U.S. patent application number 11/595063 was filed with the patent office on 2007-03-08 for device and method for reading multilayer optical disk.
This patent application is currently assigned to THOMSON LICENSING S.A.. Invention is credited to Holger Hofmann, Hartmut Richter.
Application Number | 20070053266 11/595063 |
Document ID | / |
Family ID | 8170348 |
Filed Date | 2007-03-08 |
United States Patent
Application |
20070053266 |
Kind Code |
A1 |
Hofmann; Holger ; et
al. |
March 8, 2007 |
Device and method for reading multilayer optical disk
Abstract
Device and method for reading multilayer optical disk The
invention refers to a device and a method for scanning an optical
data carrier (1) by means of a main scanning beam (3) being
reflected by the data carrier (1) and being detected by means of a
photodetector (10), the data carrier (1) having first and second
layers (11,12) to be scanned, the second layer (12) being scanned
through the first layer (11), light passing the first layer (11)
being shaded dependent on local properties of the first layer (11).
An object of the invention is to propose a device having increased
performance even for use with multi layer disks (1) having a layer
(11) causing light fluctuations on a layer (12) to be scanned, as
well as a respective method. According to the invention such device
is provided with means (2,7,17) for keeping an output signal (D) of
the photodetector (10) independent of variations of shading.
Inventors: |
Hofmann; Holger;
(Neustetten, DE) ; Richter; Hartmut;
(Villingen-Schwenningen, DE) |
Correspondence
Address: |
THOMSON LICENSING INC.
PATENT OPERATIONS
PO BOX 5312
PRINCETON
NJ
08543-5312
US
|
Assignee: |
THOMSON LICENSING S.A.
|
Family ID: |
8170348 |
Appl. No.: |
11/595063 |
Filed: |
November 9, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10416263 |
May 8, 2003 |
7158471 |
|
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11595063 |
Nov 9, 2006 |
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Current U.S.
Class: |
369/53.1 ;
G9B/7.024; G9B/7.033; G9B/7.034; G9B/7.039; G9B/7.1; G9B/7.111 |
Current CPC
Class: |
G11B 7/13 20130101; G11B
7/24088 20130101; G11B 2007/0013 20130101; G11B 7/24085 20130101;
G11B 7/1263 20130101; G11B 7/0052 20130101; G11B 20/00086 20130101;
G11B 7/00736 20130101; G11B 23/281 20130101; G11B 7/00718 20130101;
G11B 7/00745 20130101; G11B 7/24038 20130101; G11B 7/0903 20130101;
G11B 20/00586 20130101 |
Class at
Publication: |
369/053.1 |
International
Class: |
G11B 20/00 20060101
G11B020/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 10, 2000 |
EP |
00124632.1 |
Claims
1-5. (canceled)
6. Method for preparing for scanning an optical data carrier having
first and second layers to be scanned, the second layer being
scanned through the first layer, light passing the first layer
being shaded dependent on local properties of the first layer,
having the step of providing the first layer with a shading pattern
for compensating for local differences in shading effect.
7. Method according to claim 6, wherein the shading pattern is
provided only in areas having predetermined characteristics.
8. Method according to claim 6, wherein the shading pattern is
either a grey level pattern or a pit pattern, the pits of the pit
pattern being either larger than the largest data recording pits or
smaller than the smallest data recording pits.
9. Method according to claim 6, wherein the shading pattern is used
to store additional information.
10. Device for performing a method according to claim 6.
11. Optical data carrier having first and second layers to be
scanned, the second layer being scanned through the first layer,
light passing the first layer being shaded dependent on local
properties of the first layer, wherein the first layer has a
shading pattern for compensating for local differences in shading
effect.
12. Optical data carrier according to claim 11, having said shading
pattern only in areas having predetermined characteristics.
13. Optical data carrier according to claim 11, wherein the shading
pattern is either a grey level pattern or a pit pattern, the pits
of the pit pattern being either larger than the largest data
recording pits or smaller than the smallest data recording
pits.
14. Optical data carrier according to claim 11, wherein the shading
pattern is used to store additional information.
Description
[0001] The invention refers to a device and a method for reading
and/or writing dual layer or multi layer optical disks, also
referred to as scanning such optical data carriers. Use of multiple
layers is one possibility to increase storage capacity of an
optical disk. Amongst others, phase-change dual-layer recording is
regarded to be one of the most promising technologies for recording
optical disks. A dual layer disk is provided with a first layer
which is semi-transparent and partly reflective, and a second layer
which is reflective. The second layer is scanned with a beam of
light that passes through the first layer. In case of a partly
written first layer or in case of a disk according to a so-called
hard-sectorized format the first layer causes light intensity
fluctuations on the second layer due to different light
transmission in already written areas and areas not yet written to.
These light fluctuations are also referred to as shading or shading
effect in the following. According to said hard-sectorized formats
sectorization is done for example by pre-embossed markings, the
so-called pre-pits. A pre-pit area is an example of an already
written area, a recorded area is another example. Hard sectorized
formats are e.g. based on land/groove and prepit structure as e.g.
used for the known DVD-RAM standard. The light fluctuations on the
second layer mentioned above significantly reduce read out
performance and recording signal quality. This is especially the
case if phase-change recording layers are used as these are very
sensitive on recording laser power changes. However, the problem
mentioned above occurs not only with dual layer phase-change
materials, but with all multi-layer disks in combination with
materials, where information is stored in sequences that cause dark
and bright sequences in the transmitted light. Such dark and bright
sequences may have several causes, e.g. different reflectivity,
different absorption factors or different interference behaviour,
destructive/constructive, of different areas of a layer, or
differences in other properties having similar effect. The
different layers may be of a single type or of different types, as
recordable, pre-recorded or non-recorded.
[0002] It is an object of the invention to propose a device having
increased performance even for use with multi layer disks having a
layer causing light fluctuations on a layer to be scanned, as well
as a respective method.
[0003] According the invention this object is solved by the
features indicated in the independent claims. Advantageous features
are also indicated in the dependent claims.
[0004] A device according to the invention for scanning an optical
data carrier by means of a main scanning beam being reflected by
the data carrier and being detected by means of a photodetector,
the data carrier having first and second layers to be scanned, the
second layer being scanned through the first layer, light passing
the first layer being shaded dependent on local properties of the
first layer, is provided with means for keeping an output signal of
the photodetector and, if provided for, a following amplification
circuit independent of variations of shading. This has the
advantage that an output signal which lies within a certain
constant range is easy to further process, independent of the
quality or the status of the shading causing layer.
[0005] The device comprises advantageously a scanning beam
generating means having variable intensity control, the intensity
being varied in proportion to the amount of shading. Also
advantageously, it comprises an output signal amplifier having
variable gain, the gain being varied in proportion to the amount of
shading. Of course, also a combination of both features has
positive effects. The changing transmission of the semitransparent
layer or shading caused by it is detected by special means of the
optical head of the device. In case that the beam generating means
is a laser, the laser power is changed accordingly to compensate
the shading. The laser power is tuned such that--independently
whether the light is transmitted through a written area or a
non-written area, or through the data area or the prepit area--no
light intensity fluctuations occur on the recording surface during
recording and no light intensity changes occur on the detector of
the optical head during readout. For readout the compensation of
the shading is preferably done via variation of the gain of the
amplifier for the output signal of the photodetector. As the laser
power should not exceed a certain level during readout, since data
erasing could occur in this case, variation of gain is more
appropriate for readout. However, also a combination of gain change
and laser power change is advantageous e.g. because the variation
range for the gain change is smaller in this case.
[0006] Advantageously, the device further comprises a beam
generator for generating a shading detection beam, a photodetector
for detecting said beam and delay means for delaying an output
signal of said photodetector. In this way shading detection is
easily and reliably performed. The beam generator is either a light
source as a laser, a beam splitter as a semitransparent mirror, a
grating or a holographic optical element, or another appropriate
element. The shading detection beam is arranged ahead of the
scanning beam, seen in scanning direction. It thus reaches an area
of different shading earlier than the main scanning beam. This
leaves sufficient time to prepare for timely adaptation of laser
power or amplifier gain or both. Preferably the detection beam is a
preceding beam that is generated anyway for other purposes, e.g.
for use in a three-beam scanning method where a tracking error
signal is generated from side beams or using side beams to correct
a signal generated from the main beam, as it is the case for the
so-called well-known Differential Push Pull method. The distance,
at the scanned layer, between detecting beam and main scanning beam
is preferably chosen much larger then necessary for switching of
power or gain. This makes possible to increase scanning speed or to
perform additional tasks before shading occurs. Advantageously the
shading detection beam signal is delayed, e.g. via a simple delay
line or by being stored in a memory, so as to be available at the
time where it is needed to initiate switching. Preferably the delay
time is variable in order to make possible adaptation, e.g. to
different scanning speeds, different speed of switching process,
different data carrier properties or other properties that might
alter or change in time or on other circumstances. It is also
advantageously possible to use as detection beam a beam being
arranged a relatively big distance ahead of the main scanning beam,
e.g. arranged 90.degree. or 180.degree. degrees ahead, but scanning
the same or nearly the same track. It is also one of the inventive
solutions to determine shading effects by the main beam itself or a
succeeding beam and to delay the respective signal for the time of
a complete or nearly complete rotation of the disk. Another
solution according to the invention lies in the fact that the
beginning of a shading area is predicted from the location of an
indicative position on the track, the distance from which to the
shading area is known, and the disk rotation speed. For example the
pre-pit areas are arranged according to predefined rules so that
the beginning of a shading pre-pit area can be predicted from the
time elapsed after having passed the preceding one.
[0007] A method for scanning an optical data carrier according to
the invention comprises the steps of detecting variations of
shading of the second layer and changing at least one of intensity
of main scanning beam and amplification of photodetector output
signal in proportion to the detected variations. This has the
advantage, apart from those already indicated above, that it does
not require a special process for the production of dual layer
disks, but solves the object of the invention within the
device.
[0008] Preferably a method according to the invention provides for
detection of variations of shading by detecting intensity of a
shading detection beam scanning the data carrier. Preferably the
detected intensity signal is low-pass filtered. This has the
advantage that high frequent disturbances, as such caused by the
data stored on the disk or by high frequent variation of laser
power, do not occur in the low-pass filtered signal. As already
described above, it is advantageous to delay the intensity signal
in dependency on the distance between shading detection beam and
scanning beam, or in dependency on a known distance between a first
indicative position and a second shading position.
[0009] According to the invention a method for preparing for
scanning an optical data carrier having first and second layers to
be scanned, the second layer being scanned through the first layer,
light passing the first layer being shaded dependent on local
properties of the first layer, provides for compensation of local
differences in shading effect by providing the first layer with a
shading pattern. That means that areas of the first layer that have
a low shading effect are intentionally provided with a shading
causing or increasing pattern. This pattern is designed such that
it does not have an influence of reading from or writing to the
first layer but ascertains sufficient shading for not having
intensity fluctuations on the second layer. An advantage of this
method is that it is to be performed only once, at the first
writing on the first layer. All non-written areas may be provided
then with a shading pattern and all pre-pit areas or similar areas
are also provided with sufficient shading. Of course, the disk may
also be pre-formatted during production or before being provided to
the customer, so that direct access to both or all of the layers is
possible without suffering from fluctuations.
[0010] Although the complete disk may be provided with a shading
pattern according to the invention, it is especially advantageous
to provide a shading pattern only in areas having predetermined
characteristics, as the pre-pit areas or other areas having
pre-embossed patterns. In case of a phase-change disk the
transmission properties of the pre-pit areas are changed, during
first recording of the semi-transparent first layer, to the same
level as the recorded data area. Depending on the initial
phase-change state of the disk, e.g. un-initialized disk with
amorphous recording layer or initialized disk with crystallized
layer, this can be done by erasing or writing a certain pattern in
the prepit area. Certain in this case means that the shading
pattern, of course, is to be chosen such as not disturb the readout
of the pre-pit information.
[0011] According to the invention the shading pattern is either a
grey level pattern or a pit pattern the pits of which are larger
than the largest data recording pits or a pit pattern the pits of
which being smaller than the smallest data recording pits. The
certain recorded or erased pattern is e.g. a grating structure,
where adjacent tracks are
amorphous-crystalline-amorphous-crystalline-etc. with
bright-dark-bright-dark-etc. stripes. It is to be noted that
different phase-change materials and/or layer stack designs exist.
High-to-low or low-to-high materials/stacks means here that the
ground state has high reflectivity or low reflectivity,
respectively. However, ground state is usually supposed to be
crystalline. Averaged over several tracks the reflectivity or, more
important, the transmission is the same or nearly the same for
pre-pit areas as for the data areas, where dark and bright data
pits are recorded. The grating structure according to the
invention, i.e. where the pattern is either a grey one or the pits
of the shading pattern are either much larger or much smaller than
the data pits or the pre-pits, assures that it does not degrade the
readout of the pre-embossed pre-pits. Also a grating structure
having stripes arranged in track direction but having a much
smaller width lie within the scope of the invention.
[0012] Advantageously the shading pattern is also used to store
additional information. Especially in the case of shading pits
length or distance variations of these pits or different
distinguishable grey levels may be used to store information as
recording parameters, indication of the device, date of recording,
copyright information, encryption parameters etc. Even if such
information should, at present, not being readable by means of mass
produceable devices, it might be applicable for copyright issues or
for use with future devices being able to detect much larger or
smaller pits.
[0013] Data carriers being provided with a shading pattern
according to the invention as well as devices performing a method
according to the invention also lie within the scope of the
invention. This is also the case for combinations of features not
especially named herein or changes of design lying within the scope
of a skilled person. Further variants and advantages of the
invention are also included in the following description of
preferable embodiments using figures.
[0014] In the figures:
[0015] FIG. 1 shows a device according to the invention
[0016] FIG. 2 shows a track structure with shading pattern
according to the invention
[0017] FIG. 3 shows another track structure with shading pattern
according to the invention
[0018] FIG. 4 shows a different type of shading pattern according
to the invention
[0019] FIG. 5 shows a further type of shading pattern according to
the invention
[0020] FIG. 1 shows a device according to the invention in a
diagrammatic view. The optical data carrier is in this embodiment a
disk 1, being shown in a partial cross sectional view. Dimensions
are not to scale in order to more clearly show relevant features.
Disk 1 is provided with a first recording layer 11 and a second
recording layer 12. First layer 11 is a semitransparent layer while
second layer 12 is a reflecting layer. Transparent cover layer 13
and intermediate layer 14 are provided but not described in detail
here. At the right part of layer 11 a stack of layers 15 is
indicated, as instead of a single layer 11 also several layers may
be arranged above layer 2. However, only a single layer 11 will be
described here, as application to a stack of layers 15 is a task
within the scope of a skilled person.
[0021] The device comprises as light source a semiconductor laser 2
generating a scanning beam 3. The laser 2 is provided to be driven
with different power. This enables the device to perform recording,
adaptation to different types of disk 1 and adaptation according to
the invention as described below. Beam 3 is collimated by a
collimator 4, passes through a beam splitter 5 and is focused by an
objective lens 6 onto the second layer 12. A grating 7 is provided
in the path of beam 3 for generating, apart from the main scanning
beam 3, secondary beams 3', 3'', of which one is used as a shading
detection beam 3'. Shading detection beam 3' is arranged thus that
it precedes main scanning beam 3 on the disk 1 in direction of
scanning. Beam 3' is shown separately from main beam 3 at the lower
part of FIG. 1 in an exaggerated way, as well as side beam 3''.
Beams 3, 3', 3'' are reflected by second layer 12, directed by beam
splitter 5 to a focussing lens 8 and focussed by lens 8 onto a
photodetector 10.
[0022] Photodetector 10 comprises three detector areas 9, 9', 9''
onto which beams 3, 3', 3'' fall, respectively. Detector areas 9,
9', 9'' are each subdivided into 2 or more detector elements in
order to generate detector signals D necessary for performing
control functions as focus control or tracking control and for
generating a data signal according to known methods. By way of
example, the astigmatism method may be implemented as focussing
method, using detector area 9, the differential push pull method
DPP, using all detector areas 9, 9', 9'', may be implemented as
tracking method. Elements necessary to perform these functions are
known to the skilled person and are thus not depicted here.
[0023] Output signal D' of detector area 9' is supplied to a
low-pass filter 16, the low-pass filtered signal DL is provided to
an amplifier 17' having variable gain G' and output as shading
detection signal SD. As an alternative solution, shading detection
signal SD is supplied to a delay element 18 having a variable delay
time T. The output signal of delay element 18 is the delayed
shading detection signal SD'. Similar amplifiers 17, 17'' having
also variable gain G, G'' are provided to amplify output signals of
detector areas 9, 9'', respectively. In most cases it will be
appropriate to use identical gain for all three amplifiers,
however, for some detection methods different gains might be
preferable.
[0024] Scanning beam 3 is focussed onto the second layer 12 as a
small scanning spot 22 while its cross section with the first layer
11 is an enlarged spot 21. This enlarged spot 21 covers an area of
dark and bright areas and is thus shaded by the dark areas. The
ratio between dark and bright areas is constant in case of a
completely written area covered by spot 21. It is not constant in
case also non-written areas fall within spot 21. Thus, different
shading occurs in such cases, the intensity of the light forming
scanning spot 22 fluctuates as the shading does.
[0025] During scanning disk 1, a forthcoming obscurity due to
shading caused by the semi-transparent first layer 11 is probed by
an additional, leading spot, here by a scanning spot 22' of shading
detection beam 3'. This spot 22' is ahead of the main spot 22. The
separation distance between main spot 22 and leading spot 22' is
designed to allow timely laser power change or amplifier gain
switching respectively, if shading occurs. Since the detection
signal caused by shading is of low frequency, it is low-pass
filtered and is thus not degraded by the data signal or rf laser
modulation, both having high frequency. In case that so-called
differential push-pull tracking method is applied, the leading spot
22' of the three beams 3, 3', 3'' is preferably used for detecting
shading caused by the first layer 11.
[0026] That means that the device according to the invention
detects changes in the reflected intensity of shading detection
beam 3' before the main readout beam is affected. This time
difference is sufficient to switch gain for the main beam at
exactly the time when the main beam intensity changes. This means
that there is practically no change in the electronical readout
signal of the main beam although shading is present.
[0027] FIG. 2 shows, in a diagrammatic way, a small part of the
track structure of the first layer 11 of a multi layer disk 1 in
top view. Several tracks 19 are arranged in parallel, the centre 20
of each track 19 is indicated by a dotted line. Indicated by a
circle is the enlarged spot 21 of main scanning beam 3. Each track
is provided with data markings, also named pits 23, 24, indicated
here by ellipsoids of similar width perpendicular to the track
direction but of different length in track direction. At the left
and the right side of FIG. 2 there are depicted data areas 26 which
extend much longer to the respective sides than indicated here.
Between the data areas 26 there is arranged a header area 25. In
case of a hard-sectorized disk 1, the header area 25 is provided
with pre-embossed, permanent pits, so-called pre-pits 23. The
pre-pits 23 are permanent, i.e. they cannot be written or deleted
by a recording device. In the example shown in FIG. 2, pre-pits 23
are arranged only on one half of a track, either the one adjacent
to the data area 26 on the left or to the one on the right. The
pits 24 in the data area 26 are arranged on every track, provided
that the layer 11 is already recorded. In the example shown the
data area 26 consists of tracks 19 having a pre-embossed
land-and-groove structure. Each land 27 is indicated in the drawing
by a white background colour while each groove 28 is indicated with
a dark background colour. Although this dark background is similar
to the one in which the pits 23, 24 are coloured, the grooves do
not have a shading effect as they do not prevent light from passing
through them to the same amount as the pits 24 do. Pits 24 are
recordable that means they are not present on a non-recorded layer
and they may be erased from a recorded layer. In any case, pits 24
of a data area 26 cause a difference in light reflected by them,
usually described as dark and bright difference, i.e. they cause
shading of a layer below their own layer. This is different for the
pre-pits 23 which do not induce shading as they do not have a
similar shading effect as pits 24. In the data area 25, according
to the invention, there is provided a shading pattern 29 consisting
of pits much larger in length than the maximum length of pits 23,
24.
[0028] During the first recording of the semitransparent layer 11,
the header area 25 is provided with a dark-bright pattern 29, which
makes it in the far-field domain as transparent or reflective as
the recorded data area 26. That means there is a similar shading
effect on second layer 12 caused by the header areas 25 as well as
by the data areas 25 of first layer 11. The shading pattern 29 is
here a grating structure with alternating dark/bright stripes,
parallel to the prepits 23. However, any other structure of shading
pattern that does not disturb readout of the prepit information, is
also applicable.
[0029] Two cases for providing a pattern in the prepit area as
described above are described in the following:
[0030] First, layer 11 is assumed to be completely non-initialized,
which means that the recordable material of the semitransparent
recording layer 11 is not in the crystalline ground state. In this
case the recording layer needs to be initialized before the first
recording. That means that the complete data area 26 has to be
transformed to be in the crystalline ground state. It is completely
initialized and the prepit area is provided with an alternating
dark-bright pattern as described above. There are several
possibilities to perform initialization, e.g. in a commercial drive
using standard erase power, like continuous laser power, or in a
special initializer equipment during the manufacturing process. The
applied erase process with constant laser power does not disturb
the readout of the prepit information. Another solution, not
explicitely shown in FIG. 2, is not to write a dark-bright pattern
29, but a homogeneous grey level, which has the same optical
transmittance as the recorded data area 26. This is possible for
some materials having respective properties, using moderate laser
power, i.e. at a power level that lies between erase power and
maximum laser power.
[0031] Secondly, layer 11 is assumed to be completely initialized,
that means the recording layer 11 is in the crystalline ground
state. In this case only the header area 25 needs to be provided
with a dark-bright pattern as described above. This is done
preferably by applying high writing laser power to write an
amorphous pattern in the header area 25. For some materials the
writing power is to be held constant, for other materials the
writing power needs to be high frequency modulated, depending on
respective material properties. High frequency modulation of the
layer power may disturb readout of the prepits 23. In case of an
embossed header area as depicted in FIG. 2 this problem is solved
by writing only in the area not provided with prepits 23. In the
areas provided with prepits 23 laser read power is applied. Since
areas free of prepits 23 and areas provided with prepits 23
alternate in the embossed header area 25, in the far-field domain
the reflectivity or transmission is homogeneously changed.
[0032] FIG. 3 shows a similar part of first layer 11 as shown in
FIG. 2. Same parts are indicated by same reference numbers and only
referred to if different to FIG. 2. The pre-pits 23 in the header
area 25 are arranged off-centre, in the example exactly at an
intermediate position between two adjacent track centres 20. For
example according to the DVD-RAM standard such off-centred
so-called wobble pits are used as pre-pits 23. In this case the
Sector ID is repeated twice, in each of the off-centred prepit
areas. For such DVD-RAM type embossed header area, the same shading
pattern as described with regard to FIG. 1 may be applied, but
system redundancy is decreased. It is therefore preferrable to
apply writing laser power only on one half of the track in the
header area 25, as shown in FIG. 2. In this case 50% of the prepits
23 may be unreadable during writing of the shading pattern.
However, this usually is still sufficient to correctly readout the
sector ID.
[0033] FIG. 5 shows another way to provide for a shading pattern 29
in case of off-centred pre-pits 23. For simplicity reasons neither
pre-pits 23 nor pits 24 are shown, but only the different types of
track 19, land 27 and groove 28. Different to FIG. 2 and 3, several
data areas 26 and several header areas 25 are shown. It can be seen
that for each track 19 there is provided a shading pattern 29 in a
complete header area 25, while the following header area 25 of the
same track is completely free of shading pattern. That means
according to this alternative solution only every second header
area 25 is read completely and a shading pattern 29 is written to
the header areas 25 between. Here, too, system redundancy is
slightly reduced.
[0034] FIG. 4 shows a small part of a header area 25 with three
adjacent tracks 19, two of them having pre-pits 23 in the figure.
The shading pattern consists here of tiny pits 30, being of much
smaller axial length, seen in axial direction of the track 19, than
the pre-pits 23. These very small pits 30 are not detectable by the
limited resolution of the optics usually provided for or the
limited modulation transfer function (MTF) usually used. The size
of a focused spot 32 of scanning beam 3 on this layer 11 is
indicated here. It can be seen that it is too large to detect a
single one of tiny pits 30.
[0035] It is to be noted that the invention refers mainly to
shading effect. Shading effect denotes the low frequency change of
the transmission properties of a first layer through which a second
layer is scanned. These transmission properties change depending on
if the first layer is recorded or unrecorded. In contrast thereto,
high frequency variation of the second layer readout intensity
occurs caused by varying spacial distribution of pits or other data
markings of the first layer through which the scanning light beam
passes before and/or after scanning the second layer. Such high
frequency variations are often referred to as interlayer
crosstalk.
[0036] Although, according to the invention, no special process for
dual layer disk manufacturing is required, it is of course
advantageous to produce disks 1 already provided with a shading
pattern. Especially in case that a grey pattern is used, this can
be easily performed in a simple additional production step or in a
slightly changed initialization process. In case of changing of
transmission property of the semi-transparent header area 25 during
first recording process no special optical head is required. To
increase storage capacity of optical disks, multi-layer recording
on phase-change materials is a very promising technology. In
combination with continuous or soft-sectorized formats like DVD+RW
or DVD-RW no shading problem on the deeper second layer 12 occur,
if the semitransparent first layer 11 is recorded completely.
However, in case of hard-sectorized, land/groove formats the
embossed header area 25 of the semi-transparent first layer 11
causes light fluctuations on the second layer 12. According to the
invention several methods to avoid or to compensate light intensity
fluctuations on the second layer 12 caused by an inhomogeneous, in
terms of reflection and transmission, semi-transparent first layer
11 are proposed. Inhomogenities mainly occur in combination with
non-continuous land/groove formats with embossed header areas 25.
According to one solution the multilayer disk 1 is pre-processed in
the recording device before or during the first recording session.
In this case the standard disk manufacturing process can be
used.
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