 |
Claims  |
|
|
What is claimed is:
1. A digital camera apparatus, comprising:
(a) imaging means for converting optical information to pixel data and
generating a digital image therefrom;
(b) input signal processing means, operatively coupled to said imaging
means, for compressing said digital image, dividing said digital image
into a plurality of data pages, and formatting said plurality of data
pages;
(c) a holographic storage and retrieval subsystem including;
(i) an optical storage medium;
(ii) a signal beam directed at said optical medium;
(iii) means for encoding said plurality of data pages into said signal
beam;
(iv) a reference beam directed at said optical medium; and
(v) means for controlling said reference beam;
(d) storage control means, operatively coupled to said input signal
processing means and said holographic storage and retrieval subsystem, for
directing operation of said means for controlling said reference beam to
multiplex said plurality of data pages within said optical medium; and
(e) user interface means, operatively coupled to said imaging means and
said storage control means, for adding non-image data to said plurality of
data pages.
2. The digital camera apparatus of claim 1, further comprising retrieval
control means, operatively coupled to said holographic storage and
retrieval subsystem, for further directing operation of said means for
controlling said reference beam to recover said multiplexed data pages
from said optical medium.
3. The digital camera apparatus of claim 2, further comprising output
signal processing means, operatively coupled to said retrieval control
means, for interpreting said formatting of said plurality of data pages,
assembling said data pages into image data and decompressing said image
data.
4. The digital camera apparatus of claim 3, further comprising a display
operatively coupled to said output signal processing means.
5. The digital camera apparatus of claim 2, further comprising means for
positioning said optical storage medium, said means for positioning said
optical medium operatively coupled to said storage control means and said
retrieval control means.
6. The digital camera apparatus of claim 2, further comprising a detector
operatively coupled to said retrieval control means via a detector
interface unit, said detector positioned to detect light of said reference
beam diffracted from said optical storage medium.
7. The digital camera apparatus of claim 1, wherein said optical storage
medium is removable.
8. The digital camera apparatus of claim 1, wherein said means for encoding
said plurality of data pages comprises a spatial light modulator, said
spatial light modulator operatively coupled to said storage control means
via a spatial light modulator interface unit.
9. The digital camera apparatus of claim 1, wherein said means for
controlling said reference beam comprises a movable beam deflector, said
movable beam deflector operatively coupled to said storage control means
via a deflection unit, said beam deflector positioned in said reference
beam.
10. The digital camera apparatus of claim 1, wherein said input signal
processing means further comprises:
(a) means for adding redundant data error to said plurality of data pages
for error correction coding; and
(b) means for applying modulation coding to said plurality of data pages.
11. The digital camera apparatus of claim 1, wherein said signal beam and
said reference beam are incident on a single side of said optical storage
medium.
12. The digital camera apparatus of claim 1, wherein said signal beam and
said reference beam are incident on opposite sides of said optical storage
medium.
13. A method for recording digital holographic images, comprising:
(a) capturing a subject image;
(b) acquiring non-image data;
(c) coupling said subject image with said non-image data;
(d) digitizing said subject image and non-image data to form a digitized
subject image;
(e) compressing said digitized subject image and said non-image data;
(f) dividing said digitized subject image and said non-image data into at
least one digital image page;
(g) loading said digital image page into a spatial light modulator;
(h) encoding said digital image page into a signal beam;
(i) providing a reference beam; and
(j) storing said digital image page in an optical storage medium.
14. The method of claim 13, wherein said storing said digital image page
comprises:
(a) directing said signal beam into said optical storage medium;
(b) directing a reference beam into said optical storage medium; and
(c) recording interference of said signal beam and said reference beam in
said optical storage medium.
15. The method of claim 13, further comprising formatting said digital
image page.
16. The method of claim 15, wherein said formatting comprises data
interleaving, designation of data sections, and defining of pixel regions.
17. The method of claim 13, further comprising performing error correction
coding on said digital image page.
18. The method of claim 17, further comprising performing modulation coding
on said digital image page.
19. The method of claim 17, wherein said performing error correction coding
comprises adding redundant bits to said digital image page, said redundant
bits configured to allow correction of bit errors according to said error
correction coding.
20. A digital holographic camera apparatus operable in the manner of a
conventional film camera, said digital holographic camera apparatus
comprising:
(a) an image capture unit configured to generate a digital image from a
subject image;
(b) a non-image data acquisition unit for acquiring non-image data;
(c) an input signal processing unit, operatively coupled to said image
capture unit and said non-image data acquisition unit, and configured to
compress said digital image and format said digital image and non-image
data into a plurality of data pages;
(d) a holographic storage and retrieval unit including;
(i) an optical medium;
(ii) a signal beam directed at said optical medium;
(iii) a spatial light modulator positioned in said signal beam and
configured to encode said plurality of data pages into said signal beam;
(iv) a reference beam directed at said optical medium; and
(v) a beam deflector positioned to control incidence angle of said
reference beam with respect to said optical medium; and
(e) a storage control unit, operatively coupled to said input signal
processing unit and said holographic storage and retrieval unit, said
storage control unit configured to control said beam deflector for
multiplexing said plurality of data pages within said medium.
21. The digital camera apparatus of claim 20, further comprising a
retrieval control unit, operatively coupled to said holographic storage
and retrieval subsystem, and configured to further direct operation of
said beam deflector to recover said multiplexed data pages from said
optical medium.
22. The digital camera apparatus of claim 21, further comprising a output
signal processing unit, operatively coupled to said retrieval control
unit, and configured to interpret said formatting of said plurality of
data pages, assemble said data pages into image data and decompress said
image data.
23. The digital camera apparatus of claim 20, wherein said optical storage
medium is removable.
24. A digital holographic image reader comprising:
(a) a holographic storage and retrieval subsystem including;
(i) a reference beam;
(ii) a removable optical storage medium positioned in said reference beam
for capturing multiple images with variable image resolution as
multiplexed data pages, at least one of said data pages having a first
resolution and at least one of said data pages having a second resolution
higher than said first resolution;
(iii) means for controlling said reference beam; and
(iv) a detector positioned in said reference beam after said optical
storage medium;
(b) retrieval control means, operatively coupled to said holographic
storage and retrieval subsystem, for directing operation of said means for
controlling said reference beam to recover said multiplexed data pages
from said optical medium; and
(c) output signal processing means, operatively coupled to said retrieval
control means, for interpreting formatting of data pages recorded in said
optical storage medium and assembling said data pages into image data and
decompressing said image data. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to digital cameras. More specifically, the
present invention relates to a digital camera system employing digital
holographic storage.
2. The Prior Art
Digital cameras are known in the art. Most such cameras employ
charge-coupled device (CCD) or complementary metal oxide semiconductor
(CMOS) imager arrays for generating image data which are then stored in
digital semiconductor memory for later readout and viewing.
Holographic camera systems that use analog image storage are known in the
art. U.S. Pat. No. 5,101,397 to Banjo discloses a method for recording
signals on and reading signals from film. U.S. Pat. No. 5,144,461 to Horan
discloses a portable holographic recording apparatus. U.S. Pat. No.
4,707,053 to Gurevich et al. discloses a recording device. U.S. Pat. No.
5,515,183 to Hashimoto discloses a real-time holography system. U.S. Pat.
4,735,474 to Allon discloses a photograph booth with automatic holographic
camera. U.S. Pat. No. 4,376,950 to Brown et al. discloses a
three-dimensional television system using holographic techniques.
BRIEF DESCRIPTION OF THE INVENTION
According to a first aspect of the present invention, a digital holographic
camera device, medium, reader, and system for recording photographs
digitally and transferring the data to a computer or other digital device
is disclosed. Preferably, a storage device, which comprises a removable
cartridge containing a recording medium, is placed inside a digital
holographic camera. Alternatively, a fixed medium may be employed. The
medium is situated inside the camera in an appropriate manner so that
digital data can be recorded onto it holographically. The medium comprises
a photosensitive material capable of digital holographic recording, e.g. a
photopolymer or photorefractive crystal. The medium can also comprise
other layers, such as a substrate and protective layers.
Generally, picture data are first converted to a bit stream, which is then
recorded onto the medium digitally using holographic storage techniques.
The data can be buffered in solid-state memory prior to holographic
recording. The holographic recording subsystem of the present invention
can comprise, for example, a low-power laser and a spatial-light
modulator. Multiple holograms can be recorded using techniques that
include angular multiplexing, wavelength multiplexing, phase encoded
multiplexing, and related multiplexing techniques that allow the storage
of multiple holograms at the same spatial location. Additionally, multiple
holograms can be recorded at multiple spatial locations. This can be
facilitated, e.g., by including a media advance mechanism in the storage
device, by designing the cartridge to facilitate media advance by the
camera, or by optical beam steering. Linear and rotary advance mechanisms
are suitable. The storage device may also take the form of a monolithic
card.
The digital holographic camera according to the present invention can be
designed for data recording only. The camera can also be designed to have
readout capability to output recorded data directly. In the first case,
the removable holographic storage device can be transferred to a dedicated
reader. The removable holographic storage media cartridge, when used, is
designed to transfer from the camera to the reader in a manner that keeps
the media intact during the transfer i.e., light tight. The reader can
have a media delivery mechanism, a low-power laser, an imaging lens as
needed, and a detector array such as a CCD array or a CMOS detector array.
Certain output configurations require an output imaging lens, and others
do not. The reader can connect to a computer as a peripheral device or may
be integrated into a computer. The reader can also be integrated directly
into other devices, such as a printer dedicated to printing out
photographs, so that the media can be read without the use of a computer.
In addition to photo data, the high capacity afforded by holographic data
storage allows a host of other types of data to be stored digitally
integrated with the photo data. Examples include voice annotation,
date/location/exposure conditions/etc., and other information that
describes the circumstances of the photo, as well as other data not
necessarily related to the circumstances of the photo, such as music.
The data can be organized using variable formats in which a portion of the
storage media records data using low resolution pixels and the rest of the
media uses higher resolution pixels. An advantage of this process is that
the level of resolution of a large section of the material can be
identified by a dedicated low resolution region, so that as higher
resolution cameras, media, and readers are introduced, low resolution
media are compatible with high resolution media. The system of the present
invention can use switch-out program cards to control operation, e.g.
annotation, resolution control.
Using one or more flip-mirrors or additional beam splitters, the output
light can alternately be directed to a view finder to view the recorded
data.
From a user's perspective, the digital holographic camera of the present
invention can operate in a manner similar to a conventional film camera.
The capacity of each cartridge can be based on current film packaging,
which emphasizes film speed and count. For example, cartridges can be
differentiated according to resolution in a manner similar to the
distinction between high-speed and low-speed film, which generally have
lower and higher resolution, respectively. By providing predetermined
picture counts, e.g. 24 or 36, required capacity is based on count and
resolution. The picture count can potentially be increased beyond levels
available for typical film cartridges (e.g. 48, 64) in order to increase
the attractiveness of the technology. Capacities on the order of 128
Mbytes would provide a distinct advantage over present capacities of 8-16
Mbytes for current flash memory cameras.
The present invention provides the ability to use low-cost, high-capacity
media in a variety of form factor and data formats.
The present invention further provides the ability to incorporate
high-capacity annotation, such as voice annotation, in digital form.
Additional advantages of the present invention include use of a digital
page-based system in a digital camera; ability to play back data in the
camera or separately outside the camera; possibility to reuse some camera
components for image acquisition and display for page-based holographic
storage.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
FIG. 1 is a block diagram of a digital holographic camera system according
to a presently contemplated embodiment of the present invention.
FIGS. 2A ands2B are diagrams of alternate embodiments of media and media
cartridges that may be used with the digital holographic camera system of
the present invention.
FIG. 3 is a block diagram of an example of a digital holographic camera
including holographic storage and retrieval according to the present
invention.
FIG. 4 is a diagram of an illustrative digital holographic storage
subsystem suitable for recording images in the camera of FIG. 3 according
to the present invention.
FIG. 5 is a diagrammatic representation of a page of digital holographic
image data such as would be produced by the system of the present
invention.
FIGS. 6A through 6D are diagrams of alternate embodiments of digital
holographic media recording subsystems that may be usefully employed in
the present invention.
FIGS. 7A and 7B are flow diagrams illustrating the storage process for
digital holographic images according to the present invention.
FIG. 8A is a block diagram of an illustrative embodiment of a digital
holographic media reader according to the present invention.
FIG. 8B is a block diagram of an illustrative embodiment of a digital
holographic media reader according to the present invention further
including a subsystem that utilizes the images.
FIGS. 9A through 9E are block diagrams of illustrative alternate
embodiments of holographic retrieval subsystems to read out image data
according to the present invention.
FIGS. 10A and 10B are flow diagrams illustrating the readout process for
digital holographic images according to the present invention.
FIG. 11 is a block diagram of a digital holographic camera according to the
present invention further including an electronic storage subsystem.
FIG. 12 is a block diagram of an alternate embodiment of a digital
holographic camera according to the present invention.
FIG. 13 is a diagram of an illustrative integrated image capture and
holographic and retrieval subsystem for use in the digital holographic
camera of FIG. 12.
FIG. 14 is a diagram of a holographic retrieval subsystem that may be
employed with a digital holographic camera according to the present
invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Persons of ordinary skill in the art will realize that the following
description of the present invention is illustrative only and not in any
way limiting. Other embodiments of the invention will readily suggest
themselves to such skilled persons.
Referring first to FIG. 1, a generalized block diagram of the digital
holographic camera and reader system 10 of the present invention is
illustrated. According to a presently preferred embodiment of the
invention, the camera system 10 includes a camera 12 employing a digital
holographic storage system as will be further disclosed herein. Camera 12
utilizes a holographic device 14 for recording images. The medium 14
illustrated in FIG. 1 is a removable medium, although persons of ordinary
skill in the art will appreciate that cameras according to the present
invention could also employ fixed (non-removable) media.
A reader 16 is also provided and includes a holographic playback system as
will be further disclosed herein. The reader is equipped to display images
stored in removable media transferred thereto from camera 12 and may also
be provided with an output that may be used to transmit image data to an
external device 18, such as a computer, printer, internet or
telecommunications port, or some other information appliance.
Although a fixed medium can be employed in the camera 12 according to the
present invention, particularly if the medium is erasable, it is generally
desirable to have removable medium as illustrated in FIG. 1. The removable
medium 14 of FIG. 1 (or a fixed medium) can comprise a photopolymer on a
substrate or a flexible delivery material. Photosensitive crystals can
also be used. The medium may be rigid or flexible and can take on several
forms including a roll of photopolymer film, a cube or slab of
photopolymer, photorefractive, or other photosensitive material; a card, a
small disk, or tape. The overall form factor is flexible. The media may
take on a form factor similar to ordinary film and digital storage
options. In embodiments where the medium is removable, a holder to receive
the medium and maintain it in a mechanically stable state is disposed in
the camera at an appropriate position.
As shown in FIGS. 2A and 2B, the holographic storage device can comprise a
medium 14 that may be optionally mounted in a cartridge 20. The media
cartridge 20, when used, can protect the medium from factors, such as
light, in the external environment that can affect it. In FIG. 2A the
medium 14 is shown at two different stages of insertion into cartridge 20,
which is formed from a suitable material such as a plastic material. As
will be appreciated by persons of ordinary skill in the art, the medium 14
can reside in the cartridge when not in use, thus protecting it from
environmental factors such as light. The medium can be removed from or can
extend from the cartridge during use in the camera. In FIG. 2B, the medium
14 is in the form of a flexible strip and cartridge 20 may comprise a
container similar to the convention 35 mm or other form-factor
photographic film canister. A take-up device 22 is employed to advance the
position of medium 14 for sequential exposures in the manner known for
ordinary photographic film. In the camera, the media and take-up mechanism
can also be moved in a direction perpendicular to the take-up direction.
According to one aspect of the present invention, the medium 14 can be
stored in a light-tight cartridge to insure that it remains intact during
storage and during transfer to and from the camera and reader.
According to other aspects of the present invention, the medium 14 can be
configured with a portion of the recording area dedicated to media
management and content management. This region can be used for purposes
such as identification of recording zones, resolution characteristics for
backward compatibility, compatible media, material properties for exposure
control, selection of media portion, fiducial marks for media alignment in
an external reader, and data on recorded zones to allow switching between
partially used media.
According to yet other aspects of the present invention, holographic
storage devices are configured with fixed image count and a fixed
resolution. In order for users to easily identify with the media, this
capacity can be defined in a manner similar to that used with current
ordinary photographic film packages. For example, ordinary film speeds of
ASA 50-100 and below correspond to bright light, high-resolution pixel
definition, and therefore higher capacity per image. Ordinary film speeds
of ASA 400-800 and above correspond to high speed, low light,
low-resolution pixel definition. Similar designations can be applied to
packaged holographic media. Media capacity can be based on resolution,
e.g., a media cartridge may offer either 24 or 36 exposures at one of two
certain predetermined resolutions. Exposure count can also be pushed to 48
or 64 or higher to emphasize the capacity advantages of digital
holographic cameras. Storage devices can be configured for variable image
resolution so that low-resolution and high-resolution images can reside on
the same medium.
Once the medium 14 is loaded, the camera 12 may be used to capture multiple
digital photographic images. The photographic images can all be produced
in the same format or they can be of different formats.
Once a desired number of digital photographic images have been recorded,
the medium 14 may be unloaded from the camera and loaded into the reader
16. The reader 16 can be an external device, and can be connected, for
example, to an output device 18 such as a computer as indicated in FIG. 1.
The reader 16 can also connect directly to other devices. Alternatively,
the reader 16 can be embedded into a device that serves a specific
function. Non-exhaustive examples of suitable devices that could
incorporate an embedded reader include computers, printers, a storage
device with an Internet port or a telecommunications port, an information
appliance, etc. Thus, an advantageous feature of the system 10 of the
present invention is the possibility of utilizing a subsystem comprising a
holographic reader and a device that performs a specific function using
data retrieved by the reader.
Referring now to FIG. 3, a block diagram of an illustrative embodiment of a
digital holographic camera 12 of FIG. 1 is presented. The camera 12 of
FIG. 3 may be used for both recording and readout of digital holographic
data and will generally require more components and incur greater cost
than a system that provides only recording capability. In this and
subsequent block diagrams, arrows indicate the general flow of
information. A person skilled in the information systems control art will
understand that some information signal flow will be bi-directional.
As may be seen from FIG. 3, camera 12 includes an image-capture subsection
30, comprising lens 32 and image detector 34. Although only one image
detector 34 is depicted in FIG. 3, persons of ordinary skill in the art
will appreciate that one or more CCD or CMOS image detectors may be
employed in the digital holographic camera of the present invention in
conjunction with a prism to divide the image into red, green, and blue
components for the individual detectors as is known in the art. Other
color separation techniques, such as providing a single image detector
with color filtered pixels, can also be employed with the present
invention.
Lens 32 focuses an image of a subject 36 on imaging array 34 located at the
focal plane of the lens. The image capture subsystem is representative of
a variety of subsystems available in digital image cameras; the image
capture unit captures the subject image and digitizes the image.
The image capture unit 38 comprises circuitry used in conjunction with the
image detector 34 to read pixel data captured by the detector 34 and
output the data in three-color grey-scale format. Such circuitry is well
known in the art and may be provided by the vendor along with the image
detector 34.
The user interface unit 40 functions to determine if there is space left to
record data, to capture an image and to add non-image data to the recorded
data. The optional non-image data acquisition unit 42 obtains
supplementary data of the types described herein for inclusion with the
stored image data and presents the supplementary data to user interface
unit 40. Design of actual circuitry to implement the functions of user
interface unit 40 and non-image data acquisition unit is a trivial
exercise for a skilled digital designer.
The input signal processing unit 44 prepares the digital data prepared by
the image capture unit 38 for transfer to the holographic storage system,
e.g. by dividing the subject image into a stream of coded digital image
pages. The input signal processing unit 44 is configured to carry out the
steps of compressing the image, dividing the image into data pages,
formatting the data pages, adding redundant data for error correction
coding, and applying modulation codes. These functions are well known in
the art, and the design of particular circuitry for performing these
functions is a matter of routine circuit design. Exemplary encoding and
decoding techniques for use with digital holographic storage of digital
data are disclosed in U.S. Pat. Nos. 5,450,218 and 5,727,226, incorporated
herein by reference.
According to another aspect of the present invention, it is contemplated
that additional digital information can be recorded with the digital
photo. A non-exhaustive illustrative list of examples of such data
include, digital voice annotation, circumstances surrounding the photo
(i.e., location, and exposure (f-stop, etc.)); and resolution information.
Such additional digital information can be stored digitally integrated
with photo data. Another example would be to record music for a
music/photo combination.
The storage control unit 46 obtains the processed image and other digital
data from input signal processing unit 44 and transfers that data to the
holographic storage and retrieval subsystem 48. The storage control unit
46 several tasks with respect to the holographic storage and retrieval
system 48, including sending data to an SLM, controlling the reference
beam and positioning the medium. As will be appreciated by persons of
ordinary skill in the art, the imager capture unit 38, the input signal
processing unit 44, and/or the storage control unit 46 may utilize a
buffer to facilitate efficient data transfer.
The retrieval control unit 50 receives data from the holographic storage
and retrieval subsystem 48. The retrieval control unit 50 performs several
tasks with respect to the holographic storage and retrieval system 48,
including collecting data from a detector, controlling the reference beam,
and positioning the medium. As will be appreciated by persons of ordinary
skill in the art, the retrieval control unit 50, the output signal
processing unit 52, the optional display 56, and/or the output port 54 may
utilize a buffer to facilitate efficient data transfer.
The output signal processing unit 52 can carry out several steps to decode
data retrieved by retrieval control unit 50, including interpreting page
formats, interpreting modulation coding, decoding error corrected
segments, assembling images from data, and decompressing images. The
output data, which can also include non-image data, can be directed to an
output port 54 or an optional display 56.
Referring now to FIG. 4, a diagram of an exemplary holographic storage and
retrieval subsystem according to the present invention is shown. The
general operation of a holographic storage system, such as that of FIG.
4A, is described in U.S. Pat. No. 5,450,218. Briefly, for holographic
storage systems in general, coherent light from a laser is divided into
two paths, a signal beam path and a reference beam path. These paths may
be separately controlled by shutters, as required. A signal beam is
encoded using a spatial light modulator (SLM), which can be controlled by
an SLM interface unit. The signal beam is generally collimated before
being encoded with an SLM. The SLM can be, for example, a digital
micromirror device or an array of liquid crystal cells, which are well
known. Such devices generally define a matrix of pixels. The SLM interface
unit loads data onto the SLM. As is well known to persons of ordinary
skill in the art, the reference beam angle can be controlled using
standard optics and deflectors and can be controlled by a beam-positioning
unit. The reference beam and the signal beam are simultaneously incident
on a photosensitive medium capable of recording holograms. Persons of
ordinary skill in the art will also recognize that the subsystem of FIG. 4
may be modified to accommodate a digital micromirror device that operates
in reflection mode.
Numerous holographic materials are suitable for use in the present
invention and include, without limitation, photopolymers, photorefractive
crystals, and photosensitive glasses. Photorefractive crystals and glass
tend to have substantial thickness, and photopolymers tend to be thin.
Photopolymers are presently preferred for permanent storage. For
information on photopolymers see, for example, Lessard and Manivannan
(ed.), Selected Papers on Photopolymers, SPIE Milestone Series, V. MS-114,
SPIE Optical Engineering Press, Bellingham, Washington, 1995. See also
U.S. Pat. No. 5,759,721 to Dahl et al.
Because the signal and reference beams are arranged to be mutually coherent
on the photosensitive medium, they generate an interference pattern that
then alters the complex index of refraction of the material, recording a
hologram.
Multiple holograms can be stored in the same physical location in the
medium 12 by changing the properties of the reference beam. Such changes
can be achieved using a variety of techniques, such as angle multiplexing,
in which holograms can be stored using reference beams with different
incident angles (see, e.g., U.S. Pat. No. 5,450,218); wavelength
multiplexing, in which different holograms can be recorded using different
laser wavelengths (see, e.g., U.S. Pat. No. 5,440,669); shift
multiplexing, in which a complex reference beam can be used to record
different holograms different positions of the medium, where the
illuminated portions of the medium overlap substantially (see, e.g., U.S.
Pat. No. 5,671,073); and phase-encoded multiplexing, in which the
reference beam is encoded with a spatial-light modulator that controls the
phase front of the reference beam. A review of several of such known
multiplexing techniques may be found in "Optical memories implemented with
photorefractive media," L. Hesselink and M. C. Bashaw, Optical and Quantum
Electronics 25, S611-S661.
In addition, holograms can be stored at different locations as well.
Locations can be selected by modifying reference beam optics to include
beam-position manipulation elements to position the light beams to
different locations (see, e.g., U.S. Pat. No. 5,550,779) or by moving the
medium in its holder (see, e.g., U.S. Pat. No. 5,519,517). For angular
multiplexing, the angle of the reference beam is controlled by the beam
control unit. The position of the medium in its holder can be controlled
by a media positioning unit. The medium may include fiducial marks to
assist in positioning, as described, for example, in U.S. Pat. No.
5,519,517. Persons of ordinary skill in the art will be able to include
hardware to detect fiducial marks.
Data are represented as follows. Generally, a channel bit represents the
lowest level of data storage in a data storage device. In traditional
storage devices, a channel bit can be represented by the changes in the
property of a material. In magneto-optical storage, for example, the
channel bit can be represented by a magnetic domain orientation or the
point of transition from one domain to another. In a holographic storage
system, a channel bit can be represented by the values of pixels; for
example, "on" and "off" pixels can represent channel bits "1" and "0",
respectively. Groups of data bits can be represented by groups of channel
bits using modulation codes, which are well known in the art. See, for
example, U.S. Pat. No. 5,450,218 and U.S. Pat. No. 5,727,226 for
descriptions of modulation codes. Data bits can be modified with error
correction codes to improve the bit-error rates of a holographic storage
system. Error correction coding and modulation encoding are generally
performed before loading data onto an SLM display. Modulation decoding and
error correction decoding are generally performed after retrieving data
from the detector elements.
To retrieve a hologram, the reference beam is incident on the medium. The
reference beam used to retrieve a hologram generally corresponds to the
reference beam used to record a particular hologram. For angle
multiplexing, different reference beam angles are used to retrieve
different holograms. The reference beam diffracts off of the hologram to
generate the output signal beam, which is then incident on a detector; the
detector generally comprises a matrix of pixels. The detector can be
controlled by a detector interface unit, which retrieves data from the
detector. The detector can be, for example, a CCD detector array or a CMOS
detector array, which are well known.
In the holographic storage and retrieval system depicted in FIG. 4A, laser
60 enters beam expander 62. The output of beam expander 62 is fed to beam
splitter 64 which splits the expanded beam into two paths shown at
reference numerals 66 and 68.
Beam 68 is a reference beam and is deflected by deflector 70, controlled by
deflection unit 72, and passed through lenses 74 and 76 to holographic
medium 78. The spacing between SLM 82 and lens 86 and lens 86 and medium
78 may both be about f, where f is the focal length of the lens 86. This
arrangement generally provides the highest storage density. The medium may
be slightly displaced from the focal length f to avoid bright features in
the focus of the signal beam in the medium. Alternatively, persons of
ordinary skill in the art may use a phase mask placed adjacent to the SLM
to avoid bright features in the focus of the signal beam at medium 78.
Beam 68 is deflected by mirror 80 and passed through spatial light
modulator (SLM) 82, controlled by SLM interface unit 84. The SLM interface
unit passes data from the storage control unit 46 of FIG. 3 to the SLM 82.
| | |
