U.S. patent application number 13/470635 was filed with the patent office on 2012-09-06 for holographic recording system having a relay system.
This patent application is currently assigned to INPHASE TECHNOLOGIES, INC.. Invention is credited to ERNEST Y. Chuang, Kevin R. Curtis, Jan Hoogland, Yunping Yang.
Application Number | 20120224237 13/470635 |
Document ID | / |
Family ID | 40242565 |
Filed Date | 2012-09-06 |
United States Patent
Application |
20120224237 |
Kind Code |
A1 |
Chuang; ERNEST Y. ; et
al. |
September 6, 2012 |
HOLOGRAPHIC RECORDING SYSTEM HAVING A RELAY SYSTEM
Abstract
According to one aspect and example, a holographic recording
system includes a light source, an object for modulating an object
beam from the light source, and a relay system adapted to magnify
(positive or negative) the modulated beam (e.g., an image of the
object) to an output image. The object may include an SLM operable
to modulate the object beam with an information layer comprising a
plurality of data pages. The output image is directed to the
holographic storage medium, where a reference beam is also
directed, to record the resulting interference pattern.
Additionally, the relay system converges the output image to an
output Fourier plane, which may be disposed within the holographic
storage medium. A filter may be placed at an intermediate Fourier
plane located prior to the output Fourier plane, and a phase mask
may be placed at the position of the output image.
Inventors: |
Chuang; ERNEST Y.; (Boulder,
CO) ; Curtis; Kevin R.; (Longmont, CO) ;
Hoogland; Jan; (Honolulu, HI) ; Yang; Yunping;
(Los Angeles, CA) |
Assignee: |
INPHASE TECHNOLOGIES, INC.
|
Family ID: |
40242565 |
Appl. No.: |
13/470635 |
Filed: |
May 14, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11132002 |
May 17, 2005 |
8199388 |
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13470635 |
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10658055 |
Sep 8, 2003 |
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11132002 |
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60429012 |
Nov 22, 2002 |
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Current U.S.
Class: |
359/35 |
Current CPC
Class: |
G03H 2223/19 20130101;
G11B 7/28 20130101; G03H 2001/0415 20130101; G03H 2001/0428
20130101; G11B 7/0065 20130101; G11B 7/26 20130101 |
Class at
Publication: |
359/35 |
International
Class: |
G03H 1/04 20060101
G03H001/04 |
Claims
1. A holographic recording system, comprising: a light source; an
object for modulating an object beam produced by the light source;
a filter comprising an aperture for polytopic multiplexing; a relay
system for relaying an image of the object to an output image,
wherein output of the relay system produces both the output image
and an output Fourier plane, wherein an intermediate Fourier plane
is formed in a path of the object beam prior to the output Fourier
plane of the relay system and the filter is disposed at the
intermediate Fourier plane location, and wherein the relay system
is operable with the light source for directing the object beam to
interfere with a reference beam at a holographic storage medium for
recording holograms by polytopic multiplexing.
2. The system of claim 1, further including a holographic storage
medium, wherein the output image is directed to the holographic
storage medium.
3. The system of claim 2, wherein the output image is directed to
intersect a reference beam and record a hologram associated with
the object.
4. The system of claim 1, wherein the object includes an SLM or
data mask.
5. The system of claim 1, wherein an intermediate Fourier plane is
formed in a path of the object beam prior to the output Fourier
plane of the relay system.
6. The system of claim 5, further including a filter disposed at
the intermediate Fourier plane location.
7. The system of claim 1, wherein the filter includes an aperture
and at least one dimension of the aperture is between 0.5 to 4.0
times the Nyquist aperture size for polytopic multiplexing.
8. The system of claim 1, further including a phase mask disposed
at the location of the object.
9. The system of claim 1, further including a phase mask disposed
at the location of the output image.
10. The system of claim 1, wherein the output image includes a real
or virtual image and is formed before the output Fourier plane.
11. The system of claim 1, wherein the output image is a real image
formed after the output Fourier plane.
12. The system of claim 1, wherein an output beam divergence of the
relay system is greater than a beam divergence of the illumination
to the object.
13. The system of claim 1, wherein the output numerical aperture of
the relay system is greater than 0.2.
14. The system of claim 1, wherein the relay system further
magnifies the image of the object to the output image, and the
magnification of the output image is between 0.1 and 10.0 times the
size of the object.
15. The system of claim 1, further including directing the object
beam and a reference beam to interfere at a holographic storage
medium for recording holograms by one or more of polytopically
multiplexing, angle multiplexing holograms, wavelength
multiplexing, shift multiplexing, correlation multiplexing,
confocal multiplexing, or peristrophically multiplexing.
16. A holographic recording system, comprising: a light source; an
object for modulating an object beam produced by the light source;
and a relay system adapted to produce an output image of the
object, wherein output of the relay system produces both the output
image and an output Fourier plane, wherein an intermediate Fourier
plane is formed in a path of the object beam prior to the output
Fourier plane of the relay system and a filter is disposed at the
intermediate Fourier plane location, the filter including an
aperture for polytopic multiplexing, and wherein the relay system
is operable with the light source for directing the object beam to
interfere with a reference beam at a holographic storage medium for
recording holograms by polytopical multiplexing.
17. The system of claim 16, further including a holographic storage
medium, wherein the output image is directed to the holographic
storage medium.
18. The system of claim 17, wherein the output image is directed to
intersect a reference beam and record a hologram associated with
the object.
19. The system of claim 16, further including a phase mask disposed
at the location of the output image.
20. The system of claim 16, wherein an intermediate Fourier plane
is formed in the object path prior to the output Fourier plane of
the relay system.
21. The system of claim 20, further including a filter disposed at
the intermediate Fourier plane location.
22. The system of claim 16, wherein the filter includes an aperture
and at least one dimension of the aperture is between 0.5 to 4.0
times the Nyquist aperture size for polytopic multiplexing.
23. The system of claim 16, wherein the relay system is further
adapted to magnify the object the output image relative to the
object.
24. The system of claim 23, wherein the magnification is between
0.1 and 10.0.
25. (canceled)
26. (canceled)
27. (canceled)
28. (canceled)
29. A method for recording holographic storage media, comprising:
illuminating an object to produce a modulated object beam; relaying
the modulated object beam with a relay system to provide an output
modulated object beam comprising an output Fourier transform
pattern of the modulated object beam; directing the output
modulated object beam to a holographic storage medium; propagating
a reference beam to the holographic storage medium to record a
resulting interference pattern between the output modulated object
beam and the reference beam in the holographic storage medium,
wherein the reference beam comprises a plane wave; forming an
intermediate Fourier plane prior to the output Fourier transform
pattern of modulated object beam; disposing an aperture at the
intermediate Fourier plane location; recording holograms via
polytopic multiplexing by translating the holographic storage
medium with respect to the reference beam and output modulated
object beam between holograms, wherein a beam waist of the output
modulated object beam is positioned outside of the holographic
storage medium when recording each of the holograms, wherein at
least two holograms; and further including forming an intermediate
Fourier plane prior to the output Fourier plane.
30. The method of claim 29, further including disposing an aperture
at the intermediate Fourier plane location.
31. (canceled)
32. (canceled)
33. (canceled)
34. (canceled)
35. (canceled)
36. (canceled)
37. A method for recording holographic storage media, comprising:
illuminating an object to produce a modulated object beam; relaying
the modulated object beam with a relay system to provide an output
modulated object beam comprising an output Fourier transform
pattern of the modulated object beam; directing the output
modulated object beam to a holographic storage medium; propagating
a reference beam to the holographic storage medium to record a
resulting interference pattern between the output modulated object
beam and the reference beam in the holographic storage medium,
wherein the reference beam comprises a plane wave; forming an
intermediate Fourier plane prior to the output Fourier transform
pattern of modulated object beam; disposing an aperture at the
intermediate Fourier plane location; recording holograms via
polytopic multiplexing by translating the holographic storage
medium with respect to the reference beam and output modulated
object beam between holograms, wherein a beam waist of the output
modulated object beam is positioned outside of the holographic
storage medium when recording each of the holograms, wherein at
least two holograms; and further including recording holograms by
one or more of polytopically multiplexing, angle multiplexing
holograms, wavelength multiplexing, shift multiplexing, correlation
multiplexing, confocal multiplexing, or peristrophically
multiplexing.
38. The system of claim 1, wherein an object beam intersects a
reference beam to record a hologram with a holographic storage
medium, and the hologram is adapted for readout from the
holographic storage medium without imaging optics.
39. The system of claim 16, wherein an object beam intersects a
reference beam to record a hologram with a holographic storage
medium, and the hologram is adapted for readout from the
holographic storage medium without imaging optics.
40. The method of claim 29, wherein a beam waist associated with
the output modulated object beam is positioned outside of the
holographic storage medium when recording the holograms.
41. The method of claim 40, further comprising disposing an
aperture at the beam waist of each hologram during readout to
filter out neighboring holograms.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional of U.S. patent application
Ser. No. 11/132,002, entitled "HOLOGRAPHIC RECORDING SYSTEM HAVING
A RELAY SYSTEM," filed May 17, 2005, allowed, which claims priority
to a Continuation-in-part of earlier filed U.S. patent application
Ser. No. 10/658,055, entitled, "METHODS FOR IMPLEMENTING PAGE BASED
HOLOGRAPHIC ROM RECORDING AND READING," filed Sep. 8, 2003,
abandoned, which claims benefit of earlier filed provisional
application U.S. Ser. No. 60/429,012, entitled "A METHOD FOR
IMPLEMENTING A PAGE BASED HOLOGRAPHIC ROM," filed on Nov. 22, 2002,
of which are incorporated herein by reference in their
entirety.
BACKGROUND
[0002] 1. Field
[0003] The invention relates generally to holographic data storage
media and systems, and more particularly to methods and systems for
recording and/or reading holographic storage media.
[0004] 2. Description of Related Art
[0005] Holographic data storage systems store information or data
based on the concept of a signal beam interfering with a reference
beam at a holographic storage medium. The interference of the
signal beam and the reference beam creates a holographic
representation, i.e., a hologram, of data elements as a pattern of
varying refractive index and/or absorption imprinted in a volume of
a storage or recording medium such as a photopolymer or
photorefractive crystal. Combining a data-encoded signal beam,
referred to as an object beam, with a reference beam can create the
interference pattern at the storage medium. A spatial light
modulator (SLM) or lithographic data mask, for example, may create
the data-encoded signal beam. The interference pattern induces
material alterations in the storage medium that generate the
hologram.
[0006] The formation of the hologram in the storage medium is
generally a function of the relative amplitudes and polarization
states of, and phase differences between, the signal beam and the
reference beam. The hologram is also dependent on the wavelengths
and angles at which the signal beam and the reference beam are
projected into the storage medium. After a hologram is created in
the storage medium, projecting the reference beam into the storage
medium interacts and reconstructs the original data-encoded signal
beam. The reconstructed signal beam may be detected by using a
detector, such as a CMOS photo-detector array or the like. The
recovered data may then be decoded by the photo-detector array into
the original encoded data.
[0007] A basic holographic system is illustrated in FIG. 1. The
holographic storage system includes a light source 110, for
example, a laser for providing a coherent beam of light. A beam
splitter 114 is positioned to split the laser beam into an object
beam and a reference beam. The object beam is directed to an SLM or
data mask 116 where it is encoded with information as a
two-dimensional image and directed to the recording storage medium
124 by mirror 118 and lens 120 where it interferes with the
reference beam directed via mirror 130. A complex interference
pattern is recorded in the storage medium 124 where the object beam
and reference beam interact. After a first image or layer is
recorded, the system may be modified to enable additional images to
be recorded in storage medium 124. For example, by modifying the
angle and/or wavelength of the reference beam, successive images
may be recorded in the storage medium 124.
[0008] A particular image-may be retrieved from recording medium
124 with a reference beam similar to the original reference beam
used to store the image. The light is diffracted by storage medium
124 according to the stored hologram and the two-dimensional image
that was stored in recording medium 124 is directed by lens 126 to
photo-detector array 128.
[0009] Holographic Read Only Memory (Holographic ROM or HROM)
storage media are well known. In the past, holographic information
has been recorded in disc format HROM in an incremental manner by
successively aligning different locations on the HROM with an
object beam and a reference beam to record successive data bits.
Different information can be recorded at each successive location
by changing the information imparted through a spatial light
modulator (SLM) or successive data masks, for example. U.S. Pat.
No. 6,272,095, entitled, "Apparatus and Method for Storing and/or
Reading Data on an Optical Disk," by Liu et al. describes several
examples of illustrative prior recording techniques, and is
incorporated herein by reference in its entirety. Moreover,
multiple holograms can be stacked in virtual image layers through
wavelength multiplex, angle multiplex, shift multiplex, confocal
multiplex, or other multiplex techniques, for example. Each
hologram in a stack may comprise a page of information, where a
"page" is a collection of bits or of pixel data stored together,
e.g., as a 2048.times.2048 array or a 10.times.10 array. U.S. Pat.
No. 6,322,933, entitled, "Volume Track Definition for Data Storage
Media Used to Record Data by Selective Alteration of a Format
Hologram," by Daiber et al. describes several examples of
illustrative prior volume recording techniques, and is incorporated
herein by reference in its entirety. Additionally, another
reference that describes recording techniques and the bitwise
retrieval of an HROM includes "Holographic ROM System for High
Speed Replication," presented by Ernest Chuang et al. of Sony
Corporation at the Optical Data Storage Conference, Jul. 8, 2002,
in Hawaii, USA.
[0010] One shortcoming of such methods for the recording of
holograms is that significant time can be required to incrementally
record information on a location-by-location or bit-by-bit basis.
Therefore, improved recording processes have been proposed in which
an entire holographic image or "layer" of information is recorded
simultaneously across an entire storage medium. An exemplary method
includes shining a planewave beam through a transmissive optical
medium, e.g., a transmissive lithographic data mask, encoded with
information so as to create a planewave object/signal beam. The
encoded planewave object beam illuminates one surface of the
storage medium. A conical planewave reference beam, for example,
may illuminate an opposite surface of the holographic storage
medium. The object beam and the reference beam interfere within the
holographic storage medium to create an information layer within
the storage medium.
[0011] Readout of the data stored on the media may be subsequently
achieved using the same reference beam used to record the data or
by using a conjugate readout beam (i.e., similar to the original
reference beam propagated in the opposite direction) to reconstruct
a backwards-propagating signal beam that retraces the path of the
original recording. The stored information is readout in a bitwise
fashion, e.g., a bit at a time or a few bits at a time in a serial
fashion as described in the Sony Corporation approach referenced
above. A pickup lens of a system may focus a real image of the
data, created using the readout beam, onto a suitable detector as
the holographic medium and/or the drive translate and/or rotate
with respect to each other.
[0012] One shortcoming of the recording methods described above is
that information typically is readout a bit at a time or a few bits
at a time in a serial fashion. Thus, there is a need for recording
holographic storage media that more readily permits relatively
higher, parallel readout rates. Specifically, there is a need for
parallel readout of at least a page of information at a time, e.g.,
many bits in parallel. In addition, the bit-by-bit readout
architecture generally limits the implementation to spinning disks
in order to get reasonable transfer rates. There is a need for
page-wise readout that may allow for new more compact, portable
formats such as storage cards to be practical. Another shortcoming
of the prior proposed recording methods includes the alignment and
time necessary to align and record multiple data masks in
holographic storage media, e.g., HROM media.
BRIEF SUMMARY
[0013] In one aspect of the present invention, methods and systems
for recording holographic storage media are provided. In one
example of this aspect, a method includes illuminating an object
(e.g., a data mask or SLM) with an object beam (often referred to
as a "signal" beam) and recording the resulting modulated object
beam in a holographic storage medium. In one example, the data mask
includes an information layer that is divided into multiple data
pages. A reference beam is further propagated to the holographic
storage medium to record the multiple data pages of the information
layer in a parallel fashion with the holographic storage
medium.
[0014] In another aspect, a method and system for recording
holographic media where an object image is magnified is provided.
In one example, a system includes a light source, an object for
modulating an object beam from the light source, and a relay system
adapted to magnify or demagnify the modulated beam (e.g., an image
of the object) to an output image. The output image is directed to
the holographic storage medium, where a reference beam is also
directed, to record the resulting interference pattern.
Additionally, the relay system converges the output image to an
output Fourier plane, which may be disposed within the holographic
storage medium. In one example, a filter may be placed at an
intermediate Fourier plane located prior to the output Fourier
plane. Further, a phase mask may be placed at the position of the
output image in one example. The object may include an SLM operable
to modulate the object beam with an information layer comprising a
plurality of data pages. In other examples, the relay system may be
used without magnifying the object beam, and may provide the
benefits of an intermediate Fourier plane or phase mask disposed at
the output image.
[0015] In another aspect described herein, methods and system for
recording holographic media and/or holographic master data masks
are provided. In one example of this aspect, a method includes
storing at least one information layer in a holographic master data
mask, illuminating the holographic master data mask to reconstruct
the stored information layer and direct an image of the information
layer to a holographic storage medium, and propagating a reference
beam to the holographic storage medium to record the information
layer therein.
[0016] In another example, a method for recording holographic media
and/or holographic master data masks includes recording at least a
first hologram (e.g., an information layer) with a first
holographic medium (e.g., a "submaster") and recording at least a
second hologram with a second holographic medium (e.g., a second
"submaster"). The first hologram and the second hologram from the
first and second holographic media are then sequentially
reconstructed and stored with a single holographic master medium
(e.g., a "master"). The holographic master medium may then be used
to record the stored first and second holograms into additional
holographic media, for example, into HROM devices or the like. The
exemplary method of using holographic submasters to create a
holographic master data mask may provide benefits in terms of
diffraction efficiencies of stored information layers and increased
storage capacity of the final holographic storage device.
[0017] In another aspect of the present invention, various methods
for reading holographic media are provided. In one example, a
method for reading holographic storage media includes providing a
reference beam and detecting stored information in a holographic
storage medium with a detector placed at a distance from the
holographic storage medium. In one example, the holographic storage
medium includes at least one information layer divided into a
plurality of data pages stored therein and adapted to be detected
at the distance of the detector (e.g., lenses or other optical
elements may not be needed for readout).
[0018] The present invention is better understood upon
consideration of the detailed description below in conjunction with
the accompanying drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 illustrates an exemplary holographic recording and
reading system;
[0020] FIG. 2A illustrates an exemplary data mask including
multiple data pages;
[0021] FIG. 2B illustrates an exemplary stack of information layers
having multiple data pages in each layer;
[0022] FIG. 3 illustrates an exemplary holographic data recording
system for creating holographic master data masks or holographic
media;
[0023] FIG. 4 illustrates an exemplary system for detecting
holographic data directly with a detector;
[0024] FIGS. 5A and 5B illustrate an exemplary system for recording
a master holographic data mask;
[0025] FIG. 6 illustrates an exemplary system for imaging a master
holographic data mask to holographic storage media;
[0026] FIG. 7 illustrates an exemplary system for reading
holographic storage media including a holographic optical
element;
[0027] FIG. 8 illustrates an exemplary system and method for
reading images from holographic storage media;
[0028] FIGS. 9A and 9B illustrate exemplary systems and methods for
recording and reading holographic storage media;
[0029] FIG. 10 illustrates an exemplary system and method for
recording holographic storage media including a relay system to
provide a magnified output image of an object;
[0030] FIGS. 11A-11C illustrate exemplary lens configurations for
imaging an object;
[0031] FIGS. 12A-12C illustrate exemplary relay systems for imaging
and magnifying an object;
[0032] FIG. 13 illustrates an exemplary simulation of a relay
system producing a magnified output image;
[0033] FIGS. 14A and 14B illustrate exemplary methods for forming
holographic storage media from a holographic master data mask
according to reflection and transmission geometry;
[0034] FIGS. 15A and 15B illustrate an exemplary method for
recording holograms to a plurality of holographic submaster media
and copying the holograms into a single holographic master
medium;
[0035] FIGS. 16A-16C illustrate exemplary positioning of data masks
and holographic media for manufacturing holographic media from a
master holographic data mask and a plurality of submaster
media;
[0036] FIG. 17 illustrates an exemplary method for recording and
manufacturing holographic media; and
[0037] FIG. 18 illustrates an exemplary method for recording and
manufacturing holographic media.
DETAILED DESCRIPTION
Definitions
[0038] Methods and systems are provided for holographic storage
media recording and reading including, for example, holographic
read only media or HROM media. The following description is
presented to enable any person of ordinary skill in the art to make
and use the various examples. Descriptions of specific 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
invention. Thus, the present invention is not intended to be
limited to the examples described and shown, but is to be accorded
the widest scope consistent with the principles and features
disclosed herein.
[0039] In one aspect of the present invention, data masks are used
to record virtual layers or holographic images having a plurality
of data pages centered at different locations on a holographic
storage medium such that readout may be achieved in a page-wise
fashion. In one example, parallel recording of multiple data pages
in each layer allows for the fast replication of HROM media with
increased storage capacity, and page-wise readout of HROM media
allows for faster read rates than bitwise readout. The increased
storage capacity is due in part to increased diffraction efficiency
by recording multiple data pages in the storage media in parallel.
The data transfer rates are increased in part because of parallel
detection of an entire data page without moving the media as in
serial storage devices such as CDs, magnetic disks, and the like.
The examples are particularly suited for making high performance
card storage devices; however, the examples are applicable to
various storage device media and configurations such as discs and
tape media as will be appreciated by those of ordinary skill in the
art.
[0040] In another aspect of the present invention, one or more data
masks are stored in a holographic medium to create a holographic
master data mask (sometimes referred to herein as a "master" or
"master holographic medium") for use in recording holographic
storage media. In one example, a holographic master data mask is
fabricated with multiple information layers or data layers and used
to record or copy one or more of the stored data layers to a
holographic storage media such as HROM media. The holographic
master data mask may be multiplexed to retrieve the various data
masks with improved alignment and speed between recording different
layers without the need to move and align a plurality of data
masks. Additionally, the holographic master data mask may be placed
at or near a quasi Fourier transform plane of the stored data masks
using a VanderLugt setup or other transforming optical system.
Holographic master data masks may be recorded at or near Fourier
planes or image planes of the optical system. The Fourier transform
location in these systems may filter out high order transform
components during recording the holographic master data mask, and
reduce interference between stored data pages when recording an
HROM or the like, thereby improving readout performance
[0041] In another aspect of the invention, holographic recording
methods and systems for forming holographic master media using a
plurality of submaster holographic media are provided. In one
example, a method includes recording a set of desired holograms
sequentially into a plurality of separate holographic media, e.g.,
a plurality of "submasters." Thereafter, the holograms recorded in
the submasters are copied to a second holographic storage medium, a
"master." The master holographic medium may be used as a master
data mask or the like to replicate further holographic storage
media. The exemplary method may provide increased storage capacity
and hologram diffraction efficiencies in a holographic medium over
conventional methods and systems.
[0042] In another aspect of the invention, a holographic recording
method and system include an optical relay system disposed in the
object beam path to magnify or demagnify an input object image to
an output object image. The magnification of the relay system may
be adjusted to magnify the object image size to a desired output
image size to be stored and later reconstructed from a suitable
holographic medium. For example, the object image may be magnified
to a desired pixel size and pitch varying from that of the
object.
[0043] Additionally, exemplary methods and systems including a
relay system lend themselves to further advantages (including
examples that relay the object image without magnification). For
example, a phase mask may be placed at the output image plane.
Further, in one example, the light path for the output image of the
relay system converges to an output Fourier plane and an
intermediate Fourier plane may be included prior to the output
Fourier plane. A filter (e.g., including an aperture for polytopic
multiplexing) may be placed at the intermediate Fourier plane,
thereby allowing the output Fourier plane to be placed inside the
storage medium.
[0044] In another aspect of the invention, where one or more data
mask are stored in a holographic medium or a holographic master
data mask, the data pages are multiplexed in stacks using
multiplexing techniques such as wavelength or angle multiplexing.
Additionally, the data pages may be stored spatially overlapping
using polytopic multiplexing. Polytopic multiplexing is described,
e.g., in U.S. Patent Application 60/453,529, filed on Mar. 10,
2003, and entitled, "A METHOD FOR OVERLAPPING HOLOGRAMS USING
LOCATION BASED FILTERING TO SEPARATE OUT THE SIGNAL," which is
hereby incorporated by reference as if fully set forth herein.
[0045] The following description includes various examples and
aspects of the present invention. As will be recognized by those of
ordinary skill in the art various examples may be used alone or in
combination with various other methods and systems depending on the
particular application.
[0046] Volume holography can be used to multiplex multiple layers
in the same holographic storage medium. For instance, a first
transmissive mask medium or data mask may be encoded with a first
information layer, and a second transmissive mask medium may be
encoded with a second information layer. Information from the first
and second transmissive mask media can be multiplexed onto the
holographic storage medium through angle or wavelength multiplex
techniques, for example. In this manner, multiple transmissive mask
media, each encoded with different information, can be used to
produce multiple information layers. That is, stacks of holograms
can be created in which information stored at different layers in a
given-stack are created by different or successive transmissive
mask media. The use of the term "layer," in a stack is a logical
explanation or term as the physical reality of volume holographic
recording is that all holograms in a stack exist in the same or
nearly the same volume and not in physically separate layers in the
media.
[0047] In one exemplary recording method, multiple data pages
included in a single hologram image or information layer are
recorded in parallel in at least a portion of a holographic storage
medium. Further, multiple layers may be recorded to the holographic
storage medium to create multiple stacks of data pages (often
referred to as "books" of data pages).
[0048] FIG. 2A illustrates an exemplary data mask 200 including a
plurality of data pages 210. Each data page 210 may include an
array of pixels numbering in the hundreds of pixels or more. For
example, each data page 210 may include a 2048 by 2048 array of
pixels, but various sizes and shapes of each data page 210 are
possible. Each data page may be separated within data mask 200 and
storage media by a distance equal to a few microns or larger.
Alternatively, if polytopic multiplexing is used, for example, the
data pages may overlap within the storage media as described in
greater detail below.
[0049] FIG. 2B illustrates an exemplary representation of hologram
images or virtual information layers 202 having multiple data pages
212 of information that may be stored in a holographic storage
medium by data mask 200. Each layer 202 may be stored onto a
portion or all of a holographic medium at the same time such that
multiple data page 212 of a common layer are recorded in parallel.
Further, multiple layers 202 may be aligned with data pages 212
forming a stack 214 of data pages 212, where one stack 214 is
indicated generally by the arrow. During readout, a detector may be
aligned with a stack 214 of one or more data pages 212 in a storage
medium and each data page 212 detected or readout successively in a
page-wise fashion. For example, each layer 202 and data page 212 of
a stack 214 may be multiplexed into the media by angle, wavelength,
or other suitable methods and readout accordingly. When an entire
stack 214 has been read, the detector and/or media may be
translated and/or rotated to align the detector with another stack
214 and page-wise readout continued. In another example described
below, referred to as polytopic multiplexing, data pages 210 may
spatially overlap when stored.
[0050] Storing an information layer 202 including multiple data
pages 212, across a holographic storage medium may more effectively
utilize the dynamic range of holographic media, thereby increasing
the storage capacity of the media. Generally, the storage capacity
and diffraction efficiency decrease as the number of holograms, in
this instance, information layers 202, are stored in similar
locations of the volume medium. For example, diffraction efficiency
is proportional to 1/m.sup.2, where m is the number of holograms
stored therein. When multiple data pages are recorded in a medium
in parallel as part of a single image, however, the diffraction
efficiency reduction occurs only once for each layer 202 of
multiple data pages 212, i.e., m=1, rather than a reduction for
each data page 212 recorded within the holographic storage medium.
Typically, recording all data pages 212 in layer 202 in parallel
results in or allows for higher storage density due to improved
uniformity of exposure of the medium and better use of the medium
volume. In contrast, if each data page 212 is recorded by itself in
a serial fashion, the process will use more dynamic range of the
medium than when each data page 212 is recorded at the same time,
i.e., in parallel. The difference in dynamic range is due, at least
in part, to overlap of the signal beam and reference beam exposing
more medium with propagation through the medium. This results in
more effective exposures to record the same information during
serial recording and uses a greater dynamic range of the medium
than parallel recording. In addition, localized serial recording of
data pages 212 may create localized bulk index changes that can
lower the signal-to-noise ratio (SNR) of the system compared to
parallel recording an entire layer 202.
[0051] Additionally, faster readout times may be achieved with the
exemplary methods. During readout of a holographic storage medium
with multiple data pages 212 in each layer 202, a system may align
a detector with a data page 212 or a stack 214 of data pages 212
and readout the data pages 212 in a page-wise fashion, e.g., where
the array of information stored in each data page 212 is readout in
parallel. In one example, a detector is aligned with a stack 214
and the entire stack 214 of data pages 214 is readout page-wise
(through multiplexing, etc.). The detector may then move to another
stack 214 of the storage medium to readout data pages 212. Reading
in a page-wise fashion may substantially increase the readout rate
compared to methods and systems where readout is performed in a
serial or bitwise fashion.
[0052] Further, detecting an entire data page 212 in parallel
allows for page-wise error correction methods. For example,
detecting an image of a data page 212 allows for various error
correction and channel modulation techniques to recover detected
signals and decode them into the storage information with greater
accuracy. For example, U.S. patent application Ser. No. 10/305,769
entitled "MICRO-POSITIONING MOVEMENT OF HOLOGRAPHIC DATA STORAGE
SYSTEM COMPONENTS," describes several error correction methods and
techniques and is hereby incorporated by reference in its
entirety.
[0053] FIG. 3 illustrates a portion of an exemplary system for
recording data mask 200 including multiple data pages onto
holographic storage medium 220 where the multiple data pages are
advantageously recorded in parallel. The exemplary systems shown
and described herein may include various additional or different
optical elements such as lenses, prisms, apertures, filters, beam
splitters, gratings, and the like as will be recognized by those of
ordinary skill in the art. For clarity, however, such additional
features have been omitted from the description.
[0054] In this example, data mask 200 is illuminated by an object
beam 242 provided by light source 236 and interacts with a
reference beam 240 provided by light source 236. Light source 236
may include any suitable light source such as a laser or other
coherent light source. Further, object beam 242 and reference beam
240 may be provided by the same or different light sources. The
exemplary recording method is shown in a reflection geometry
holography process configuration (object and reference beam
entering the media from opposite sides), where the plane wave
reference beam interferes with the data encoded object beam inside
the holographic storage medium 220. The system illustrated may be
configured in various manners and include various other features
not explicitly described herein as will be recognized by those of
ordinary skill in the art. For example, it should be readily
recognized that transmission geometry holography processes may also
be used to store the information of data mask 200, where the plane
reference wave beam is provided from the same side of the medium
220 as the data mask 200. Additionally, the optical system in the
object beam path may include a single lens 230 as shown, no lenses,
or a plurality of lenses.
[0055] In this example, data mask 200 is illuminated by object wave
242 from light source 236 and is propagated in a transmissive
system to image plane 222 that is a distance d from the holographic
storage medium 220 center. The distance d can be positive, zero, or
negative, i.e., on the other side of storage medium 220 shown in
dotted lines. Image plane 222 may also be located within storage
medium 220. One potential advantage of holographically implementing
multiple layers (image planes) is that the layers do not have to
physically be on the storage medium. For example, with current
multilevel CD, near-field, or high na systems, the layers are on or
inside the media. By imaging or placing the data mask 200 of the
data, the layers can appear to a detector or drive as being located
outside of the storage medium 220, inside storage medium 220, or on
storage medium 220 depending on where the data mask 200 image plane
222 lies during recording. Thus, the usual problem of compensating
for spherical aberration due to the change in the amount of
substrate that the light travels through may not be necessary and
in some instances may be detected without a pickup lens or lenses.
In addition, by placing the image plane 222 of the data outside of
the storage medium 220, various media defects or contamination
issues such as dust, scratches, and the like may be mitigated.
[0056] Further, when imaging, a lens system 230 can change the
magnification of the mask to enlarge or shrink. In one example,
lens 230 demagnifies the image by ten. Lens system 230 may include
one or more optical elements including lenses, lens arrays, prisms,
beam splitters, and the like. Further, data masks may be recorded
without lenses by placing the data masks 200 near to the media, for
example, at a distance d from medium 220 and recording at this
location.
[0057] Data mask 200 may be propagated to a holographic storage
medium in any suitable manner. In the case of multiple data masks
200, successive data masks 200 and 201 may be stored in the
holographic storage media utilizing a procedure in which each data
mask 200, 201, and so on, is effectively multiplexed for readout
purposes. In one example for recording multiple pages of data mask
200 in parallel, data mask 200 is Fourier transformed onto the
medium 220 with a lens array including a lens element for each data
page 210.
[0058] Methods for recording multiple data pages of data mask 200
in parallel may further include a VanderLugt imaging system. For
example, a VanderLugt imaging system generally includes or is
characterized by having lens 230 before the data mask 200 such that
the data mask 200 is illuminated by a converging beam. FIG. 5A,
described below, includes an exemplary VanderLugt imaging
configuration. The VanderLugt system is preferred in that it allows
the storage medium 220 to be in or near the Fourier transform
plane, and when used, e.g., with phase conjugate readout the image
plane is generated without the need of a lens. Storage medium 220
may also be placed near the Fourier transform plane to enable an
aperture to filter out high order transform components during
recording. Further, the Fourier transform plane may be contained
within the media, or near the media depending on the particular
application. If the Fourier transform plane is within the media,
the Fourier plane filtering may be done at another Fourier plane in
the relay system. In addition, phase masks or other elements to
lower the DC spot energy relative to the rest of the orders
intensity may be included to allow for better recording of the
information.
[0059] In one exemplary method of wavelength multiplexing, the
wavelength of the beams 240, 242 may be changed and a new data mask
201 with data pages that have different information stored.
Additionally, data mask 201 may include a holographic data mask
having one or more layers may be used (as will be described below).
In one example, a tunable laser device may be included, for
example, where the wavelength may be varied by tuning the
temperature of a diode as described in U.S. patent application Ser.
No. 10/346,399, entitled, "System and Method for Bitwise Readout
Holographic ROM," which is hereby incorporated by reference as if
fully set forth herein. Additionally, a series of discrete laser
sources with varying wavelengths may be used. Other methods for
implementing various wavelengths in a system include techniques
developed for telecom applications. For example, these include
electroabsorptive modulation and the use of a
micro-electro-mechanical systems (MEMs) structure on a laser cavity
to change the wavelength. Central wavelengths of interest can be
anywhere between 350-950 nm, and practical tuning ranges of
interest can be as small as 1 nm. A preferred range of tenability
is between approximately 5 nm and 15 nm. The readout laser may
include a continuous wave or pulsed beam. A pulsed beam is
generally more efficient in using energy. Angle multiplexing may
also be achieved by varying the angle of incidence of the reference
beam. Additionally, phase code, polytopic, shift, correlation, or
other known multiplexing methods may also be used alone or in
combination.
[0060] In another example, confocal multiplexing may be implemented
by varying the image plane 222 distance d between recording
successive data masks 200 and 201. Distance d may be varied through
a repositioning device controller 232 that translates lens 230 in a
direction perpendicular to data mask 200, along the optical axis of
lens 230 to vary the location of d between successive layers. The
repositioning device may include any suitable device capable of
translating lens 230 (or multiple lenses) and controlled by a
suitable microprocessor. This could also be implemented by moving
the mask between successive layers. On readout, the detector may be
similarly varied by a repositioning device to image and detect the
stored images. For example, changing the wavelength for wavelength
multiplexing or changing the location of the detector for confocal
multiplexing. Additionally, for confocal multiplexing it is
generally desirable to have a filter such as an array of pinholes
at the image plane, i.e., on the detector array or at an
intermediate image plane, to filter out noise from the other
confocally stored holograms. These, and other multiplexing
techniques well known in the art, can be implemented alone or in
any combination to achieve high-density storage and capacity within
storage medium 220.
[0061] An advantage of confocal multiplexing includes that the data
mask 200 may be moved over a distance, e.g., 100 microns or more,
without significant loss of image quality and without magnification
change, provided that the optical system is close to telecentric.
By filtering out other data pages on a per pixel basis with a
pinhole filter similar to confocal microscope, the multiple pages
in the stack can be recorded at different depths. This can
significantly increase the density that may be stored in the
holographic medium 220. Confocal multiplexing may also be combined
with Bragg based, e.g., angle, wavelength, shift, and/or momentum
based multiplexing techniques, e.g., fractal, aperture,
peristrophic, to increase storage density.
[0062] Data mask 200 may include a phase mask, amplitude mask, or
any combination of amplitude and phase mask, where phase and/or
amplitude represents data. For example, binary information can be
stored as (1,0) or (1,-1,0), and detected as 1,0 on any suitable
detector such as a CCD image sensor, CMOS image sensor, or other
suitable sensor such as an area array sensor. Data mask 200 may
contain information similar to that found on conventional DVD
disks, including light and dark spots corresponding to is and Os,
data tracks, and servo patterns in the data.
[0063] Data mask 200 may be fabricated through any suitable
lithographic method as known in the art. Alternatively, an SLM or
holographic storage medium may be used to encode an object beam. In
one example, one or more data masks 200 having a layer with
multiple pages may be holographically stored in a holographic
medium as a holographic master data mask and used to store data
into the holographic storage medium, e.g., an HROM. The
holographically store layer may be imaged to the HROM, and in the
case of multiple pages, multiplexed to the HROM to store each
hologram having multiple pages as will be described in greater
detail below.
[0064] In one example, the holographic storage media 220 may
include a card medium such as a high performance card storage
device or the like. Alternatively, the storage medium 220 may
include disc media similar to or different from conventional CD or
DVD media, tape, or any other convenient format in which optical
media may be fabricated. Generally, the holographic storage medium
220 includes a photopolymer on or between protective substrates.
The protective substrates are advantageously selected from glass,
sapphire, polycarbonate, poly(methyl methacrylate) or PMMA,
plastic, quartz, or other suitable material that is generally
transparent to the wavelength of light being used, and which has
adequate mechanical properties in the holographic storage system.
Alternatively, the storage medium may be enclosed at least
partially within a cartridge or other protective structure.
[0065] The photopolymer layer may include a photopolymer material
that is capable of recording the desired interference pattern. One
exemplary holographic medium includes Tapestry.TM.. media
manufactured by InPhase Technologies, but any volumetric media
capable of recording holograms may be used. Other examples of
photopolymers and storage media include those described in U.S.
Pat. No. 6,482,551, which is hereby incorporated herein in its
entirety by reference. It is also desirable that the optical
quality of the media be fairly high, e.g., <4 wavelength of
power per cm.sup.2, to achieve good image reconstruction and good
Bragg selectivity. U.S. Pat. No. 5,932,045 describes an exemplary
method for fabricating exemplary media and is incorporated herein
in its entirety by reference.
[0066] FIG. 4 illustrates an exemplary system for reading
holographic storage media, for example, storage medium 220 recorded
using the system of FIG. 3. Readout of data stored in storage
medium 220 may be achieved in many different ways. For example, a
phase conjugate reference beam can be used with a reference wave
the size of a single page or stack of holograms (for a plane wave
reference beam this may include a plane wave in the opposite
direction). A reference beam 260 may be introduced and deflected to
the media by a beamsplitter 270. Beamsplitter 270 may include a
standard beamsplitter, a polarizing beamsplitter, or a holographic
optical element that redirects the reference beam 260 to storage
medium 220. The use of an aperture or angle filter (to just pass
the zeroth order or some fraction of the zeroth order) between
beamsplitter 270 and storage medium 220 may be used to get an
appropriate sized beam and filter out the other data pages. Such a
filter or aperture could also be part of the media structure
itself.
[0067] In one example, where the image was stored outside the
storage medium 220 at distance d, a suitable detector 250 can be
placed at the image plane distance and the data page detected
without the need for an optical lens or other optical elements.
Repositioning device controller 252 may be used to vary the
distance between detector 250 and storage medium 220 through a
suitable repositioning device to read different data pages stored
at different distances d in the stack by translating detector 250.
Alternatively or additionally, repositioning device controller may
also translate the position of storage medium 220 through a
suitable repositioning device.
[0068] Further, detector 250 and storage medium 220 are capable of
moving parallel to each other to align detector 250 with other data
pages and stacks of storage medium 220. For example, the other
stacks of holograms may be accessed by moving detector 250 and the
reference beam illumination to the stack that is desired.
Conversely, the media could be moved to the stack location or a
combination of head and media movement as in Compact Disk (CD) or
Digital Versatile/Video Disk (DVD) systems.
[0069] The cost and/or compactness of the exemplary system, for
example, a read drive without a pick-up lens or the like, may be
greatly improved in this manner. It should be understood that the
exemplary system depicted may include various other devices and
elements including optics such as a lens or the like. For example,
various lenses, apertures, filters, gratings, and the like may be
used to image data pages to the detector as are known in the
art.
[0070] In one example, detector 250 includes the same number of
pixels as a single data page stored in storage medium 220. Detector
250 may include an appropriate pixel size depending in part on the
data mask pixel size used to store the data page and the optical
system magnification, if any. In other examples, the detector 250
may include more pixels of a smaller size than the stored image
allowing detector 250 to over sample the stored image. Detector 250
may include any suitable detector such as a CMOS image sensor, a
CCD image sensor, or the like that are sensitive to optical
information. Further, a decoding unit coupled to detector 250 may
determine the corresponding values of the recovered data.
[0071] In various examples, a detector may also include a line
detector CCD or CMOS line sensor. A line detector can be scanned
(physically moved across the image) to readout an entire data page,
which could be the size of the storage medium or any faction of the
storage medium. The data pages could be multiplexed and readout in
a page wise fashion and detected with a line camera. The data pages
can also be detected in a similar fashion with a CMOS or CCD camera
or other such camera that contains an array of pixels to detect the
reconstructed data page.
[0072] The holograms in a stack, i.e., data pages multiplexed at
the same location of storage medium 220, can all be readout by
changing the reference beam in the manner used to store them. For
example, changing the wavelength for wavelength multiplexing or
changing the location of the detector for confocal multiplexing.
Additionally, for confocal multiplexing it is desirable to have a
filter such as an array of pinholes at the image plane, i.e. on the
detector array or at an intermediate image plane, to filter out
noise from the other confocally stored holograms. In an example
where a lens or other optical elements are used during readout, the
filter may be at an intermediate image plane location before the
detector.
[0073] In another example for reading holographic storage media,
e.g., HROM media, various data pages may be readout without using
the phase conjugate readout. This can be done with and without
lenses in the drive as well. For example, with reference again to
FIG. 3, if a hologram or layer is stored at a distance-d (image on
the opposite side of storage medium 220 shown in dotted lines) then
the same reference beam shown in FIG. 3 may be used to readout the
layers of data. The holograms will appear as images a distance-d
from the media (other side of media). Thus the detector can be
place at location-d. Various optical elements including pick-up
lenses may also be used to image or transform as desired and is
well understood by those of ordinary skill in the art.
[0074] Various examples herein may include multiplexing multiple
pages to get to high storage density in a holographic storage media
or holographic master data mask. For example, wavelength, angle,
confocal (storing at different distances from center of media),
peristrophic, polytopic, and the like. Further, one or more of the
multiplexing technique may be combined to increase storage density,
e.g., (wavelength, confocal), (angle, confocal), (wavelength,
polytopic) or all three. Additionally, data page(s) and layer(s)
may be combined with visible image(s) for security purposes and the
like.
[0075] Other exemplary holographic data storage systems that may be
used in conjunction with various aspects of the present invention
include those described in U.S. Pat. No. 5,920,536 entitled,
"Method and Apparatus for Holographic Data Storage System," U.S.
Pat. No. 5,719,691, entitled, "Phase Correlation Multiplex
Holography," and U.S. Pat. No. 6,191,875 entitled, "Process for
Holography Using Reference Beam Having Correlated Phase Content,"
all of which are incorporated herein by reference in their
entirety.
[0076] In another aspect of the present invention, one or more data
masks are recorded in a holographic master data mask. The
holographic master data mask may then be used to record multiple
holographic layers in a holographic storage media, such as HROM
storage media or the like. For example, the holographic master data
mask may be used to holographically record an entire information
layer across a holographic storage medium. Each layer may be
divided into multiple data pages of information to be recorded in
parallel. Multiple layers may be multiplexed onto a medium using
wavelength, angle, peristropic, confocal, polytopic, phase-code, or
other multiplex techniques. In producing the holographic master
data mask, data pages and stacks of data pages can be carefully
aligned and recorded in the holographic master data mask, and the
alignment maintained by using the holographic master data mask to
replicate holographic storage media. For example, rather than
aligning successive data masks, the various data masks to be
recorded may be aligned and then recorded into a holographic master
data mask by wavelength or angle multiplexing. The correct data
mask may be recalled with the appropriate wavelength or angle when
it is desired to be imaged and stored. This makes the interchange
of data masks very quick and easy during the replication process
allowing for fast cycle times. On readout of the media, the
alignment of the pages and stacks of pages created by the
holographic master data mask may be increased such that readout is
improved with minimal time for mechanical alignment and servo in
the drive between different pages of a stack. Additionally, careful
alignment of layers or between stacks of data page in a layer may
be used to detect fake, unauthorized, or pirated storage media. For
example, by intentionally varying the alignment or holding a
particular alignment between page stacks or between layers in a
predetermined way, a signature of a valid storage medium may be
generated in the medium itself
[0077] FIG. 5A illustrates an exemplary system for recording data
masks into the master holographic data mask. In this example,
holographic master data mask 520 is placed near a quasi Fourier
transform plane of data mask 500 to be stored therein. Further, the
exemplary system may include a VanderLugt imaging setup to achieve
the quasi Fourier transform placement. A polytopic filter may also
be included in the exemplary system.
[0078] A plane wave illuminates data mask 500, which may include an
SLM, lithographic data mask, or other suitable object, and stores a
hologram to a portion of holographic master data mask 520. In this
instance, the plane wave passes through a lens 530, e.g., a
converging lens. A plane wave reference beam is also incident on
the holographic master data mask 520 and may be varied, e.g.,
wavelength or angle, to record multiple data masks in holographic
master data mask 520. The system may include a transmission or
reflection geometry. An aperture, angle filter, or phase mask may
also be incorporated into the system of FIG. 5A.
[0079] Holographic master data mask 520 and/or the imaging system
may be moved to spatially multiplex or polytopically multiplex
multiple data pages 510 across the media as shown in FIG. 5B. For
example, the system may align one or more masks 500 at a first
spatial position to record a stack of data pages 510 in holographic
master data mask 520 though one or more multiplexing techniques.
The system may then be repositioned to record another stack of data
pages 510 and so on to complete holographic master data mask 520.
Stored data pages 510 may be imaged to a holographic medium with
the same plane wave reference used to record.
[0080] Placing holographic master data mask 520 near a quasi
Fourier transform plane of data mask 500 may reduce interference
between pages in the same stack during recording and when data
pages 510 are readout, for example, from holographic master data
mask 520 and recorded with an HROM medium. This, in turn, may
reduce interference between pages of the same stack during readout
of manufactured HROM media recorded using the holographic master
data mask 520. More specifically, during readout from holographic
master data mask 520, a Fourier filter may be advantageously
disposed in the Fourier transform plane of the holographic master
data mask 520, close to a surface of an HROM medium or the like to
filter out high order transform components. For example, a 4-F
system may be used to image into a VanderLugt imaging system with a
Fourier filter in the Fourier transform plane of the 4-F system. A
4-F system generally includes two lenses that are separated by the
sum of their focal lengths, with two dimensional input and output
planes located one focal length in front of the lens pair and one
focal length behind the lens pair. Using a 4-F system the Fourier
plane may also be placed inside the media.
[0081] Another advantage of recording HROM media in or near the
Fourier transform plane includes increased locational tolerance of
the HROM media and a detector during readout. The increased
locational tolerance due, at least in part, since the Fourier
transform plane is generally shift invariant. For example, it is
possible for the Fourier transform plane to be out of position,
e.g., by +/-100 microns, in any of x, y, z, and still recover an
image aligned to a detector which has much smaller pixels, e.g.,
.about.10 microns. The shift invariance of the Fourier transform
plane makes the tolerances greater for readout than conventional
systems. In addition, because the information in the Fourier
transform plane is uniformly distributed, media defects such as
scratches and dust are not as severe a problem for data recovery as
when in the image plane.
[0082] In one example, the system as described with reference to
FIGS. 5A and 5B includes a phase mask in the object beam and uses
phase conjugate readout, where the phase mask may provide a degree
of piracy protection. For a good quality reconstruction of the
stored information, the phase mask is desirably the same or nearly
the same phase mask as used when recording, and disposed at the
same or nearly the same location in the system. Knowledge of the
phase mask and phase mask location allows for protection of the
stored information. Further, such protection could be made customer
specific, e.g., by recording information for customer X with one
phase mask (readable with a specific drive) and customer Y with
another phase mask (readable with a different drive). Customer X
would not be able to read customer Y's media without a customer Y
drive (or knowledge of the particular phase mask and location used
in recording).
[0083] In another aspect, an exemplary method for recording
holographic master data masks includes using confocal multiplexing
where each data mask is imaged to a different distance d from the
center of the master storage medium. For example, by varying the
distance of data mask 500 shown in FIG. 5A or the image of data
mask 500 relative to holographic master data mask 520 between
recording different layers. Readout may be confocally detected with
a filter similar to methods performed with confocal microscopes.
Exemplary descriptions of confocal detection are described in U.S.
Pat. Nos. 5,202,875, 6,111,828, and 6,272,095, all of which are
hereby incorporated by reference in their entirety. Confocal
multiplexing techniques may be advantageously combined with volume
holographic multiplexing techniques as described above. One
potential advantage of holographically implementing multiple layers
(image planes) is that the layers do not have to physically be on
the storage medium. For example, with current multilevel CD,
near-field, or high na systems, the layers are on or inside the
media. By imaging or placing the data mask of the data, the layers
can appear to a detector or drive as being located outside of the
storage medium, inside the medium, or on the medium depending on
where the data mask image plane lies during recording. Thus, the
usual problem of compensating for spherical aberration due to the
change in the amount of substrate that the light travels through
may not be necessary and in some instances may be detected without
a pickup lens or lenses.
[0084] An adjustable lens system as shown in FIG. 3, and described
above, for example, may be used for recording to the holographic
master data mask 520 as shown in FIG. 5A. Further, the holographic
master data mask 520 may be readout by a suitable pickup lens that
may translate to image different data pages or layers stored at
various distances d. Alternatively, the image and detector planes
may be moved to record at varying distances. The holographic master
data mask 520 may then be used to holographically record multiple
layers on HROM media using confocal multiplexing. Similarly, a
holographic storage medium, such as an HROM medium, may be recorded
and readout with adjustable lens systems at varying distances from
the medium.
[0085] Copying volume holograms from a holographic master, e.g.,
produced as described with reference to FIGS. 3 and 5A, is
illustrated in FIGS. 14A and 14B for reflection geometry and
transmission geometry (respectively). Generally, a holographic
master 1400 is placed adjacent to a target medium 1420 (e.g., an
HROM device) for replication. A reference beam is directed to
holographic master 1400 and target medium 1420. The reference beam
is sufficient to reconstruct stored holograms in holographic master
1400 and simultaneously store the holograms in target medium 1420.
In particular, target medium 1420 is positioned relative to
holographic master 1400, such that light from both the diffracted
holograms as well as the large reference beam used to read out
holographic master 1400 interfere inside target medium 1420,
resulting in the same set of holograms being recorded into target
medium 1420. There may be some positional offset due to
displacement between holographic master 1400 and target medium 1420
during copying. Further, in the case of transmission geometry, as
shown in FIG. 14B, the reference beam incident on target medium
1420 may be partially distorted or depleted by the hologram
diffraction of holographic master 1400.
[0086] A plurality of holograms intended for target medium 1420 can
be contained in a single master holographic medium, such as master
medium 1400. If the master holographic medium 1400 is recorded by a
sequential page-recording process (e.g., as described above with
respect to FIG. 3), the holograms may have low diffraction
efficiencies for a high capacity storage device, e.g., below 1%.
While relatively weak holograms (e.g., having low diffraction
efficiencies) may be detectable in a holographic readout system,
they are generally inefficient or undesirable for use in a mass
replication process. Generally, optimum holographic recording
occurs when the object beam and reference beam have near equal
intensities. For example, if the object beam intensity from the
master holograms is less than 1% of that of the reference beam, the
efficiency of the recording in the target medium will be poor,
resulting in an inefficient use of the dynamic range of the medium,
long replication cycle times, weak holograms that may reduce the
readout transfer rate, and the like.
[0087] FIGS. 15A and 15B illustrate an exemplary method and system
for manufacturing holographic media, including holographic master
data masks, which may provide for improved storage capacity and
improved hologram diffraction efficiency over previous methods. The
exemplary method generally includes a two-step or two-process
method for manufacturing a holographic master, wherein a set of
holograms or information layers are recorded across a plurality of
holographic submasters in a first process, and the set of holograms
are recorded or transferred from the holographic submasters to a
holographic master in a second process. The master may then be used
to manufacture target devices such as HROM devices or the like to
include some or all of the set of holograms.
[0088] With particular reference to FIG. 15A, a desired set of
information layers are recorded sequentially to multiple
holographic media or submasters 1520a-c in a first process. For
example, at least a first hologram is recorded to a first
holographic medium 1520a, at least a second hologram is recorded to
a second holographic medium 1520b, and so on. In particular, a
light source 1510 provides an object beam directed by beam splitter
1514 and mirror 1518 to a data mask 1500. A lens 1526 directs the
object beam illuminate data mask 1500 and to converge to a Fourier
plane near the first submaster 1520a. Various optical elements and
relay systems (e.g., as described herein or otherwise), may be
included between light source 1510 and the position of submaster
1520a.
[0089] Light source 1510 further directs a reference beam to
submaster 1520a via mirror 1512. A first portion of a set of
holograms desired in a final holographic master medium may be
stored in the first submaster 1520a. The first portion of holograms
may include a plurality of holograms stored in submaster 1520a
through any suitable multiplexing processes. A second submaster
1520b may then be suitable positioned to record a second portion of
a set of desired holograms, followed by submaster 1520c and so on
until all desired holograms are stored in holographic
submasters.
[0090] The example shown in FIG. 15A is for a reflection geometry
and recording near a Fourier plane, but the method can be
generalized for either transmission or reflection geometry and a
variety of recording configurations and multiplexing methods as
will be recognized by those of ordinary skill in the art. For
example, the holograms may be multiplexed with one or a combination
of methods, such as spatial, polytopic, angle, or wavelength
multiplexing methods.
[0091] The set of holograms stored in holographic submasters
1520a-c may then be reproduced and recorded to a single holographic
master 1520 (see FIG. 15B) in a second process, which may be used
as a holographic master data mask, e.g., to record some or all of
the holograms in one or more target devices such as HROM devices or
the like. A master holographic medium (in this example, holographic
master 1520) manufactured in this manner may be particularly suited
for replicating many distribution copies (e.g., HROM devices or the
like). For example, limiting the number of holograms recorded to
each submasters 1520a-c may preserve stronger diffraction
efficiencies in each hologram stored in holographic master
1520.
[0092] As illustrated in FIG. 15B, light source 1510 directs a
reference beam via mirror 1512 to illuminate both submaster 1520a
and master 1520. The holograms stored in submsater 1520a are
reconstructed and interfere with the reference beam, thereby stored
in master 1520. The system illustrated in FIG. 15B for transferring
holograms from submasters 1520a-c to master 1520 may be the same or
different than that of FIG. 15A.
[0093] In one example, a plurality of holograms stored in submaster
1520a are simultaneously read-out by the reference beam, such that
light from the diffracted holograms interferes with the reference
beam inside holographic master 1520, resulting in a plurality of
stored holograms being transferred or copied to holographic master
1520 simultaneously. In one example, this is repeated for the
necessary number of reference beams (for example, with different
angles or wavelengths) to transfer all of the holograms from
submaster 1520a to master 1520.
[0094] Submaster 1520b may then be appropriately positioned
adjacent master 1520 and a similar process as with submaster 1520a
repeated. It is noted that some alignment processes may be
necessary to ensure consistency in the position between different
submasters (e.g., 1520a-c) within a desired tolerance; however,
alignment processes are only used in the process to produce the
holographic master 1520. This is generally preferable to the
alternative case where swapping and alignment is needed for
multiple data masks or holographic masters during a replication
process. Accordingly, with an exemplary holographic master medium
as described, replication of additional devices can be produced
from a single master, where the proper alignment of the holograms
has been previously set during the submaster 1520a-c to master 1520
transfer.
[0095] Additionally, the exemplary methods allow more flexibility
in the positioning of the final master holographic medium and the
replicated or target media relative to the object beam path. With
single step mastering, where all holograms are sequentially
recorded in the master, efficient use of the media dynamic range is
desired to achieve strong holograms for further replication.
Therefore, it is generally optimal to record the holographic master
as close as possible to a Fourier plane of the object beam, where
the overlap between adjacent spatial holograms is minimized This
means, however, that the replicated media must be disposed
elsewhere (i.e., away from the Fourier plane), because the
holographic master and replicated media cannot share the same
physical space during replication. For compact playback drive
systems, for example, it may be desirable to have the replicated
media very close to the Fourier plane of the diffracted hologram to
filter the signal (such as for polytopic multiplexing), while
keeping beam paths as short as possible.
[0096] Accordingly, in the first submastering process (shown
generally in FIG. 15A) of the present example, which is generally
sensitive to overlap of adjacent holograms due to the sequential
recording of multiple holograms, submasters 1520a-c can be recorded
at the Fourier plane to increase the efficient use of the media
dynamic range. In the second process (shown generally in FIG. 15B),
the master 1520 can be flexibly positioned such that the position
does not coincide with the position of submasters 1520a-c or the
desired position of the replicated media (e.g., the target). This
provides increased freedom for the position of the final replicated
media because each of the two mastering steps can be performed with
either forward or conjugate readout of the source media.
[0097] FIGS. 16A-16C illustrates exemplary positioning of a
holographic submaster 1520a, holographic master 1520, and target
medium 1620 in an exemplary mastering and replication processes.
The exemplary method includes recording a hologram or information
layer with submaster 1520a positioned at the Fourier plane of the
object beam as shown in FIG. 16A. The hologram is then transferred
or copied from submaster 1520a to master 1520 by a forward
reference beam as shown in FIG. 16B. Replication of the hologram
from master 1520 to target medium 1620 is shown in FIG. 16C, where
target medium 1620 is positioned near the Fourier plane of the
diffracted hologram from master 1520, in a position that would
generally not be possible with one step mastering due to the
positional overlap with submaster 1520a.
[0098] Additionally, for the case of transmission geometry, each
process of the exemplary method may be performed with the source
(e.g., submaster 1520a in FIG. 16B and master 1520 in FIG. 16C) and
the target medium (e.g., master 1520 in FIG. 16B and target medium
1620 in FIG. 16C) positioned to ensure that a portion of the
reference beam incident on the target media is a portion of the
reference beam that is undepleted by the hologram diffraction, as
shown in FIG. 17. Such a system allows greater control over
relative intensities of the object beam and reference beam used to
record the target media.
[0099] Alignment of the source media and target media in the
various examples described herein may be carried out with various
methods known in the art. One exemplary method for aligning data
pixels in a holographic recording system is described in co-pending
U.S. patent application Ser. No. 11/069,007, filed Feb. 28, 2005
and entitled "Processing Data Pixels in a Holographic Data Storage
System," which is incorporated by reference as if fully set forth
herein. These and other alignment processes are possible both
during recording and during readout of holograms.
[0100] The process of storing a set of holograms across multiple
holographic submaster media and then reconstructing and storing the
set of holograms in a single holographic master may allow for
stronger holograms (e.g., greater diffraction efficiency) to be
achieved in the master and replicated media from the master. Two
factors that generally affect the strength (e.g., diffraction
efficiency) of holograms in a storage medium include the modulation
depth with which the holograms were recorded within the medium and
the number of overlapping hologram exposures in a unit of volume of
the medium.
[0101] The modulation depth m of the index grating of a hologram
can be written as,
m = 2 I S I R I S + I R ##EQU00001##
where I.sub.s and I.sub.R are the object (or signal) beam and
reference beam intensities during recording. The modulation depth
has a maximum value of 1 when the intensities of the object bean
and reference beam are equal. If the intensities are imbalanced,
the modulation depth decreases. For example if the intensity ratio
is 100:1, the modulation depth is about 0.2, meaning that only
about 20% of the dynamic range of the media is effectively
contributing to the hologram index modulation. The rest of the
dynamic range is essentially wasted as if illuminated with a
uniform beam of light. Accordingly, strong holograms with strong
diffraction efficiency in a holographic master are important for
efficient replication, and in turn producing strong replicated
holograms.
[0102] Diffraction efficiency .eta. is related to the dynamic range
(M/#) of the medium and the number of hologram exposures M
overlapped in the media volume. This can be written,
.eta. .apprxeq. ( M / # M ) 2 ##EQU00002##
[0103] If we consider a specific example of recording with a
combination of angle and polytopic multiplexing, as illustrated
generally in FIG. 18, the number of overlapping exposures M can be
further broken down into two components, the number of pages per
book (or stack) N.sub.page, and the spatial overlap factor
N.sub.overlap of neighboring books.
.eta. .apprxeq. ( M / # N page N overlap ) 2 ##EQU00003##
[0104] Accordingly, exemplary methods and systems using multiple
submaster media to record a holographic master medium may increase
or maximize the diffraction efficiency in a number of ways. With
reference to FIGS. 15A and 15B, in the first process of recording
multiple submasters 1520a-c, each submaster 1520a-c may include a
fraction of the total number of pages of each book (or stack), but
all of the book locations. For example, 30 pages could be divided
into 3 submasters 1520a-c with 10 pages each. This effectively
multiplies the available dynamic range, M/#, by the number of
submasters utilized. Additionally, because submasters 1520a-c can
be recorded at the Fourier plane, where the object beam size is
smallest, the spatial overlap, N.sub.overlap, can be minimized or
reduced. For high NA systems, even recording at a small offset from
the Fourier plane can raise the overlap factor dramatically.
[0105] In the second process (e.g., transferring holograms to the
master medium), a broad planewave reference beam with a certain
incidence angle may be used to transfer all hologram pages with
that reference angle from all books in the particular submaster
1520a-c to master 1520 simultaneously. Only one holographic master
1520 is used, so there is no multiplier for the dynamic range M/#.
However, the spatial overlap, N.sub.overlap, effectively becomes 1,
because spatially neighboring (polytopic multiplexed) pages are not
exposed separately in this step. All spatial holograms with the
same reference angle are recorded in parallel to the master 1520,
effectively as one hologram.
[0106] The desirability of the exemplary methods may be illustrated
by considering a polytopic-multiplexed holographic system using a
1.5 mm thick recording medium with a dynamic range (M/#) of 45,
which is typical for the current state of the art holographic
recording medium. Using a lens of reasonably high NA, e.g., around
0.50, to Fourier transform the object beam, and spacing adjacent
hologram stacks by a Nyquist area factor of 1.2 (or 1.08 linear
factor), the overlap factor N.sub.overlap may be about 20, even
when recording exactly at the Fourier plane of the object beam.
Recording with the medium even 4-5 mm offset from the Fourier
plane, N.sub.overlap quickly increases to 100 or more due to the
fast divergence of the high NA transform. If each hologram stack
contains N.sub.page=40 holograms (such as by angle or wavelength
multiplexing), we can estimate the hologram diffraction
efficiencies by the above equation to be about 0.3% when recorded
exactly at the Fourier plane, and only about 0.01% when recorded a
few millimeters off the Fourier plane. If, in addition to recording
at the Fourier plane, the hologram is further divided into pages of
5 submasters as described herein (effectively raising the available
M/# to 5.times.45=225), the diffraction efficiencies can be raised
up to about 8%.
[0107] With the 2-step mastering method described herein, the
holograms stored in these multiple submasters are the reconstructed
and recorded to a single holographic master. Achieving
approximately 8% diffraction efficiency from the submasters, the
modulation depth for recording in the master is about 0.5, such
that the effective M/# available in the master is half the original
value of 45. The overlap factor can be ignored when copying the
polytopically multiplexed pages in parallel to the master such that
40 pages can be recorded to the master with an increased
diffraction efficiency of over 30%. This higher diffraction
efficiency allows even higher modulation depth for the replication
process, such that the diffraction efficiencies could be amplified
yet again to the final replicated copies, theoretically even close
to 100%.
[0108] In contrast, a master produced without the use of
holographic submasters as described, e.g., using a single
holographic medium and recording the holograms page by page, the
diffraction efficiencies of the holograms in the master will be
well under 1% at reasonably high data storage densities (e.g., 0.3%
in the above example, even when recording at the Fourier plane). At
0.3%, the modulation depth for replicating from such a master will
be about 0.1, meaning that only 10% of the M/# of the master media
is effectively available. For 40 pages, the diffraction
efficiencies of the final replicated holograms will then be limited
to under 2% diffraction efficiency.
[0109] The diffraction efficiency of the final replicated media is
important for enabling fast data transfer rates in the playback
drive, especially in low-power consumer devices where the available
readout laser power may be limited. Media produced by the method of
this invention may have increased hologram diffraction by over an
order of magnitude in intensity, with a corresponding reduction in
the required integration time of the detector, and hence increase
in achievable transfer rate. Accordingly, in one example, the
average diffraction efficiency of a set of holograms recorded in a
master (or the target media) is greater than 5%, where the areal
storage density of the holographic recording material is greater
than 10 bits/um.sup.2. In another example, the average diffraction
efficiency of the set of holograms is higher than (M/N).sup.2,
where M is the dynamic range (M/#) of the storage medium without
the plurality of holograms (e.g., the blank medium prior to
recording) and N is the average number of overlapping holograms
recorded the storage medium.
[0110] Accordingly, recording a desired set of holograms to more
than one piece of holographic media, e.g., multiple submasters, in
a first process may allow stronger holograms (e.g., greater
diffraction efficiencies) to be recorded in each individual
submaster than if they were all recorded sequentially into a single
medium. The stronger holograms generally allows more optimal beam
intensity ratio between the object and reference beams for
transferring the holograms from the multiple submasters to a
master, and more efficient use of the dynamic range of the master.
Also, because the second process transfers many holograms in
parallel from the submaster to the master, the effective number of
hologram recordings is reduced. The combination of these benefits
may allow hologram strengths to be amplified from the submasters to
the holographic master.
[0111] With reference now to FIG. 6, an exemplary system and method
for recording with a holographic master data mask 520 (which may be
manufactured as described with reference to FIG. 5 or as described
with reference to FIGS. 15A and 15B) to store data in holographic
storage medium 620, e.g., HROM storage media. The holographic
master data mask 520 may be illuminated with a plane wave beam and
multiplexed depending on how the data masks where stored therein,
e.g., wavelength multiplexed, to image various data masks onto
holographic storage medium 620. The holographic master data mask
520 is thus the data source that is recorded in the replicated
holographic storage medium 620. The plane wave, encoded by a data
mask of holographic master data mask 520, may be imaged, relayed,
or propagated to holographic storage medium 620 by lens 630. Lens
630 may include a high performance photolithographic lens.
Alternatively, the holographic master data mask 520 may be placed
in proximity to the storage medium 620 and recorded without a lens
(natural beam propagation). Holographic storage medium 620 is
further illuminated with a plane wave reference beam to interfere
with the encoded beam and store the data from holographic master
data mask 520. It should be recognized that various other systems
and configurations are possible and contemplated to record with
holographic master data mask 520.
[0112] An advantage of a Fourier transform plane in recording the
holographic master data mask 520 includes that large pixels can be
used and a small hologram may be recorded in the holographic master
data mask 520 and then copied into the replicated holographic media
620. Additionally, an advantage of a VanderLugt imaging system is
that it can form an image outside the storage medium 620 and with
the information layer in the quasi Fourier plane (small) that may
be readout with a phase conjugate reference beam or with a standard
reference beam. The present VanderLugt system is illustrative only
and any system that results in a Fourier Transform or quasi Fourier
Transform plane will include similar advantages. A VanderLugt
system is desirable in part because if a phase conjugate beam is
used for readout, lenses are not required in a drive.
[0113] FIG. 7 illustrates a phase conjugate readout of holographic
storage medium 620 created with the exemplary system of FIG. 6,
e.g., including a VanderLugt imaging system. A detector 250 may be
placed opposing holographic storage medium 620. In this example, a
Holographic Optical Element (HOE) grating 780 is placed adjacent
detector 250. The reference beam illuminates HOE grating 780 from
the side and is directed from HOE grating 780 to the storage medium
620 thereby providing the phase conjugate beam 790. A phase
conjugate image 792 is reflected back to detector 250 from storage
medium 620 for readout. Use of HOE grating 780 or other similar
grating to introduce the readout reference beam into the optical
path allows for readout in a relatively compact system.
[0114] In other examples, standard beamsplitters, polarizing
beamsplitters, waveplates, gratings, and the like at various angles
may also be used to introduce the reference beam into the system as
will be recognized by those of ordinary skill in the art.
[0115] Another exemplary method for recording holographic storage
media that may be readout without the use of a lens includes
storing multiple image planes or layers at a distance from the
holographic storage medium located outside of the storage medium as
described above with reference to FIG. 3. A detector may be
advantageously placed at the distance the image was stored and
detect a portion or the entire stored image, e.g., an entire data
page or more in parallel. In one example, each storage layer of a
holographic storage medium is imaged to the same distance `d` from
the data mask as shown in FIG. 3. In an example where the
holographic storage medium includes a holographic master data mask,
each level of a holographic storage medium recorded with the
holographic master data mask, e.g., an HROM, may be recorded such
that the holographic master data mask is imaged to the same
distance d from the HROM.
[0116] The exemplary method does not require a lens during readout
of the holographic image from the storage medium. Specifically, a
detector may be placed at distance d from the holographic storage
medium, and an image may be readout directly to the detector
without the need for a lens. A further advantage includes that an
entire page of data can be readout to a detector in parallel. By
imaging off the media on or before the image plane, the normal
holographic reconstruction is then naturally imaged without the
need for a lens. For example, if an image plane of the system is
before the media and no lenses are disposed there between, a phase
conjugate wave may be used to generate an image without a lens at
that image plane for readout. If the drive has a detector at that
image plane the pages in that stack can then be detected and the
information in that stack of pages readout. Other stacks are
addressed by moving the camera relative to the media to the other
stacks that were recorded on to the media during replication. In
other examples, the use of a phase conjugate wave may be used for
readout with a lens or optical imaging elements.
[0117] Another exemplary recording method for holographic storage
media such as HROM media or holographic master data masks includes
polytopic multiplexing. Polytopic multiplexing, described in the
above referenced U.S. Patent Application No. 60/453,529, which is
incorporated by reference herein, is an exemplary method to
increase the layer density by making the individual data page
stacks smaller spatially. Polytopic multiplexing may be used in
conjunction with wavelength, angle, fractal, or other holographic
multiplexing techniques. Polytopic multiplexing methods allow
holograms to be spatially multiplexed onto a holographic material
with partial spatial overlap between neighboring holograms and/or
stacks of holograms. Each individual stack of data pages may
additionally take advantage of an alternate multiplexing scheme
such as angle, wavelength, phase code, peristrophic, or fractal
multiplexing. Generally, the holograms are separated by an amount
equal to the beam waist of the data beam writing the hologram.
Unlike more traditional approaches, however, the beam waist is
intentionally placed outside of the holographic media such that
there is significant beam overlap between stacks inside the media.
Upon reconstruction, the data pages and its neighboring data pages
will all be readout simultaneously, however, an aperture (filter)
placed at the beam waist of the reconstructed data filters out the
neighbors that are readout such that only the desired data is
detected. Polytopic multiplexing may be implemented with the beam
waist inside of the media with the aperture(s) placed at another
Fourier plane or by using an angle filter that filters out the
neighboring data page reconstructions.
[0118] The holograms can be multiplexed by combinations of the
standard multiplexing techniques as well as by polytopic
multiplexing. Significant increases in storage density may be
achieved particularly for thick media and high numerical aperture
optical systems where spot sizes are relatively small. Previous
techniques were limited in their ability to spatially separate
stacks of holograms due to significant amounts of beam divergence.
High numerical aperture optics, despite the small beam waist,
expand very quickly. The spatial separation of stacks in the prior
art was generally limited by the size of the beam in the media
after expansion and not limited by the spot size. Up to this point,
there has generally been a tradeoff between media thickness (and
therefore dynamic range) and the lens numerical aperture.
Therefore, there is saturation in achievable capacity that can be
obtained for any holographic storage system using the approaches
given by the prior art. The polytopic methods described here allow
a system to fully utilize high numerical aperture lenses and
independently choose the media thickness and therefore gain in both
bit density and media dynamic range.
[0119] FIG. 8 illustrates an exemplary readout system using a phase
conjugate reference beam that may filter or block unwanted
reconstructions. As shown in FIG. 8, a block 820 at the beam waist
802 will block out the unwanted reconstructions as reference beam
806 is incident on holographic storage medium 800. FIG. 8 shows a
Fourier transform arrangement, including a detector 830, Fourier
transform lenses 832, 834, a beam splitter 836, and SLM 838. It
should be recognized, however, that this system could be used with
an imaging system as well.
[0120] In this example the full multiplexing (angle, wavelength,
etc) of the stack can be used and the stacks can be placed a
minimal distance from each other that is not determined by the
thickness of the medium 800. This allows for increased use of thick
films (>100 microns) and is particularly useful for films
greater than 400 microns to many millimeters.
[0121] In addition, the stacks do not have to be recorded fully at
one time. For material reasons it may be desirable to partially
fill the neighboring stacks and then come back and completely fill
a stack as the neighborhood of the stack is recorded. See, e.g.,
U.S. patent application Ser. No. 09/588,908, "Process for
Holography Involving Skip-Sorted Hologram Storage," which is hereby
incorporated by reference as if fully set forth herein.
[0122] The exemplary method may also be used with high numerical
aperture lenses or holographic optical elements that function as
lenses. The holograms can be recorded in a reflection or
transmission holography geometry process. It should be further
recognized by those of ordinary skill in the art that the exemplary
system and polytopic methods may be employed without a phase
conjugate reference beam.
[0123] FIGS. 9A and 9B illustrate another exemplary recording and
reading method. In this example, polytopic multiplexing is used
with phase conjugate readout beam to enhance high order filtering,
where the data encoded signal beam is focused in front of the
medium 900 as opposed to behind the medium 900 as previously
discussed. The focus location is where the beam block 920 is now
placed. This has the additional benefit of blocking unwanted higher
orders diffracting from a SLM or data mask. The beam then
propagates into the media and is holographically recorded with the
reference beam as shown in FIG. 9A. Multiplexed holograms may then
be recorded at one location by a Bragg multiplexing method (angle,
wavelength, etc). Media 900 is moved by an amount given by the
dimension of the beam waist and then another stack may be recorded.
Upon readout, the phase conjugate readout is used as the input
reference beam and this reconstructs a backwards-propagating signal
beam that retraces the path of the original recording to detector
950. The block 920 separates out the unwanted reconstructions from
the desired signal beam.
[0124] The exemplary methods may be implemented with Fourier plane
or image plane filtering at or near the planes. It may also be
implemented with a single lens, two or more lens, or with a
VanderLugt imaging system that has no lens when reading out. This
may also be relayed with a quasi Fourier plane filter in the relay
and the Fourier plane of the hologram plane near or in the storage
media.
[0125] A detector for detecting a stored image without a lens,
e.g., similar to that described with reference to FIGS. 4, 5A, 7,
9A, and 9B, is generally matched for certain imaging specifications
associated with the stored image, which may be determined by the
SLM used to modulate the object beam and store the image. For
example, the pixel size and pitch of the SLM or data mask are
matched to a sensor array of the detector for a pixel-matched
readout. Alternatively, the pixel size and pitch may correspond to
a desired ratio for an over-sampled readout. Typically, CMOS
detector technology can currently provide arrays with pixel pitch
on the order of 6.mu.m, whereas SLM devices typically have pixel
pitches of 10.mu.m or higher, such that finding an exact match of
SLM and camera devices is not always possible or cost effective.
Accordingly, in some applications it may be desirable to magnify an
object image to an output image size to be stored with a suitable
holographic medium; for example, to match a desired read out size
for a desired detector characteristic, sampling characteristic, or
the like.
[0126] FIG. 10 illustrates an exemplary holographic recording
system that includes a relay system 1030 for relaying an input
object (e.g., a data mask or other means for modulating an object
beam) to a magnified or demagnified output image. In this
particular example, relay system 1030 is included in the object
beam path to reduce the size of the object image, e.g., a data mask
1000, to a desired magnified image size for storage in holographic
medium 1020. The hologram may thereafter be reconstructed and
readout at the magnified size. Further, the output image of relay
system 1030 may converge to a Fourier plane, where the output image
may be real or virtual, and may be on either side of the output
Fourier plane. The exemplary system including relay system 1030 may
further facilitate an intermediate Fourier plane disposed prior to
the output Fourier plane where a filter or aperture may be placed,
for example.
[0127] Generally, an object beam is propagated from light source
1010 (e.g., a coherent light source) and directed via beamsplitter
1014 and mirror 1018 to lens 1026. Lens 1026 directs the object
beam to data mask 1000 to encode the object beam with an
information layer, which may include one or more pages of data as
described herein. Additionally, lens 1026 directs the object beam
to an intermediate Fourier plane positioned prior to relay system
1030 and the output Fourier plane, where filter 1002 may be placed
(as discussed in greater detail below).
[0128] Relay system 1030 is configured in this example, to
negatively magnify the modulated objected beam to a smaller output
magnified image. Various magnifications or demagnifications are
possible and contemplated. For example, the magnification of the
object image may be between 0.1 and 10.0. Additionally, in some
examples, relay system 1030 may be operable to relay the object
beam without magnification (e.g., a magnification of 1.0). Such a
system may benefit from the position and configuration of the
intermediate and/or output Fourier planes (e.g., with the use of a
phase mask and/or filter).
[0129] The output image of relay system 1030 is directed to
converge at an output Fourier plane located near or within
holographic storage medium 1020. The output image interferes with a
reference beam at storage medium 1020, thereby storing the output
image within storage medium 1020. In this example, the reference
beam is directed from common light source 1010 and directed via
beamsplitter 1014 and mirror 1019 to storage medium 1020. It is
noted that FIG. 10 is drawn for the specific case of transmission
geometry, but the examples described are equally applicable for
both transmission and reflection geometry holographic recording
systems as will be recognized by those of ordinary skill in the
art.
[0130] In one example, a filter 1002 (e.g., for polytopic
multiplexing or the like) may be placed at the intermediate Fourier
plane to filter the object beam, thereby allowing the output
Fourier plane to be located inside or very close to storage medium
1020. Typically, holograms cannot be recorded at or very close to
the Fourier plane of the image if a filter (e.g., for polytopic
multiplexing) is used near the Fourier plane. See, for example, the
system described with reference to FIG. 9A. Recording away from the
minimum waist of the object beam, however, results in relatively
larger holograms, which lowers storage density or increases overlap
between neighboring holograms leading to less efficient use of the
dynamic range of the media. Thus, placing filter 1002 at the
intermediate Fourier plane allows more freedom in the placement of
the output Fourier plane with respected to storage medium 1020.
[0131] In one example, filter 1002 includes an aperture placed at
the intermediate Fourier plane. The aperture includes at least one
dimension (e.g., diameter, width, height, etc.) having a size
between 0.5 to 4.0 times the Nyquist aperture size for polytopic
multiplexing (e.g., where the Nyquist aperture size is the smallest
aperture dimension satisfying Nyquist's sampling condition such
that the spatial frequency sampling on the data mask 1000 is twice
the maximum spatial frequency allowed to pass the aperture
dimension). Those of ordinary skill in the art will recognize that
various other filters and/or aperture dimensions may be placed at
or near the intermediate Fourier plane for various other
applications and design considerations.
[0132] In one example, the illumination of data mask 1000 in the
system of FIG. 10 may have a lower Numerical Aperture (NA) than the
desired output NA of relay system 1030. This feature may be
desirable, for example, in systems employing SLMs that are designed
for a specific illumination angle, and may have deleterious effects
(such as lower contrast) if illuminated at angles which vary from
an expected angle. In one example, the output NA of the relay
system is greater than 0.2, and the NA of data mask 1000 or data
mask is less than 0.2. Further, in one example, the output image
has less than 0.5 waves of aberration and 1% distortion.
[0133] In contrast, conventional imaging systems such as a Z1-Z2,
4-F, and Vanderlugt imaging systems (shown generally in FIGS.
11A-11C respectively) do not generally perform well at high NA over
a large field of view, and are generally not optimized for
preserving the phase of the image, such as the converging shape of
the beam after the image, while at the same time maintaining low
aberrations at both the output image and Fourier planes. For
example, imaging systems are typically designed with the primary
intent of duplicating the intensity of the image, without strict
concern for its phase.
[0134] With reference again to FIG. 10, the exemplary recording
system including relay system 1030 may further allow for insertion
of a reference beam in close proximity to the output image (e.g.,
relative to a recording system without relay system 1030). The
addition of relay system 1030 allows the physical data mask 1000 to
be moved away from the storage medium 1020, which may allow
additional space in the vicinity of the magnified image. For
example, a phase mask (not shown) may be disposed in the object
path, if desired, and located precisely at the output image plane,
thereby more evenly distributing the energy at storage medium 1020
with minimum degradation to image quality. In other examples, a
phase mask may be located at data mask 1000.
[0135] It is noted that the addition of the relay system 1030 to
the recording system does not add complexity to the readout methods
and systems. For example, a readout system does not require relay
system 1030 for reconstructing holograms stored with storage medium
1020, and may be achieved as described herein with various optical
system, without a lens (e.g., a conjugate readout), or the
like.
[0136] The optical relay system shown in FIG. 10 illustrates an
exemplary relay system that creates a magnified (in this example,
demagnified) real image of the object prior to the output Fourier
plane. In other examples, the output image may be real or virtual,
and may be located on either side of the output Fourier plane. FIG.
12A-12C illustrate exemplary relay systems 1230a-c for use with a
holographic recording system to magnify an object image, such as
the holographic recording system described with reference to FIG.
10. In particular, FIG. 12A illustrates an exemplary relay system
1230a that creates a magnified real image 1201a of the object 1200a
prior the output Fourier plane. FIG. 12B illustrates an exemplary
relay system 1230b that creates a magnified virtual image 1201b of
the object 1200b prior the output Fourier plane. FIG. 12C
illustrates an exemplary relay system 1230c that creates a
magnified real image 1201c of the object 1200c after the output
Fourier plane.
[0137] Relay systems for achieving a magnified image and output
Fourier plane may be achieved by numerous constructions of optical
elements, including for example, a combination of one or more
lenses, prisms, and other suitable optical elements. FIG. 13
illustrate one exemplary lens design simulation for a relay system
with a high output NA of 0.5 and magnification of -0.5 times (i.e.,
demagnification of the object image by 50 percent), while achieving
imaging performance of 0.02 waves of error (peak to valley) and
0.02% distortion. The input NA is 0.25, half that of the output NA,
and the design allows for access to an intermediate Fourier plane
in front of the relay system, for insertion of an aperture such as
for polytopic multiplexing or the like. Those of ordinary skill in
the art will recognize that a relay system as described herein may
be achieved in a numerous manners, including various combinations
of optical elements, to achieve the desired functions.
[0138] Additionally, holographic recording systems and methods,
including a relay system as described herein, may further carry out
holographic recording via one or more of polytopic multiplexing,
angle multiplexing, wavelength multiplexing, shift multiplexing,
correlation multiplexing, confocal multiplexing, or
peristrophically multiplexing holograms.
[0139] The above detailed description is provided to illustrate
exemplary embodiments and is not intended to be limiting. It will
be apparent to those of ordinary skill in the art that numerous
modification and variations within the scope of the present
invention are possible. For example, various methods of recording
in holographic media and reading from holographic media may be used
in holographic storage systems in isolation or in combination with
other methods. Additionally, the apparatus and methods described
herein should not be limited to any particular holographic storage
system, for example, the methods and systems are generally
applicable with various system configurations and multiplexing
methods. Accordingly, the present invention is defined by the
appended claims and should not be limited by the description
herein.
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