U.S. patent application number 11/161123 was filed with the patent office on 2005-11-10 for multi-layer optical disc and system.
Invention is credited to Li, Chian Chiu.
Application Number | 20050249107 11/161123 |
Document ID | / |
Family ID | 32849993 |
Filed Date | 2005-11-10 |
United States Patent
Application |
20050249107 |
Kind Code |
A1 |
Li, Chian Chiu |
November 10, 2005 |
Multi-layer Optical Disc And System
Abstract
A multi-layer optical disc comprises multiple storage and
reference layers. The storage layers each have a distinct distance
from its reference layer. Beam portions of a read-out beam are
reflected by the storage and reference layers respectively.
Interference among the reflected beam portions is tuned to retrieve
stored information.
Inventors: |
Li, Chian Chiu; (San Jose,
CA) |
Correspondence
Address: |
LI, CHIAN CHIU
1847 BRISTOL BAY COMMON
SAN JOSE
CA
95131-3802
US
|
Family ID: |
32849993 |
Appl. No.: |
11/161123 |
Filed: |
July 24, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11161123 |
Jul 24, 2005 |
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10367510 |
Feb 14, 2003 |
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Current U.S.
Class: |
369/275.1 ;
369/112.01; 369/283; 369/53.1 |
Current CPC
Class: |
G01N 21/49 20130101;
A61B 5/0066 20130101; G01B 9/02057 20130101; G01B 9/0209 20130101;
G01B 9/0201 20130101; G01B 9/02091 20130101; G01B 9/02019 20130101;
A61B 5/0059 20130101; G01B 11/2441 20130101 |
Class at
Publication: |
369/275.1 ;
369/283; 369/053.1; 369/112.01 |
International
Class: |
G11B 007/24; G11B
007/00 |
Claims
What is claimed is:
1. A multi-layer optical data storage medium comprising: 1) a
plurality of storage layers as storage reflectors for storing data;
and 2) at least one reference layer as a reference reflector; 3)
said storage and reference layers being arranged such that there is
a distinct optical path length between said reference reflector and
each of said storage reflectors in a direction a read-out beam is
transmitted.
2. The storage medium according to claim 1 wherein at least one of
said storage layer is disposed in a discrete storage unit.
3. The storage medium according to claim 1 wherein said reference
layer is disposed in a discrete storage unit.
4. The storage medium according to claim 1 wherein at least one of
said storage reflectors has an adjustable reflectivity.
5. The storage medium according to claim 1 wherein said storage and
reference layers are arranged sharing one substrate.
6. The storage medium according to claim 1 wherein at least one of
said storage layers is disposed to function as said reference layer
for another said storage layer.
7. An optical data storage system comprising: 1) a light source for
generating a read-out beam; 2) a multi-layer optical data storage
medium comprising a plurality of storage layers for storing data
and at least one reference layer, said read-out beam being
transmitted to impinge onto said storage medium and reflected by
said storage and reference layers respectively, and said storage
and reference layers being arranged such that there is a distinct
optical path length between said reference reflector and each of
said storage reflectors in a direction said read-out beam is
transmitted; and 3) a detector for sensing interference among the
reflected beams reflected by said storage and reference layers.
8. The storage system according to claim 7, further including a
spatial phase modulator for dividing said read-out beam into a
plurality of beam portions by wavefront division and producing
phase shift on each said beam portion respectively.
9. The storage system according to claim 8, further including
tuning means for adjusting phase shift of at least one of said beam
portions.
10. The storage system according to claim 7 wherein said light
source has relatively low coherence.
11. The storage system according to claim 7, further including
optics means for focusing said read-out beam onto said medium and
said reflected beams onto said detector respectively.
12. A method for retrieving information from an optical data
storage medium, comprising: 1) causing a light source to generate a
read-out beam; 2) providing a multi-layer optical data storage
medium comprising at least one storage layer and at least one
reference layer; 3) transmitting a first beam portion of said
read-out beam through a first optical path, said first optical path
being arranged to connect said source and a detector via said
storage layer; 4) transmitting a second beam portion of said
read-out beam through a second optical path, said second optical
path being arranged to connect said source and said detector via
said reference layer; 5) adjusting path length difference between
said first and second paths; and 6) sensing interference between
said first and second beam portions by said detector.
13. The method according to claim 12 wherein said light source has
relatively low coherence.
14. The method according to claim 12 wherein said first and second
beam portions are generated using methods including wavefront
division.
15. The method according to claim 12, further including focusing
said beam portions onto said medium and said detector respectively.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a division of U.S. Ser. No. 10/367,510, filed Feb.
14, 2003.
BACKGROUND
[0002] 1. Field of Invention
[0003] This invention is related to optical data storage media and
systems, particularly to multi-layer optical data storage media and
optical storage systems using such media.
[0004] 2. Description of Prior Art
[0005] Most optical discs, including a compact disc (CD) and a
digital versatile disc (DVD), have a single storage layer for
storing information. Some discs contain double storage layers to
increase the capacity. To read double storage layers, an objective
lens is moved between two positions, which in turn moves the focal
position of a read-out beam such that the beam is focused onto each
layer respectively. Similar read-out methods are also used for more
than two storage layers. Obviously, the maximum allowable number of
storage layers in a multi-layer disc is determined by the spacing
between two adjacent storage layers and the working distance of the
objective lens. But the spacing has to be large enough to avoid
crosstalk between neighboring storage layers. Depending upon each
individual system, the spacing ranges from 30 to 80
micrometers.
[0006] In order to place storage layers more closely in an optical
disc, other methods have been proposed to read a layer without
severe crosstalk from its neighboring layers. One of them employs
techniques of optical coherence tomography (OCT), which is an
emerging technology and has great potentials in biomedical
applications.
[0007] An OCT system has a low-coherence light source which emits a
beam with a relatively short coherence length. Currently at the
heart of OCT is an amplitude division interferometer, usually a
Michelson interferometer. An OCT system splits a beam into two
beams by a beam splitter. One beam propagates to a reference
reflector along a reference optical path, and the other beam to a
sample medium along a sample optical path. The beams reflected by
the reference reflector and the sample medium are then recombined
by the beam splitter.
[0008] Due to the nature of low coherence, the combined beams
interfere with each other only when their optical path length
difference is within the beam's coherence length. The interference
intensity and pattern contrast reach a maximum when the two path
lengths are matched. For highly scattering sample media, various
sample paths yield different optical path lengths, depending upon
where a beam is reflected inside the media. Since a reference
optical path length can be adjusted to match a sample optical path
length, tuning the reference path length results in interference
between the reference beam and a sample beam which is reflected
from a layer at a depth inside the media. The interference
intensity and patterns are related to the layer's optical
properties, such as refractive index, birefringence, scattering
coefficient, etc. Coherence length of the beam determines
measurement resolution along the beam propagation direction. The
shorter the coherence length is, the higher the measurement
resolution. By combing the low coherence interference technique
with a laterally scanning mechanism, a three-dimensional image can
be constructed.
[0009] Naturally, an OCT scheme can be used to read multiple
storage layers in an optical disc. The storage layers are partially
reflective and partially transmissive, and distributed in three
dimensions. Since optical path length is of the product of a path
length and refractive index along the path, the minimum distance
between adjacent layers is of half the beam's coherence length
divided by the refractive index. For a broadband light source, its
coherence length can be in the order of 1 micrometer. Thus an
optical disc using OCT techniques can have much smaller layer
spacing and hold much more storage layers than a conventional
optical disc.
[0010] There are several references using OCT methods for a
multi-layer optical disc. See, for example, U.S. Pat. No. 5,883,875
(1999) to Coufal, et al. and U.S. Pat. No. 6,072,765 (2000) to
Rolland, et al. As a result, the multi-layer medium only contains
storage layers, while the reference reflector is built within the
OCT system. Since read-out results depend upon an optical path
length to a storage layer, medium vibration causes a change of the
optical path length and brings measurement errors. The setup
inherits drawbacks of a current OCT: Sensitivity to sample
vibration and a bulky structure due to separate sample and
reference paths.
[0011] Accordingly, there is a need for a multi-layer optical disc
which contains more storage layers, and a multi-layer optical disc
system which is able to read such a disc.
OBJECTS AND ADVANTAGES
[0012] Accordingly, several main objects and advantages of the
present invention are:
[0013] a). to provide an improved multi-layer optical disc;
[0014] b). to provide such an optical disc which comprises multiple
storage and reference layers, where each storage layer has a
distinct distance from its corresponding reference layer;
[0015] c). to provide an improved multi-layer optical disc
system;
[0016] d). to provide such a system which employs a relatively
simple and compact interference structure; and
[0017] e). to provide such a system which retrieves information
using vibration-insensitive interference between beam portions
reflected by the storage and reference layers.
[0018] Further objects and advantages will become apparent from a
consideration of the drawings and ensuing description.
SUMMARY
[0019] In accordance with the present invention, a multi-layer
optical disc and an optical disc system are constructed. The
multi-layer optical disc comprises multiple storage and reference
layers, where each storage layer and its reference layer have a
distinct spacing between them.
[0020] The optical storage system retrieves data using adjustable
interference between beam portions reflected by the storage and
reference layers. The beam portions are split by wavefront division
in a relatively simple and compact structure. Since the beam
portions are reflected by the layers within the disc, the
interference result is insensitive to disc vibration.
[0021] Abbreviations
[0022] AR Anti-reflection
[0023] CD Compact Disc
[0024] DVD Digital Versatile Disc
[0025] HR High Reflection
[0026] OCT Optical Coherence Tomography
[0027] PR Partial Reflection
DRAWING FIGURES
[0028] FIG. 1-A is a schematic diagram showing a prior-art double
layer optical disc and a read-out method.
[0029] FIG. 1-B is schematic diagram showing a prior-art
multi-layer optical disc system using an OCT scheme.
[0030] FIG. 2 is a schematic diagram illustrating an interferometer
for measurement using wavefront division according to the
invention.
[0031] FIG. 3 is a schematic diagram illustrating an embodiment of
a multi-layer optical disc system according to the invention.
[0032] FIG. 4 is a schematic cross-sectional view illustrating an
embodiment of a multi-layer optical storage arrangement according
to the invention.
[0033] FIGS. 5 and 6 are schematic cross-sectional views
illustrating embodiments of a multi-disc optical data storage
arrangement according to the invention.
REFERENCE NUMERALS IN DRAWINGS
[0034]
1 12 collimated beam 14 modulator element 16 modulator element 17
spatial phase modulator 18 beam portion 20 beam portion 21 sample
23 sample surface 26 HR reflector 30 lens system 35 beam portion 36
beam portion 50 detector 52 lens system 54 beam splitter 63
detector 71 light source 72 reflective reference layer 74 PR
storage layer 76 PR storage layer 78 PR storage layer 86 PR storage
layer 91 optical disc 93 optical disc 95 PR reference layer 96
optical disc 97 optical disc 98 PR reference layer 99 PR storage
layer 104 read-out beam 106 read-out beam 108 optical disc 110
storage layer 112 storage layer 116 optical disc 118 multi-layers
120 beam splitter 130 optical disc 132 multi-layers
DETAILED DESCRIPTION--FIGS. 1-A AND 1-B--PRIOR-ART MULTI-LAYER DISC
AND SYSTEM
[0035] FIG. 1-A shows schematically a read-out beam reads a double
layer optical disc 108 in a prior-art optical disc system. Disc 108
contains two storage layers 110 and 112. A read-out beam 104 is
focused on layer 110 initially. To read layer 112, which is beneath
layer 110, an objective lens (not shown in FIG. 1-A) of the system
is moved closer to the disc so that the read-out beam's focal
position penetrates deeper in the disc, as illustrated by another
read-out beam 106. Since the beams each interact with both layers
simultaneously, the spacing between the layers has to be larger
enough to avoid crosstalk.
[0036] FIG. 1-B shows schematically an OCT scheme is employed to
read a multi-layer disc. The OCT is basically a Michelson
interferometer comprising a low-coherence light source 71, a beam
splitter 54, an adjustable reference reflector 26, a detector 63,
and an optical disc 116 having multiple storage layers 118. A beam
from source 71 is spit into a reference beam and a sample beam by
splitter 54 through amplitude division. The reference beam is
transmitted to reflector 26, reflected by the reflector, and then
propagates to detector 63 after passing through splitter 54. The
sample beam impinges onto disc 116 and is reflected by multiple
layers 118. Then the reflected sample beam is reflected by splitter
54 and combined with the reference beam. The two beams generate a
low-coherence interference which is detected by detector 63. Since
the reflected sample beam contains multiple portions caused by
multiple reflections of the storage layers, reflector 26 is
adjusted such that the optical path length of the reference beam
matches that of a sample beam portion which is reflected by a
specific storage layer. The minimum storage layer spacing is of
half the coherence length of light source 71 divided by the
refractive index. However as discussed in the background section,
the system is bulky due to separate sample and reference paths and
because any disc movement in a direction the read-out beam travels
affects the sample beam path length, the system is also sensitive
to vibration, which makes it difficult for practical use.
[0037] FIGS. 2 and 3--Novel Interferometer and Optical Disc
System
[0038] FIG. 2 illustrates schematically an interferometer for
optical measurement. A collimated beam contains beam portions 18
and 20, which are used as sample and reference beams. The beam
portions impinge onto a surface 23 of a sample 21 after being
transmitted through a beam splitter 120. Beam portions 18 and 20
are reflected by surface 23, and then by splitter 120. Finally, the
beam portions interfere with each other when they are mixed and
focused onto a detector 50 by a lens system 52. The interference
can be tuned by adjusting phase difference between the beam
portions using a spatial phase modulator.
[0039] Compared with the setup of FIG. 1-B, the sample and
reference beams in the interferometer here are side-by-side, thus
the system becomes simpler and more compact. Furthermore, the
interference result is insensitive to sample vibration, because the
two beam portions experience the same path length change during the
vibration. The interference results may be used to profile surface
23.
[0040] The interferometer configuration of FIG. 2 can be modified
for use in a multi-layer optical disc system, as shown
schematically in FIG. 3. A collimated beam 12 enters a spatial
phase modulator 17 and is processed by modulator elements 14 and
16. When beam 12 leaves the modulator, it becomes beam portions 18
and 20 with a tunable phase difference. The beam portions are
transmitted through a beam splitter 120 and focused on a region of
multiple layers 132 of an optical disc 130 by a lens system 30.
Layers 132 comprise multiple storage and reference layers, instead
of storage layers only in a conventional optical disc. Next, the
beam portion are reflected by layers 132 and collimated by lens
system 30. Then, as in FIG. 2, the reflected beam portions are
reflected again by splitter 120 and focused onto detector 50, where
interference signals are observed. The reflected beam portions
contain multiple waves whose phase is determined by modulator 17
and layers 132. A wave reflected by the reference layer becomes a
reference beam, while a wave reflected by a storage layer becomes a
sample beam. By tuning modulator 17, the multi-wave interference is
adjusted accordingly. When beam 12 is of low-coherence,
low-coherence interference can be used to single out a sample beam
and detect the reflectivity of the corresponding storage layer as
illustrated in the following paragraphs.
[0041] The system of FIG. 3 is simpler and more compact than
systems using the current OCT structure, because the sample and
reference beams are not separate. In addition, the disc comprises
both sample and reference reflectors, thus path length difference
between sample and reference beams is insensitive to disc
vibration, so is the read-out result.
[0042] FIGS. 4-6--Multi-Layer Optical Storage Structures
[0043] A schematic cross-sectional view in FIG. 4 illustrates an
embodiment of structure and read-out method for a multi-layer
optical disc. The multi-layer optical disc contains two regions. A
first region has a single partial reflection (PR) or high
reflection (HR) reference layer 72 as a reference reflector. A
second region has PR storage layers 74, 76, and 78 as storage
reflectors. Surrounding the reference and storage layers are
low-loss transmissive materials. Stored data are represented by
either a partial reflection or a relatively low reflection of the
storage layer. Read-out beam portions 35 and 36 are created and
tuned by a spatial phase modulator (not shown in FIG. 4), and are
aligned to the two regions respectively. Portion 35 impinges onto
layer 72, while portion 36 impinges onto the storage layers.
[0044] For portion 35, it has only one reference optical path
involving one reflection from layer 72. But due to three storage
layers, beam portion 36 has three storage optical paths containing
a single reflection, and various storage optical paths containing
multiple reflections. One storage path of portion 36 has a route
from the modulator to layer 74, to layer 76, to layer 74, to layer
76, to layer 74, and finally to a detector. The spacing between
adjacent storage layers should be equal or larger than half the
beam's coherence length divided by the refractive index. For
read-out purpose, only three storage paths involving a single
reflection are needed. The three paths each have a respective
optical path length. By tuning the spatial phase modulator, the
reference optical path length can be adjusted to match any of the
storage optical path lengths. Therefore, reflectivity of the three
storage layers can be detected.
[0045] A schematic cross-sectional view in FIG. 5 illustrates an
embodiment of a multi-disc optical data storage configuration.
Again beam portions 35 and 36 work as reference and sample beams,
respectively. Optical discs 91 and 93 each contain a PR reference
layer 95 and a PR storage layer 86. Surrounding the reference and
storage layers are low-loss transmissive materials. The discs are
stacked and their reference layers and storage layers are aligned
in a direction perpendicular to the layers. Layer 95 functions as a
reference reflector for storage layer 86 in the same disc. Layer 86
stores information by having different reflectivity values. Each
disc has a distinct spacing between layers 86 and 95 in a direction
perpendicular to the layers or portions 35 and 36 travel along,
which is equal or larger than half the beam's coherence length
divided by the refractive index. Distance between layers 95 in the
discs should be large enough to avoid any unwanted
interference.
[0046] To read out data, beam portions 35 and 36 are transmitted to
impinge onto the discs. Each beam portion impinges on two layers
which are in separate discs. Consider reflected beams with only one
reflection. There are total four reflected beams, among which two
are reference beams bounced by reference layer 95 and the other two
are sample beams generated by sample layer 86. In other words,
discs 91 and 93 each create a sample and a reference beam. The
sample and reference beams have a path length difference which is
affected by the spacing between the layers. Since the phase of
portions 35 and 36 is tunable through a spatial phase modulator
(not shown in FIG. 5), the modulator can be used to compensate the
phase difference caused by the spacing and match the sample optical
path length to the reference path length for a disc. To avoid
matching two pairs of path length at the same time, the spacing
difference between layers 86 and 95 in the discs should be equal or
larger than half the beam's coherence length divided by the
refractive index. Therefore, although there are four reflected
beams from two discs in FIG. 5, it is possible to produce a
low-coherence interference between two beams from one disc.
Therefore as in FIG. 4, interference between the reflected beams
can be used to measure the reflectivity of storage layer in the
disc.
[0047] FIG. 6 illustrates another embodiment of a multi-disc
optical data storage configuration through a schematic
cross-sectional view. In the embodiment, a PR storage layer 99
overlaps a PR reference layer 98 in discs 96 and 97. Surrounding
the reference and storage layers are low-loss transmissive
materials. The two discs are stacked together. Beam portions 35 and
36 each impinge onto the four PR layers in two discs. Again the
spacing between a reference and a storage layer in each disc is
distinct, equal or larger than half the beam's coherence length
divided by the refractive index, and is used to distinguish storage
layers in the discs. And thickness of each disc is larger than the
beam's coherent length.
[0048] The overlapping layers in FIG. 6 mean that the beam portions
have the same optical path lengths inside the storage medium; in
other words, separation of beam portions becomes not necessary in
the medium. Once again, consider reflected beams having one
reflection only, since a reflected beam bounced between layers has
at least three reflections, which reduced its intensity greatly.
When portions 35 and 36 are reflected by the reflectors of the
discs, eight reflected beams are created. The phase of each
reflected beam is determined by a path through which it travels in
the storage medium and a spatial phase modulator (not shown in FIG.
6) which tunes the phase of portions 35 and 36. There are three
spacings between the four layers in FIG. 6. To avoid unwanted
interference, each spacing has to be equal to or larger than a
value, which we call value A, as discussed before. Value A is of
half the beam's coherence length divided by the refractive index.
Assume the spacing between 98 and 99 layers in disc 96 is of value
A, between 98 and 99 in disc 97 twice value A, and between 98 in
disc 96 and 99 in disc 97 at least three times of value A. Then
when the modulator adjusts the phase of portions 35 and 36, and
makes the optical path length of a beam reflected by layer 98 of
disc 96 match that of a beam reflected by layer 99 of the same
disc, it creates the only interference, since the path length
difference between any other two beams is larger than the coherence
length. The only interference happens again between beams reflected
by layers 98 and 99 of disc 97, when their path lengths are
matched. Therefore as before, the low-coherence interference method
can be used to select two beams which are reflected by a reference
and a storage layer in the same disc, and reveal the storage layer
reflectivity.
CONCLUSION, RAMIFICATIONS, AND SCOPE
[0049] Accordingly, the reader will see that a multi-layer optical
disc has multiple storage and reference layers. Each storage layer
has a distinct distance from its corresponding reference layer. And
a multi-layer optical storage system retrieves data from the
multi-layer disc using adjustable interference among beam portions
which are reflected by the storage and reference layers
respectively.
[0050] Although the description above contains many specificities,
these should not be construed as limiting the scope of the
invention but as merely providing illustrations of some of the
presently preferred embodiments. Numerous modifications,
alternations, and variations will be obvious to those skilled in
the art. For example, a single disc containing multiple reference
and storage layers can replace the multi-disc structures in FIG. 5
or 6. To replace the scheme in FIG. 4, storage layers may be
arranged in multiple discs and have a common reference reflector
attached to them. A beam may be divided into portions of any number
with any geometrical shapes by wavefront-division; for example, a
beam may be divided into a central circular portion and several
outer ring-shaped portions. The intensity ratio of one portion to
another can be of any value depending upon the interference effect
between them. For example, if a portion is intended to be reflected
by a reflector of low reflectivity, this portion should have a
larger intensity in order to improve contrast of interference
patterns. Lastly in FIG. 5, the reference layers in two discs may
be misaligned to the storage layers based on the schemes shown in
FIG. 6.
[0051] Therefore the scope of the invention should be determined by
the appended claims and their legal equivalents, rather than by the
examples given.
* * * * *