 107 is composed of a pair of positive-polarity and
negative-polarity pulses per one-layer jump, and replaces the
positive or negative pulse in accordance with the upward or
downward jumping direction. The first pulse is used to drive the
spot approximately by a jumping distance in the jumping direction,
and the next polarity inverted pulse is provided to prevent the
spot from excessively jumping. A number of pairs of pulses equal to
the number of layers over which the spot is to jump is inputted to
the driver 91. Next, the layer number is detected, and when j
becomes equal to k, the spot is positioned on the k.sub.th target
layer. When another layer is to be accessed by random access, the
layer jump may be executed similarly to the above.
[0186] FIG. 25 shows a block diagram of a second embodiment when a
k.sub.th target layer is accessed.
[0187] A focus lens 8 is raised or lowered relative to a rotating
disc 4. At this time, the foregoing AF error signal is generated.
Also, a total light amount 36 detected by a photo detector 10 and
outputted from a total light amount detecting circuit 102 has a
peak value when the focus is placed on each recording layer, as
shown in FIG. 24A. Therefore, a pulse generating circuit 98 in a
cross layer signal detecting circuit 101 detects an AF pulse 37 and
a total light amount pulse 38 by slice levels 103, 108, and the
total light amount pulse 38 is used as a gate to detect falling
edges of the AF pulse, thereby further reliably detecting a focused
point. Moreover, to recognize the direction in which the lens is
shifted relative to the disc, the cross layer pulse generator 99a
generates from these two kinds of pulses an up pulse Pa 109 and a
down pulse Pb 110 which are counted to always recognize on which
layer the lens is located.
[0188] In FIG. 25, a saw-tooth wave 10b is generated from the AF
actuator shift signal generating circuit 93 to shift the AF
actuator, resulting in shifting a focused position from the top
layer to the lowermost layer of the disc. At this time, if a
shifted amount is sufficiently larger than a vertical swinging
amount of the rotating disc, the operation of the AF actuator is
ensured. Focused points on N layers are counted by the cross layer
signal detecting circuit 101, and the top layer (n=1) and the
lowermost layer (n=N) are recognized from an upper limit of the up
pulse 109 when the lens is shifted upwardly and a lower limit of
the down pulse 110 when the lens is moved downwardly, respectively.
A switch 100 is changed over by an instruction from the upper level
controller immediately before the focus is placed on the next
target layer to close the servo loop. Such a control allows layers
to be accessed without providing a layer address.
[0189] Incidentally, when using a medium whose transmissivity and
reflectivity change upon recording information, the AF error and
total light amount signals 123, 124 are different from those shown
in FIG. 24 in the vicinity of a recorded layer as shown in FIG.
26B. This indicates that a light amount changes when the spot scans
a portion where exist marks, and returns to a normal value when the
spot passes a portion without marks. Since the signal thus
fluctuates, even a signal in a servo band also deteriorates to give
rise to fluctuations of a gain in the AF servo system and AF
offset, which results in out-of-focus on a recorded layer. In such
a case, by always holding a signal indicating a portion without
marks in signal components detected by the photo detector, the
ideal AF error signal 35 and total light amount signal 36 are
provided.
[0190] An exemplary means for implementing this method is shown in
FIG. 26B. FIG. 26B specifically shows the AF detecting circuit 89
and the total light amount detecting circuit 102 in FIGS. 23 and
25, respectively. Front and rear photo detectors 111 and 112 in the
drawing illustrating the principle of a front-to-rear differential
AF error signal detecting optical system comprise light receiving
planes 119, 120 or 121, 122. If the sizes of spots 113, 114 are
equal on the front and rear photo detector planes 111, 112, it
indicates a focused point. Sum signals of the respective detectors
are generated by preamplifiers 115, 116 which have a band in which
the spot scans strings of marks, i.e., a data recording/reproducing
frequency band. Next, signals in a scanned mark portion are
detected by sample and hold circuits 117, 118 and held therein
during a period of a servo band. A difference signal of the thus
generated signals is derived as the AF error signal 36, while a sum
signal of them is derived as the total light amount 36. The sample
and hold circuits 117, 118 may be a peak hold type which samples a
maximum point of a light amount. Alternatively, an area in which no
mark is recorded is previously provided as a sample area in a
format, and the sample and hold circuits 117, 118 may recognize
such a sample area by a sample timing bit and hold a signal in that
area.
[0191] Although in this embodiment, a front-to-rear differential
method has been shown as an out-of-focus detecting method, another
out-of-focus detecting method such as an astigmatism method or an
image rotating method may be employed.
[0192] After accessing a target layer, a positioning in the radial
direction of the disc, i.e., track positioning is performed on that
target layer. A track shift signal can be detected by a known
push-pull method by providing each layer with a guide groove 39 as
shown in FIG. 20. In this method, since diffracted lights from
grooves other than the target layer are out of focus, the phase of
light wave striking the grooves is disordered so that a uniform
light distribution is present on the photo detector, whereby no
influence is exerted on the track shift signal about the target
layer. Also, as shown in FIG. 22, if wobble pits 40 are previously
formed on each layer in the track direction, a known sample servo
method can be applied. The above described spot positioning
technique is disclosed in known patent documents JP-A-63-231738 and
JP-A-1-19535. A method of forming guide grooves and wobble pits
will be later described.
(5) Recording Control Method
[0193] Next, description will be made as to a recording control
method which achieves the principle of the three-dimensional
recording method of the present invention shown in Section (1). As
described in Section (1), in order to stably record on a k.sub.th
layer or a target layer, a recording power P (light intensity) must
be determined in consideration of the transmissivity up to the
k.sub.th layer. Thus, as shown in FIG. 6, the recording condition
setting circuit 20 employs address recognition 42 and the
transmissivity 42 up to the target k.sub.th layer. An example of
this circuit is shown in detail in FIG. 7 in a block form, and
examples of signals are illustrated in FIG. 9.
[0194] Referring to FIG. 9, when binary data 19 is recorded as
recording marks 43, recording conditions for the address
recognition 41 (l, m, k), for example, setting of recording pulse
width, recording power setting condition and so on are previously
stored in ROMs 44, 45 in consideration of the difference in
recording conditions due to a recording position and a recording
state by a data pattern, whereby a light intensity modulation
signal P(t) 47 is generated corresponding to the output of a D/A
convertor 46, and accordingly the marks in an ideal recording state
can be recorded. Such a circuit arrangement indicated by solid
lines in FIG. 7A can be applied to the following case.
[0195] When the foregoing Section (2) item (b) is employed as the
order of recording data, or when the third method for achieving (1)
item (2) and Section (2) item (a) are employed, since the
transmissivity 42 up to a target layer (.SIGMA.Tn (n=1, 2, . . . ,
k-1)) has been determined upon producing the disc, if a layer
address k is inputted, the transmissivity is handled as a known
value.
[0196] In the cases other than the above, the transmissivity up to
a target layer is not known at the time of recording. To coop with
this, circuits (47, 48) indicated by broken lines in the circuit of
FIG. 7A are added. The power setting ROM 46 has been loaded with
recording power setting values in consideration of the
transmissivity up to a k.sub.th layer when an all-layers unused
state.
[0197] The transmissivity up to the k.sub.th layer .SIGMA.Tn (n=1,
2, . . . , k-1) upon shipment of the disc (or a design value)
derived by the address recognition 42 and a transmissivity up to
the k.sub.th layer .SIGMA.Tn' (n=1, 2, . . . , k-1) 42 immediately
before recording, detected by a method, later referred to, are
inputted to a division circuit 47, while a change G in
transmissivity is inputted to a gain control circuit 48, so as to
set an optimal recording power.
[0198] An example to which this circuit arrangement can be applied
will be shown. Suppose that "the management layer for layer data"
described in Section (2) is provided and its contents have
previously been reproduced for recognition, the third method for
achieving Section (1) item <3> is applied, and the data
management referred to in Section (2) items (c), (d) is
implemented. If a layer on which recording is in progress is known,
the transmissivity up to the k.sub.th layer .SIGMA.Tn' (n=1, 2, . .
. , k-1) 42 can be derived since the transmissivity of each layer
after recording in a light spot is constant and known.
[0199] Another example is a method of previously scanning the spot
to detect a change G in transmissivity.
[0200] As the method of previously reproducing an area on which
recording is to be performed, after a reproduction check is done in
the first rotation of the disc in a recording mode, recording is
performed in the next rotation, and then a recording error check is
done in the third rotation. Another method is one which employs a
plurality of spots as shown in FIG. 8 and performs a reproduction
check by a preceding spot 49. Here, the latter method is explained
as an example. The reproduction check employs a reproduced signal
C'k(t-.tau.) derived by the preceding spot 49, where .tau.
represents the distance between the preceding spot 49 and a
recording spot 51 converted into a time. Then, the transmissivity
change G is calculated by a processor 52 as a square root of a
ratio of a reproduced signal Ck' in a state where the spot is
focused on the target k.sub.th recording layer to a reproduced
signal Ck as a design value which has previously been set upon
shipment of the disc, as shown in FIG. 7B. This calculation is
performed because the signal is reproduced by using a reflected
light, so that a change in transmissivity up to the k.sub.th layer
appears in the reproduced signal in the form of its square.
[0201] It should be noted however that the value of the reproduced
signal Ck can be detected by previously providing a non-recording
area with respect to the layer direction on a disc plane, as a
check area in a disc format, and absorbing variations among
different discs and optical variations in a disc. A highly accurate
recording power control is thereby achieved. The photo detector 10
for generating reproduced signals may be formed in the shape of
FIG. 1, as has been described in connection with the first method
for satisfying the condition in Section (1) item <3>, to
reduce the influence of lights reflected from other layers, whereby
reflected light components only from the target layer can be
detected as reproduced signal so that the transmissivity change G
can be further accurately derived. Although a recording state 53
when gain control is not performed is different from the ideal
recording state 47 as shown in FIG. 9, the ideal recording state 47
can be achieved by performing a gain control for the recording
power and recording with G.times.P(t).
(6) Reproduction Control Method
[0202] Next, the reproduction control circuit 25 shown in FIG. 6
will be described in detail with reference to the accompanying
drawings. Here, in addition to the principle of reproduction for
reducing inter-layer cross-talk by the first to third methods shown
in Section (1), a fourth method will be shown for suppressing
inter-layer cross-talk components in a data signal band which may
arise when an inter-layer distance is further reduced in order to
achieve a higher recording density or inter-layer cross-talk
components which may arise when the optical system is shifted from
an ideal state. The fourth method, in addition to the detection of
reflected light components from a target layer as shown in the
first method, detects reflected light components from adjacent
layers which particularly include a majority of inter-layer
cross-talk, and removes components mutually included in those
detected by these two methods by a calculation to extract reflected
light components of the target layer.
[0203] FIG. 17A shows the principle of the optical system employed
in the fourth method. Although the basic configuration is the same
as that shown in FIG. 1, photo detectors 54, 55 are further
positioned on focal planes of adjacent layers (k+1), (k-1) on the
light receiving plane side when the focus is placed on a k.sub.th
layer. However, since the photo detectors 54 and 55 disposed as
shown in FIG. 17A mutually shield the light, half mirrors 56, 57
are inserted in the focus system. Alternatively, beam splitters may
be inserted in place of the half mirrors as shown in FIG. 17B. The
shape of the photo detectors 10, 54 and 55 is determined to be a
circle, the diameter of which is D=(.lamda./NAI). Also, pinholes
may be used to implement these detectors as shown in FIG. 17C.
Reproduced signals detected by the respective photo detectors in
this arrangement are shown in FIG. 14.
[0204] FIG. 14 shows a reproduced signal Ck detected by the photo
detector 10; a reproduced signal C(k-1) detected by the photo
detector 55; and a reproduced signal C(k+1) detected by the photo
detector 54. These reproduced signals are generated by a circuit
shown in FIG. 13. It should be noted that if a system shown in FIG.
17 is employed, integration circuits 59, 60 and delay circuits 61,
62 shown in FIG. 13 are not necessary. As shown in FIG. 14, when
the distance between adjacent layers is shorter than the distance
between an adjacent layer which satisfies the first-third methods
and a target layer, a reproduced signal 73 without inter-layer
cross-talk which is derived when a spot 69 scans a mark string 71
on a k.sub.th layer fluctuates as a reproduced signal 72. This is
because, as the spot 69 scans on the k.sub.th layer, components of
a reproduced signal 64 detected from the (k-1).sub.th adjacent
layer by a spot 70 defocused on the (k-1).sub.th adjacent layer
which scans a mark array 74 on the (k-1).sub.th adjacent layer, and
components of a reproduced signal 63 likewise detected from the
(k+1).sub.th adjacent layer on the opposite side in the Z-direction
are included in an unneglectable degree with respect to the
reproduced signal 73. For this reason, the following equation is
calculated by a calculation circuit 66 as shown in FIG. 13.
Ck.apprxeq.CkR+.beta..times.C(k-1)R+.beta..times.C(k+1)R
C(k-1).apprxeq.C(k-1)R+.beta..times.CkR+.beta..times.C(k-2)R
C(k+1).apprxeq.C(k+1)R+.beta..times.CkR+.beta..times.C(k+2)R
(Equation 22)
where CnR represents reproduced signal components by a reflected
light only from an n.sub.th layer.
[0205] In Equation 22, .beta.<1 is satisfied. From the above
equation:
Calculation F .ident. Ck - .gamma. .times. C ( k - 1 ) - .gamma.
.times. C ( k + 1 ) .apprxeq. CkR + .beta. .times. C ( k - 1 ) R +
.beta. .times. C ( k + 1 ) R - .gamma. .times. { C ( k - 1 ) R +
.beta. .times. CkR + .beta. .times. C ( k - 2 ) R } - .gamma.
.times. { C ( k + 1 ) R + .beta. .times. CkR + .beta. .times. C ( k
+ 2 ) R } ( Equation 23 ) ##EQU00012##
[0206] Since C(k-2)R and C(k+2)R are sufficiently small and
frequency components are also low, these terms can be
neglected.
Thus,
F.apprxeq.(1-2.gamma..beta.).times.CkR+(.beta.-.gamma.).times.C(k--
1)R+(.beta.-.gamma.).times.c(k+1)R (Equation 24)
[0207] Here, if .gamma..ident..beta.<1,
F.apprxeq.(1-.beta..sup.2).times.CkR (Equation 25)
whereby the inter-layer cross-talk can be suppressed, and the
reproduced signal 68 after being processed coincides with the
reproduced signal 73 as shown in FIG. 14. As an alternative
arrangement for achieving the foregoing fourth method, an example
of employing a plurality of spots is shown below. Referring again
to FIG. 14, a spot 75 having the same spot diameter as the
defocused spot 70 scans the two adjacent layers (k-1) and (k+1)
prior to the spot 69 to detect signals to be reproduced. Note that
delay circuits 61, 62 corresponding to spot intervals are inserted,
as shown in FIG. 13, to perform calculations similar to the above.
An example of an optical system used in this arrangement is shown
in FIG. 18. Referring to FIG. 18, an optical axis is shown for
three separate cases in order to illustrate the principle of the
optical system. This principle is applicable also to an optical
system employing a focus lens 8. A means for setting the spot
diameter of a spot 75 which is focused on the upper adjacent layer
and a spot 82 which is focused on the lower adjacent layer to
(2d.times.NAF) may be a diaphragm 83 inserted as shown in FIG. 18
to reduce an effective aperture. More specifically, an effective
diameter a' may be changed to .lamda./(2d.times.NAF.sup.2).times.2.
Of course, similar effects can be produced if the numerical
apertures of the focus lenses for the two preceding spots are
reduced. That is, NAF'=.lamda./(2d.times.NAF) is employed.
[0208] While the shape of the preceding spot 75 is hitherto the
same as the spot shape 75 shown in FIG. 14 (i.e., the same shape as
that of the spot 70), a spot shape 76 shown in FIG. 15 (an
elliptical shape oblonger than the spot 75) or three spots 77, 78
and 79 shown in FIG. 16, by way of example, can produce similar
effects. For employing these spots, integration circuits 59, 69 are
inserted in the circuit of FIG. 13. Referring to FIG. 15, if a
reproduced signal detected by the preceding spot is multiplied with
a weighting function 80 derived by approximating a Gaussian
distribution, which is an intensity distribution of the spot, to,
for example, a triangle distribution and integration is performed
to this product, a reproduced signal when the spot 75 is scanning
the mark string 74 can be effectively derived. As to the spots
shown FIG. 16, a weighting function 81 may be similarly employed in
consideration of a spot intensity distribution in the
two-dimensional direction.
[0209] Now, description will be made as to a method of calculating
.beta. in a weight setting circuit 67 used in the calculation
circuit 66 shown in FIG. 13. If mark recording areas on at least
three layers including upper and lower adjacent layers are located
as a disc format such that they are not included in the same light
flux, as shown in FIG. 19, h(k-1)/hk and h(k+1)/h are set to
.beta.(-1) and .beta.(+1) as shown in FIG. 14, whereby weights for
the upper and lower adjacent layers can be derived.
[0210] While in the embodiments so far described, description has
been made as to a case where recording/reproduction is performed
basically on a single layer, it is also possible to simultaneously
recording/reproducing on two or more layers by using a plurality of
spots and focusing these spots on each layer. In other words,
parallel recording/reproduction is enabled, whereby a data transfer
rate can be increased. A means for forming a plurality of spots may
be a plurality of optical heads 22 which are located on a single
disc or a single head having a plurality of light source
incorporated therein. Also, by employing light sources which
generate lights having different wavelength from each other as a
plurality of light sources, a recording layer to be recorded can be
selected by a wavelength, and separate reproduction is also enabled
by a wavelength filter.
(7) Embodiment of Disc Structure and Disc Producing Method
[0211] On the surface of a disc-shaped chemical tempered glass
plate having a diameter of 130 mm and a thickness of 1.1 mm, a
replica substrate 401 is produced by a photo polymerization method
(2P method). Formed on the replica substrate 401 is a UV cured
resin layer having tracking guide grooves at intervals of 1.5 .mu.m
and prepits (referred to as a header section) in the form of uneven
pits in elevated portions between grooves at the start of each of
17 sectors formed by equally dividing the disc plane for
representing layer addresses, track addresses, sector addresses and
so on.
[0212] The structure of the disc will be explained with reference
to FIG. 11. On the replica substrate 40, an antireflection film 402
made of silicon nitride (SiN) was formed in a thickness of about 50
nm by using a sputtering apparatus which provides good uniformity
and reproductivity of film thickness. Next, a recording film 403
composed of In.sub.54Se.sub.43Tl.sub.3 was formed in a thickness of
10 nm in the same sputtering apparatus. On this recording film 403,
a UV cured resin layer 404 having tracking guide grooves and
prepits representing layer addresses, sector addresses, track
address and so on was formed in a thickness of 30 .mu.m in
consideration of a heat insulation effect associated with other
layers by the 2P method which uses a transparent frame such that
the light is incident from the frame side.
[0213] Subsequently, an SiN antireflection film 405 was formed in a
thickness of about 50 nm in the sputtering apparatus, and on this
layer a recording film 406 composed of In.sub.54Se.sub.43Tl.sub.3
was formed in a thickness of 10 nm. Further on this film a UV cured
resin layer 407 having tracking guide grooves and prepits
representing layer addresses, sector addresses, track address and
so on was formed in a thickness of 30 .mu.m by the 2P method.
Further on this layer, an SiN antireflection film 408 of silicon
nitride was formed in a thickness of about 50 nm in the same
sputtering apparatus, and a recording film 409 composed of
In.sub.54Se.sub.43Tl.sub.3 was formed in a thickness of 10 nm on
this antireflection film 408.
[0214] In the same manner, an SiN antireflection film 402'; an
In.sub.54Se.sub.43Tl.sub.3 recording film 403'; a UV cured resin
layer 404'; an antireflection film 405'; an
In.sub.54Se.sub.43Tl.sub.3 recording film 406'; a UV cured resin
layer 407'; an SiN antireflection film 408'; and an
In.sub.54Se.sub.43Tl.sub.3 recording film 409' were sequentially
formed on a like replica substrate 401'. The two discs thus
produced were bonded by a bonding agent layer 410 with the layers
409 and 409' being directed inwardly. The thickness of the bonding
agent layer is about 50 .mu.m. The disc thus produced allows
recording/reproduction to be performed on a single disc from both
sides.
[0215] While in the foregoing example of the disc production, guide
grooves 39 for push-pull tracking has been explained, the wobble
pits 40 used for a sample servo method can be likewise formed by a
similar method to that for forming the prepits.
[0216] The disc produced in this embodiment is such that a change
in atomic arrangement of atoms constituting the recording films is
caused by irradiation of a laser light to change optical constants,
and data is read out utilizing the difference in reflectivity. The
change in atomic arrangement refers to a phase change between
crystalline and non-crystalline.
[0217] In the disc immediately after forming the recording films,
the recording film constituting elements are not sufficiently
reacted so that the recording films are in a non-crystalline state.
When this disc is used as a Write Once type medium, a recording
laser light is irradiated to the recording films to perform
crystallization recording. Alternatively, the recording films are
previously heated by irradiation of an Ar laser light, flash anneal
or the like, such that each element is sufficiently reacted and
crystallized, and thereafter a recording laser light with a high
power density is irradiated to the recording films to perform
non-crystallization recording. Here, a range of a laser power
suitable to the crystallization recording should be above a
temperature causing crystallization and below a temperature causing
non-crystallization. On the other hand, when this disc is used as
an overwritable type medium, the recording films are previously
heated by irradiating an Ar laser thereto, subjected to flash
anneal or the like to sufficiently react and crystallize each
element, and thereafter, a recording laser light which is modulated
between a laser power suitable for crystallization and a laser
power suitable for non-crystallization is irradiated onto the
recording films to overwrite data thereon.
[0218] The disc was rotated at 1800 rpm, the light (wavelength is
780 nm) from a semiconductor laser maintained at a power level (1
mW) with which recording was not performed was converged by a lens
disposed in a recording head and irradiated onto a recording film
on the first layer through the substrate, and a reflected light was
detected, thereby driving the head such that the center between the
tracking grooves was always coincident with the center of a light
spot. By forming a recording track between two adjacent grooves,
the influence of noise generated from the grooves can be avoided.
Automatic focusing was performed so as to focus the light spot on
the recording film while thus continuing the tracking, to perform
recording/reproduction. When the light spot passed a recording
portion, the laser power was decreased to 1 mW, and the tracking
and automatic focusing was still continued. It should be noted that
the tracking and automatic focusing is maintained also during a
recording operation. This focusing enables the light spot to be
independently focused on the respective recording layers 403, 406
and 409 of the disc.
[0219] A case where recording was performed sequentially on
recording films from the substrate side toward the lowermost layer
will be shown, assuming that the disc constructed as described
above is rotated at a linear velocity of 8 m/s (rotational speed:
1800 rpm, radius: 42.5 mm). First, the focus was placed on the
recording film 403 which was irradiated with a recording pulse with
a recording frequency at 5.5 MHz and a period of 90 ns to record on
this film 403. A recording power dependency of a reproduced signal
intensity at this time is shown below:
TABLE-US-00001 Reproduced Signal Intensity Recording Power (mW)
(mV) 6 30 7 100 8 160 9 210 10 250 11 280 12 300 14 310
[0220] Then, after recording on the recording film 403, the light
spot was focused on the recording film 406 to perform recording
thereon. A recording power dependency of a reproduced signal
intensity at this time is shown below:
TABLE-US-00002 Reproduced Signal Intensity Recording Power (mW)
(mV) 7 25 8 95 9 155 10 205 11 245 12 275 13 295 15 305
[0221] After recording on the recording films 403 and 406, the
light spot was focused on the recording film 409 to perform
recording thereon. A recording power dependency of a reproduced
signal intensity at this time is shown below:
TABLE-US-00003 Reproduced Signal Intensity Recording Power (mW)
(mV) 8 20 9 90 10 150 11 200 12 240 13 270 14 290 16 300
[0222] Also, the results will be shown below, when, after recording
a signal at 3 MHz on the recording film 403, a signal at 4 MHz on
the recording film 406 and a signal at 5 MHz on the recording film
409, the light spot was focused on the recording films 403, 406 and
409 to read reproduced signals therefrom.
[0223] The reproduced signals were analyzed by a spectra analyzer,
and, as a measuring condition, the resolution frequency width was
selected to be 30 kHz. The following table shows measurement
results of CN ratios (ratio of noise components to carrier
components) of the reproduced signals at a carrier frequency.
TABLE-US-00004 3 MHz 4 MHz 5 MHz Recording Film 403 55 dB 23 dB 6
dB Recording Film 406 25 dB 53 dB 21 dB Recording Film 409 10 dB 23
dB 51 dB
[0224] It will be understood from the above table that highly
reliable signals were reproduced from each layer with the CN ratio
of not less than 50 dB and inter-layer cross-talk from adjacent
recording layers below 25 dB.
[0225] Next, another disc was produced, where thin films composed
of Ge.sub.14Sb.sub.29Te.sub.57 were formed in a thickness of 2 nm
as the recording films 403, 406 and 409, thin films of ZnS were
formed in a thickness of 50 nm as the antireflection films 402, 405
and 408, and the rest of the structure was completely the same as
the foregoing disc. This disc features that the transmissivity of
recorded layers decreases. For this reason, recording is performed
from the substrate side.
[0226] The measurement was done under the condition that the disc
structured as described above was rotated at a linear velocity of 8
m/s (rotational speed: 1800 rpm, the radius: 42.5 mm), and
recording was performed sequentially from the lowermost layer
toward upper layers. First, the focus was placed on the recording
film 409 to record thereon by irradiating a recording pulse with a
recording frequency at 5.5 MHz and a duration being 90 ns. The
recording power dependency of the reproduced signal intensity at
that time is shown in the following table.
TABLE-US-00005 Reproduced Signal Intensity Recording Power (mW)
(mV) 7 15 8 85 9 145 10 195 11 235 12 265 13 285 15 295
[0227] After recording on the recording film 409, the focus was
placed on the recording film 406 to record thereon. The recording
power dependency of the reproduced signal intensity at that time is
shown in the following table.
TABLE-US-00006 Reproduced Signal Intensity Recording Power (mW)
(mV) 7.5 20 8.5 90 9.5 150 10.5 200 11.5 240 12.5 270 13.5 290 15.5
300
[0228] After recording on the recording films 409 and 406, the
focus was placed on the recording film 403 to record thereon. The
recording power dependency of the reproduced signal intensity at
that time is shown in the following table.
TABLE-US-00007 Reproduced Signal Intensity Recording Power (mW)
(mV) 8 25 9 95 10 155 11 205 12 245 13 275 14 295 16 305
[0229] Also, the measurement results will be shown in the following
table as to the CN ratios at a carrier frequency of reproduced
signals which were read from the recording films 403, 406 and 409
by placing the focus thereon, after signals at 3 MHz, 4 MHz and 5
MHz had been recorded on the recording films 403, 406 and 409,
respectively.
TABLE-US-00008 3 MHz 4 MHz 5 MHz Recording Film 403 54 dB 24 dB 7
dB Recording Film 406 26 dB 52 dB 22 dB Recording Film 409 11 dB 24
dB 50 dB
[0230] As shown in the above table, highly reliable signals were
reproduced from each layer with the CN ratio of not less than 50 dB
and inter-layer cross-talk from adjacent recording layers below 25
dB.
[0231] When a plastic disc of polycarbonate or acrylic resin made
by injection molding was used as the substrate other than the
chemical tempered glass used in the above embodiment, similar
results were obtained.
[0232] Also, when Ge--Sb--Te composition, Ge--Sb--Te-M (M
represents a metal element) composition, In--Sb--Te composition,
In--Sb--Se composition, In--Se-M (M represents a metal element)
composition, Ga--Sb composition, Sn--Sb--Se composition,
Sn--Sb--Se--Te composition and so on were used as the recording
film other than the foregoing In--Se--Tl composition, similar
results were likewise obtained.
[0233] Further, other than the foregoing recording film utilizing a
phase change between crystalline and non-crystalline, An In--Sb
composition utilizing a crystalline-to-crystalline phase change or
the like may be used as a recording film to derive similar
results.
[0234] Particles of Bi-substituted garnet
(YIG(Y.sub.3Bi.sub.3Fe.sub.10O.sub.24)) of 20 nm in diameter were
dispersed in an organic binder and spin coated to produce a
recording film on a substrate similar to that shown in FIG. 11. The
Bi-substituted garnet of 20 nm in diameter was produced by a
coprecipitation method. The used organic binder was that with the
refractivity equal to 25. A film thickness of the spin coated
recording film was about 1.5 .mu.m, and the reflectivity (R),
transmissivity and absorptivity (k) thereof were R=8%, T=12% and
K=80%, respectively, at a wavelength of 530 nm. Since the volume
ratio of the Bi-substituted garnet in the binder was about 60%, a
rotating angle of plane of polarization of a reflected light was
about 0.8.degree.. A method of stacking a multiplicity of layers
with UV cured resin layers inserted between the layers, a method of
bonding two discs, and a recording/reproducing method were similar
to those of the foregoing embodiment. However, the wavelength of a
light source is selected to be .lamda.=530 nm.
[0235] Next, explanation will be given of an example where an
experiment was made on recording/reproduction using the information
recording medium structured as shown in FIG. 12. FIG. 12A shows
part of a cross-sectional view of an information recording medium;
and FIG. 12B shows a cross-sectional view of part of a recording
layer.
[0236] A laser light guide groove with a track pitch being 1.5
.mu.m was formed in a UV cured resin layer 412 of 50 .mu.m in
thickness on a disc-shaped glass substrate 411 with a diameter of
13 cm and a thickness of 1.2 mm. Next, a recording layer 413 was
stacked by a vacuum vapor deposition method. The recording layer
413 comprises two Sb.sub.2Se.sub.3 layers 414 of 8 .mu.m in
thickness sandwiching a Bi layer 415 of 3 .mu.m in thickness, as
shown in FIG. 10B. Further, on the recording layer 413, two pairs
of a UV cured resin layer 412 of 30 .mu.m in thickness formed with
laser light guide groove and the recording layer 413 were stacked.
In other words, three recording layers were provided. On the top, a
UV cured resin layer of 100 .mu.m in thickness was provided for the
purpose of protecting the recording layers. It is assumed that the
recording layers are referred to as a first recording layer, a
second recording layer and a third recording layer from the
substrate side.
[0237] A track groove was selected to be U-shaped one, and the
widths of a land portion and a groove portion were both selected to
be 0.75 .mu.m. Measured reflectivities of the first, second and
third recording layers were 8.5%, 5.8% and 4.4%, respectively.
Recording was performed by irradiating each recording layer with a
laser light of not less than 6.0 mW. The reflectivities of laser
light irradiated portions on the first, second and third recording
layers were 18.5%, 13.0% and 9.4%, respectively.
[0238] The change in reflectivity between the recorded and
unrecorded recording layers is caused by the alloying of the
recording layers. Specifically explaining, when part of recording
layer made up of two Sb.sub.2Se.sub.3 layers and a Bi layer is
heated by the irradiation of the recording laser light, a diffusion
reaction occurs between Se and Bi, which results in alloying.
Consequently, an area with different optical constants, i.e., a
recording point is formed on the recording layer. It should be
noted that in the recording layer composed of Sb.sub.2Se.sub.3 and
Bi, the alloying causes the reflectivity and the transmissivity to
increase and the absorptivity to decrease.
[0239] Although not performed in this embodiment, if a land portion
is irradiated with a continuous laser light before recording, the
land portion is alloyed, with the result that an average
transmissivity per recording layer is increased by 10%. Therefore,
since the reflectivities before and after recording are increased,
this is convenient to tracking and so on. If recording is performed
on both the land portion and the groove portion, an average
transmissivity per recording layer can be likewise increased. The
recording layer is not limited to a combination of Sb.sub.2Se.sub.3
and Bi, but may be of any combination as long as alloying is caused
by temperature rise.
[0240] Since the present invention provides light spot focusing
optical system, a disc structure, and a light detecting optical
system which enable stable recording and reproducing in recording
and reproducing processes, a coding method for suppressing
particularly problematic inter-layer cross-talk, a cross-talk
canceling method, a three-dimensional data format, a disc producing
method associated with the data format, a three-dimensional access
method, highly reliable data can be recorded and reproduced by
focusing a light spot on each layer of a multi-layer structured
disc.
* * * * *