U.S. patent application number 12/643718 was filed with the patent office on 2010-04-22 for method and apparatus for phase-encoded homogenized fourier transform holographic data storage and recovery.
This patent application is currently assigned to STX Aprilis, Inc.. Invention is credited to Joby Joseph, David A. Waldman.
Application Number | 20100097675 12/643718 |
Document ID | / |
Family ID | 34437314 |
Filed Date | 2010-04-22 |
United States Patent
Application |
20100097675 |
Kind Code |
A1 |
Waldman; David A. ; et
al. |
April 22, 2010 |
METHOD AND APPARATUS FOR PHASE-ENCODED HOMOGENIZED FOURIER
TRANSFORM HOLOGRAPHIC DATA STORAGE AND RECOVERY
Abstract
An apparatus for writing and reading holograms, comprising a
spatial light modulator (SLM) operable in phase mode, having a
plurality of pixels for generating an object beam that overlaps
with a reference beam; a holographic recording medium (HRM) in the
path of the object beam; and a first lens element disposed in the
path of the object beam between the SLM and the HRM; wherein the
HRM is disposed at or near the Fourier transform plane of the first
lens element.
Inventors: |
Waldman; David A.; (Concord,
MA) ; Joseph; Joby; (New Delhi, IN) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD, P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Assignee: |
STX Aprilis, Inc.
Maynard
MA
|
Family ID: |
34437314 |
Appl. No.: |
12/643718 |
Filed: |
December 21, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12217755 |
Jul 8, 2008 |
7652809 |
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12643718 |
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10962202 |
Oct 8, 2004 |
7411708 |
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12217755 |
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60532795 |
Dec 24, 2003 |
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60509983 |
Oct 8, 2003 |
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Current U.S.
Class: |
359/24 ;
359/29 |
Current CPC
Class: |
G03H 2225/32 20130101;
G03H 1/16 20130101; G03H 1/26 20130101; G03H 1/265 20130101; G11B
7/0065 20130101 |
Class at
Publication: |
359/24 ;
359/29 |
International
Class: |
G03H 1/16 20060101
G03H001/16; G03H 1/22 20060101 G03H001/22; G03H 1/28 20060101
G03H001/28 |
Claims
1. A method of reading a phase-encoded hologram, comprising:
directing a reference beam at a location in a holographic recording
medium where a phase-encoded Fourier transform hologram or
fractional Fourier transform hologram was recorded, thereby
reconstructing said phase-encoded hologram, said reconstructed
hologram comprising images of edges of pixels, said images
corresponding to transitions between pixels recorded by light
wavefronts having different phases; detecting the reconstructed
hologram with a detector having resolution sufficient to detect the
edges of pixels; and assigning a value of "0" or "1" to each pixel
based on the image of the edges of pixels, said image corresponding
to transitions between pixels recorded by light wavefronts having
different phases.
2. The method of claim 1 wherein assigning a value of "0" or "1" to
each pixel includes oversampling, whereby each pixel of the SLM is
detected by more than one pixel of the detector.
3. The method of claim 1 wherein assigning a value of "0" or "1" to
each pixel includes image processing.
4. The method of claim 1 wherein assigning a value of "0" or "1" to
each pixel includes detecting the gradient of intensity of the
reconstructed hologram.
5. The method of claim 1 further including translating the detector
in the plane of the detector.
6. The method of claim 1 wherein assigning a value of "0" or "1" to
each pixel includes detecting at least one fiducial mark
corresponding to a pixel of a known state and assigning a value of
"0" or "1" to each pixel relative to the detected fiducial
mark.
7. A method of reading holograms, comprising: directing a reference
beam at a selected location in a holographic recording medium where
a phase-encoded Fourier transform hologram or fractional Fourier
transform hologram was recorded, reconstructing a first object beam
and directing the first object beam to a detector; illuminating a
phase spatial light modulator (SLM) displaying a uniform data page,
thereby forming a second object beam and directing the second
object beam to said detector, thereby producing an interference
pattern between the first object beam and the second object beam at
the detector that reproduces an amplitude-modulated data page; and
detecting the amplitude-modulated data page.
8. The method of claim 7 wherein the light wavefront reflected from
or transmitted through any pixel of the SLM has a phase of .phi.=0
or .pi..
9. The method of claim 7 wherein the intensity of the second object
beam is substantially equal to the intensity of the first object
beam.
10. A method of recording a homogenized hologram, comprising:
recording a uniform data page hologram; and recording a Fourier
transform hologram or a fractional Fourier transform hologram at
the same storage location.
11. The method of claim 10 wherein the Fourier transform hologram
or a fractional Fourier transform hologram is a phase-encoded
homogenized hologram.
12. The method of claim 10 wherein the light wavefront reflected
from or transmitted though any pixel of the SLM has a phase of
.phi.=0 or .pi..
13. The method of claim 11 wherein the uniform data page hologram
and the homogenized hologram are recorded using the same reference
beam.
14. The method of claim 11 wherein the uniform data page hologram
and the homogenized hologram have substantially the same
diffraction efficiency.
15. The method of claim 13 wherein a first uniform data page
hologram and a first homogenized hologram are recorded using a
first reference beam, and further wherein a second uniform data
page hologram and second homogenized hologram are recorded at the
same storage location, or a substantially overlapped storage
location using a second reference beam.
16. The method of claim 11 wherein the uniform data page hologram
and the phase-encoded homogenized hologram are recorded with
substantially the same diffraction efficiency.
17. The method of claim 11 wherein a uniform data page hologram and
more than one phase-encoded homogenized holograms are recorded at
the same storage location.
18. The method of claim 11 wherein the uniform data page hologram
and the homogenized hologram are recorded using different reference
beams.
19. The method of claim 18 wherein the uniform data page hologram
and the phase-encoded homogenized hologram are recorded with
substantially the same diffraction efficiency.
20. The method of claim 18 wherein two or more homogenized
holograms are recorded, each homogenized hologram being recorded
with a different reference beam.
21. A method of reading a homogenized hologram, comprising:
directing a reference beam at a selected location in a holographic
recording medium where a phase modulated Fourier transform hologram
or fractional Fourier transform hologram and a uniform
phase-encoded data page hologram were recorded using the same
reference beam, thereby reconstructing a first object beam, used to
record a first homogenized hologram, and a second object beam, used
to record the uniform data page hologram, thereby producing an
interference pattern between the first and second object beams that
reproduces an amplitude-modulated data page; and detecting the
amplitude-modulated data page with a light detector.
22. A method of reading a homogenized hologram, comprising:
directing a first reference beam at a selected location in a
holographic recording medium where a phase-encoded Fourier
transform hologram or fractional Fourier transform hologram and a
uniform phase-encoded data page hologram were recorded using the
first and a second reference beams, thereby reconstructing a first
object beam used to record a first phase-encoded Fourier transform
hologram or fractional Fourier transform hologram; directing the
second reference beam at the selected storage location in the
holographic recording medium, thereby reconstructing a second
object beam used to record the uniform data page, thereby producing
an interference pattern between the first and second object beams
that reproduces an amplitude-modulated data page; and detecting the
said amplitude-modulated data page with a light detector.
23. The method of 22 wherein the intensities of the first and
second reference beams are adjusted so that the intensities of the
first and second reconstructed object beams are substantially the
same.
24. The method of 22 wherein the phases of the first and second
reference beams are substantially the same.
25. A method of searching a holographic recording medium for a
specified content, comprising: illuminating a spatial light
modulator (SLM) displaying at least one search pattern
corresponding to a selected content, thereby forming a search beam;
directing the search beam to one or more storage locations on a
holographic recording medium where at least one phase-encoded
Fourier transform hologram or fractional Fourier transform hologram
is recorded, thereby producing at least one reconstructed reference
beam when the one or more storage locations contain at least one
phase-encoded Fourier transform hologram or fractional Fourier
transform hologram that contains selected content of the search
pattern; and detecting the at least one said reconstructed
reference beam with one or more light detectors.
26. The method of claim 25 wherein the SLM is operable in a phase
mode.
27. A method of searching a holographic recording medium for a
specified content, comprising: illuminating a spatial light
modulator (SLM) operable in phase mode and displaying at least one
search pattern corresponding to a selected content, thereby forming
a search beam; directing the said search beam to one or more
storage locations on a holographic recording medium where at least
one Fourier transform hologram or fractional Fourier transform
hologram is recorded, thereby producing at least one reconstructed
reference beam when the one or more storage locations contain at
least one Fourier transform hologram or fractional Fourier
transform hologram that contains selected content of the search
pattern; and detecting the at least one reconstructed reference
beam with one or more light detectors.
28. A method of recording a homogenized hologram, comprising:
illuminating a spatial light modulator (SLM) operable in phase
mode, said SLM having a plurality of pixels, each pixel of the SLM
that operates in phase mode being in at least either a first state
or a second state, the number of pixels of the SLM in the first
state is about equal to the number of pixels in the second state;
controllably changing the state of selected pixels of the SLM,
thereby changing the polarization of a light wavefront reflected
from or transmitted through each pixel of the SLM, thereby forming
an output beam; directing the output beam at a polarization filter,
thereby changing the phase of a light wavefront reflected from or
transmitted through each pixel of the SLM by .phi., depending on
the state of the pixel, and thereby producing a phase encoded
object beam; directing the phase encoded object beam through a
first lens element disposed in the path of the object beam between
the SLM and a holographic recording medium (HRM), wherein the HRM
is so disposed at or near the Fourier transform plane of the first
lens element; and directing a reference beam at the HRM so as to
overlap the phase encoded object beam at a selected storage
location in the HRM, thereby producing an interference pattern at
the HRM and recording a hologram; wherein the reference beam
overlaps the object beam at or near the Fourier transform plane of
the first lens element, and further wherein one or more holograms
are recorded at the storage location by one or more multiplexing
methods, and wherein the multiplexed holograms are fully or
partially overlapped at the storage location.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/217,755, filed Jul. 8, 2008, which is a
divisional of U.S. patent application Ser. No. 10/962,202, now U.S.
Pat. No. 7,411,708, filed Oct. 8, 2004, which claims the benefit of
U.S. Provisional Application No. 60/532,795, filed on Dec. 24, 2003
and U.S. Provisional Application No. 60/509,983, filed on Oct. 8,
2003. The entire teachings of the above applications are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] One important potential use of volume holograms is in
digital data storage. The three dimensional nature of a volume
hologram, which refers to the storage of each bit as a hologram
extending throughout the entire volume of the recording medium,
renders volume holograms suitable for use in high capacity digital
data storage. A group of bits can be encoded and decoded together
as a two dimensional array of bits referred to as a page.
[0003] In Holographic Data Storage (HDS) systems the optics
configuration used for recording and reading typically comprises a
Fourier Transform (FT) geometry that uses a 4f optical imaging
system which for recording includes a spatial light modulator (SLM)
or other optical encoding device that displays information.
[0004] Mutually coherent object and reference beams form an
interference pattern in the volume of their overlap. A hologram is
recorded when light-induced changes in the volume of their overlap
in the storage medium, such as photopolymerization, produce a
record of the resulting interference pattern. The essential
elements and arrangement of the 4f optical design for recording
holograms are: an SLM that encodes the object beam, a lens element
L1, having an effective focal distance f1 and which is located at a
distance f1 from the SLM in the optical path of the encoded object
beam, for relaying a 2-D Fourier transform of the spatial pattern
of the encoded object beam to a plane that is one focal distance
from the lens and a holographic recording medium (HRM), located at
a distance f1 from the lens element L1 in the optical path of the
encoded object beam relayed by the lens element L1.
[0005] Reconstruction of a hologram is accomplished by firstly
illuminating the hologram with a reference beam that is
substantially the same as the reference beam used for recording the
hologram, and secondly imaging the resultant diffracted light
(reconstructed object beam) onto the detector array with a second
lens element of the said. 4f optical design. The following elements
are used for reading a reconstructed hologram: a lens element L2
for relaying an object beam reconstructed by directing a reference
beam at a storage location in the HRM, having an effective focal
distance f2 and which is located at a distance f2 from the HRM in
the optical path of a reconstructed object beam; and a light
detector, located at a distance f2 from the lens element L2 in the
optical path of the reconstructed object beam directed by lens
element L2. In the aforementioned 4f optical design f1=f2, but in
other optical configurations it may be advantageous for f2>f1
and thereby improve SNR of the reconstructed holograms by use of an
optical relay system. Additionally, when magnification or
demagnification is preferred in the optical configuration then f2
is not equal to f1.
[0006] Typically, an SLM operates in an amplitude mode, whereby the
date page appears as a two-dimensional array of dark and bright
pixels. When the Fourier transform (FT) of such an
amplitude-modulated data page is obtained using a lens element L1,
a strong high intensity direct current (DC) spike, that corresponds
to the 0.sup.th order diffraction, appears at the center of the
Fourier transform at the Fourier plane (focal plane) due to the
constructive interference of the light from the SLM pixels in the
bright state. When large numbers of holograms are multiplexed in
the medium, co-locationally or substantially overlapped, this
intense DC peak will quickly saturate the dynamic range of a
recording material (i.e. deplete the available photopolymerizable
component), preventing the efficient use of the medium, and,
additionally, will result in significant non-uniformity in the
modulation depth of the recording for spatial frequencies of the
interference pattern of the holographic recording corresponding to
the DC and "alternative current" (AC) components of the FT, and
thereby cause poor fidelity in the reconstructed data pages. The
intense DC peak at the center and the typically large amplitude
distribution in the AC components of the FT spectrum can also
result in nonlinear grating formation, increasing the noise level
in the reconstructed data page.
SUMMARY OF THE INVENTION
[0007] The instant invention relates to methods and apparati that
can be used for recording and retrieving phase-encoded Fourier
transform and fractional Fourier transform holograms.
[0008] In one embodiment, the present invention is an apparatus for
writing and reading holograms comprising a spatial light modulator
(SLM) operable in phase mode, having a plurality of pixels, each
for generating an object beam that overlaps with a reference beam;
a holographic recording medium (HRM) in the path of the object
beam; and a first lens element disposed in the path of the object
beam between the SLM and the HRM. The HRM is disposed at or near
the Fourier transform plane of the first lens element.
[0009] In another embodiment, the present invention is a method of
recording a homogenized hologram, comprising illuminating a spatial
light modulator (SLM) operable in phase mode, said SLM having a
plurality of pixels, each pixel of the SLM being in either a first
state or a second state; controllably changing the state of
selected pixels of the SLM, thereby changing the polarization of a
light wavefront reflected from or transmitted through each pixel of
the SLM, thereby forming an output beam; directing the output beam
at a polarization filter, thereby changing the phase of a light
wavefront reflected from or transmitted through each pixel of the
SLM by .phi., depending on the state of the pixel, and thereby
producing a phase encoded object beam; directing the phase encoded
object beam through a first lens element disposed in the path of
the object beam between the SLM and a holographic recording medium
(HRM), wherein the HRM is disposed at or near the Fourier transform
plane of the first lens element; and directing a reference beam at
the HRM so as to overlap the reference beam and the phase encoded
object beam at a selected storage location in the HRM, thereby
producing an interference pattern at the HRM and recording a
hologram.
[0010] In another embodiment, the present invention is a method of
reading a phase-encoded holograms comprising directing a reference
beam at a location in a holographic recording medium where a
phase-encoded Fourier transform hologram or fractional Fourier
transform hologram was recorded, thereby reconstructing said
phase-encoded hologram, said reconstructed hologram comprising
images of edges of pixels, said images corresponding to transitions
between pixels recorded by light wavefronts having different
phases; detecting the reconstructed hologram by a detector having
resolution sufficient to detect the edges of pixels; and assigning
a value of "0" or "1" to each pixel based on the images of the
edges of pixels, said images corresponding to transitions between
pixels recorded by light wavefronts having different phases.
[0011] In another embodiment, the present invention is a method of
reading holograms, comprising directing a reference beam at a
selected location in a holographic recording medium where a
phase-encoded Fourier transform hologram or fractional Fourier
transform hologram was recorded, reconstructing a first object beam
and directing the first object beam to a detector; illuminating a
phase spatial light modulator (SLM) displaying a uniform data page,
thereby forming a second object beam and directing the second
object beam to said detector, thereby producing an interference
pattern between the first object beam and the second object beam at
the detector that reproduces an amplitude-modulated data page; and
detecting the amplitude-modulated data page.
[0012] In another embodiment, the present invention is a method of
recording a homogenized hologram, comprising recording a uniform
data page hologram; and recording a Fourier transform hologram or a
fractional Fourier transform hologram at the same storage
location.
[0013] In another embodiment, the present invention is a method of
reading a homogenized hologram, comprising directing a reference
beam at a selected location in a holographic recording medium where
a phase modulated Fourier transform hologram or fractional Fourier
transform hologram and a uniform phase-encoded data page hologram
were recorded using the same reference beam, thereby reconstructing
a first object beam, used to record a first homogenized hologram,
and a second object beam, used to record the uniform data page
hologram, thereby producing an interference pattern between the
first and second object beams that reproduces an
amplitude-modulated data page; and detecting the
amplitude-modulated data page with a light detector.
[0014] In another embodiment, the present invention is a method of
reading a homogenized hologram, comprising directing a first
reference beam at a selected location in a holographic recording
medium where a phase-encoded Fourier transform hologram or
fractional Fourier transform hologram and a uniform phase-encoded
data page hologram were recorded using the first and a second
reference beams, thereby reconstructing a first object beam used to
record a first phase-encoded Fourier transform hologram or
fractional Fourier transform hologram; directing the second
reference beam at the selected storage location in the holographic
recording medium, thereby reconstructing a second object beam used
to record the uniform data page, thereby producing an interference
pattern between the first and second object beams that reproduces
an amplitude-modulated data page; and detecting the said
amplitude-modulated data page with a light detector.
[0015] In another embodiment, the present invention is a method of
searching a holographic recording medium for a specified content,
comprising illuminating a spatial light modulator (SLM) displaying
at least one search pattern corresponding to a selected content,
thereby forming a search beam; directing the search beam to one or
more storage locations on a holographic recording medium where at
least one phase-encoded Fourier transform hologram or fractional
Fourier transform hologram is recorded, thereby producing at least
one reconstructed reference beam when the one or more storage
locations contain at least one phase-encoded Fourier transform
hologram or fractional Fourier transform hologram that contains
selected content of the search pattern; and detecting the at least
one said reconstructed reference beam with one or more light
detectors.
[0016] In another embodiment, the present invention is a method of
recording multiplexed holograms, comprising recording a first
phase-encoded Fourier Transform hologram or a fractional FT
hologram with a first reference beam; and recording a second
phase-encoded Fourier Transform hologram or a fractional FT
hologram with a second reference beam at the same location or at a
substantially overlapped location on a holographic recording
medium, wherein the angle between the first and the second
reference beams is less than the angular separation between the
primary diffraction peak and the first null of the Bragg angle
selectivity curve of the first or the second holograms.
[0017] In another embodiment, the present invention is a method of
recording multiplexed holograms, comprising recording a first
phase-encoded Fourier Transform hologram or a fractional FT
hologram with a first wavelength; and recording a second
phase-encoded Fourier Transform hologram or a fractional FT
hologram with a second wavelength at the same location or at a
substantially overlapped location on a holographic recording
medium, wherein the difference between the first and the second
wavelengths is less than the wavelength separation between the
primary diffraction peak and the first null of the Bragg wavelength
selectivity curve of the first or the second holograms.
[0018] The instant invention is particularly useful for holographic
data storage applications where the areal density of stored
information exceeds about 16 Gbits/inch.sup.2 and where use of
content addressable memory is desirable. The invention can
substantially increase the usable data density that can be stored
in holographic media, such as relatively thin photopolymers, by
firstly providing for recording with uniform modulation depth at
the Fourier transform plane which improves the fidelity and
efficiency of the recording and diminishes the requirements for
laser power during recording, and, secondly, by providing for
multiplexed holograms so that the angle, shift and/or wavelength
increment between holograms is smaller than the corresponding
increment of angle, shift or wavelength between the primary
diffraction peak and first null of the Bragg selectivity of the
multiplexed holograms. The invention substantially decreases
cross-correlation in content addressable searching, and also
increases the correlation signal strength that is obtained from
content addressable searches when a small stored sub-image is to be
identified and located within a larger stored image.
[0019] The present invention makes it possible for a full page or
partial page content addressable searching to be performed directly
on holograms recorded with data pages displayed in either amplitude
or phase mode. The search method of the instant invention is
substantially independent of the fidelity of the holographically
recorded information. Another advantage of searching for data using
a phase SLM is that it facilitates searching the holograms without
causing cross-correlation that typically degrades the
signal-to-noise (SNR) of the search results and thereby improves
discrimination of the search.
[0020] Another embodiment of the present invention facilitates edge
enhanced reconstruction of the phase data pages recorded as
holograms, by providing for detection of said edge enhanced
features of the reconstructed phase data pages and/or providing for
assignment of said features to, by way of example, binary "1" and
"0" values, so as to reconstruct the original amplitude data page
information.
[0021] In still another embodiment the present invention provides
for reconstruction of the original data recorded as multiplexed
phase data page holograms by implementing holographic
interferometry during the read out process of said holograms.
[0022] A further advantage of the method and apparatus of the
present invention is the ability to multiplex recorded holograms
and search such holograms for content, in a manner such that the
areal density of the holographically recorded information can be
increased substantially as compared to conventional methods. This
results in substantially higher capacity, higher data transfer
rates, and higher speed data search.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0024] FIG. 1 shows a 4f optical system suitable for use in the
present invention.
[0025] FIG. 2 shows an alternative embodiment of a 4f optical
system suitable for use in the present invention.
[0026] FIG. 3 shows an optical scheme used to record image plane
holograms.
[0027] FIG. 4 shows an optical scheme used to record a fractional
Fourier transform hologram.
[0028] FIG. 5 is a schematic diagram of an amplitude mode of SLM
operation.
[0029] FIG. 6 is a schematic diagram of a phase mode of SLM
operation.
[0030] FIG. 7(a) shows a representative amplitude modulated data
page.
[0031] FIG. 7(b) shows MATLAB simulation results of a Fourier
transform of the data page shown in FIG. 7(a).
[0032] FIG. 7(c) shows MATLAB simulation results of Fourier
transform of the data page in 0 and .pi. phase mode.
[0033] FIG. 8 is a schematic representation of a data recovery
process that uses interferometry.
[0034] FIGS. 9(a) through 9(c) show experimental results of
DC-removed Fourier transform (FT) of a data page. FIG. 9(a) shows
data page. FIG. 9(b) shows an FT (magnified) of amplitude data
page. FIG. 9(c) shows FT (magnified) of phase data page, showing
reduced DC peak.
[0035] FIGS. 10(a) and 10(b) show experimental results of edge
detection method. FIG. 10(a) shows holographic edge reconstruction
of 4.times.4 pixel data page. FIG. 10(b) shows holographic edge
reconstruction of 8.times.8 pixel data page.
[0036] FIGS. 11(a) through (d) show experimental results of
interferometric recovery of data page. FIG. 11(a) shows direct
amplitude image (SLM in amplitude mode). FIG. 11(b) shows data
recovered through real-time interferometry. FIG. 11(c) shows phase
data page (SLM in phase mode), concentric circular rings and
darkened annular region near periphery of image caused by
interference effects associated with the cover glass of CMOS
sensor. FIG. 11(d) shows data recovered through double-exposure
interferometry.
[0037] FIGS. 12(a) through (f) show the Results of "content
addressed" search for holographic recording of thirty (30) binary
data pages with an SLM operated in either phase mode during
recording as shown in FIG. 12(a) or amplitude mode as shown in
FIGS. 12(b) and (c)-(f). FIG. 12(a) shows search result for blank
page with SLM operating in phase mode; FIG. 12(b) shows search
result for 15.sup.th data page with SLM operating in amplitude mode
for displaying data page search pattern, showing substantial cross
correlations; FIGS. 12(c)-(f) Search result for 15.sup.th data page
with SLM operating in phase mode for displaying search pattern for
100%, 75%, 50% and 25% of the content area of the 15.sup.th data
page, respectively, showing near to zero cross correlations.
[0038] FIG. 13 is a representative angular or wavelength
selectivity curve of a recorded hologram.
DETAILED DESCRIPTION OF THE INVENTION
[0039] A description of preferred embodiments of the invention
follows.
[0040] The disclosed invention is an apparatus for holographic data
storage (HDS) systems that comprises moderately to high numeric
aperture components, such as one or more lenses and/or one or more
mirrors, and a spatial light modulator (SLM) that is operable to be
used in phase mode for the signal beam, said system operable to
achieve moderate to high areal density of stored information, and
said stored information comprising multiplexed holograms. The
apparatus and method of this invention operates to achieve high
areal density, defined as greater than about 25 bits/.mu.m.sup.2,
with acceptable SNR, defined as SNR corresponding to a raw
bit-error-rate (BER) of .ltoreq.10E-2, for stored information that
comprises multiplexed holograms, and where acceptable SNR for
information that is stored so as to be searchable corresponds to a
BER can be >10E-2. A spatial light modulator operable in 0 and
.pi. phase modes, or alternatively in other phase modes, is
optionally provided during recording to substantially remove the dc
peak from the Fourier transform (FT) at the recording plane (i.e. a
plane where the holographic recording medium is disposed), thereby
providing for homogenizing the amplitude distribution in said
Fourier transform spectrum. A spatial light modulator operable in 0
and .pi. phase modes, or alternatively in other phase modes, is
additionally provided so the HDS system can perform full or partial
content addressable searching, and the acceptable raw BER of the
holographically stored data for the apparatus and method of this
invention can be greater than 10E-2 for said searching method.
[0041] FIG. 1 shows a 4f system suitable for using in the present
invention. The signal beam 102, generated by a source 103 and beam
shaping optics (BSO), passes through a spatial light modulator
(SLM) 104, having a plurality of pixels 106, arranged in a two
dimensional array. SLM 104 operates to encode signal beam 102 with
data information that is to be recorded. SLM 104 can be operable in
either an amplitude mode or a phase mode, as will be explained
below in greater detail. Generally, SLMs can operate by
transmission, as SLM 104 shown in FIG. 1, or by reflection, as
shown in FIG. 2 for SLM 204. Referring to FIG. 1, plane 107 of SLM
104 is located at one focal length, f.sub.1, from a first Fourier
transform lens element 108, said lens having a large enough
diameter (clear aperture) to accept the field of the light that
diffracts or reflects from the SLM 104.
[0042] As used herein, the term "lens element" refers to one or
more elements having optical power, such as lenses, that alone or
in combination operate to modify an incident beam of light by
changing the curvature of the wavefront of the incident beam of
light. Lens element 108, for example, can comprise more than one
lens.
[0043] In the 4f optical arrangement shown in FIG. 1, first Fourier
transform lens element 108 operates to relay a 2-D Fourier
transform of the spatial pattern of the SLM-encoded signal beam 110
to plane 112 (Fourier transform or focal plane) that is one focal
distance f1 away from lens element 108 and, additionally, is two
focal distances (2f1) from SLM 104. Holographic recording material
114 can be located at or near plane 112.
[0044] A second Fourier transform lens element 116, having
effective focal length f2, is positioned at a distance f2 from
plane 112. In one embodiment, f2 can be equal to f1. Lens element
L2 operates to perform a second Fourier transform, causing an
inverted image of the spatial pattern of SLM 104 to appear at plane
118, which is one focal distance f2 behind second lens element 116.
When reference beam 120a or 120b impinges upon the recorded
hologram at an incident angle that was used to carry out recording
of a hologram(s), then the light diffracted from the hologram(s)
forms reconstructed optical beam 122, which passes into second lens
element 116, is relayed to plane 118. In the embodiment where f1 is
equal to f2, plane 118 is thus four focal distances f1 away from
SLM 104. Plane 118 corresponds to the correlation plane at the
detector array (light detector) 124. Light detector 124 has a
plurality of pixels 126 that can be of the same or of different
linear dimensions (x',y') as dimensions (x, y) of pixels 106 of SLM
104.
[0045] Three types of hologram recording methods are distinguished
based on the positioning of the recording media. In Fourier
transform holograms, shown in FIG. 1, the media is placed at the
focal plane (plane 112) of a lens element disposed in the path of
the object beam encoded by an SLM. The other two types are image
plane holograms and Fresnel zone holograms (also referred to as
fractional FT holograms). Shown in FIG. 3, image plane holograms
are recorded when SLM 302 is the first element in a 4f optical
configuration which uses lens elements 304 and 306 to image a data
page displayed on SLM 302 to holographic recording medium 308,
which is the last element in the configuration. The third type of a
holographic recording method, Fresnel zone or fractional FT plane
holograms are shown in FIG. 4. In this scheme, holographic
recording medium 414 is offset from Fourier transform plane 412 by
a distance l. While any of the described methods can be used to
record a hologram according to using methods and apparati of the
present invention, the fractional FT method is preferred.
[0046] Principles of operation of SLM devices operable in amplitude
mode and in phase mode will now be explained with references to
FIG. 5 and FIG. 6, respectively.
[0047] A device referred herein as "spatial light modulator (SLM)"
comprises a plurality of pixels onto which a light beam impinges
and either reflects from the SLM (reflection-type SLM) or is
transmits through the SLM (transmittal type SLM). (SLM 104 depicted
in FIG. 1 is a transmittal-type SLM. FIG. 2 shows a reflection-type
SLM 204.) SLMs contemplated can, by way of example, be one of the
following: a nematic liquid crystal SLM, a twisted nematic liquid
crytal SLM, a ferroelectric liquid crystal SLM, a cholesteric SLM,
an SLM that uses arrays of photoelastic crystals for polarization
modulation, an SLM that uses controllable micro gratings such as
Silicon Light Machines Grating Light Valve (GLV) technology or a
device that uses a means to dynamically control refractive or
optical path length.
[0048] An example of an SLM suitable for use in the present
invention is a ferroelectric liquid crystal (FLC) SLM, such as
Lightcaster.TM. (1280.times.768 pixels) produced by Displaytech,
Inc. The description of FIGS. 5 and 6 use an FLC SLM as a
non-limiting example. One skilled in the art of holography will be
readily able to adopt any other type of an SLM device for the
purposes of the instant invention.
[0049] Amplitude mode of operation is schematically depicted in
FIG. 5. Coherent beam 502, produced by source 504 is passed through
a linear polarizer 506 resulting in input beam 508 having
polarization 510 (here, vertical). Each pixel 512 of the FLC SLM
can have one of the two states of its constituent liquid crystals
that orient said crystals 45 degrees apart, thereby forming FLC
axes 514a and 514b, oriented at 45.degree. angle. Even though both
axes 514a and 514b are shown in FIG. 5, it should be understood
that each pixel 512 can have its crystals oriented to form either
axis 514a or axis 514b, but not both. Controllably applying voltage
to each pixel changes its optical polarization properties by
switching between liquid crystal states and thus FLC axes 514a and
514b.
[0050] As used herein, the term "optical polarization properties"
refers to the ability of a material to change the direction of
polarization of the light wavefront impinging onto such material
and either reflected or transmitted through such material.
[0051] The SLM is positioned so that one of the two axes, here axis
514a, is oriented in a direction parallel to the direction of
linear polarization 510 of the input beam 508. Upon being
transmitted through, as shown in FIG. 5, or reflected from pixel
512, the direction of linear polarization 510 of the input beam 508
is rotated by twice the angle between the polarization axis and FLC
axes 514a or 514b. In the example shown in FIGS. 5 and 6, the FLC
SLM is a reflection type SLM. Accordingly, when input beam 508 is
reflected from pixel 512, either output beam 516a, having linear
polarization 518a (vertical), or output beam 516b, having linear
polarization 518b (horizontal), is produced, depending on the state
of each pixel 512. Thus rotation of polarization 510 is 0.degree.
when pixel 512 is in the state having axis 514a and 90.degree. when
pixel 512 is in the state having axis 514b.
[0052] Output beams 516a or 516b are then directed through linear
polarizer 520, which transmits only the light from the pixels whose
that has polarization direction coincided with that of linear
polarizer 520. In the embodiment, where a reflection-type SLM is
used, linear polarizer 520 and linear polarizer 506 can be one and
the same. As a result, the observer sees either black (0) or
illuminated (1) pixels depending on their state of polarization of
each pixel.
[0053] The amplitude encoding scheme described above is a binary
amplitude scheme. Other schemes can be used with the methods of the
instant invention that use a "grey scale" of amplitude, whereby the
amplitude detected by the light detected can vary continuously
between zero and the amplitude of input beam 508.
[0054] Phase mode of operation of an SLM device is shown in FIG.
6.
[0055] Coherent beam 602, produced by source 604 is passed through
linear polarizer 606 resulting in input beam 608 having
polarization direction 610 (here, vertical).
[0056] The SLM is positioned so that one of the two axes, here axis
614a, is oriented at an angle -22.5.degree. to the direction of
linear polarization 610 of the input beam 608. Upon being reflected
from pixel 612, as shown in FIG. 6, direction of linear
polarization 610 of input beam 608 is rotated by twice the angle
between the polarization axis and FLC axes 614a or 614b.
Accordingly, when input beam 608 is reflected from pixel 612,
either output beam 616a, having linear polarization 618a, or output
beam 616b, having linear polarization 618b, is produced, depending
on the state of each pixel 612.
[0057] Output beams 616a or 616b are then directed through
polarizing element 620, which can be a polarizing beam splitter or
a waveplate and thereby selects a polarization direction.
Polarizing element 620, which, in the example of FIG. 6, is a
polarizing beam splitter, transmits the p-polarized light (here,
horizontally oriented) and reflects the s-polarized light (here
vertically oriented). As a result, the observer after polarizing
element 620 sees either the light having a phase of .phi..sub.0=0
(reflected light from 620) or the light having the phase
.phi..sub.1 (transmitted light from polarizing element 620) that is
different from .phi..sub.0=0 by +/-.pi./2, depending on the state
of each pixel, and thus the phase difference between light
reflected from pixel 612 for the two states is .pi. for the light
reflected by polarizing element 620.
[0058] The scheme described above refers to a phase encoding where
a pair of phases (0, .pi.) are used. Other schemes can be employed,
using pairs (0, .phi.=2.pi..times.n/m), where n is a whole number
or zero and m is a natural number.
[0059] In general, a binary data page used for holographic storage
will be similar to a binary random image, due to the various
modulation codes used while creating such a data page. When such a
modulation coded binary digital data page with equal number of 0s
and 1s is displayed on the phase SLM, and when its Fourier
transform is, for example, obtained by a lens, destructive
interference of fields at the dc region, from the 0 and .pi. pixels
or other phase modes of the SLM leads to a Fourier spectrum with no
dc peak.
[0060] FIG. 7(a) shows a representative amplitude modulated data
page. FIG. 7(b) shows MATLAB simulation results of a Fourier
transform of the data page shown in FIG. 7(a). As seen in FIG.
7(c), the dc peak can be fully removed by using (0, .pi.) phase
mode for representing binary data pages. Consequently, at the FT
plane there is a substantially homogenized Fourier spectrum without
the presence of the high intensity dc peak.
[0061] As used herein, the term "homogenized hologram" refers to a
hologram recorded with a phase-modulated (encoded) object beam
resulting in substantial removal of the DC peak from the Fourier
transform at the recording plane that is at or near the Fourier
transform plane, and hence homogenizing the amplitude distribution
in said Fourier transform spectrum. In a preferred embodiment, a
data page to be holographically recorded contains equal number of
transparent or opaque pixels (when displayed by an SLM operating in
amplitude mode) or equal number of pixels in each polarization
state (when displayed by an SLM operating in phase mode).
[0062] However, when the data page is recorded with the SLM
operable in the phase mode, the reconstructed data cannot be
recovered directly by the standard application of the second
Fourier transform to provide for imaging of the hologram
diffraction onto the CCD or CMOS detector.
[0063] One embodiment of the present invention for recovery of data
is herein referred to as the method of "edge detection".
Conventionally, during the reconstruction of a hologram, the object
beam arm is blocked and the hologram(s) is illuminated by the
reference beam that was used to record the hologram. In one
embodiment of the method and apparatus of this invention the
hologram is read out by use of the reference beam in the
conventional manner, with the object arm blocked. This method
reconstructs the phase data page. However, since the dc peak was
absent at the FT plane, holographic recording has occurred only for
the ac components of the FT, and, consequently, said reconstruction
provides only an "edge enhanced" reconstruction of the data page
corresponding to the ac frequency components, as shown in FIGS.
10(a) and 10(b) for two levels of oversampling (SLM pixel size for
data page is larger than detector pixel size), 4.times.4 and
8.times.8, respectively, which corresponds to transitions between
pixels with difference phases. In one embodiment, the method and
apparatus of this invention provides for recovery of data by use of
a high-resolution detector that obtains the data from said edge
enhanced image by identification of said transitions. Various means
of identification of said transitions so as to recover the data are
contemplated by the present invention. By way of example, a means
of identification of said transitions can be assignment of the
detected signal, located at the periphery of pixels, to a binary
value of either 0 or 1 for the pixel. It is preferable to use over
sampling in reconstruction and detection of the phase data pages,
such that x/1 and y/1 pixels in the detector, where x and y are
integers equal to two or greater, correspond to each pixel in the
field of said optical encoding device, so as to provide for
improved differentiation of said phase transitions. In cases where
over sampling is not used, or in cases where the fill factor of the
detector pixels is less than 100%, such as for high resolution
detectors, then it is alternatively preferable to shift the
detector by an amount, such as 1/2 of a pixel dimension in x or y
directions, so as to provide for improved differentiation of said
phase transitions. One experienced in the art will be able to
choose the optimum shift amount for a particular detector. In an
alternative embodiment the method and apparatus of this invention
provides for detection of said edge-enhanced image and,
additionally, for use of image processing for detection and
analysis of edges and or lines, such as can perform edge extraction
and/or edge contouring of digital images, which, by way of example,
can be implemented using software available from Adobe Systems,
Inc. or ArcSoft, Inc. or from vendors of machine vision software
such as Cognex Corporation or Adept Technology, Inc., or other
suitable image capture and analysis software that can perform
analysis of the edge enhanced image by use of methods, that by way
of example, can perform gradient edge enhanced detection preferably
on a pixel level. In one embodiment of this method one or more
fiducial markings, or otherwise known marking or pattern, recorded
in the phase data page are used to determine a reference position
for a "1" or "0" edge delineation in said reconstruction of the
phase data page, and every other transition of phase change in the
page is thereby assigned to the corresponding "1" or "0" binary
value to provide for reconstruction of the original amplitude data
page.
[0064] Various multiplexing methods, such as angular (planar angle
or out-of-plane angle), spatioangular, azimuthal, shift (shift
in-plane or shift out-of-plane), wavelength, phase-code, speckle,
and related methods, as well as combinations thereof, are used to
store multiple pages co-locationally within the same volume or in
partially overlapping volumes.
[0065] Another embodiment of the present invention for recovery of
data is herein referred to as the method of Real-time holographic
interferometry. The invention further provides for reconstruction
of the original data recorded as multiplexed holograms by providing
for holographic interferometry during the read out process. In one
embodiment of the method of Real-time holographic interferometry of
the present invention, the object beam is kept `ON` during the
illumination of the hologram by the reference beam during
reconstruction of the stored data from a predetermined storage
location. In this manner, the recorded object beam is reconstructed
by the reference beam, said reconstruction containing a wavefront
corresponding, by way of example, to the distribution of 0 and .pi.
pixels or other phase modes of the optical encoding device used
during recording. Concurrently, on the phase SLM or said optical
encoding device, a uniform page (i.e. for example, all pixels in 0
phase mode or .pi. phase mode or other phase modes) is displayed
and propagated to the stored data at the predetermined storage
location. The new object beam from the phase SLM or said optical
encoding device, which in a preferred embodiment is adjusted such
that the intensity of this object beam is substantially equal to
the intensity of the holographically reconstructed object beam
obtained from illumination with the Reference beam, operates to
form an interference pattern with the holographically reconstructed
object beam at the common image plane on the CCD or CMOS detector.
In this manner the reconstructed intensity pattern from the said
interferometric method produces an image that corresponds to the
original amplitude data page notwithstanding effects such as due to
the bit error rate of the image. FIG. 8 depicts, by way of example,
that 0 and 0 phase pixels produce constructive interference (binary
1) and 0 and .pi. pixels produce destructive interference (binary
0). This embodiment of the present invention further produces
better contrast in the recovered data page, because constructive
interference gives 4 times the intensity.
[0066] Another embodiment of the present invention for recording
and recovery of data is herein referred to as the method of Double
exposure holographic interferometry. In this embodiment recording
of a blank data page (i.e. full field super pixel), in addition to
the recording of the phase data page, is carried out at or near the
FT plane for each Reference beam condition used to multiplex
holograms recorded co-locationally or substantially overlapped.
When recording a blank data page, that is paired with a phase data
page for each reference beam condition, it is preferable during
recording of the blank data page to attenuate the dc component of
the FT of the object beam such that its intensity is about equal to
the intensity of the reference beam [I.sub.obj
(DC)/I.sub.ref].about.1 used during recording of the blank data
page. Upon reconstruction with a selected Reference beam condition,
the presence of the blank page hologram in combination with the
phase page hologram provides for an interference of the two
resultant diffraction wavefronts thereby providing for
reconstruction of the original amplitude data page. It is
preferable that the diffraction efficiency of each of the blank
data page holograms would be substantially identical to that of
each of the respective data page phase holograms that each is
superimposed with, and, consequently, this method would preferably
use an optimized recording schedule.
[0067] Still another embodiment of the present invention for
recording and recovery of data is herein referred to as the method
of Double reference beam interferometry. In said embodiment
recording of a blank data page (i.e. full field super pixel) with a
unique Reference beam condition is carried out at or near the FT
plane, preferably with use of an attenuation of the dc component of
the FT, as mentioned above, such that the ratio of I.sub.obj
(DC)/I.sub.ref.about.1. The presence of this blank hologram among
the overlapped multiplexed holograms provides for a diffraction
wavefront, upon reconstruction with the correct reference beam,
which can further provide for creating an interference pattern with
the diffraction wavefronts from all of the other phase data pages.
This embodiment thus requires use of two Reference beam conditions
concurrently: one for the aforementioned blank page, and one for
the selected phase data page that is to be reconstructed. Overlap
of the two diffraction patterns so as to be substantially identical
to when they were recorded is a required condition, so as to
reconstruct the amplitude representation of the selected data page.
The intensity of the reference beam for reconstruction of the blank
data page can be adjusted relative to that of the reference beam
used to reconstruct each of the data page phase holograms such that
the diffracted intensities are substantially the same. The two
reference beams at the time of readout need preferably to be in
phase (0 or multiples of 2.pi.) as the diffracted beams of each
travel in the same direction, and this, if needed, can be adjusted
at the reconstruction events.
[0068] Other embodiments of the method and apparatus of this
invention for recording and recovery of data are contemplated and
as such the extent of the invention is not limited to the specifics
described herein. By way of example, use of a shear plate that
operates to provide an interference condition at a defined angle
between a recovered wavefront and a sheared wavefront can provide
for reconstruction of an amplitude image from which the original
amplitude data page can be obtained.
[0069] Other embodiments of the method and apparatus of the present
invention relate to Content address based data search. In one such
embodiment holographic recording is carried out at the FT plane or
fractional FT plane by conventionally displaying the data page on
an SLM or other optical encoding device operating in the amplitude
mode. In the present invention, content addressable searching is
performed by displaying the search data patterns on an optical
encoding device (SLM) that is operating in phase mode. Correlation
matching is described, for example, in B. J. Goertzen and P. A.
Mitkas, Opt. Engineering, Vol 35, No. 7, pp. 1847-1853 (1995) and
G. W. Burr, S. Kobras, H. Hanssen, and H. Coufal, Appl. Optics,
Vol. 38, No. 32, pp. 6779-6784 (1999) and G. W. Burr, SPIE, Vol.
5181, pp 70-84 (2003). In one embodiment, correlation matching
comprises the steps of displaying a search pattern on the SLM in
phase mode, relaying with a lens element an object beam comprising
the phase search pattern to one or more recorded data page
holograms in a storage location in the holographic recording medium
and thereby illuminating the recorded holograms with said search
object beam, and detecting a reconstructed diffracted reference
beam from one or more data page holograms containing content of the
search pattern, wherein the amount the amount of power in a
diffracted reference beam is proportional to the degree of
correlation between the input search pattern and the associated
data page hologram. Advantageously, the inventive method of
searching allows correlation matching when the size of the
contiguous region of matched pixels is reduced from a fully matched
data page to a smaller grouping of matched pixels. In this
embodiment correlation matching can also be achieved for the case
when the matched pixels in the search pattern are translated or
rotated from their original location in either x or y directions.
The amount of such translation or rotation of the search pattern
depends on parameters such as thickness of the recording material,
Bragg selectivity, multiplexing method, etc. and those experienced
in the art will be able to determine appropriate conditions.
[0070] Another embodiment of the method and apparatus of the
present invention also relates to Content address based data
search. In this embodiment holographic recording is done at FT or
fractional FT plane by displaying the data page on an SLM or other
optical encoding device operating substantially in the phase mode,
and content addressable searching is performed by displaying the
selected search data pages or patterns on said optical encoding
device that is also operating in phase mode and propagating the
pattern in phase mode to the storage locations. In said embodiment,
correlation matching can be readily and advantageously achieved
when the size of the contiguous region of matched pixels is
diminished from a fully matched data page to a smaller grouping of
matched pixels that corresponds to less than about 50% of the total
data page, more advantageously correlation matching can also be
achieved when the area of said smaller grouping of matched pixels
corresponds to less than about 10% of the total data page, and even
more advantageously correlation matching can be achieved when the
area of said smaller grouping of matched pixels corresponds to less
than about 5% of the total data page. In said embodiment
correlation matching can also be achieved for the case when the
matched pixels in the search pattern have been translated or
rotated from their original location in either x or y directions.
The amount of such translation or rotation of the search pattern
depends on parameters such as thickness of the recording material,
Bragg selectivity, multiplexing method, etc. and those experienced
in the art will be able to determine appropriate conditions.
[0071] Another embodiment of the method and apparatus of the
present invention relates to Content address based data search as
well as the manner in which hologram are multiplexed so as to
achieve advantageously high areal density for stored information,
high data rates, and high search rates. Information can be
advantageously stored at high areal density in thin media so as to
substantially mitigate geometric constraints that are encountered
when the Reference beam is incident at an oblique angle.
[0072] Referring to FIG. 13, the detected signal strength of the
reconstructed object beam of a recorded hologram (I) varies with
either the wavelength .lamda. of the reference beam used to
reconstruct the hologram, or angle .phi. between such a reference
beam and the optical axis of the object beam, said axis in this
example being normal to the surface of the holographic recording
medium according to the function depicted in FIG. 13. The curve
shown in FIG. 13 is referred to as a Bragg selectivity (in this
example angular or wavelength) or detuning curve for address-based
retrieval. As shown in FIG. 13, the highest signal strength of the
detected hologram is achieved at wavelength .lamda..sub.0 or angle
.phi..sub.0 that correspond to the position of the primary or
principal diffraction peak (I.sub.0). The first subsidiary maximum
(having intensity I.sub.1) is separated from the primary
diffraction by the 1.sup.st null or minimum and is separated from
the second subsidiary maximum by the 2.sup.nd null or minimum. In
one embodiment of the present invention, the an angle between
reference beams used to record two or more multiplexed holograms is
less than angular separation between a primary diffraction peak and
the 1.sup.st null of a holographic angle selectivity curve. In
another embodiment, a difference in wavelength between reference
beams used to record the multiplexed holograms is less than wave
length separation between a primary diffraction peak and the 1st
null of a holographic wavelength selectivity curve. In still
another embodiment, a difference in position used to record shift
multiplexed holograms is less than the increment of position
between a primary diffraction peak and the 1st null of a
holographic shift selectivity curve.
[0073] In accordance with the present invention, the holograms are
multiplexed co-locationally or substantially overlapped in a manner
such that the angular or wavelength or positional increments for
recording and/or recovering information from said holograms
correspond to less than the respective increment between the
primary diffraction peak and the first null or minima of the
corresponding Bragg selectivity curve. Holograms recorded in this
manner can be readily differentiated during content addressable
search, described above, when the optical encoding device (SLM) is
operable in phase mode. Preferably, said increment is significantly
less than the increment between the primary diffraction peak and
the first null of the Bragg selectivity curve, such as 1/5.sup.th
to about 1/25.sup.th of the increment so as to provide for
increasing the multiplexing factor (i.e. number of co-locationally
multiplexed holograms or overlapped multiplexed holograms recorded
across the diameter of one storage location) by at least a value of
about 5 to 25. This improvement is at least comparable to the
improved multiplexing factor that can otherwise be achieved by
combining multiplexing methods, such as planar-angle and azimuthal
or shift in tangential and shift in radial directions, but
advantageously the opto-mechanical system for recording and/or
reading would be simplified by comparison to what is required when
combining multiplexing methods.
[0074] By way of example, for conventional planar-angle
multiplexing of digital data pages an increment in angle of the
reference beam for multiplexing is typically equal to the increment
in angle between the primary diffraction peak and the second null
or minima of the Bragg selectivity curve (see FIG. 13) so as to
provide for reasonably good SNR. When a search of stored
information is carried out with the SLM operable in phase mode,
then this increment can be substantially reduced by a factor of 10
to 20 or more during recording of the multiplexed holograms, with
limitations, by way of example, being the NA of the lens that may
be used to redirect the ensemble of reference beams generated by
the multiplexed holograms to a digital detector, the size and
number of the detectors which by way of example can be photo diodes
or CMOS detectors, distance between the detector(s) and the media,
etc. A related aspect of this invention is that the areal density
of the holographically recorded information can be increased
substantially as compared to conventional methods and thereby
advantageously provides for substantially higher capacity per unit
thickness of the recording media, higher data rates, and higher
speed data search. Another related aspect of the present invention
is that the requirements for BER and SNR exhibited by the
multiplexed holograms can be relaxed without compromising the
operation of content addressable searching, and consequently the
overhead associated with encoding the data with modulation codes
and error correction codes can be reduced. The present invention
provides for recording on the order of a terabytes of information
in the form factor of a DVD and with a thin recording material that
has a thickness of only about 400 microns. By the method of the
present invention, raw optical search rates with disk media
rotating, by way of example, at 1000 rpm can be on the order of at
least about 100 Gbytes/sec.
EXEMPLIFICATION
Example 1
Recording a Homogenized Phase-Encoded Data Page and Reading the
Recorded Hologram Using Edge Enhanced Reconstruction with 4.times.
and 8.times. Oversampling at Detector
[0075] Recording was carried out using a Coherent 315M DPSS laser
emitting at 532 nm. A classical 4f optical configuration was
implemented using spatially filtered coherent reference and signal
beams and conventional doublets (f=70 mm) for all optics. Matched
power densities were implemented for the signal and reference beams
at the recording plane for recording carried out with a Displaytech
ferroelectric LCD SLM (1280.times.768 pixels) in 0-pi phase mask
conditions at Fourier Transform plane. The SLM can be operated in
binary amplitude (0 and 1) as well as binary phase (0 and .pi.)
modes by rotation of the SLM by 22.5 degrees. Alternatively,
amplitude and phase mode operations of the SLM were achieved by
rotating a half-wave plate kept in front of it. During the binary
phase mode operation the Displaytech SLM, in conjunction with the
polarizing beam splitter in front of it, imparts -.pi./2 and
+.pi./2 phase shifts on the light reflected from the pixels. A
Photobit MV02 CMOS camera (512.times.512 pixels, 16 .mu.m pixel
pitch) was used as the detector device. Aprilis CROP photopolymer
material of 50 .mu.m to 400 .mu.m thickness was used as the
holographic recording medium.
[0076] A balanced (i.e. having a substantially equal number of the
opaque and transparent pixels) 6-8 modulation coded binary data
page comprising regularly spaced fiducial markings was displayed on
the SLM. FIG. 9(a) shows a part of said data page with said pixel
grouping for an example of said fiducial marking FIG. 9(b) shows
the Fourier power spectrum as captured by the CMOS camera when it
was positioned at the FT plane, for the case when the SLM is
operated in the amplitude mode. The presence of the high intensity
dc peak at the center, as seen in FIG. 9(b) typically requires that
the recording take place at a plane that is either in front of or
is behind the exact FT plane, or some other beam conditioning
technique may be employed. FIG. 9(c) shows the Fourier power
spectrum as captured by the CMOS camera when it was positioned at
the FT plane, for the case when the SLM is operated in the 0-.pi.
phase mode. FIG. 9(c) shows that the high intensity dc peak is
substantially absent, due to the destructive interference between
the light from 0 and .pi. pixels of the SLM when a balanced binary
data page is presented to said SLM.
[0077] After displaying the data page on said SLM operating in
phase mode, a hologram is recorded in the photopolymer material
which is positioned at the FT plane. When the hologram is read-out
in the conventional manner by the reference beam, with the object
arm being blocked, the resultant reconstruction is the phase data
page. The dc peak, however, was absent at the FT plane, as
holographic recording occurred only for the ac components of the
Fourier spectrum. The resultant reconstruction is thus an edge
enhanced reconstruction of the data page exhibiting the phase
transitions, as shown in FIGS. 10(a) and 10(b) for the case of
4.times.4 and 8.times.8 over sampling, respectively, on the binary
data page presented to the SLM. If the value of "0" or "1" is known
for any specific pixel location in the reconstructed data page,
such as would be the case for known fiducial markings, then by edge
detection means every pixel can be assigned to a "0" or "1" and,
accordingly, the original data page can be reconstructed.
Example 2
Reconstruction of an Amplitude Data Page by Interference of the
Reconstructed Phase-Modulated Data Page and an Imaged Uniform Page
Displayed on the SLM
[0078] When the phase hologram is recorded as in Example (1) then
reconstruction of the original amplitude data page from the phase
image is accomplished by using a real-time holographic
interferometric method. In one embodiment, a blank (uniform) page
is displayed on the phase SLM. With the object beam being kept
`ON`, the data page hologram was read-out by concurrent
illumination with the reference beam using the correct reference
beam angle for said data page hologram. As shown in FIG. 8,
interference between the holographically reconstructed data page
and the blank (uniform) page from the SLM reproduces the original
data page, shown in FIG. 11(a), in amplitude mode for detection by
the CMOS camera. The recovered data page as captured by said CMOS
camera of Example (1) is shown in FIG. 11(b). FIG. 11(c) shows the
phase image as captured directly by said CMOS camera without the
recording material at the FT plane.
Example 3
Reconstruction of an Amplitude Data Page by Interference of the
Reconstructed Phase-Modulated Data Page and a Blank Co-Locationally
Recorded Page
[0079] When the phase hologram is recorded so as to use the method
of Double exposure holographic interferometry for data recovery, a
blank data page (i.e. full field super pixel), in addition to the
recording of the phase data page, was carried out near the FT
plane. Upon reconstruction with the Reference beam, the presence of
the blank page hologram in combination with the phase page hologram
provided for an interference of the two resultant diffraction
wavefronts thereby providing for reconstruction of the original
amplitude data page as shown in FIG. 11(d).
Example 4
Degree of Correlation Matching for Search Pattern Having 100% of
the Area of the Data Page Versus 0% of the Area of the Data Page
for Multiplexed Data Page FT and Fractional FT Holograms Recorded
in Binary Versus Amplitude Mode with High and Low Modulation
Depth
[0080] The extent of correlation matching was evaluated for
holograms recorded with several different types of recording
conditions. The ratio of the intensity of the Reference beam
reconstruction was determined when using a fully matched data page
presented to the SLM versus 15 different non-matched data pages
that were similarly presented to said SLM. Each of the 15 different
non-matched data pages comprised a balanced random distribution of
(1s) and (0s) to simulate a standard data page. Poor correlation
matching was achieved and significant cross correlation was
exhibited when one data page was recorded with the Displaytech SLM
operated in amplitude mode and with the recording material
positioned at a distance of about 10% behind the Fourier Transform
plane. When multiple (15 to 30) amplitude data pages were
multiplexed with planar angle multiplexing at the Fourier Transform
plane then good correlation matching was exhibited and cross
correlation was nominal.
[0081] In case (i) FT plane recording of a data page was carried
out with said Displaytech LCD SLM operating in 0-.pi. phase
conditions using the setup of Example (1), and the recording
material (400 microns thick Aprilis CROP photopolymerizable
material) was positioned at the Fourier Transform plane. The
incident power densities of the reference and signal beams were
matched at the recording plane.
[0082] In case (ii) FT plane recording of a data page was carried
out with said Displaytech LCD SLM operating in amplitude mode using
the setup of Example (1), and the recording material (400 microns
thick Aprilis CROP photopolymerizable material) was positioned at
the Fourier Transform plane. The incident power densities of the
reference and dc portion of the signal beams were matched at the
recording plane.
[0083] In case (iii) FT plane recording of a data page was carried
out with said Displaytech LCD SLM operating in amplitude mode using
the setup of Example (1), and the recording material (400 microns
thick Aprilis CROP photopolymerizable material) was positioned at
the Fourier Transform plane. The incident power densities of the
reference and signal beams were mismatched at the recording plane
such that I.sub.Ref/I.sub.ACmax=1 where I.sub.AC is the intensity
distribution of the ac components of the FT of the object beam.
Accordingly, high modulation depth was achieved for holographic
recording of the ac components, and low modulation depth for the DC
component. This approach is opposite to the conventional method
where the intensity of the Reference beam is typically much greater
than I.sub.AC in order to achieve good fidelity for the recorded
data page.
[0084] In case (iv) fractional FT plane recording of a data page
was carried out with said Displaytech LCD SLM operating in
amplitude mode using the setup of Example (1), and the recording
material (400 microns thick Aprilis CROP photopolymerizable
material) was positioned at a distance equal to about 10% of the
focal distance behind the Fourier Transform plane. The incident
power densities of the reference and the average of the signal
beams were approximately matched at the recording plane.
[0085] In case (v) fractional FT plane recording of a data page was
carried out with said Displaytech LCD SLM operating in 0-.pi. phase
mode using the setup of Example (1), and the recording material
(400 microns thick Aprilis CROP photopolymerizable material) was
positioned at a distance equal to about 10% of the focal distance
behind the Fourier Transform plane. The incident power densities of
the reference and signal beams were matched at the recording
plane.
[0086] Results for the ratio of the intensity of the Reference beam
reconstruction using the matched data page at the SLM versus 15
different non matched data pages comprising a balanced random
distribution of (1s) and (0s) were as follows.
Case (i)
[0087] Intensity ratio=(80-82)/(2.1-2.5) where correlation with
each of the 15 non matched data pages results in a small variation
in the dc signal strength but in each case the dc level is
substantially diminished relative to the value achieved for the
matched data page. This ratio should only be limited by diffraction
efficiency of the recorded data page. By way of example, for larger
diffraction efficiency a ratio of 148/0.5 was achieved.
Case (ii)
[0088] Intensity ratio=(62)/(60) where correlation with each of the
15 non-matched data pages results in a small variation in the dc
signal strength but in each case the dc level is substantially
similar to the value achieved for the matched data page.
Case (iii)
[0089] Intensity ratio=(53)/(5.35) where correlation with each of
the 15 non-matched data pages results in a small variation in the
dc signal strength but in each case the dc level is moderately
diminished relative to the value achieved for the matched data
page.
Case (iv)
[0090] Intensity ratio=(97)/(20-22) where correlation with each of
the 15 non-matched data pages results in a small variation in the
dc signal strength but in each case the dc level is moderately
diminished relative to the value achieved for the matched data
page.
Case (v)
[0091] Using 0-pi phase and a shift of the media position to 10% of
focal length behind FT plane; Intensity ratio=(60)/(0.9-1.1) where
correlation with each of the 15 non-matched data pages results in a
small variation in the dc signal strength but in each case the dc
level is again substantially diminished relative to the value
achieved for the matched data page.
Example 5
Correlation Signal Strength Relates to Contiguous Area for Search
Pattern Relative to 100% of the Area of a Data Page
[0092] The size of the contiguous region of matched pixels of the
data page in a content addressable search correlates with the
magnitude of the intensity ratio of the correlation for the case
when the total number of matched pixels is kept constant for case
(i) of Example 4. For example, when the contiguous region of
matched pixels is 50% of the width of the whole data page and along
the entire length of the data page (i.e. 50% of the area is matched
from left to right), independent of where the 50% matched portion
is positioned horizontally along the full data page, then the
intensity ratio of the correlation is diminished from 56/1.2
observed for a fully matched data page to 25/1.2. When the
contiguous region of matched pixels that is equal to 50% of the
width of the entire data page, however, is split into two regions
oriented along the entire length of two opposing edges of the data
page, and these are separated by a center region of random pixels
extending along the entire length and equal to about 50% of the
total pixels of the data page, then the intensity ratio of
correlation is diminished further to a value slightly larger than
when 25% of the data page is matched in a contiguous region.
Example 6
Improved Discrimination for Phase Content Search of Balanced
Co-Locational Binary Amplitude Data Pages, Multiplexed with Angle
Increments Less than the Angle Difference Between the Primary
Diffraction Peak and First Minima, Compared to Amplitude Search of
Balanced Co-Locational Binary Amplitude Data Pages
[0093] The effect of the size of the contiguous region of matched
pixels of the data page in a content addressable search was
additionally examined for the case of having recorded multiplexed
amplitude data pages where each page was a balanced random encoded
binary page and multiplexing was carried out at less than Bragg
selectivity conditions for both amplitude and phase recording.
Thirty (30) 262 kbit substantially phase data pages were
multiplexed co-locationally using planar angle multiplexing with an
angle increment of the Reference beam that was reduced to about 1/5
of the full width half height of the Bragg detuning curve,
corresponding to 10 times denser than with conventional
planar-angle multiplexing. In another location, thirty (30) 262
kbit amplitude data pages were multiplexed co-locationally using
planar angle multiplexing with an angle increment of the Reference
beam that was reduced to about 1/3 of the full width half height of
the Bragg detuning curve, corresponding to 6 times denser than with
conventional planar-angle multiplexing. The holograms were recorded
using the optical configuration of FIG. 2, as described in Example
(1), and exposure times were scheduled to achieve similar
diffraction efficiency for each multiplexed data page. Content
addressable searching of the co-locational amplitude data pages was
implemented firstly with the search pattern presented to the SLM in
substantially phase mode for the case when the angle increment was
reduced to provide for a factor of 10 times the conventional
density, and, secondly, in amplitude mode for the case when the
angle increment was reduced to provide for a factor of 6 times the
conventional density. The search pattern for FIG. 12(a)
corresponded to a blank page presented to the SLM operating in
phase mode. The entire ensemble of reconstructed Reference beams is
exhibited for the 30 multiplexed holograms that were recorded at 10
times conventional density, due to the residual dc component that
was present as a consequence of the SLM not being operated fully in
phase mode. The search pattern for 8(b) was in amplitude mode and
corresponded to one fully matched page out of the 30 that were
co-locationally multiplexed in amplitude mode. FIG. 12(b) shows
substantial cross correlation and nominal differentiation in the
intensity of the reconstructed Reference beams. Thirdly, content
addressable searching of the co-locational amplitude data pages was
implemented with the search pattern presented to the SLM in phase
mode. The search pattern again corresponded to one fully matched
page out of the 30 that were co-locationally multiplexed. FIG.
12(c) shows negligible evidence of cross correlation and correct
identification of the Reference beam corresponding to the data page
of the matched search data. Additionally, content addressable
searching of the co-locational amplitude data pages was implemented
with the search pattern presented to the SLM in phase mode, but
with the search data being reduced to 75%, 50%, and 25% of the
fully matched data page for one of the co-locationally multiplexed
data pages. FIGS. 12(d), 12(e) and 12(f) show correlation matching
for the 75%, 50%, and 25% cases, respectively, and also exhibit
negligible evidence of cross correlation.
Example 7
Degree of Correlation Matching for Multiplexed Phase Data Page when
the Area of the Contiguous Search Pattern is Varied Between 100%
and about 5% of the Area of the Data Page
[0094] The degree of correlation matching was determined for a
phase data page recorded with conditions of case (i) of Example (3)
when the contiguous grouping of matched pixels used for a content
addressable search by the method of Example (3) was varied between
about 5.45% and 100% of the total data page area.
[0095] When about 30.times.550 out of 550.sup.2 pixels (i.e. 5.45%)
of original area of the page is matched in a contiguous region, and
the remaining .about.94.5% of the page is unmatched and is,
additionally, randomly distributed maintaining the balanced
modulation code, the intensity ratio of the correlation declined to
a value of about 6.8/1.2, as compared to 56/1.2for a fully matched
area.
[0096] When about 60.times.550 out of 550.sup.2 pixels (i.e. 10.9%)
of original area of the page is matched in a contiguous region, and
the remaining .about.89% of the page is unmatched and is,
additionally, randomly distributed maintaining the balanced
modulation code, the intensity ratio of the correlation declined to
a value of about 6.8/1.2, as compared to 56/1.2 for a fully matched
area.
[0097] When about 120.times.550 out of 550.sup.2 pixels (i.e.
21.8%) of original area of the page is matched in a contiguous
region, and the remaining .about.78% of the page is unmatched and
is, additionally, randomly distributed maintaining the balanced
modulation code, the intensity ratio of the correlation declined to
a value of about 16/1.2, as compared to 56/1.2 for a fully matched
area.
[0098] When about 50% of original area of the page (i.e.
225.times.550 pixels in both directions) is matched in a contiguous
region, and the remaining 50% of the page is unmatched and is,
additionally, randomly distributed maintaining the balanced
modulation code, then the intensity ratio of the correlation
decreased from the fully matched value of 56/1.2 to 25/1.2.
[0099] When about 320.times.550 out of 550.sup.2 pixels (i.e.
58.2%) of original area of the page is matched in a contiguous
region, and the remaining .about.42% of the page is unmatched and
is, additionally, randomly distributed maintaining the balanced
modulation code, the intensity ratio of the correlation declined to
a value of about 40/1.2, as compared to 56/1.2 for a fully matched
area.
[0100] When about 420.times.550 out of 550.sup.2 pixels (i.e.
76.4%) of original area of the page is matched in a contiguous
region, and the remaining .about.23.6% of the page is unmatched and
is, additionally, randomly distributed maintaining the balanced
modulation code, the intensity ratio of the correlation declined to
a value of about 49/1.2, as compared to 56/1.2 for a fully matched
area.
[0101] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *