U.S. patent application number 13/875071 was filed with the patent office on 2014-07-24 for dynamic aperture holographic multiplexing.
This patent application is currently assigned to AKONIA HOLOGRAPHICS LLC. The applicant listed for this patent is AKONIA HOLOGRAPHICS LLC. Invention is credited to Kenneth E. ANDERSON, Fredric R. ASKHAM, Mark R. AYRES, Bradley Jay SISSOM.
Application Number | 20140204437 13/875071 |
Document ID | / |
Family ID | 51207473 |
Filed Date | 2014-07-24 |
United States Patent
Application |
20140204437 |
Kind Code |
A1 |
AYRES; Mark R. ; et
al. |
July 24, 2014 |
DYNAMIC APERTURE HOLOGRAPHIC MULTIPLEXING
Abstract
Systems and methods for dynamic aperture holographic
multiplexing are disclosed. One example process may include
recording a set of holograms in a recording medium by varying both
the reference beam angular aperture and the signal beam angular
aperture. The angular aperture of the signal beam may be
dynamically changed such that the closest edge of each signal beam
angular aperture is selected to be a threshold angle different than
the angular aperture of the reference beam used to record it. In
some examples, the dynamic aperture holographic multiplexing
process may include dynamic aperture equalization to reduce
cross-talk, to improve error correction parity distribution for
improved recovery transfer rate, to provide multiple locus aperture
sharing for increased recording density, and to provide
polarization multiplexed shared aperture multiplexing for increased
transfer rate in both recording and recovery.
Inventors: |
AYRES; Mark R.; (Boulder,
CO) ; ANDERSON; Kenneth E.; (Boulder, CO) ;
ASKHAM; Fredric R.; (Loveland, CO) ; SISSOM; Bradley
Jay; (Boulder, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
AKONIA HOLOGRAPHICS LLC |
Longmont |
CO |
US |
|
|
Assignee: |
AKONIA HOLOGRAPHICS LLC
Longmont
CO
|
Family ID: |
51207473 |
Appl. No.: |
13/875071 |
Filed: |
May 1, 2013 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61755893 |
Jan 23, 2013 |
|
|
|
Current U.S.
Class: |
359/11 ;
359/22 |
Current CPC
Class: |
G03H 2001/267 20130101;
G11B 7/1381 20130101; G03H 1/2645 20130101; G03H 1/265 20130101;
G11B 7/00772 20130101; G03H 1/0248 20130101; G11B 7/0065
20130101 |
Class at
Publication: |
359/11 ;
359/22 |
International
Class: |
G03H 1/26 20060101
G03H001/26 |
Claims
1. A method for recording a set of multiplexed holograms, the
method comprising: recording a first hologram of the set of
multiplexed holograms to a recording medium using a first signal
beam angular aperture and a first reference beam; and recording a
second hologram of the set of multiplexed holograms to the
recording medium using a second signal beam angular aperture and a
second reference beam, wherein the second signal beam angular
aperture is varied in at least one characteristic from the first
signal beam angular aperture.
2. The method of claim 1, wherein the first hologram and the second
hologram each comprise a data page of pixel information.
3. The method of claim 1, wherein the first signal beam angular
aperture and the second signal beam angular aperture vary in one or
more of shape, size, and position.
4. The method of claim 1, further comprising: recording a third
hologram of the set of multiplexed holograms to the recording
medium using a third signal beam angular aperture and a third
reference beam, wherein the third signal beam angular aperture is
varied in at least one characteristic from the first signal beam
angular aperture and the second signal beam angular aperture; and
recording a fourth hologram of the set of multiplexed holograms to
the recording medium using a fourth signal beam angular aperture
and a fourth reference beam, wherein the fourth signal beam angular
aperture is varied in at least one characteristic from the first
signal beam angular aperture, second signal beam angular aperture,
and the third signal beam angular aperture.
5. The method of claim 4, wherein: an edge of the first signal beam
angular aperture is separated from an angular aperture of the first
reference beam by a first angle; an edge of the second signal beam
angular aperture is separated from an angular aperture of the
second reference beam by a second angle; an edge of the third
signal beam angular aperture is separated from an angular aperture
of the third reference beam by a third angle; and an edge of the
fourth signal beam angular aperture is separated from an angular
aperture of the fourth reference beam by a fourth angle.
6. The method of claim 5, wherein the first angle, the second
angle, the third angle, and the fourth angle are substantially
equal.
7. The method of claim 5, wherein: the first angle and the third
angle are substantially equal; the second angle and the fourth
angle are substantially equal; and the first angle and the third
angle are different than the second angle and the fourth angle.
8. The method of claim 1, wherein using the first signal beam
angular aperture comprises using a signal beam with an angular
range.
9. The method of claim 1, wherein at least a portion of an angular
locus of a set of reference beams used to record the set of
multiplexed holograms overlaps at least a portion of an angular
locus of a set of signal beams used to record the set of
multiplexed holograms.
10. The method of claim 1, wherein a first portion of the set of
multiplexed holograms are used to store error parity data and a
second portion of the set of multiplexed holograms are used to
store input data, wherein the holograms of the first portion are
smaller than the holograms of the second portion.
11. A system for recording a set of multiplexed holograms, the
system comprising: an aperture sharing element configured to output
a modulated signal beam and a reference beam; a recording medium;
and a controller configured to: cause the recording of a first
hologram of the set of multiplexed holograms to the recording
medium by causing the aperture sharing element to output a first
signal beam having a first signal beam angular aperture and a first
reference beam having a first reference beam angular aperture; and
cause the recording of a second hologram of the set of multiplexed
holograms to the recording medium by causing the aperture sharing
element to output a second signal beam having a second signal beam
angular aperture and a second reference beam having a second
reference beam angular aperture, wherein the second signal beam
angular aperture is varied in at least one characteristic from the
first signal beam angular aperture.
12. The system of claim 11, wherein the aperture sharing element
comprises a spatial light modulator, and wherein causing the
aperture sharing element to output the first signal beam having the
first signal beam angular aperture and the first reference beam
having the first reference beam angular aperture comprises
controlling the spatial light modulator to output the first signal
beam having the first signal beam angular aperture and the first
reference beam having the first reference beam angular
aperture.
13. The system of claim 11, further comprising: a laser source for
generating a beam; a beam directing device coupled to receive the
beam; and a spatial light modulator coupled to receive the beam,
wherein: causing the aperture sharing element to output the first
signal beam having the first signal beam angular aperture comprises
controlling the spatial light modulator to cause the aperture
sharing element to output the first signal beam having the first
signal beam angular aperture; and causing the aperture sharing
element to output the first reference beam having the first
reference beam angular aperture comprises controlling the beam
directing device to output the first reference beam having the
first reference beam angular aperture
14. The system of claim 11, wherein the controller is further
configured to: cause the recording of a third hologram of the set
of multiplexed holograms to the recording medium by causing the
aperture sharing element to output a third signal beam having a
third signal beam angular aperture and a third reference beam
having a third reference beam angular aperture, wherein the third
signal beam angular aperture is varied in at least one
characteristic from the first signal beam angular aperture and the
second signal beam angular aperture; and cause the recording of a
fourth hologram of the set of multiplexed holograms to the
recording medium by causing the aperture sharing element to output
a fourth signal beam having a fourth signal beam angular aperture
and a fourth reference beam having a fourth reference beam angular
aperture, wherein the fourth signal beam angular aperture is varied
in at least one characteristic from the first signal beam angular
aperture, second signal beam angular aperture, and the third signal
beam angular aperture.
15. The system of claim 14, wherein: an edge of the first signal
beam angular aperture is separated from an angular aperture of the
first reference beam by a first angle; an edge of the second signal
beam angular aperture is separated from an angular aperture of the
second reference beam by a second angle; an edge of the third
signal beam angular aperture is separated from an angular aperture
of the third reference beam by a third angle; and an edge of the
fourth signal beam angular aperture is separated from an angular
aperture of the fourth reference beam by a fourth angle.
16. The system of claim 15, wherein the first angle, the second
angle, the third angle, and the fourth angle are substantially
equal.
17. The system of claim 15, wherein: the first angle and the third
angle are substantially equal; the second angle and the fourth
angle are substantially equal; and the first angle and the third
angle are different than the second angle and the fourth angle.
18. The system of claim 11, wherein the first signal beam angular
aperture comprises an angular range of the signal beam.
19. The system of claim 11, wherein at least a portion of an
angular locus of a set of reference beams used to record the set of
multiplexed holograms overlaps at least a portion of an angular
locus of a set of signal beams used to record the set of
multiplexed holograms.
20. The system of claim 11, wherein a first portion of the set of
multiplexed holograms is used to store error parity data and a
second portion of the set of multiplexed holograms is used to store
input data, wherein the holograms of the first portion are smaller
than the holograms of the second portion.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 61/755,893, filed Jan. 23, 2013, the entire
disclosure of which is hereby incorporated by reference in its
entirety for all purposes as if put forth in full below.
BACKGROUND
[0002] 1. Field
[0003] The present disclosure relates generally to holography and,
more specifically, to holographic multiplexing.
[0004] 2. Related Art
[0005] Holography is a technique for storing both phase and
amplitude information of light by recording the interference
pattern generated between a coherent object beam and a reference
beam as a hologram in a photosensitive medium. During recovery, a
probe beam (which is a replica of the reference beam) illuminates
the hologram, and a diffracted beam (which is a replica of the
object beam) may be generated. In the original "in line"
configuration described by Dennis Gabor in "A new microscopic
principle," Nature 161, 777 (1948), the object and reference beams
shared an optical axis, creating a diffracted "ambiguity" beam from
the conjugate interference term, as well as resulting in a
superposition of the diffracted beam with the probe beam. However,
as described in E. N. Leith and J. Upatnieks, "Reconstructed
wavefronts and communication theory," J. Opt. Soc. Amer. 52,
1123-30 (1962), an "off-axis" configuration--one in which the
object and reference beams have axes with different angles of
incidence--would naturally allow for the separation of the
diffracted beam from the other components. Such beams might be said
to issue from separate, rather than shared, apertures in angle
space. Off-axis holography subsequently became the dominant
configuration, and is used for virtually all holographic systems,
including holographic data storage systems.
[0006] Holography is attractive for digital data storage because
many holograms may be written into the same volume (or overlapping
volumes) of a thick recording medium using a process known as
multiplexing, which is described by G. Barbastathis and D. Psaltis,
"Volume holographic multiplexing methods," in Holographic Data
Storage, H. J. Coufal, D. Psaltis, and G. Sincerbox, eds. Springer
(2000), pp. 21-62. Many different holographic multiplexing
techniques have been developed. For example, using angle
multiplexing, described by F. H. Mok, "Angle-multiplexed storage of
5000 holograms in lithium niobate," Opt. Lett. 18, 915-917 (1993),
one may record hundreds or thousands of different holograms in the
same volume of media by using collimated (plane wave) reference
beams that differ slightly from each other by their angle of
incidence. Each hologram may record a different object beam (or
signal beam) that has been modulated with a different digital data
pattern. During recovery, the hologram may be illuminated by a
probe beam. Due to the Bragg effect, only a hologram recorded with
a reference beam angle at the same angle of incidence as the probe
beam will produce substantial diffraction. Each signal beam may
thus be reconstructed independently, allowing the digital data to
be recovered without cross-talk from the rest of the multiplexed
holograms.
[0007] Other holographic multiplexing techniques, such as
wavelength multiplexing described by D. Lande, J. F. Heanue, M. C.
Bashaw, and L. Hesselink, "Digital wavelength-multiplexed
holographic data storage system," Opt. Lett. 21, 1780-1782 (1996),
shift multiplexing described by D. Psaltis, A. Pu, M. Levene, K.
Curtis, and G. Barbastathis, "Holographic storage using shift
multiplexing," Opt. Lett. 20, 782-784 (1995), and polytopic
multiplexing described by K. Anderson and K. Curtis, "Polytopic
multiplexing," Opt. Lett. 29, 1402-1404 (2004), have been
developed. These multiplexing techniques may be used alone or in
combination with other multiplexing techniques to increase the
amount of data stored in a recording medium.
[0008] Other features of the recording geometry may be varied to
record data in the recording medium. For example, a page-oriented
system is one in which the signal beam is modulated as a
two-dimensional array of pixels, the modulation typically being
imparted by a spatial light modulator (SLM). A Fourier architecture
is one in which the recording medium is placed at or near an
optical Fourier plane of the page image. A monocular system is one
in which both the reference and signal beams pass through a single,
shared objective lens before illuminating the recording medium, as
described in U.S. Pat. No. 7,742,209, "Monocular holographic data
storage system architecture," Jun. 22, 2010.
SUMMARY
[0009] Methods for recording a set of multiplexed holograms are
provided. One example method may include: recording a first
hologram of the set of multiplexed holograms to a recording medium
using a first signal beam angular aperture and a first reference
beam; and recording a second hologram of the set of multiplexed
holograms to the recording medium using a second signal beam
angular aperture and a second reference beam, wherein the second
signal beam angular aperture is varied in at least one
characteristic from the first signal beam angular aperture.
[0010] In one example, the first hologram and the second hologram
may each comprise a data page of pixel information. In another
example, the first signal beam angular aperture and the second
signal beam angular aperture may vary in one or more of shape,
size, and position.
[0011] In one example, the method may further include: recording a
third hologram of the set of multiplexed holograms to the recording
medium using a third signal beam angular aperture and a third
reference beam, wherein the third signal beam angular aperture may
be varied in at least one characteristic from the first signal beam
angular aperture and the second signal beam angular aperture; and
recording a fourth hologram of the set of multiplexed holograms to
the recording medium using a fourth signal beam angular aperture
and a fourth reference beam, wherein the fourth signal beam angular
aperture may be varied in at least one characteristic from the
first signal beam angular aperture, second signal beam angular
aperture, and the third signal beam angular aperture.
[0012] In one example, an edge of the first signal beam angular
aperture may be separated from an angular aperture of the first
reference beam by a first angle; an edge of the second signal beam
angular aperture may be separated from an angular aperture of the
second reference beam by a second angle; an edge of the third
signal beam angular aperture may be separated from an angular
aperture of the third reference beam by a third angle; and an edge
of the fourth signal beam angular aperture may be separated from an
angular aperture of the fourth reference beam by a fourth angle. In
another example, the first angle, the second angle, the third
angle, and the fourth angle may be substantially equal. In yet
another example, the first angle and the third angle may be
substantially equal; the second angle and the fourth angle may be
substantially equal; and the first angle and the third angle may be
different than the second angle and the fourth angle.
[0013] In one example, using the first signal beam angular aperture
may include using a signal beam with an angular range. In another
example, at least a portion of an angular locus of a set of
reference beams used to record the set of multiplexed holograms may
overlap at least a portion of an angular locus of a set of signal
beams used to record the set of multiplexed holograms.
[0014] In one example, a first portion of the set of multiplexed
holograms may be used to store error parity data and a second
portion of the set of multiplexed holograms may be used to store
input data, wherein the holograms of the first portion may be
smaller than the holograms of the second portion.
[0015] Systems for recording a set of multiplexed holograms are
also provided
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The present application can be best understood by reference
to the following description taken in conjunction with the
accompanying drawing figures, in which like parts may be referred
to by like numerals.
[0017] FIGS. 1(a) and 1(b) illustrate real and k-space
distributions of holographic recording terms.
[0018] FIG. 2(a) illustrates an example holographic data storage
system having a monocular architecture.
[0019] FIG. 2(b) illustrates an example angular aperture map of the
monocular system of FIG. 2(a).
[0020] FIG. 3(a) illustrates an example angular aperture map of a
monocular system.
[0021] FIG. 3(b) illustrates an example angular aperture map of a
monocular system implementing dynamic aperture holographic
multiplexing.
[0022] FIG. 4(a) illustrates an example graph showing the
relationship between user capacity and the number of multiplexed
holograms, and the relationship between the average number of
recorded pixels per hologram and the number of multiplexed
holograms.
[0023] FIG. 4(b) illustrates an example graph showing the
relationship between the reference angle and the multiplexed
hologram number, and the relationship between the angular spacing
and the multiplexed hologram number.
[0024] FIG. 5 illustrates a system diagram for an example
holographic data storage system for performing dynamic aperture
holographic multiplexing.
[0025] FIG. 6(a) illustrates an example k-space distribution of
holographic recording terms obtained using angle multiplexing.
[0026] FIG. 6(b) illustrates an example k-space distribution of
holographic recording terms obtained using angle multiplexing and
dynamic aperture holographic multiplexing.
[0027] FIG. 7 illustrates an example k-space distribution of
holographic recording terms obtained using dynamic aperture
equalization.
[0028] FIGS. 8(a) and 8(b) illustrate example angular aperture maps
using multiple locus aperture sharing.
[0029] FIG. 9 illustrates a system diagram for an example collinear
holographic data storage system for performing dynamic aperture
holographic multiplexing.
[0030] FIG. 10 illustrates an example SLM pattern and angular
aperture map for collinear recording.
[0031] FIGS. 11(a)-(c) illustrate an example scheme for performing
dynamic aperture holographic multiplexing using a collinear
system.
[0032] FIG. 12 illustrates an example process for performing
dynamic aperture holographic multiplexing.
[0033] FIG. 13 illustrates an example computing system.
DETAILED DESCRIPTION
[0034] The following description is presented to enable a person of
ordinary skill in the art to make and use the various embodiments.
Descriptions of specific devices, techniques, and applications are
provided only as examples. Various modifications to the examples
described herein will be readily apparent to those of ordinary
skill in the art, and the general principles defined herein may be
applied to other examples and applications without departing from
the spirit and scope of the various embodiments. Thus, the various
embodiments are not intended to be limited to the examples
described herein and shown, but are to be accorded the scope
consistent with the claims.
[0035] Various embodiments are described below relating to dynamic
aperture holographic multiplexing. One example dynamic aperture
holographic multiplexing process may include recording a set of
holograms in a recording medium by varying both the angular
aperture of a reference beam and the angular aperture of a signal
beam. The angular aperture of the signal beam may be dynamically
changed such that the closest edge of each signal beam angular
aperture is selected to be a threshold angle different than the
reference beam angular aperture used to record it. Thus, at least a
portion of the reference beam locus (e.g., the aggregate coverage
of the individual reference beam angular apertures) may be shared
with the signal beam locus (e.g., the aggregate coverage of the
individual signal beam angular apertures), resulting in a greater
number of holograms being recorded in the same volume of recording
medium than obtainable without the use of dynamic aperture
holographic multiplexing. In some examples, the dynamic aperture
holographic multiplexing process may include dynamic aperture
equalization to reduce cross-talk, to improve error correction
parity distribution for improved recovery transfer rate, to provide
multiple locus aperture sharing for increased recording density,
and to provide polarization multiplexed shared aperture
multiplexing for increased transfer rate in both recording and
recovery.
[0036] In some examples, the processes for dynamic aperture
holographic multiplexing may be combined with other multiplexing
techniques, such as angle multiplexing, polytopic multiplexing, and
the like. In one example, a page-oriented, monocular, Fourier
geometry may be used to perform dynamic aperture holographic
multiplexing. However, dynamic aperture holographic multiplexing
may similarly be used with other architectures, such as collinear
holography systems, common aperture holography systems, and the
like.
k-Space Formalism for Holography
[0037] Holographic recording and diffraction can be analyzed using
k-space formalism, as described in M. R. Ayres, "k-Space
Formalism," in K. Curtis, L. Dhar, W. L. Wilson, A. Hill, M. R.
Ayres, Holographic Data Storage: From Theory to Practical Systems,
John Wiley & Sons, Ltd. (2010), pp. 26-31. In k-space,
propagating optical waves and holographic gratings may be
represented by three-dimensional Fourier transforms of their
distributions in real space. For example, a collimated
monochromatic reference beam can be represented in real space and
k-space by
E r ( r ) = A r exp ( k r r ) E r ( k ) = A r .delta. ( k - k r ) ,
( 1 ) ##EQU00001##
[0038] where E.sub.r ({right arrow over (r)}) is the optical scalar
field distribution at all {right arrow over (r)}={x,y,z} 3D spatial
vector locations, and its transform E.sub.r({right arrow over (k)})
is the optical scalar field distribution at all {right arrow over
(k)}={k.sub.x,k.sub.y,k.sub.z} 3D spatial frequency vectors.
A.sub.r is the complex amplitude of the field, and {right arrow
over (k)}.sub.r is a vector whose length indicates the spatial
frequency of the light waves, and whose direction indicates the
direction of propagation. In some examples, all beams may be
composed of light of the same wavelength, so all optical k-vectors
may have the same length (e.g., |{right arrow over
(k)}.sub.r|=k.sub.n). Thus, all optical propagation vectors may lie
on a sphere of radius k.sub.n. This construct is known as the
k-sphere.
[0039] The other important k-space distribution is that of the
holograms themselves. Holograms for data storage usually include
spatial variations of the index of refraction within the recording
medium, typically denoted .DELTA.n({right arrow over (r)}).
Ideally, this index modulation pattern is proportional to the
spatial intensity of the recording interference pattern, i.e.,
.DELTA.n({right arrow over (r)}).varies.|E.sub.s({right arrow over
(r)})+E.sub.r({right arrow over (r)})|.sup.2=|E.sub.s({right arrow
over (r)}).sup.2+|E.sub.r({right arrow over
(r)})|.sup.2+E.sub.s*({right arrow over (r)})E.sub.r({right arrow
over (r)})+E.sub.s({right arrow over (r)})E.sub.r*({right arrow
over (r)}), (2)
[0040] where E.sub.s({right arrow over (r)}) is the spatial
distribution of the signal beam field. The final term in this
expansion, E.sub.s({right arrow over (r)})E.sub.r*({right arrow
over (r)}), is the signal-bearing (data band) term. Thus we can
write
E s ( r ) E r * ( r ) E r ( k ) E s ( k ) , ( 3 ) ##EQU00002##
[0041] where is the 3D cross-correlation operator. This is to say,
the product of one field and the complex conjugate of another in
the spatial domain become a cross-correlation of their respective
Fourier transforms in the frequency domain.
[0042] FIGS. 1(a) and 1(b) illustrate example distributions for a
Fourier angular-multiplexing geometry. In particular, FIG. 1(a)
shows a cross-section of the recording beams E.sub.s({right arrow
over (k)}) 102 and E.sub.r({right arrow over (r)}) 104 in real
space. The cross-hatched region 106 indicates where the beams
intersect within the recording layer 108, and thus where the
data-bearing holographic fringes are located. The narrow waist 110
of this region 106 corresponds to the Fourier plane.
[0043] FIG. 1(b) illustrates these same distributions in k-space.
Since E.sub.s({right arrow over (k)}) 112 and E.sub.r({right arrow
over (k)}) 114 represent monochromatic optical fields, they may be
confined to arcs along the k-sphere. Note that while E.sub.r({right
arrow over (r)}) 104 shows only a single collimated reference beam,
the dots in the arc of E.sub.r({right arrow over (k)}) 114
represent multiple reference beams used to write an
angle-multiplexed stack of holograms. Note also that while
E.sub.r({right arrow over (k)}) 114 may be confined largely to the
plane of the figure, E.sub.s({right arrow over (k)}) 112 may extend
out of the figure plane to subtend a page-shaped region (or
"patch") on the surface of the sphere. In FIG. 1(b), it can be seen
that the data band 116 distribution can be constructed graphically
from the cross-correlation of the signal patch E.sub.s({right arrow
over (k)}) 112 with the reference arc E.sub.r({right arrow over
(k)}) 114. The conjugate data band 118 may be similarly constructed
by reversing the order of the operands.
[0044] The internal structure of the data bands is also indicated.
The entire data band (along with the conjugate data band)
represents the k-space locus of the holographic fringes for all of
the holograms in an angle-multiplexed hologram stack, and each
hologram occupies an E.sub.s({right arrow over (k)}) 112
patch-shaped layer within each of the bands. Each layer has a
slight thickness (determined by the Bragg selectivity imparted by
the medium thickness) and may be packed in a nested fashion similar
to the layers of an onion within the data band to maximize density.
It should be noted that while FIG. 1(b) depicts only 14 layers,
hundreds or more may be present in an actual implementation. Each
hologram/layer may thus occupy a different (substantially disjoint)
region of k-space, such that there is little to no cross-talk from
other holograms during reconstruction.
Monocular System Architecture
[0045] FIG. 2(a) illustrates an example holographic data storage
system 200 having a monocular architecture. A monocular
architecture is a configuration that may employ a very high
numerical aperture (NA) objective lens in order to maximize storage
density. In this configuration, both the reference and signal beams
may pass through the objective lens. The example system 200 shown
in FIG. 2(a) is similar to a conventional, off-axis architecture in
that it is a page-oriented, angle-multiplexed, Fourier
geometry.
[0046] FIG. 2(b) illustrates the angular aperture map 210 of system
200. The x and y locations of angular aperture map 210 indicate the
external angle of incidence of beam components into the recording
medium. The SLM region 212 is represented by a black rectangle, and
the gray pixelated region 214 within the SLM region 212 and the
acceptable NA (e.g., NA=0.90) indicates the size and shape of the
data page, and thus the size and shape of the signal beam angular
aperture. It should be noted that in a Fourier architecture, an
image of the SLM may coincide with the angular aperture plane. The
arrow 216 spanning 50.degree. to 30.degree. and labeled "Ref-beam"
shows the locus of the reference beam angular apertures. In some
examples, this locus may be further subdivided into multiple (e.g.,
192) finely-spaced points corresponding to the multiple (e.g., 192)
reference beam angular apertures (e.g., angle of incidences) used
for angle multiplexing. For this example system 200, the signal
beam angular aperture may remain constant during the recording of
the multiple (e.g., 192) holograms, and the locus of the signal
beam angular aperture may be disjoint (non-overlapping) with the
locus of the reference beam angular aperture.
Dynamic Aperture Holographic Multiplexing
[0047] Using a holographic data storage system having a monocular
architecture like that shown in FIG. 2(a), overlapping holographic
recordings can be made within a recording medium. However, due to
the static relationship between the signal beam angular aperture
and the reference beam angular aperture, the recording medium is
being used inefficiently. Thus, in some examples, a data storage
system implementing dynamic aperture holographic multiplexing may
be used to increase the amount of data that can be stored in a
recording medium. Generally, dynamic aperture holographic
multiplexing involves the changing of the signal beam angle
aperture for different holograms in a multiplexed set.
[0048] To illustrate, FIG. 3(a) shows an angular aperture map of a
data storage system having a monocular architecture that does not
implement dynamic aperture holographic multiplexing. As shown in
this example, the reference beam is confined (e.g., to a scan range
of 23.degree. (-58.degree. to -35.degree.)), and the edge 304 of
the signal beam locus 302 is separated from the reference beam
locus 306 (e.g., by a minimum of 15.degree. (worst selectivity
case)) according to a representative design criterion. The
reference beam locus 306 may be subdivided into multiple (e.g.,
192) reference angles, again according to a design rule based on
constant minimum Bragg selectivity. According to a software model,
this configuration (along with other assumptions) may result in a
user capacity of 700 GB on a 120 mm disk.
[0049] In contrast, FIG. 3(b) shows an angular aperture map of a
data storage system having a monocular architecture that implements
dynamic aperture holographic multiplexing. As shown in this
example, the reference beam scan range has been expanded to
101.degree. (-58.degree. to +43.degree.). The minimum separation of
15.degree. between reference beam angular apertures of locus 316
and angular apertures of signal beam locus 312 may be achieved by
dynamically changing the signal beam angular aperture such that the
closest edge of each signal beam angular aperture is selected to be
15.degree. higher than the angular aperture of the reference beam
used to record it. The locus 316 of the reference beam angular
apertures is thus shared with the locus 312 of the signal beam
angular apertures. In some examples using dynamic aperture
holographic multiplexing, holograms may be recorded in order of
ascending reference beam angular aperture, and thus with a
shrinking signal beam angular aperture as the reference beam scans
from left to right in the figure. In other examples, the holograms
may be recorded in other orders. While specific values (e.g., of
minimum separation, reference beam scan range, and signal beam
angular aperture) were provided for the example above, it should be
appreciated that other values may be used.
[0050] In some examples, the signal beam angular aperture may be
dynamically changed by changing the subset of the SLM pixels that
are included in the holographic data page. In some examples,
regions of the signal angular aperture that are not included in the
holographic data page may be darkened to prevent their illuminating
the recording medium. In some examples, this darkening may be
accomplished by setting SLM pixels corresponding to the excluded
regions to a dark, or "off" state. In other examples, this
darkening may be accomplished using a knife edge shutter or similar
device to selectively block illumination from excluded aperture
regions while passing illumination from included regions. Such a
method might be used if, for example, the SLM employed does not
produce an appropriate dark pixel state. In still other examples,
this darkening may be accomplished by a beam shaping device that
dynamically redirects light from dark regions to illuminated
regions.
[0051] Employing the same reference beam angular spacing design
rule that was used in the preceding discussion of FIG. 3(a), the
number of holograms multiplexed may be increased from 192 to 834
using dynamic aperture holographic multiplexing. Because the size
of the signal beam angular aperture may decrease as the reference
angular aperture moves, the amount of data per hologram may also
decrease. Nevertheless, the total amount of data that may be
recorded in the recording medium may increase substantially.
Applying the same model used for the example shown in FIG. 3(a),
the maximum user capacity may increase from 700 GB to 1.7 TB, which
is an improvement of over 240%.
[0052] FIGS. 4(a) and 4(b) illustrate example graphs showing
relationships between various storage attributes with respect to
the multiplexed hologram number when using dynamic aperture
holographic multiplexing. In particular, the solid line of FIG.
4(a) indicates the user capacity in TB as a function of the number
of multiplexed hologram (pages) according to the model. The dashed
line of FIG. 4(a) shows the average number of recorded pixels per
hologram, which is a useful proxy for the transfer rate that may be
achieved by a real device. The points indicated by the circle and
the x indicate the capacity and transfer rate of a conventional
system, respectively. Because of the diminishing increases in
capacity and decreasing transfer rate, a designer may elect to
record only a subset of these holograms in a real system. For
example, if 500 holograms were used instead of 834, the device
would achieve over 1.5 TB of capacity with a transfer rate penalty
of only 17% compared to the conventional system.
[0053] FIG. 4(b) shows the angle (solid line) and angular spacing
(dashed line) of the reference beams produced by the model. The
figure illustrates the increased angular density that may be
available in the middle of the scan range to a design employing a
constant minimum Bragg selectivity spacing rule. However, any
spacing rule may be employed without departing from the scope of
the present disclosure.
[0054] FIG. 5 illustrates an example holographic data storage
system 500 for performing dynamic aperture holographic
multiplexing. System 500 generally includes a laser source 502, a
beam directing device (e.g., a galvanometer) 504 for directing the
output of the laser source 502, a polarizing beam splitter ("PBS")
506 in conjunction with a thin strip half-wave plate ("Strip HWP")
508 that may collectively act as an aperture sharing element, a
detector 522 for performing the recovery operation, an SLM 520 for
modulating the signal beam 512, a polytopic aperture 518, a high NA
objective lens 510 through which both the reference beam 516 and
signal beam 512 may pass, and a recording medium 514. While not
shown, SLM 520 may receive an output from laser source 502 and
modulate that output to generate the signal beam. System 500 may
further include a processor (not shown) for controlling laser
source 502, beam directing device 504, and SLM 520.
[0055] System 500 may include an aperture sharing element to
combine the reference 516 and signal beam 512 paths in the regions
that are shared between the two. In the illustrated example of FIG.
5, the aperture sharing function may be performed using a passive
method utilizing PBS 506 in conjunction with Strip HWP 508. In
particular, PBS 506 may serve to combine the beams 512 and 516 in
orthogonal polarization states, for example, with the signal beam
512 in the linearly-polarized "s" state (with respect to the PBS
506 hypotenuse), and the reference beam 516 in the "p" state. Strip
HWP 508 may be placed in the back focal plane of the objective lens
510 where the reference beam 516 comes to a focus. This plane may
also be an image plane of the SLM 520. Strip HWP 508 may be aligned
with the transit path of the reference beam 516, but may be
sufficiently narrow that it occludes only a few rows of SLM pixels.
In some examples, the occlusion may be confined to a single row. To
account for this occlusion, the occluded SLM pixel(s) may be easily
omitted from the SLM page data format with negligible loss of
capacity. The birefringent axes of the Strip HWP 508 may be
oriented so as to rotate the reference beam polarization to the "s"
state, thus allowing the reference beam to interfere with the
signal beam and enabling the recording of holographic fringes. The
fringes so written are disjoint in k-space. In other examples,
passive aperture sharing may be accomplished using a thin strip
mirror that reflects only the reference beam, thus allowing the
beams to be combined in the same polarization state. The larger
schematic in FIG. 5 shows the assembly with the reference beam 516
near the start of its scan range where the SLM page is large,
whereas the inset 524 to the right shows the beam positions near
the end of the scan range where the SLM page is small. In some
examples, beam directing device 504 and SLM 520 may be used to
adjust the angle of incidence of reference beam 516 and the signal
beam angular aperture of signal beam 512, respectively.
[0056] While passive aperture sharing methods are described above,
in other examples, an active aperture sharing element employing a
switchable element, such as a MEMS-actuated micro-mirror array, to
dynamically select the desired beam source for each region of the
shared aperture, may be used. In yet other examples, a single SLM
may be used to generate both signal and reference beams, and may
thus itself be considered to be an active aperture sharing element.
Moreover, other architectures, potentially employing other methods
of either passive or active aperture sharing, may additionally or
alternatively be used.
K-Space Separability
[0057] FIGS. 6(a) and 6(b) illustrate the results of a k-space
analysis for system 500 using a method similar to that described
above with respect to FIGS. 1(a) and 1(b). FIG. 6(a) illustrates a
cross-section of k-space distributions for a monocular system. In
contrast, FIG. 6(b) shows the analogous k-space distributions that
may result using dynamic aperture holographic multiplexing as
described above with respect to FIGS. 3-5. As is evident in FIG.
6(b), the layers representing individual holograms within the data
band continue to nest in disjoint regions of k-space despite the
fact that the loci of E.sub.s({right arrow over (k)}) and
E.sub.r({right arrow over (k)}) are no longer disjoint over the
multiplexed set. The overall volume occupied by the data band and
conjugate data band is larger than in that shown in FIG. 6(a),
reflecting the increased multiplexing density of the present
disclosure.
Dynamic Aperture Equalization
[0058] As shown in FIG. 6(b)Error! Reference source not found., the
distributions of the holograms are packed more densely near the
origin (e.g., low spatial frequency) than they are further away
from it. This is a manifestation of the lower Bragg selectivity
exhibited by gratings of lower frequency compared to those of
higher frequency, which is well-known among those skilled in the
art. For this reason, cross-talk between holograms may be higher in
page regions corresponding to lower grating frequencies, and these
worst-case regions tend to limit the achievable density of angle
multiplexing. Thus, in some examples, dynamic aperture equalization
may be performed to mitigate this effect.
[0059] In some examples, dynamic aperture equalization may be
performed by interleaving data page sizes. For example, the edge of
the signal beam angular aperture may be changed every other
hologram so that only the odd (or alternatively even) numbered
holograms have the lowest allowable frequency components. In the
example described with respect to FIG. 3(b), the angular apertures
of the signal beams for odd holograms may be separated from the
angular apertures of the reference beams by 15.degree. as
described, while the angular apertures of the signal beams for even
(or alternatively odd) holograms may instead be separated from the
angular apertures of the reference beams by 40.degree.. The
resulting k-space distributions are illustrated in FIG. 7, which
shows a considerable decrease in hologram packing density in
comparison to FIG. 6(b). Interleaving data page sizes in this way
may allow for the angular separation between reference beams to be
decreased, leading to a considerable recording density increase.
This increase may come at the cost of reduced transfer rate, as the
average page size is smaller. Page size interleaving may present
diminishing returns at higher reference angles, and might be
stopped at these high angles rather than pursuing these diminishing
returns.
[0060] In other examples, dynamic aperture equalization may be
performed with or without shared aperture multiplexing.
Additionally, interleaving patterns of different lengths (not just
odd/even), and patterns that are not cyclical may also be
performed. In general, any technique that equalizes the k-space
modulation distribution may be performed and may be referred to as
dynamic aperture equalization.
Error Correction Parity Distribution
[0061] Holographic storage devices typically employ error
correcting codes in order to achieve robust data recovery in the
presence of recovery errors. For example, systematic codes may be
used to append parity data to the input data to allow for
reconstruction when some part of the whole cannot be recovered.
Examples of systematic codes include low density parity check
(LDPC) codes and Reed-Solomon codes.
[0062] In some examples using dynamic aperture holographic
multiplexing, the parity portion of the data recorded may be
preferentially distributed to some subset of the data pages, while
input data may be preferentially distributed to some other subset.
In one example, parity data may be preferentially distributed to
smaller data pages, while input data may be preferentially
distributed to larger ones. Distributing the parity data in this
way advantageously improves the recovery transfer rate because in
the event of error-free recovery of the input data, the parity data
residing on the smaller data pages need not be recovered. When used
in a dynamic aperture system, the parity pages may be selectively
distributed to lower data rate (smaller) pages.
Multiple Locus Aperture Sharing
[0063] In some examples, regions of the aperture may be shared
multiple times. Multiple sharing of the signal and/or reference
angular apertures can be used to access grating space that is
inaccessible to the "singly shared" methods discussed above.
Multiple sharing in this context is distinct from the "sharing" of
an underlying multiplexing scheme, such as the angle multiplexing
described above.
[0064] In one example, multiple locus aperture sharing may include
double sharing and may be performed with the dynamic aperture
holographic multiplexing described above. FIGS. 8(a) and 8(b)
illustrate one example arrangement for double aperture sharing. In
particular, FIG. 8(a) shows an aperture map comparable to the
aperture sharing of FIG. 3(b) with two modifications: 1) the
aperture map has been rotated by -45.degree.; and 2) a second edge
("Edge 2") 804 has been added to the signal beam angular aperture
locus 802. This arrangement may then be used to affect aperture
sharing in the manner described above, with Edge 1 808 leading the
reference beam angular aperture locus 806 by some amount
(e.g.,)15.degree.. Furthermore, the new signal edge, Edge 2 804,
may be dynamically changed so that its y-component is always
slightly less than the y-component of the reference beam. Then, by
applying the k-space formalism described above, it is apparent that
the k.sub.y component of the data band grating distribution so
generated will always be less than zero (e.g., the data band lies
in the negative k.sub.y half of k-space). The conjugate data band,
conversely, has a k.sub.y component greater than zero.
[0065] FIG. 8(b) is similar to FIG. 8(a), except that the
distributions have been flipped about the x-axis. By similar
k-space formalism analysis, it is apparent that the data band
grating distribution for this example will lie entirely in the
positive k.sub.y half of k-space (and the conjugate band will lie
in the negative half). One may verify that the distributions of
both data bands and both conjugate data bands are mutually disjoint
by construction, and therefore both sets of holograms may be
multiplexed into the same volume of recording medium. Such a system
may achieve a recording density of somewhat less than double that
of the example described with respect to FIGS. 3-5, with a somewhat
decreased average number of pixels per hologram (and, hence,
transfer rate).
[0066] While a specific locus shared aperture example is provided
above, it should be appreciated that other multiple locus shared
aperture schemes may be used. The multiple locus hologram
distributions may or may not be symmetric in k-space, and three,
four, or even more distributions may be employed. The method may be
practiced in combination with multiplexing methods other than
angular multiplexing and/or polytopic multiplexing.
Polarization Multiplexed Shared Aperture Multiplexing
[0067] In some examples in which multiple locus shared aperture
techniques are used, multiple locus multiplexing may be performed
simultaneously, rather than sequentially, by employing
substantially orthogonal polarization states for the recording or
recovery of two shared apertures simultaneously. In some examples,
the shared apertures of FIGS. 8(a) and 8(b) may each be recorded or
recovered with reference and signal light in the linearly-polarized
"s" state with respect to their own reference beams. Since these
polarizations are substantially orthogonal to each other, a
recording operation may not produce significant interference
fringes between reference A and signal B, or between reference B
and signal A. Thus the two distributions may be recorded
simultaneously. To perform the simultaneous recording, the
holographic data storage system (e.g., system 500) may include two
separate SLMs and reference scanners, as well as another aperture
sharing element based on polarization (such as a PBS) used to
combine the beams. The system may further include two detectors to
perform the recovery operation, and the new aperture sharing
element may direct light of each polarization toward the
appropriate detector. Polarization multiplexing would thus achieve
a factor of two increase in transfer rate for both recording and
recovery compared to the equivalent non-polarization multiplexed
system.
Collinear Dynamic Aperture Multiplexing
[0068] The examples described above relate to systems employing
angle and polytopic multiplexing. However, it should be appreciated
that the present disclosure may also be applied to other system
architectures. For example, dynamic aperture holographic
multiplexing may similarly be applied to a collinear holography
system, such as that described in H. Horimai, X. Tan, and J. Li,
"Collinear holography," Appl. Opt. 44, 2575-2579 (2005). According
to Horimai et al., "[t]he unique feature of this technology is that
2-D page data are recorded as volume holograms generated by a
coaxially aligned information beam and a reference beam, which are
displayed simultaneously by one SLM and interfere with each other
in the recording medium through a single objective lens." FIG. 9
illustrates an example collinear holographic data storage system
900. System 900 generally includes a laser source 902 (e.g., green
or blue), an SLM 904 for producing a reference beam and a signal
(information) beam, a polarizing beam splitter (PBS) 906, a
dichroic mirror 908, quarter-wave plate (QWP) 910, objective lens
912, recording media 914, laser source 916 (e.g., red),
photo-detector 918, ring mask 920, land CMOS or CCD sensor 922.
System 900 may further include a processor (not shown) for
controlling laser sources 902 and 916, SLM 904, and other
components of the system. Additional lenses and/or reflectors may
also be included in system 900, as shown in FIG. 9.
[0069] During the write process, a combined image of the signal
beam and the reference beam, as shown in the angular aperture map
of FIG. 10, may be produced by SLM 904. The loci of the reference
and signal components are labeled, showing the annular reference
pattern 1002 surrounding a central data page 1004. The p-polarized
output of SLM 904 may pass through PBS 906 and may then be incident
on QWP 910. The p-polarized beams may be converted to a circularly
polarized state by QWP 910 and may be focused in the holographic
recording media 914 by objective lens 912.
[0070] During the read process, only the outer reference beam may
be generated by SLM 904 and passed through PBS 906, QWP 910, and
objective lens 912 onto holographic recording media 914. A
reconstructed signal beam may be produced and may be reflected back
through objective lens 912 and passed through QWP 908, where it may
be converted from a circularly polarized state to an s-polarized
state. The reconstructed signal beam may be then reflected by PBS
906 and detected using CMOS or CCD sensor 922. Laser source 916 may
be used for optical servo control to adjust the focal point of the
objective lens 912.
[0071] A collinear system similar or identical to that shown in
FIG. 9 may be modified to benefit from dynamic aperture holographic
multiplexing. For example, the modified SLM patterns of FIG. 11 may
be generated by shifting the position of the signal beam angular
aperture to the edge of the SLM (e.g., in the directions of
0.degree., 120.degree., and 240.degree. for FIGS. 11(a), 11(b), and
11(c), respectively) and modifying the position of the reference
beam angular aperture to accommodate these shifts. In some
examples, a threshold angle difference may be maintained between
edges of the signal beam angular aperture and the reference beam
angular aperture. The threshold angle may be the same or different
for each of the modified patterns.
[0072] Modifying the collinear system in this way may
advantageously provide at least two benefits:
[0073] 1) Though the k-space hologram distributions generated by
the three patterns are substantially overlapping, the overall
volume of the data bands and conjugate data bands of the holograms
so multiplexed may be larger than in the conventional case. This
may result in a higher theoretical recording density.
[0074] 2) According to a theoretical analysis as described in T.
Shimura, M. Terada, Y. Sumi, R. Fujimura, and K. Kuroda,
"Inter-page cross-talk noise in collinear holographic memory,"
Joint Int. Symp. on Opt. Memories and Opt. Data Storage, Waikoloa,
Hi., July (2008), paper TuPO4, inter-page cross-talk noise in
collinear holography goes as an incoherent sum of contributions
from the multiplexed pages. The k-space hologram distributions for
conventional collinear holograms are completely overlapping, but
the distributions of, e.g., FIGS. 11(a) and 11(b) are only
partially overlapping. Thus the cross-talk contributions between
the differing SLM patterns of FIGS. 11(a) and 11(b) should be lower
than in the conventional case, leading to increased signal-to-noise
ratio and hence increased recording density.
[0075] Collinear holography relies on a correlation effect for
holographic multiplexing. In contrast to angle multiplexing where
individual holograms occupy disjoint regions of k-space, individual
holograms in collinear recording are broadly distributed and
densely overlapped with other holograms, leading to cross-talk
expressions such as that of Shima et al. Dynamic aperture
holographic multiplexing described herein serves to slightly reduce
the overlap of these distributions, and thus serves to slightly
reduce cross-talk by driving the design toward a more disjoint
k-space partitioning scheme. Other variations of this technique may
be implemented under the scope of the present disclosure.
Dynamic Aperture Holographic Multiplexing Process
[0076] FIG. 12 illustrates an exemplary process 1200 that can be
used to perform dynamic aperture holographic multiplexing. In some
examples, process 1200 may be performed by a holographic data
storage system similar or identical to system 500 or 900. In other
examples, a system similar to system 500, but modified as described
with respect to FIGS. 8(a) and 8(b), may be used.
[0077] At block 1202, a first hologram may be recorded to a
recording medium using a first signal beam angular aperture and a
first reference beam having a first reference beam angular
aperture.
[0078] In one example, using a system similar or identical to that
shown in FIG. 5, a reference beam may be generated by a laser
source (e.g., laser source 502) and directed toward an aperture
sharing element, such as a PBS in combination with a thin strip
half-wave plate (e.g., PBS 506 and Strip HWP 508), by a beam
directing device (e.g., galvanometer 504). A signal beam may also
be generated and modulated to contain data using an SLM (e.g., SLM
520). The signal beam may be directed toward the aperture sharing
element, where the reference and signal beam paths may be combined
in the regions that are shared between the two. For example, the
PBS may combine the reference and signal beams in orthogonal
polarization states (e.g., signal beam in the linearly-polarized
"s" state with respect to the PBS hypotenuse and the reference beam
in the "p" state). The thin strip half-wave plate may be positioned
in the back focal plane of an objective lens (e.g., objective lens
510) where the reference beam comes to a focus. This plane may also
be an image plane of the SLM. The output of the thin strip
half-wave plate may pass through the objective lens before entering
a recording medium (e.g., medium 514) to record the first
hologram.
[0079] In another example, using a collinear system similar or
identical to that shown in FIG. 9, reference beam may be generated
by a laser source (e.g., laser source 902) and directed toward an
SLM (e.g., SLM 904). The SLM may generate both a reference beam and
a signal beam, which may then pass through a PBS (e.g., PBS 906)
and be incident on a QWP (e.g., QWP 910). The QWP may convert the
p-polarized reference and signal beams to a circularly polarized
state and may be focused in a recording medium (e.g., recording
medium 914) by an objective lens (e.g., objective lens 912) to
record the first hologram.
[0080] In some examples, the reference beam angular aperture and
the edge of the signal beam angular aperture nearest the reference
beam angular aperture may be separated by a threshold angle.
[0081] In one example, using a system similar or identical to that
shown in FIG. 5, the separation between the reference beam angular
aperture and the signal beam angular aperture may be set by
adjusting the orientation of the beam deflecting device (e.g.,
galvanometer 504) reflecting the output of the laser source and/or
by causing the SLM to change the size and position at which the
signal beam enters the aperture sharing element (e.g., the PBS).
For example, the signal beam angular aperture may be dynamically
changed by changing the subset of the SLM pixels that are included
in the holographic data page. In some examples, regions of the
signal angular aperture that are not included in the holographic
data page may be darkened to prevent their illuminating the
recording medium. In some examples, this darkening may be
accomplished by setting SLM pixels corresponding to the excluded
regions to a dark, or "off" state. In other examples, this
darkening may be accomplished using a knife edge shutter or similar
device to selectively block illumination from excluded aperture
regions while passing illumination from included regions. Such a
method might be used if, for example, the SLM employed does not
produce an appropriate dark pixel state. In still other examples,
this darkening may be accomplished by a beam shaping device that
dynamically redirects light from dark regions to illuminated
regions. In some examples, the threshold angle separating the
angular apertures of the reference and signal beams may be selected
such that the edge of the signal beam angular aperture may be
15.degree. higher than the angular aperture of the reference beam
used to record it. However, other differences in angles may be
used.
[0082] In another example, using a collinear system similar or
identical to that shown in FIG. 9, the separation between the
reference beam angular aperture and the signal beam angular
aperture may be set by causing the SLM (e.g., SLM 904) to shift the
signal beam angular aperture to the edge of the SLM (e.g., in the
directions of 0.degree., 120.degree., or 240.degree. as shown in
FIGS. 11(a), 11(b), and 11(c), respectively) and to shift the
reference beam angular aperture to accommodate the shifted signal
beam angular aperture. This may be done in a manner that results in
a threshold angle separation between the reference beam angular
aperture and the edge of the signal beam angular nearest the
reference beam angular aperture. In some examples, a separation of
approximately 1, 2, 4, 8, or other number of degrees may be created
between the annular reference pattern (e.g., annular reference
pattern 1002) and the flat edges of the central data page (e.g.,
central data page 1004) and a separation of less than 1, 2, 4, or
other number of degrees may be created between the annular
reference pattern and the corners of central data page.
[0083] At block 1204, a second hologram may be recorded to the
recording medium using a second signal beam angular aperture and a
second reference beam having a second reference beam angular
aperture. It should be appreciated that the second reference beam
may be similar to the first reference beam used to record the first
hologram at block 1202, except that a characteristic of the first
reference beam may be modified to generate the second reference
beam at block 1204. Similarly, the second signal beam angular
aperture may be similar to the first signal beam angular aperture
used to record the first hologram at block 1202, except that a
characteristic of the first signal beam angular aperture may be
modified to generate the second signal beam angular aperture at
block 1204.
[0084] In one example, using a system similar or identical to that
shown in FIG. 5, the reference beam may be modified so as to change
its angular aperture (e.g., angle of incidence with respect to the
recording medium). This may be performed by, for example, adjusting
the orientation of the beam directing device reflecting the output
of the laser source. Additionally, the signal beam angular aperture
may be modified such that the reference beam angular aperture and
the edge of the signal beam angular aperture nearest the reference
beam angular aperture may be separated by a threshold angle. This
may be accomplished by causing the SLM to change the size and
position at which the signal beam enters the aperture sharing
element, as described above. In some examples, the threshold angle
may be the same or substantially the same (e.g., within a tolerance
threshold) as the threshold angle used at block 1202 (e.g., the
edge of the signal beam angular aperture may be 15.degree. higher
than the angular aperture of the reference beam used to record it).
Thus, the signal beam angular aperture may be modified by the same
or substantially the same (e.g., within a tolerance threshold)
amount as the angular aperture of the reference beam used to record
it. In other examples, the threshold angle may be different than
the threshold angle used at block 1202. For example, the threshold
angle may be selected such that the edge of the signal beam angular
aperture may be 40.degree. higher than the angular aperture of the
reference beam used to record it. However, other differences in
angles may be used.
[0085] In another example, using a system similar or identical to
that shown in FIG. 9, the SLM (e.g., SLM 904) may shift the signal
beam angular aperture to a different edge of the SLM (e.g., in the
directions of 0.degree., 120.degree., or 240.degree. as shown in
FIGS. 11(a), 11(b), and 11(c), respectively, or other direction)
and may shift the reference beam angular aperture to accommodate
the shifted signal beam angular aperture. This may be done in a
manner that results in a threshold angle separation between the
reference beam angular aperture and the edge of the signal beam
angular aperture nearest the reference beam angular aperture. The
threshold angle may be the same as or different than the threshold
angle used at block 1202.
[0086] At block 1206, a third hologram may be recorded to the
recording medium using a third signal beam angular aperture and a
third reference beam having a third angular aperture. It should be
appreciated that the third reference beam may be similar to the
first reference beam used to record the first hologram at block
1202, except that a characteristic of the first reference beam may
be modified to generate the third reference beam at block 1206.
Similarly, the third signal beam angular aperture may be similar to
the first signal beam angular aperture used to record the first
hologram at block 1202, except that a characteristic of the first
signal beam angular aperture may be modified to generate the third
signal beam angular aperture at block 1206.
[0087] In one example, using a system similar or identical to that
shown in FIG. 5, block 1206 may be performed in a manner similar to
that of block 1204, except that the reference beam angular aperture
may be different than that used in any of the previous recordings
(e.g., first and second recordings made at blocks 1202 and 1204)
and that the signal beam angular aperture may be modified such that
the reference beam angular aperture and the edge of the signal beam
angular aperture nearest the reference beam angular aperture may be
separated by a threshold angle. In some examples where the
threshold angles used at blocks 1202 and 1204 were the same (or at
least substantially the same), the threshold angle used at block
1206 may also be the same (e.g., 15 degrees) or substantially the
same. In other examples where the threshold angles used at blocks
1202 and 1204 were different, the threshold angle used at block
1206 may be the same (or at least substantially the same) as that
used at block 1202 (e.g., 15 degrees). In this way, dynamic
aperture equalization may be achieved by interleaving data page
sizes to reduce cross-talk between holograms.
[0088] In another example, using a system similar or identical to
that shown in FIG. 9, block 1206 may be performed in a manner
similar to that of block 1204, except that the reference beam
angular aperture may be different than that used in any of the
previous recordings (e.g., first and second recordings made at
blocks 1202 and 1204) and that the signal beam angular aperture may
be modified such that the reference beam angular aperture and the
edge of the signal beam angular aperture nearest the reference beam
angular aperture may be separated by a threshold angle. In some
examples where the threshold angles used at blocks 1202 and 1204
were the same (or at least substantially the same), the threshold
angle used at block 1206 may also be the same or substantially the
same. In other examples where the threshold angles used at blocks
1202 and 1204 were different, the threshold angle used at block
1206 may be the same (or at least substantially the same) as that
used at block 1202. In this way, dynamic aperture equalization may
be achieved by interleaving data page sizes to reduce cross-talk
between holograms.
[0089] At block 1208, a fourth hologram may be recorded to the
recording medium using a fourth signal beam angular aperture and a
fourth reference beam having a fourth angular aperture. It should
be appreciated that the fourth reference beam may be similar to the
first reference beam used to record the first hologram at block
1202, except that a characteristic of the first reference beam may
be modified to generate the fourth reference beam at block 1208.
Similarly, the fourth signal beam angular aperture may be similar
to the first signal beam angular aperture used to record the first
hologram at block 1202, except that a characteristic of the first
signal beam angular aperture may be modified to generate the fourth
signal beam angular aperture at block 1208.
[0090] In one example, using a system similar or identical to that
shown in FIG. 5, block 1208 may be performed in a manner similar to
that of block 1204, except that the reference beam angular aperture
may be different than that used in any of the previous recordings
(e.g., first, second, and third recordings made at blocks 1202,
1204, and 1206) and that the signal beam angular aperture may be
modified such that the reference beam angular aperture and the edge
of the signal beam angular aperture nearest the reference beam
angular aperture may be separated by a threshold angle. In some
examples where the threshold angles used at blocks 1202, 1204, and
1206 were the same (or at least substantially the same), the
threshold angle used at block 1208 may also be the same (e.g., 15
degrees) or substantially the same. In other examples where at
least some of the threshold angles used at blocks 1202, 1204, and
1206 were different, the threshold angle used at block 1208 may be
the same or substantially the same as that used at block 1204
(e.g., 40 degrees). In these examples, the threshold angles for the
first and third holograms may be the same or substantially the
same, while the threshold angles for the second and fourth
holograms may be the same or substantially the same. However, the
threshold angle for the first and third holograms may be different
than the threshold angle for the second and fourth holograms. In
this way, dynamic aperture equalization may be achieved by
interleaving data page sizes to reduce cross-talk between
holograms.
[0091] In another example, using a system similar or identical to
that shown in FIG. 9, block 1208 may be performed in a manner
similar to that of block 1204, except that the reference beam
angular aperture may be different than that used in any of the
previous recordings (e.g., first, second, and third recordings made
at blocks 1202, 1204, and 1206) and that the signal beam angular
aperture may be modified such that the reference beam angular
aperture and the edge of the signal beam angular aperture nearest
the reference beam angular aperture may be separated by a threshold
angle. In some examples where the threshold angles used at blocks
1202, 1204, and 1206 were the same (or at least substantially the
same), the threshold angle used at block 1208 may also be the same
or substantially the same. In other examples where at least some of
the threshold angles used at blocks 1202, 1204, and 1206 were
different, the threshold angle used at block 1208 may be the same
or substantially the same as that used at block 1204. In these
examples, the threshold angles for the first and third holograms
may be the same or substantially the same, while the threshold
angles for the second and fourth holograms may be the same or
substantially the same. However, the threshold angle for the first
and third holograms may be different than the threshold angle for
the second and fourth holograms. In this way, dynamic aperture
equalization may be achieved by interleaving data page sizes to
reduce cross-talk between holograms.
[0092] In some examples, additional holograms may be recorded in a
manner similar to that described with respect to blocks 1204, 1206,
and 1208. Each additional hologram may be recorded using a
reference beam that is different than any of those previously used
to record holograms and a signal beam that has been dynamically
adjusted accordingly, as described above. In some examples, the
threshold angle offset between the reference beam angular aperture
and the edge of the signal beam angular aperture nearest the
reference beam may be the same or substantially the same as the
threshold angles used in each of the previous recordings. In other
examples, the threshold angle offset may be interleaved such that
even numbered holograms may use the same or substantially the same
threshold angle and odd numbered holograms may use the same or
substantially the same threshold angle (different from the angle
used for the even numbered holograms) in order to perform dynamic
aperture equalization to reduce cross-talk between holograms. In
yet other examples, other non-uniform distributions of threshold
angles may be used to generate the holograms.
[0093] In some examples, process 1200 may include the use of error
correcting codes. In these examples, some of the data pages or
holograms may be used to store parity information, while the other
data pages are used to store input data. For example, the smaller
data pages may be used to store the parity information, while the
remaining data pages may be used to store input data. This may
advantageously improve the recovery transfer rate because in the
event of error-free recovery of the input data, the parity data
residing on the smaller data pages need not be recovered.
[0094] In some examples, process 1200 may include multiple locus
aperture sharing, as discussed above. In these examples, regions of
the aperture may be shared multiple times. For example, process
1200 may include double sharing, as described above with respect to
FIGS. 8(a) and 8(b).
[0095] In some examples in which multiple locus shared aperture
techniques are used in process 1200, multiple locus multiplexing
may be performed simultaneously, rather than sequentially, by
employing substantially orthogonal polarization states for the
recording or recovery of two shared apertures simultaneously. In
some examples, the shared apertures of FIGS. 8(a) and 8(b) may each
be recorded or recovered with reference and signal light in the
linearly-polarized "s" state with respect to their own reference
beams. Since these polarizations are substantially orthogonal to
each other, a recording operation may not produce significant
interference fringes between reference A and signal B, or between
reference B and signal A. Thus the two distributions may be
recorded simultaneously. To perform the simultaneous recording, the
holographic data storage system (e.g., system 500) may instead
include two separate SLMs and reference scanners, as well as
another aperture sharing element based on polarization (such as a
PBS) used to combine the beams. The system may further include two
detectors to perform the recovery operation, and the new aperture
sharing element may direct light of each polarization toward the
appropriate detector.
[0096] FIG. 13 depicts computing system 1300 with a number of
components that may be used to perform the above-described
processes. The main system 1302 includes a motherboard 1304 having
an input/output ("I/O") section 1306, one or more central
processing units ("CPU") 1308, and a memory section 1310, which may
have a flash memory card 1312 related to it. The I/O section 1306
is connected to a display 1324, a keyboard 1314, a disk storage
unit 1316, and a media drive unit 1318. The media drive unit 1318
can read/write a non-transitory computer-readable storage medium
1320, which can contain programs 1322 and/or data.
[0097] At least some values based on the results of the
above-described processes can be saved for subsequent use.
Additionally, a non-transitory computer-readable medium can be used
to store (e.g., tangibly embody) one or more computer programs for
performing any one of the above-described processes by means of a
computer. The computer program may be written, for example, in a
general-purpose programming language (e.g., Pascal, C, C++, Java)
or some specialized application-specific language.
[0098] Although only certain exemplary embodiments have been
described in detail above, those skilled in the art will readily
appreciate that many modifications are possible in the exemplary
embodiments without materially departing from the novel teachings
and advantages of this disclosure. For example, aspects of
embodiments disclosed above can be combined in other combinations
to form additional embodiments. Accordingly, all such modifications
are intended to be included within the scope of this
disclosure.
* * * * *