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Claims  |
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What is claimed is:
1. A method of holographic recording in a photorefractive medium comprising
the steps of:
recording a plurality of servo patterns within an image space in the
medium;
recording a plurality of data pages within the image space in the medium;
and providing a closed loop position feedback system for reconstructing
the data pages, wherein each servo pattern provides continuous position
feedback information and each pattern is generated by illuminating the
medium with a servo reference beam and a servo object beam, each servo
reference beam and servo object beam being incident upon a face of the
medium at a servo reference angle, the servo reference angle defining the
angle between the servo reference beam and the servo object beam and being
approximately half of a minimum angular spacing of the medium.
2. The method of claim 1 wherein each of the data pages is generated by
illuminating the medium with a data page reference beam and a data page
object beam, each data page reference beam and data page object beam being
incident upon the face of the medium at a data page reference angle, the
data page reference beam defining the angle between the data page
reference beam and the data page object beam.
3. The method of claim 1 further comprising the step of converting the
plurality of servo patterns into a plurality of permanent holograms that
cannot be erased by subsequent illumination of the medium.
4. The method of claim 3 wherein the step of converting the plurality of
servo patterns further comprises the step of heating the medium.
5. The method of claim 1 wherein the plurality of servo patterns provide
means for reconstructing one of the data pages with substantially no
crosstalk from other data pages.
6. The method of claim 1 wherein each of the servo patterns is located
around the periphery of the image space in the medium.
7. The method of claim 1 wherein the servo block object beam and the servo
block reference beam are generated with a coherent monochromatic light
source.
8. The method of claim 2 wherein the data page object beam and the data
page reference beam are generated with the coherent monochromatic light
source.
9. The method of claim 1 further comprising the step of recording an image
space identifier within each of a plurality of image spaces in the medium,
each identifier providing means for distinguishing each of the image
spaces.
10. A method of holographic recording in a photorefractive medium
comprising the steps of:
recording a first servo pattern in the medium, the first servo pattern
being defined by a first interference grating, the first grating being
generated by illuminating the medium with a first reference beam and a
first object beam to effect the migration of charges in the medium, the
first servo block reference beam and the first servo block object beam
being incident upon a face of the medium at a first servo block reference
angle, the first servo block reference angle defining the angle between
the first servo block reference beam and the first servo block object
beam;
recording a second servo pattern in the medium, the second servo pattern
being defined by a second interference grating, the second grating being
generated by illuminating the medium with a second reference beam and a
second object beam to effect the migration of charges in the medium, the
second servo pattern reference beam and the second servo pattern object
beam being incident upon the face of the medium at a second servo pattern
reference angle, the second servo pattern reference angle defining the
angle between the second servo pattern reference beam and the second servo
pattern object beam;
recording a third servo pattern in the medium, the third servo pattern
being defined by a third interference grating, the third grating being
generated by illuminating the medium with a third reference beam and a
third object beam to effect the migration of charges in the medium, the
third servo pattern reference beam and the third servo pattern object beam
being incident upon the face of the medium at a third servo pattern
reference angle, the third servo pattern reference angle defining the
angle between the third servo pattern reference beam and the third servo
pattern object beam;
recording a fourth servo pattern in the medium, the fourth servo pattern
being defined by a fourth interference grating, the fourth grating being
generated by illuminating the medium with a fourth reference beam and a
fourth object beam to effect the migration of charges in the medium, the
fourth servo pattern reference beam and the fourth servo pattern object
beam being incident upon the face of the medium at a fourth servo pattern
reference angle, the fourth servo pattern reference angle defining the
angle between the fourth servo pattern reference beam and the fourth servo
pattern object beam;
recording a fifth servo pattern in the medium, the fifth servo pattern
being defined by a fifth interference grating, the fifth servo pattern
being generated by illuminating the medium with the fifth reference beam
and the fifth object beam to effect the migration of charges in the
medium, the fifth servo pattern reference beam and the fifth servo pattern
object beam being incident upon the face of the medium at a fifth servo
pattern reference angle, the fifth servo pattern reference angle defining
the angle between the fifth servo pattern reference beam and the fifth
servo pattern object beam;
recording a first data page in the medium, the first data page being
defined by a first data page interference grating, the first data page
grating being generated by illuminating the medium with a first data page
reference beam and a first data page object beam to effect the migration
of charges in the medium, the first data page reference beam and the first
data page object beam being incident upon the face of the medium at a
first data page reference angle, the first servo pattern reference angle
defining the angle between the first data page reference beam and the
first data page object beam;
recording a second data page in the medium, the second data page being
defined by a second data page interference grating, the second data page
grating being generated by illuminating the medium with a second data page
reference beam and a second data page object beam to effect the migration
of charges in the medium, the second data page reference beam and the
second data page object beam being incident upon the face of the medium at
a second data page reference angle, the second data page reference angle
defining the angle between the second data page reference beam and the
second data page object beam; and
recording a third data page in the medium, the third data page being
defined by a third data page interference grating, the third data page
grating being generated by illuminating the medium with a third data page
reference beam and a third data page object beam to effect the migration
of charges in the medium, the third data page reference beam and the third
data page object beam being incident upon the face of the medium at a
third data page reference angle, the third data page reference angle
defining the angle between the third data page reference beam and the
third data page object beam,
wherein adjacent servo patterns are recorded at angular increments of
approximately one-half of a minimum angular spacing of the medium.
11. The method of claim 10 further comprising the step of converting each
of the servo patterns into holograms that cannot be erased by subsequent
illumination.
12. The method of claim 10 wherein the second and third servo patterns
provide continuous closed loop position feedback information during
reconstruction of the first data page.
13. The method of claim 10 wherein the fourth and fifth servo pattern
provide continuous closed loop position feedback information during
reconstruction of the second data page.
14. The method of claim 10 wherein each of the servo patterns is defined by
one of five patterns.
15. The method of claim 14 wherein each of the five patterns is recorded
about a periphery of the image space.
16. The method of claim 14 wherein each of the five patterns is defined by
a five spot arrangement.
17. The method of claim 10 wherein the reference beams, object beams, data
page reference beams and data page object beams are each propagated with a
same wavelength.
18. The method of claim 10 wherein adjacent data pages are recorded at an
angular spacing of a minimum angular spacing of the medium.
19. A method for retrieving holograms recorded within an image space in a
photorefractive medium comprising the steps of:
recording position feedback information within the image space in the
medium, the position feedback information being defined by a plurality of
patterns, each pattern being generated by illuminating the medium with a
servo reference beam and a servo object beam, each servo reference beam
and servo object beam being incident upon a face of the medium a servo
block reference angle, the servo reference angle defining the angle
between the servo reference beam and the servo object beam;
converting each of the plurality of patterns into permanent spatially
varying index of refraction patterns in the medium that cannot be erased
by subsequent illumination;
recording a plurality of data pages within the image space in the medium,
each of the data pages being defined by illuminating the medium with a
data page reference beam and a data page object beam to effect the
migration of charges in the medium, each data page reference beam and data
page object beam being incident upon the face of the medium at a data page
reference angle, the data page reference beam defining the angle between
the data page reference beam and the data page object beam;
reconstructing one of the data pages and position feedback information on a
detector array by propagating the data page reference beam at the face of
the medium at a reference angle; and
adjusting the data page reference angle in response to the position
feedback information.
20. The method of claim 19 wherein a servo block reference angle defined
between each servo reference beam and each servo object beam remains
fixed.
21. The method of claim 19 wherein a data page reference angle defined
between each data page reference beam and each data page object beam
remains fixed.
22. A method for retrieving holograms recorded within a plurality of image
spaces in a photorefractive medium comprising the steps of:
recording a plurality of patterns within the image spaces in the medium,
each pattern containing position feedback information being defined by a
servo reference beam and a servo object beam, each servo reference beam
and servo object beam being incident upon a face of the medium at a servo
reference angle, the servo reference angle defining the angle between the
servo reference beam and the servo object beam;
converting each of the plurality of patterns into permanent spatially
varying index of refraction patterns in the medium that cannot be erased
by subsequent illumination;
recording an image space identifier within each of the image spaces, each
identifier providing means for distinguishing each of the image spaces;
recording a plurality of data pages within the image spaces in the medium,
each of the data pages being defined by a data page reference beam and a
data page object beam, each data page reference beam and data page object
beam being incident upon the face of the medium at a data page reference
angle, the data page reference beam defining the angle between the data
page reference beam and the data page object beam;
reconstructing a desired data page and position feedback information from
an image space, on a detector array, by propagating the data page
reference beam at the face of the medium at a reference angle; and
continuously adjusting the reference angle in response to the position
feedback information until the desired data page is reconstructed with a
minimum crosstalk.
23. The method of claim 2 wherein the reconstructing of data pages
comprises the step of projecting a desired reference beam at the medium at
a reference angle.
24. The method of claim 23 wherein the closed loop position feedback system
continuously adjusts the reference angle, in response to position feedback
information, until a desired data page is reconstructed with a minimum
crosstalk. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention relates to the field of holographic storage systems and
methods. More particularly, this invention relates to a method for
recording data in and reconstructing data from a photorefractive medium in
the form of holograms, including a permanent hologram containing position
feedback information.
BACKGROUND OF THE INVENTION
The potential of volume holographic storage in photorefractive medium for
large digital storage capacity, fast data transfer rates and short access
times has been considered for some time. Recent developments in materials
and holographic storage components have made the promise of data storage
capacity in the magnitude of terabytes, transfer rates exceeding 1
gigabyte per second and random access times less than 100 micro seconds
closer to being realized.
Photorefractive materials have the property of developing light induced
changes in their index of refraction. Holographic storage can be
accomplished by propagating and recording an image-bearing light beam and
a reference beam into a photorefractive medium. The resulting optical
interference pattern causes a spatial index of refraction to be modulated
throughout the volume of the medium. In a photo-refractive medium such as
LiNbO.sub.3 (lithium niobate), the spatial index of refraction gratings
are generated through the electro-optic effect as a result of an internal
electric field generated from migration and trapping of photoexcited
electrons. When the medium is illuminated with a beam identical to the
reference beam used to generate the refractive index grating, the beam
will defract in such a way as to reproduce the original image bearing
wavefront.
In a typical holographic storage system, shown in FIG. 1, a coherent
monochromatic beam, projected from a light source 11, may be split into an
object beam 24 and a reference beam 22 by a beam splitter 12. The object
beam 24 is converted to an optical signal with a Spatial Light Modulator
(SLM) 14. Through reducing optics 16 and 17, the object beam 24 and the
reference beam 22 converge on and illuminate a photorefractive crystal 18,
generating a volumetrically distributed interference pattern in the
crystal 18 which is recorded in the form of a refractive index grating,
otherwise known as a hologram. The recorded hologram may be reproduced by
illuminating the crystal 18 with the identical reference beam 22 and
imaging the defracted optical signal onto a detector array 19, which
converts the optical signal back into an electrical signal.
Multiple holograms, each corresponding to a data page, may be written and
stored in the crystal 18, using various forms of multiplexing, e.g.
angular, wavelength, etc. Using angular multiplexing, each hologram is
written with a reference beam incident at a different angle. The angles
vary depending on the physical geometry and material of the crystal.
Typically, angles may differ by a magnitude of about 50 micro radians. The
angle may be changed either by mechanically translating the crystal 18
while keeping the object to reference angle constant or by changing the
angle of incidence of the reference beam on the crystal by steering the
reference beam angle with the reducing optics 16 and 17. With wavelength
multiplexing, each hologram is generated with the reference beam fixed at
some angle while changing the wavelength of the light source for each data
page.
One limitation inhibiting the potential advantages of holographic recording
is the metastable (impermanent) nature of recorded holograms. When
holograms are serially but coextensively recorded in the same volume of
crystal, commonly referred to as "a stack" of recordings, subsequently
recorded holograms tend to non-uniformly reduce the diffraction efficiency
of previously written holograms. Thus, a "write" process destroys the
previously recorded nearby holograms by fractionally reducing the
previously recorded hologram's intensity over many write cycles.
Similarly, a "read" process of exposing an area to a reference beam
illumination will also cause a redistribution of the charges which make up
the recorded hologram. This has led to the development of techniques for
fixing and developing more permanent holograms. For example, holograms
generated by electron charge patterns may be made permanent by heating the
crystal, which results in redistributing the ions which cancel the space
charge variation in the crystal. The crystal is thereafter cooled,
trapping the ions and forming a permanent ionic grating to generate the
index variation.
Another limitation inhibiting the potential application of holographic
recording is cross talk during hologram retrieval, which limits the
information density and storage capacity of a crystal. Because of the
Bragg-selective nature of a readout, a stored image or data page may be
reproduced independently from other pages of the stack of recordings. As
discussed, retrieval is accomplished by illuminating the medium with a
reference wavelength identical to the one used in recording that image.
However, although Bragg-selectivity ensures that an image associated with
a particular reference wavelength is reconstructed with the highest
efficiency, other stored images may also be reconstructed with less
efficiency and distortions due to Bragg-mismatch. To avoid this form of
crosstalk, the angular or wavelength separation between holograms must
precisely correspond to the zeros of the sinc function associated with the
Bragg matching condition. Any deviation from the ideal angle degrades the
signal-to-noise ratio (SNR). The consequence is that either the maximum
resolution of the image or the storage capacity of the system is reduced.
Many approaches have attempted to overcome this capacity limitation.
However, none have utilized a closed loop position feedback system during
data page retrieval to maximize SNR of the recorded signal by reducing
crosstalk and precise angle positioning. Further, none have utilized a
closed loop position feedback system combining permanent and metastable
holograms within the same recording area. Thus, a hitherto unsolved need
has remained for a method of holographic recording in a photorefractive
medium which provides a position feedback system for maximizing SNR of the
recorded signal by reducing crosstalk and is applicable to both angular
and wavelength multiplexing.
SUMMARY OF THE INVENTION WITH OBJECTS
A general object of the present invention is to provide a method of
holographic recording in a photorefractive medium which overcomes
limitations and drawbacks of the prior art.
Specifically, an object of the present invention is to provide a method of
holographic recording in a photorefractive medium having a position
feedback system which maximizes SNR of the recorded signal by reducing
crosstalk.
Another object of the present invention is to provide a method of
holographic recording in a photorefractive medium wherein the position
feedback system is applicable for both angular and wavelength
multiplexing.
One more object of the present invention is to provide a method of
holographic recording in a photorefractive medium having a position
feedback system by combining permanent and metastable holograms.
In accordance with principles of the present invention, a holographic
recording method first records a plurality of servo blocks in a
photorefractive medium, such as an LiNbO.sub.3 crystal. Each servo block
is defined by a five spot pattern. The servo blocks are generated by the
simultaneous illumination of the crystal by an object and a reference beam
on a same area of the crystal. The angle of incidence on a face of the
crystal, by the reference beam, defines a reference angle. The servo
blocks provide position feedback during reconstruction of data pages
stored in the crystal, enabling data pages to be reconstructed with
maximum SNR. The servo blocks are further recorded at reference angle
increments of half the minimum angular separation of the crystal, which is
determined by the physical dimensions of the crystal. The servo blocks are
then made non-erasable (fixed) using methods known by those skilled in the
art, e.g. by heating the crystal. Each of the five spots is recorded in
the same image space as data pages so that position feedback information
is retrieved along with a data page during hologram retrieval.
Data pages are then recorded in the same image space of the crystal in the
same manner, i.e. by simultaneously illuminating the crystal with an image
bearing object beam and a reference beam. The data pages are recorded at
reference angle increments of the minimum angular spacing of the crystal.
During a data page retrieval, position feedback information from the servo
blocks is communicated to a reflector positioner, e.g. a voice coil motor.
The positioner rotatably adjusts the angular position of a reflecting
mirror to fine tune the reference angle of the reference beam thereby
maximizing SNR of the recorded signal by reducing crosstalk.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a typical prior art holographic recording
system, using a photorefractive crystal.
FIG. 2 is a flow diagram of steps for achieving the holographic recording
method in accordance with the present invention.
FIG. 3 illustrates one embodiment of a servo block for providing position
feedback for the holographic recording method in accordance with
principles of the present invention.
FIGS. 4a-4e illustrate five patterns of the servo block of FIG. 3, in
accordance with principles of the present invention.
FIG. 5 is a plot of data page amplitude as a function of angle of incidence
of the reference beam.
FIG. 6 is a plot of servo block amplitude as a function of angle of
incidence of the reference beam.
FIG. 7 is a plot of difference of position functions of FIG. 6 as a
function of angle of incidence.
FIG. 8 is a plot of linearized positioning function as a function of angle
of incidence of the reference beam.
FIG. 9 illustrates servo block sinc image intensities as a function of
angle of incidence of the reference beam.
FIG. 10 is a schematic diagram of the holographic recording system for
recording and reconstructing holograms in accordance with the present
invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
FIG. 2 shows a flow diagram of the steps for achieving the method of
holographic recording in accordance with the present invention. These
steps include determining minimum angular spacing of the crystal 100,
recording servo blocks in the crystal 200, heating the crystal to fix the
servo blocks 300, quenching the crystal back to room temperature 400,
recording data pages in the crystal 500, and reconstructing recorded data
pages 600.
The first step 100, prior to recording any holograms, is determining a
minimum angular spacing required to define separate holographic recordings
in the photorefractive crystal such that crosstalk is minimized. The
details in deriving an equation for determining the minimum angular
spacing, are known to those skilled in the art. Specifically, such details
are discussed and described in an article by John H. Hong et al. entitled
"Volume holographic memory systems: techniques and architectures", Optical
Engineering, Vol. 34, No. 8, August 1995, the article being incorporated
herein by reference. Hong et al. defines the minimum angular spacing
.theta., by the equation
.theta.=.lambda.cos.theta..sub.o /nLsin(.theta..sub.r +.theta..sub.o)(1)
where .lambda.=wavelength of the signal, n=refractive index of the crystal,
L=thickness of the crystal, .theta..sub.r =angle of incidence of the
reference beam with respect to the z-axis, and .theta..sub.o =angle of
incidence of the object beam with respect to the z-axis. .theta..sub.r
=may be determined based on the geometry of the reducing optics and the
crystal. Applying equation (1) to the embodiment illustrated in FIG. 4,
where .theta..sub.o =0 and .theta..sub.r is approximately 33 degrees,
Equation (1) would be reduced to
.theta.=1.88.lambda./nL (2)
Once the minimum angular spacing is determined, the servo blocks may then
be recorded, in the conventional manner, known by those skilled in the
art. Servo block s are recorded with a preferred holographic recording
system 30, illustrated in FIG. 10. The system 30 includes a signal
producing light source 31, a beam splitter 32 which splits a signal 43
into object beam 44 and reference beam 42, a rotatable reflecting mirror
33 having two (2) degrees of freedom for changing the reference angle
.theta..sub.r, a reflecting mirror 35, an SLM 34 for converting an
electrical signal to an optical signal by modulating the object beam 44,
reducing lenses 36 and 37, a photorefractive crystal 38 for recording
holograms, a detector array 39 and a voice-coil motor (VCM) 41 for
rotating reflecting mirror 33 in response to position feedback information
detected by array 39. The SLM 34 has an approximately 1.0".times.0.8"
viewing area, providing an approximately 640.times.480 pixel area for
modulating the object beam. The detector array 39 may be any known in the
art e.g. a charge coupled device (CCD) having an approximately
0.5".times.0.4" viewing area, providing an approximately 1134.times.486
pixel area. The crystal 38 is Fe--LiNb O.sub.3 and disk shaped,
approximately 2 mm thick and 70 mm diameter.
In accordance with conventional holographic recording practices, each servo
pattern is recorded in the crystal 38 by illuminating a servo pattern
bearing object beam 44 with a reference beam 42, at a particular reference
angle, to form an interference grating in the crystal 38.
In one preferred embodiment, each servo block is defined by a five spot
pattern, shown in FIG. 3. The intensity of each of the five spots A, B, C,
D, and E and the combination thereof represent angular positions of the
reference beam and provide position feedback information. As depicted in
the FIG. 3 embodiment, the five spots are located around the outer
periphery of the data area, which is further represented in FIG. 3 as a
symmetrical four sided area. In this embodiment, the spots are arranged as
shown to maximize the distance between the pairs of spots which define
each of the five servo blocks, thereby optimizing the amount of
retrievable data area. The servo block of FIG. 3 is shown with the
reference beam having one axis of freedom , x-axis, as represented by Ax,
Bx, Cx, Dx, and Ex being highlighted. A reference beam indexed in the
Y-axis would be represented by Ay, By, Cy, Dy, and Ey being highlighted.
Those skilled in the art will understand that other variations of the five
patterns may be used, including other arrangements, spot locations, and
number of spots.
The servo blocks for one axis of freedom, x-axis, are defined by five
patterns, A-B, B-C, C-D, D-E and E-A, as illustrated in FIGS.4a-4e. Each
pattern defines a variation of the five spots. For example, pattern A-B is
defined by spots A and B being shaded, pattern B-C defined by spots B and
C being shaded, pattern C-D defined by spots C and D being shaded, pattern
D-E defined by spots D and E being shaded, and pattern E-A defined by
spots E and A being shaded. The shaded spots represent intensified images
while the non-shaded spots represent non-intensified images. Each spot is
recorded at 0.5.theta.increments, starting with spot A recorded at
-0.75.theta., spot B at -0.25.theta., and so forth. As will be explained
herein below, each of the patterns shown in FIGS. 4a-4e presents feedback
information to enable retrieval of each data page with maximum SNR.
FIG. 6 shows a graphical representation of the image intensity of the five
spots (A,B,C,D,E) of a servo block, as a function of angle of incidence,
i.e. reference angle, .theta..sub.r. Thus, as the reference beam is
indexed through a range of angles, the intensity of each of the five spots
within each servo block varies, as illustrated in FIG. 9. As illustrated
therein, each longitudinal band, B1, B2, etc., corresponds to the
intensity of each spot as a function of angle of incidence. In addition,
each angle of incidence provides a "snapshot" of the servo block, i.e. the
relative intensities of each of the five spots. For example, at the angle
of incidence of 0.25.theta., the intensity of spot C would be strongest,
while spots B and D would exhibit lesser intensified images.
Once the servo blocks are recorded in the crystal, like any other
metastable holograms recorded in a photorefractive medium, the servo
blocks may be erased over time, by subsequent illuminations. To "fix" the
servo blocks, so as not to be erasable by subsequent illumination, the
crystal 3 | | |
