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Claims  |
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What is claimed is:
1. A volume holographic memory comprising:
a) a disk comprised of photorefractive medium, said disk having an outer
edge and a central opening;
b) object beam optics configured to direct an object beam through the outer
edge of said disk; and
c) reference beam optics configured to direct a reference beam through the
outer edge of said disk;
d) wherein said object beam and said reference beam cooperate within said
photorefractive medium to sequentially form a plurality of volume
holograms therein, said disk spinning as the holograms are formed; and
e) wherein the object beam and the reference beam are directed into the
central opening of said disk after being transmitted therethrough.
2. The volume holographic memory as recited in claim 1 further comprising
an angle multiplexer for varying the angle at which at least one of the
object beam and the reference beam is directed through the outer edge of
the disk to facilitate the formation of at least one set of angle
multiplexed holograms at a desired common location within said disk.
3. The volume holographic memory as recited in claim 2 wherein said angle
multiplexer directs the reference beam through the outer edge of the disk.
4. The volume holographic memory as recited in claim 2 wherein said angle
multiplexer comprises a galvanometer mirror.
5. The volume holographic memory as recited in claim 4 further comprising a
sensor for sensing the position of the reference beam after it has been
transmitted through said disk so as to provide an indication of said
galvanometer mirror's position.
6. The volume holographic memory as recited in claim 5 wherein said sensor
comprises a one-dimensional CCD array.
7. The volume holographic memory as recited in claim 1 further comprising a
sensor for measuring the intensity with which the holograms are formed.
8. The volume holographic memory as recited in claim 7 herein said sensor
comprises a one-dimensional CCD array.
9. The volume holographic memory as recited in claim 1 wherein said object
beam optics and said reference beam optics are configured to define an
interferometer comprising:
a) a reflector disposed upon a translation stage; and
b) a sensor for sensing interference fringes resulting from combining the
object and reference beams.
10. The volume holographic memory as recited in claim 9 wherein:
a) said reflector comprises a reflecting spatial light modulator;
b) said translation stage comprises a piezoelectric translation stage; and
c) said sensor comprises a one-dimensional CCD array.
11. The volume holographic memory as recited in claim 1 wherein said disk
comprises iron-doped LiNbO.sub.3.
12. The volume holographic memory as recited in claim 1 wherein said disk
is approximately 6 centimeters in diameter and approximately 1.2
centimeters thick.
13. The volume holographic memory as recited in claim 1 wherein said disk
is configured such that the object and reference beams contact a
substantial portion of upper and lower surfaces thereof during write
operations, so as to facilitate dissipation of photovoltaic charges
generated within said disk.
14. The volume holographic memory as recited in claim 1 wherein said disk
comprises:
a) an upper surface having a first groove formed therein; and
b) a lower surface having a second groove formed therein;
c) wherein said first and second grooves define a generally hourglass-like
cross-section of said disk.
15. The volume holographic memory as recited in claim 14 wherein said disk
further comprises a conductive layer formed upon the upper and lower
surfaces thereof to facilitate dissipation of photovoltaic charges within
said disk.
16. The volume holographic memory as recited in claim 14 wherein said disk
further comprises fillets formed within said first and second grooves to
inhibit stress crack formation within said disk.
17. A volume holographic memory comprising:
a) a Pockels cell polarization rotator for rotating the polarization of a
laser beam to a desired orientation;
b) a beam splitter for separating the laser beam from the Pockels cell into
separate object and reference beams of desired intensities;
c) a disk comprised of photorefractive medium and configured to spin about
an axis thereof, said disk having an outer edge and a central opening
formed therein;
d) object beam optics configured to direct an object beam through the outer
edge of said disk, said object beam optics comprising:
i) a reflecting spatial light modulator for modulating the object beam;
ii) first object beam expansion optics for imaging the object beam upon
said spatial light modulator;
iii) a beam splitter for directing the laser beam onto said spatial light
modulator and for transmitting the modulated object beam reflected from
said spatial light modulator;
iv) second object beam Fourier transform optics for forming the Fourier
transform of the object beam within said disk such that the object beam
enters said disk through the outer edge thereof and exits said disk at the
central opening thereof;
v) a FLC polarization rotator for rotating polarization of the object beam
to a desired orientation prior to its entering said disk;
e) reference beam optics configured to direct a reference beam through the
outer edge of said disk, said reference beam optics comprising:
i) a galvanometer mirror for varying an angle at which the reference beam
is incident upon the outer edge of said disk;
ii) reference beam imaging optics for imaging the reference beam within
said disk such that the reference beam enters said disk through the outer
edge thereof and exits said disk at the central opening thereof, said
reference beam imaging optics having a focus;
iii) a pressure cell disposed at the focus of said reference beam imaging
optics to prevent air ionization due to high energy density at the focus
of said reference beam imaging optics;
f) a translation stage upon which said spatial light modulator is disposed
for varying the path length of the object beam path by one-half wavelength
thereof to facilitate erasure of holograms stored within said disk;
g) a first beam reflector disposed within the central opening of said disk
for reflecting said reference beam;
h) a phase conjugator receiving the reference beam from said first beam
reflector and reflecting a conjugate reference beam back to said disk such
that a conjugate object beam is formed thereby;
i) a FLC phase rotator for rotating the phase of the reference beam
reflected by the first beam reflector disposed within the opening of said
disk to a desired orientation;
j) a beam splitter for directing the reference beam from the disk onto the
phase conjugator and for directing the conjugate reference beam from the
phase conjugator to the disk;
k) focusing optics for focusing the reference beam within the phase
conjugator;
l) a two-dimensional CCD array for translating the conjugate object beam
into an electronic signal representative of a stored hologram;
m) a beam splitter for transmitting the object beam during write and erase
operations and for directing the conjugate object beam to the
two-dimensional CCD array during read operations;
n) a one-dimensional CCD array for sensing the intensity of a diffracted
object beam after it has been transmitted through the disk so as to
determine the amplitude of a plurality of the holograms stored therein;
o) focusing optics for focusing the object beam upon the one-dimensional
CCD array;
p) a beam splitter for directing the object beam to said one-dimensional
CCD array;
q) a FLC polarization rotator for rotating the polarization of the object
beam to a desired orientation to facilitate reflection by said beam
splitter for directing the object beam to said one-dimensional CCD array;
r) wherein amplitude of hologram stored within said disk is measured by
sensing the intensity of the object beam with said one-dimensional CCD
array after the object beam has been diffracted by the hologram;;
s) wherein the difference in path lengths between the object beam path and
the reference beam paths is measured by sensing interference fringes
generated at said one-dimensional CCD array by the object beam after its
being diffracted by a plane-wave hologram stored in said disk and
reference beam; and
t) wherein calibration of said galvanometer mirror is performed by sensing
the positions of the laser beam reflected thereby via said one-dimensional
CCD array.
18. A method for reading volume holograms, the method comprising the steps
of:
a) spinning a disk comprised of photo-refractive medium about an axis
thereof, said disk having an outer edge;
b) directing a reference beam through the outer edge of the disk;
c) directing the reference beam onto a phase conjugator after the reference
beam has passed through the disk, the phase conjugator directing a
conjugate reference beam back through the disk, so as to form a conjugate
object beam; and
d) sensing the conjugate object beam and converting the conjugate beam into
an electronic signal representative thereof.
19. The method as recited in claim 18 further comprising the step of
varying the angle at which at least one of the object beam and the
reference beam is directed through the outer edge of the disk.
20. The method as recited in claim 19 wherein the step of varying the angle
at which at least one of the object beam and reference beam is directed
through the outer edge of the disk comprises varying the angle at which
the reference beam is directed through the outer edge of the disk.
21. The method as recited in claim 20 wherein the step of directing the
reference beam through the outer edge of the disk comprises directing the
reference beam through the outer edge of the disk with a galvanometer
mirror.
22. A volume holographic memory comprising:
a) a disk comprised of photorefractive medium, said disk having an outer
edge and a central opening;
b) object beam optics configured to direct an object beam through the outer
edge of said disk;
c) reference beam optics configured to direct a reference beam through the
outer edge of said disk; and
d) a phase conjugator for directing a conjugate reference beam back through
said disk after the reference beam has previously passed therethrough, so
as to form a conjugate object beam to facilitate read-out;
e) wherein said object beam and said reference beam cooperate within said
photorefractive medium to sequentially form a plurality of volume
holograms therein, said disk spinning as the holograms are formed.
23. The volume holographic memory as recited in claim 22 further comprising
a reflecting element for directing the reference beam from the central
opening of said disk to the phase conjugator and for directing the
conjugate reference beam from the phase conjugator back into said disk.
24. A volume holographic memory comprising:
a) a disk comprised of photorefractive medium, said disk having an outer
edge and a central opening;
b) object beam optics configured to direct an object beam through the outer
edge of said disk;
c) reference beam optics configured to direct a reference beam through the
outer edge of said disk;
d) optics for imaging the reference beam within said disk, said imaging
optics having a focus thereof; and
e) a pressure cell disposed at the focus of said focusing optics to prevent
air ionization.
f) wherein said object beam and said reference beam cooperate within said
photorefractive medium to sequentially form a plurality of volume
holograms therein, said disk spinning as the holograms are formed.
25. A volume holographic memory comprising:
a) a disk comprised of photorefractive medium, said disk having an outer
edge and a central opening;
b) object beam optics configured to direct an object beam through the outer
edge of said disk;
c) reference beam optics configured to direct a reference beam through the
outer edge of said disk;
d) a beam splitter for splitting a laser beam into separate object and
reference beams; and
e) a Pockels cell polarization rotator for rotating the polarization of the
laser beam from which the object and reference beams are formed to a
desired orientation, so as to determine the relative intensities of the
laser beams which define the object and reference beams;
f) wherein said object beam and said reference beam cooperate within said
photorefractive medium to sequentially form a plurality of volume
holograms therein, said disk spinning as the holograms are formed.
26. The volume holographic memory as recited in claim 25 further comprising
a spatial light modulator for modulating the object beam.
27. The volume holographic memory as recited in claim 26 wherein said
spatial light modulator comprises a reflecting spatial light modulator.
28. A volume holographic memory comprising:
a) a disk comprised of photorefractive medium, said disk having an outer
edge and a central opening;
b) object beam optics configured to direct an object beam through the outer
edge of said disk;
c) reference beam optics configured to direct a reference beam through the
outer edge of said disk; and
d) a reflecting element disposed upon a translation stage for varying a
path length of one of the object and reference beams.
e) wherein said object beam and said reference beam cooperate within said
photorefractive medium to sequentially form a plurality of volume
holograms therein, said disk spinning as the holograms are formed.
29. The volume holographic memory as recited in claim 28 wherein said
translation stage comprises a piezoelectric translation stage.
30. A volume holographic memory comprising:
a) a disk comprised of photorefractive medium, said disk having an outer
edge and a central opening;
b) object beam optics configured to direct an object beam through the outer
edge of said disk;
c) reference beam optics configured to direct a reference beam through the
outer edge of said disk; and
d) a reflecting spatial light modulator mounted upon a piezoelectric
translation stage for both modulating the object beam and varying the path
length of the object beam;
e) wherein said object beam and said reference beam cooperate within said
photorefractive medium to sequentially form a plurality of volume
holograms therein, said disk spinning as the holograms are formed.
31. A volume holographic memory comprising:
a) a disk comprised of photorefractive medium, said disk having an outer
edge and a central opening;
b) object beam optics configured to direct an object beam through the outer
edge of said disk;
c) reference beam optics configured to direct a reference beam through the
outer edge of said disk; and
d) a coupling prism for coupling the object and reference beams to said
disk;
i) a first planar surface for receiving the object beam;
ii) a second planar surface for receiving the reference beam; and
iii) a curved surface for transmitting both the object beam and the
reference beam to said disk, the curved surface defining a gap having
substantially constant width intermediate said coupling prism and said
disk;
e) wherein said object beam and said reference beam cooperate within said
photorefractive medium to sequentially form a plurality of volume
holograms therein, said disk spinning as the holograms are formed.
32. A holographic memory read-out device comprising:
a) a disk comprised of photorefractive medium and configured to spin about
an axis thereof, said disk having an outer edge and a central opening
formed therein;
b) reference beam optics configured to direct a reference beam through the
outer edge of said disk; and
c) a phase conjugator for directing a conjugate reference beam back through
the disk after the reference beam has previously passed therethrough so as
to form a conjugate object beam to facilitate read-out;
c) wherein said reference beam facilitates reconstruction of a holographic
image stored within said disk. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention relates generally to holographic storage devices and
more particularly to a method and device for storing a plurality of volume
holograms within a spinning, disk-shaped, photorefractive medium.
BACKGROUND OF THE INVENTION
Holographic techniques for storing images are well known. Such techniques
are commonly used to store images in a wide variety of different
applications. Additionally, various methodologies for utilizing such
holographic techniques to store digital data for use in computer systems
are currently being explored.
The technique for forming holograms comprises splitting the highly coherent
output beam of a laser into separate reference and object beams. The
reference beam is directed onto the holographic storage medium, e.g., a
photorefractive material, while the object beam is directed onto the
object whose image is to be stored. Light from the object is directed to
the photorefractive medium wherein an interference pattern is formed due
to the interaction of the reference beam with the object beam.
When utilized in digital data storage applications, the object beam
typically passes through a spatial light modulator, e.g., a liquid crystal
shutter matrix, rather than being reflected off an object, in order to
form the holographic image. The spatial light modulator adds the desired
digital data to the object beam to facilitate storage of the digital data
in the hologram formed therefrom.
Regardless of the application (i.e., the storage of images or data),
subsequently directing a reference beam onto the holographic storage
medium results in the reconstruction of an image representative of the
originally illuminated object or stored digital data.
Also known are techniques for storing a plurality of such images within a
single photorefractive medium via angle-multiplexing of the reference
beam. Such angle-multiplexing is discussed in "THEORY OF OPTICAL
INFORMATION STORAGE IN SOLIDS", Applied Optics, Vol. 2, No. 4, pg. 393
(1963). The method of angle-multiplexing generally involves maintaining a
constant angle for the object beam, while varying the angle of the
reference beam for each sequential exposure, i.e., the formation of each
separate hologram. Angle-multiplexing thus allows a large number of
holograms to be stored within a common volume of photorefractive medium,
thereby greatly enhancing the storage density thereof.
Also known are techniques for storing a plurality of such holograms within
a spinning drum or disk shaped photorefractive medium. Examples of some
holographic memories which utilize drum or disk shaped medium are provided
in U.S. Pat. Nos. 3,610,722; 3,737,878; 3,848,096; 4,104,489; 4,224,480;
4,420,829; 4,449,785; 4,929,823; 5,111,445; 5,128,693; 5,285,438;
5,339,305.
However, one problem commonly associated with such contemporary disk and
drum based holographic memories is that the geometry of the system is not
optimized with respect to the crystalline structure of the storage medium.
Further, such contemporary systems do not utilize effective path-length
monitoring so as to assure the integrity of holograms within the medium
and to assure reliable read-out of a plurality of different sets of
angle-multiplexed holograms.
As such, although the prior art has recognized to a limited extend the
problem of storing volume holograms in a spinning disk medium, the
proposed solutions, to date, have been ineffective in providing a
satisfactory remedy.
SUMMARY OF THE INVENTION
The present invention specifically addresses and alleviates the
above-mentioned deficiencies associated with the prior art. More
particularly, the present invention comprises a volume holographic memory
comprising a disk comprised of photorefractive medium and configured to
spin about a central axis thereof. The spin axis is perpendicular to a
central opening formed within the disk such that the disk spins in a
manner similar to that of a contemporary CD-ROM.
Object beam optics are configured to direct an object beam through the
outer edge of the disk and reference beam optics are similarly configured
to direct a reference beam through the outer edge of the disk. The object
beam and the reference beam intersect within the photorefractive medium
wherein they cooperate so as to sequentially form a plurality of separate
volume holograms within the spinning disk. Such volume holograms may be
written to, erased from, or read from the disk while the disk is spinning,
so as to provide a fast, high density memory.
An angle multiplexer varies the angle at which either the object beam or
the reference beam, preferably the reference beam, is directed through the
outer edge of the disk. The angle multiplexer preferably comprises a
galvanometer mirror. The storage density of the photorefractive medium is
substantially enhanced via the use of such angle multiplexing.
Both the object beam and the reference beam are preferably directed into
the center opening of the disk after cooperating to form a hologram, and
are then reflected from the center opening of the disk via a reflecting
element, preferably a pair of beam splitters. During write and erase
operations, both the object and reference beams may be terminated,
preferably via beam blocks, after exiting the disk, since their task has
been completed and they are both no longer needed. During read-out
operations, one of the two beam splitters disposed within the central
opening of the disk directs the reference beam from the central opening of
the disk to a phase conjugator.
The phase conjugator forms a conjugate reference beam which is directed
back through the beam splitter and into the photorefractive medium of the
spinning disk. As the conjugate reference beam is transmitted through the
photorefractive medium, a previously stored hologram formed therein causes
the conjugate reference beam to be transformed into a conjugate object
beam which is representative of the hologram effecting such
transformation. Thus, the conjugate reference beam excites a conjugate
object beam from the stored hologram. The excited conjugate object beam is
then read by a sensor, preferably a two-dimensional array charge coupled
device (CCD), so as to provide an electrical signal representative of the
originally stored data.
As those skilled in the art will appreciate, the phase conjugator removes
distortions introduced into stored holograms in a manner which facilitates
the use of inexpensive, fast, (i.e., low f/#) object beam optics. Thus,
the effects of distortion, such as spreading of the object beam due to
undesirable diffraction, are substantially reversed by the phase
conjugator.
According to the preferred embodiment of the present invention, a
high-power pulsed laser is utilized for the write, erase, and read-out
operations. The energy density obtained by such a high-power pulsed laser,
when focused, is sufficient to cause ionization of the air in the
immediately vicinity of the focus. For this reason, a pressure cell is
preferably disposed at the focus of the reference beam optics, where the
power density is greatest, so as to inhibit such ionization. Those skilled
in the art will appreciate that air ionization is inhibited at increased
pressure.
The present invention preferably comprises a liquid Stimulated Brillouin
Scattering (SBS) phase conjugator, preferably comprised of methanol. Those
skilled in the art will appreciate that various other phase conjugating
materials are likewise suitable.
According to the preferred embodiment of the present invention, a Pockels
cell is utilized to rotate the polarization of the laser beam from which
the object and reference beams are formed to an orientation suitable for
writing, erasing, and reading of holograms, according to well-known
principles.
A spatial light modulator (SLM) is utilized for modulating, i.e., applying
digital data to, the object beam. According to the preferred embodiment of
the present invention, a 1024.times.1024 pixel reflecting type spatial
light modulator is utilized to facilitate the storage of 1.18 terabits of
data, as discussed in detail below. Those skilled in the art will
appreciate that various different resolutions of spatial light modulators
are likewise suitable.
A reflecting element, preferably the spatial light modulator, is disposed
upon a translation stage and configured to vary the path length of one of
the object and reference beams, preferably object beam, so as to
facilitate adjustment of the relative path lengths of the object and
reference beams. The translation stage preferably comprises a piezoelectro
translation stage to facilitate automatic measurement and precise control
of relative path lengths of the object and reference beams. Those skilled
in the art will appreciate that various other types of translation stages
are likewise suitable.
A sensor, preferably a one-dimensional array charged coupled device (CCD),
measures the intensity or diffraction efficiency of holograms formed
within the disk. This facilitates the formation of holograms according to
an exposure schedule wherein later formed holograms are stored at a lower
intensity than earlier formed holograms. The use of an exposure schedule
and the formation of multiple holograms within a single media is taught in
"STORAGE OF 500 HIGH RESOLUTION HOLOGRAMS IN A LiNbO.sub.3 CRYSTAL",
Optics Letters, Vol. 62, No. 8, p. 105 (1991).
The use of such an exposure schedule has been found to be helpful in
minimizing diffraction efficiency degradation. More particularly,
according to such exposure schedules, earlier stored holograms are formed
utilizing more intense object and reference beams than later stored
holograms, such that each subsequent write process tends to lower the
diffraction deficiency of the earlier stored holograms in a manner which
substantially equalizes the diffraction deficiency of all stored
holograms. Thus, according to such exposure schedules, each succeeding
hologram is stored using a lower intensity than the preceding holograms.
According to the preferred embodiment of the present invention, the object
beam optics and the reference beam optics are configured so as to define
an interferometer. The one-dimensional CCD array detects interference
fringes resulting from combining of the object and reference beams, so as
to permit measurement of the relative path lengths of the object beam path
and the reference beam path. Thus, the relative path lengths of the object
and reference beams can be adjusted via the piezoelectric translation
stage such that the object and reference beams are in a constant or
desired phase relation during write operations and are 180 degrees from
this constant or desired phase relationship during erase operations.
In order to maintain and/or duplicate the desired phase relationship of the
object and reference beams during write processes, a plurality of
plane-wave holograms are preferably formed within the spinning disk when
the phase relationship of the object and reference beams is at a desired
angle. The desired phase relationship can subsequently be reproduced by
utilizing the plane-wave holograms as diffraction gradings, so as to
define a Michaelson interferometer which is utilized to monitor the
relative path lengths of the object and reference beams, as discussed in
detail below. Preferably, such plane-wave holograms are formed at each
radial location of the spinning disk and at the top, middle, and bottom
angles for each location. However, those skilled in the art will
appreciate that various different schemes for positioning and configuring
such plane-wave holograms are likewise suitable.
A sensor, preferably the same one-dimensional array as used for measurement
of the relative path lengths of the object beam path and reference beam
path, senses the position of the reference beam after it has been
transmitted through the disk, so as to provide an indication of the
position of the galvanometer mirror, thereby facilitating calibration of
the galvanometer mirror.
The disk is preferably comprised of LiNbO.sub.3, preferably iron-doped
(LiNbO.sub.3 :Fe), and is preferably approximately six centimeters in
diameter and approximately two centimeters thick and preferably has a
central opening of approximately two centimeters in diameter.
According to the preferred embodiment of the present invention, the disk is
configured such that the reference beam and the object beam contact the
upper and lower surfaces thereof during writing and erase processes, so as
to facilitate the dissipation of electrical charges within the
photorefractive medium generated by the photovoltaic effect. To accomplish
this, grooves are preferably formed in the upper and lower surfaces of the
disk so as to define an hourglass-like cross section thereof. Fillets are
preferably formed within the groove so as to mitigate the formation of
stress cracks.
Further, a conductive coating is preferably applied to the upper and lower
surfaces of the disk a | | |
