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Description  |
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FIELD OF THE INVENTION
This Invention relates to Associative Memory Systems and in particular to
Associative Memory Systems using spatial light modulators (SLM) such as
liquid crystal light valves (LCLV).
BACKGROUND OF THE INVENTION
The speed and computational accuracy of modern digital computers are
well-known. However, all digital computers solve problems in a sequential
fashion through the use of numerical computation. While the processing
unit contained in a simple pocket calculator can easily out-perform the
human brain in number crunching tasks, digital computers are able to
accomplish this sophisticated numerical analysis only on a step-by-step
basis. Digital computers exhibit their best abilities when presented with
a serially programmable algorithm. Digital computers are not capable of
sophisticated parallel processing, such as that required when a human
undertakes the task of pattern recognition. Problems such as comparing the
fingerprint found at the scene of a crime with a data base full of
fingerprints is the sort of practical and necessary problem that arises
and yet is not easily solved by a digital computer. To the extent that
digital computers have been programmed to match the fingerprint found at
the scene of the crime with an existing fingerprint in the files, lengthy
serial searches of memory are required to digitally achieve accurate
pattern recognition.
A matrix algebra based on an associative memory model was described by J.
J. Hopfield in his paper "Neural Networks and Physical Systems with
Emergent Collective Computational Abilities," proceedings of the National
Academy of Science U.S.A., 1982, Vol. 79, pp. 2554-2558. The Hopfield
model utilizes feedback and nonlinear thresholding to force the output
pattern to be the stored pattern which most closely matches an input
pattern presented to the associative memory system. A digital emulation of
this model requires large storage and computational effort for the
manipulation of an association matrix used in the model. For example, in
order to store two-dimensional image patterns consisting of N.times.N
pixels, the model requires a matrix with N.sup.4 entries be used.
A natural implementation of an associative memory model would be one which
uses optical technology. Optical associative memory systems store
information as patterns; so that, upon the introduction of a stored
pattern to the system,. the system is able to recall the stored pattern
and perform a match. These Optical systems achieve massive parallel
processing. The ability of an optical associative memory to perform such a
function has wide application in the fields of pattern recognition and
image understanding. Used in conjunction with a laser beam, specially
treated photosensitive film or plates act as holograms. A hologram is a
frozen "picture" of an object wherein the image of the object is recorded
on the film plate as an interference pattern between a reference beam of
plane waves (which is directed only at the photographic film) and an
object wave front (which is created by reflection from the object, where
the object wave front is made by the same coherent source that produced
the reference beam). Holograms are characterized as having extremely good
spatial coherence. The light used to produce the hologram, normally a
laser beam, exhibits a high degree of temporal coherence. In order to view
the recorded holographic image, one redirects coherent light along the
same path as the reference beam which originally recorded the hologram. A
viewer views the hologram along the same line of sight that connected the
object and the hologram during its recording. Directing a new reference
beam on the hologram causes an image to appear which, in a lensless
environment, gives rise to a three-dimensional image. The lifelike
dimensionality of a lensless image produced in a hologram is due to the
fact that, unlike a photograph, a hologram stores not only amplitude
changes but also records phase changes as interference fringes resulting
from the interaction between spatially coherent object and reference
beams.
Holograms are characterized by very precise and lifelike imaging. In
addition, a hologram, when viewed from different angles, produces
different views of the recorded image. The hologram is programmable for
use in storing a plurality of images, by varying the angle of the
reference beam used to record the image. The information stored within a
hologram is recorded throughout the holographic medium; even a portion of
the hologram retains the complete record. It therefore can be seen that
holograms are quite useful in parallel processing systems. Furthermore,
holograms are inherently useful for optical pattern recognition
mechanisms.
Among the types of holograms known in the art are the volume, Fresnel, and
Fraunhofer holograms. The volume holograms have a thickness and can be
used to record either amplitude or phase modulated images without the
generation of both primary and conjugate waves that is inherent with thin
holograms. Fraunhofer holograms are characterized as holograms that record
distant objects. Larger and closer positioned objects produce Fresnel
holograms.
The Fourier transform hologram uses a lens and is adaptable for memory
storage purposes. As is well known in the numerical analysis arts, the
Fourier transform is a mathematical tool wherein any function may be
broken up into a sum of sinusoidal superimposed patterns. This manner of
dividing a function into its Fourier components is known as defining the
Fourier transform of a function. In Fourier transform holography, one
captures an object's wave front holographically, after it has undergone a
Fourier transformation. To do this, one places a photographic holographic
plate at the back focal plane of the lens. A flat object, such as a
transparency, is placed at the same distance in front of the lens as the
photographic plate is behind it. The object's wave front, when it reaches
the plate, has been Fourier-transformed by the lens. The holographic
pattern produced as an image is quite unlike the original object. If the
object is illuminated only by coherent light, such as a laser beam, and if
a reference beam is provided at an angle to the plate, the Fourier
transform can be recorded as a hologram.
Pattern recognition has used Fourier transform holograms in another fashion
to perform the operation of convolution. The best way to understand
convolution is to look at an example. If one were to convolve a first
transparency having three dots with a second transparency having one
triangle, using a holographic Fourier transform, one obtains three
triangles, one at each position of the dots. A related operation
mathematically similar to convolution is correlation. The result of
correlating two identical objects is a sharp peak at a position
corresponding to the shift value which superimposes the two objects. The
peak is greatly reduced if the two objects are not identical, making
correlation useful in pattern recognition.
To correlate two transparencies (also referred to as objects) one simply
positions a first object one focal length in front of a lens and a Fourier
transform hologram of a second object one focal length back of this lens.
A second lens is positioned in back of the Fourier transform hologram of
the second object. The correlation of the first and second objects appears
one focal length behind the second lens.
An example of optical pattern recognition using correlation would be where
in a printed page of text one could recognize a particular word or letter
at some position on a page. Wherever the particular word appears in the
text, a bright spot of light highlights the word in the correlated image.
Wherever the word occurs on the page, there will be a corresponding bright
spot of light in the correlated image called a correlation peak. Thus, the
nature of holograms an lenses combined in an optical system using a
coherent light source allows the operation of pattern recognition to
occur. Such a device has been characterized as an optical neural computer.
The term "neural" is derived from the fact that the parallel processing of
a hologram to provide an associative memory is similar to that of a human
brain's neural system in that the stored information is not localized.
Heretofore, one such optical associative memory has been proposed by
Abu-Mostafa and Psaltis in Scientific American, vol. 256, no. 3 in an
article entitled "Optical Neural Computers," at page 88 (March, 1987). In
that article an optical thresholding device and a pinhole array were used
as part of a double hologram associative memory system.
The applicants have previously disclosed (as co-inventors) in a prior
patent an associative memory system entitled "ASSOCIATIVE HOLOGRAPHIC
MEMORY APPARATUS EMPLOYING PHASE CONJUGATE MIRRORS", U.S. Pat. No.
4,739,496. Also, the applicants are co-inventors in a now pending
application "ASSOCIATIVE HOLOGRAPHIC MEMORY APPARATUS EMPLOYING PHASE
CONJUGATE MIRRORS IN A TWO-WAVE WAVE MIXING CONTRA-DIRECTIONAL COHERENT
IMAGE AMPLIFIER", U.S. Pat. No. 4,750,153. (The disclosures contained in
both patents are hereby incorporated by reference.) Hughes Aircraft
company, the assignee of this application, is also the assignee of these
two patents. These systems also employ primarily all-optical elements.
As indicated above, optical elements, such as the hologram, make excellent
associative memory storage devices. When a distorted input image is
presented to a system which includes at least one hologram (containing a
clear representation of that image), the system processes light through
its components in such a manner as to correlate and match the distorted
input image with one of the images stored on the hologram. The sharper the
correlation peaks, the better the match. All optical systems are excellent
parallel processors but generally may not be shift-invariant and
furthermore, they may exhibit optical and gain losses in the system as the
image is processed. In order to achieve a good match, an optical
associative memory must have good thresholding and gain so that the
correlation peak which reconstructs the reference beam (when the image is
to be reconstructed) is sharp and bright. Losses of light intensity in the
system are inevitable as the light is processed through an optical system
as disclosed in the above-incorporated applications or as that disclosed
in the Abu-Mostafa article, supra. Additionally, reconstruction and phase
conjugation of the reference beam in the all-optical systems described in
U.S. Pat. Nos. 4,739,496 and 4,750,153, is achieved inherently by use of
phase conjugate mirrors, (PCMs) using for example BaTiO.sub.3 material. In
such systems, thresholding is determined by physical processes in the PCMs
and is not easily alterable nor readily adjustable. Also, such optical
systems heretofore have required at least a second for the PCM to respond.
BaTiO.sub.3 -based optical techniques are relatively slow, in a computer
sense. Phase conjugate mirrors of an all-optical component system may be
used to fully reconstruct and return an image to its point of origin to
achieve pattern recognition. Non-linearities in the phase conjugate
mirrors are used to select those stored objects which exceed a threshold,
based on the overlap of computed integrals of the object input with the
stored objects. Although, experimentally, store-and-recall of two objects
with shift invariance, was achieved, the gains achieved by phase conjugate
mirrors were not enough to overcome hologram losses. Additionally, the
non-linearities of the phase conjugate mirrors were difficult to control.
It is therefore an object of this invention to provide a system which makes
use of the pattern recognition properties of a hologram but in such a
manner that optical losses are kept to a minimum, thresholding with gain
achieved, and sharp correlation of images at the hologram accomplished,
with shift invariance. U.S. Pat. Nos. 4,546,248 and 4,556,986, both issued
to Glenn D. Craig and assigned to the United States (NASA), disclose
electro-optical systems used to process images with incoherent light
sources. The systems represent attempts to vary spatially the optical gain
of signals without thresholding or enhancement of optical images. Such
references show the state of the electro-optical art, but do not in
themselves advance the achievement of the objects of this invention to
provide an associative memory system.
Liquid crystal light valves (LCLVs) suitable for use in the present
invention include, for example, those shown in the following U.S. patents
all assigned to Hughes Aircraft Company, the assignee of the present
invention:
U.S. Pat. No. 3,824,002, "Alternating Current Liquid Crystal Light Valve",
issued to T. D. Beard, on July 16, 1974; U.S. Pat. No. 4,019,807,
"Reflective Liquid Crystal Light Valve with Hybrid Field Effect Mode",
issued to Boswell et al. on Apr, 26, 1977;
U.S. Pat. No. 4,018,509, for "Optical Data Processing system with
Reflective Liquid Crystal Light Valve", issued to Boswell et al. on Apr.
19, 1977;
U.S. Pat. No. 4,378,955, for "Method and Apparatus for a Multimode Image
Display with a Liquid Crystal Light Valve", issued to Bleha et al. on Apr.
5, 1983;
U.S. Pat. No. 4,239,348, "High Resolution AC Silicon MOS-Light Valve
Substrate" issued to J. Grinberg et al. on Dec. 16, 1980;
U.S. Pat. No. 4,443,064 "High Resolution AC Silicon MOS-Light Valve
Substrate" issued to J. Grinberg et al. on Apr. 17, 1984; and,
U.S. Pat. No. 4,127,322, "High Brightness Full Color Image Light Valve
Projection System", issued to Jacobson et al., on Nov. 28, 1978.
SUMMARY OF THE INVENTION
An associative memory system capable of recalling a complete and
undistorted stored image when the memory system is provided with an input
image which is distorted (or is a part of the complete stored image or
both) is disclosed. Such an input image is hereinbelow referred to as a
"distorted image." The associative memory system of this invention
includes a holographic means for recording and reconstructing a first
object transform of a first object. Image transforming apparatus, such as
a lens, provides a second transform of a second object or set of second
objects to the holographic means. The holographic means forms the product
of the transforms of the first object, second object, and the first
reference beam used in recording the hologram. This composite product,
known as a distorted second reference beam, is transformed by a
correlation lens into a correlation function which is the transform of the
product. The correlation function is threshold limited and conjugated by
spatial light modulator means which feeds back the threshold limited
conjugate correlation back to the holographic means for reading out the
first object transform stored therein. An image then appears in the output
plane as a reconstructed first object.
In present preferred embodiment, the spatial light modulator (SLM) means
comprises a liquid crystal light valve (LCLV) means.
The liquid crystal light valve (LCLV) means is electronically controllable
and modulates a readout beam in accordance with the correlation function
incident on the photoconductor side of the LCLV. The modulated readout
beam is then fed back to the holographic means.
A second leg of the system may be provided wherein a second LCLV receives a
focused image from the hologram. This second LCLV modulates a second
readout beam using a polarizing beam splitter. When the beam splitter is
illuminated by the bright readout beam, an enhanced, and amplified object
beam resonates within the memory system, providing a truer match on the
holographic plate.
The processing of data, for pattern recognition purposes, accomplished by
the associative memory of the present inventive system may be generalized
as an associative memory system where a first body of data is transformed
and is recorded within a memory storage means, such as a hologram, using
transforms (plane or spherical waves) of a reference set of data (delta
functions) and the memory system is presented with a second incomplete
body of data (that is, the distorted input image). For example, the nature
of the transformation may be a Fourier transform. A second transform set
of data corresponding to the second body of data is provided to the
hologram. The hologram, in conjunction with a correlation device,
generates the correlations of the first and second bodies of data. Such a
generalized model also includes a liquid crystal means for conjugating and
thresholding this correlation function. This improved correlation function
is transformed as an enhanced reference beam which is then presented to
the data storage area where the data store in the system is kept. This
results in the reconstruction of the first body of data.
An associative processing occurs wherein the transformed data storage
combines and correlates these first stored and transformed set of data
with the second transformed input data and the reference data to form the
composite product set of data, whereby a first stored body of data is
associated with a second incomplete body of data. In applicant's presently
preferred embodiment, the parallel processing and pattern recognition are
accomplished optically, and thresholding and gain are accomplished
electro-optically using LCLV means.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a simplified first embodiment of one presently preferred
embodiment of the associative memory of this invention using a liquid
crystal light valve.
FIG. 2 (a) shows the configuration of the grid component 47 like the grid
of FIG. 1 in greater detail.
FIG. 2 (b) shows a plan view of the grid 33 of FIG. 1 in enlarged detail.
FIG. 3 shows a resonator associative memory system in accordance with a
preferred embodiment of this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIG. 1 the preferred embodiment of the invention of this
application is generally shown and may operate in a first recording mode
and a second reconstruction mode. This description details the associative
memory system operating in the reconstruction mode. A hologram 18 has
stored within it at least two coherent wave amplitudes generated by at
least two different objects. The hologram 18 carries within its
light-sensitive medium both phase and amplitude information with regard to
the objects stored therein. The particular hologram 18 which is described
in detail in the following description is a Fourier transform hologram.
However a Fresnel or a volume hologram can also be used. Utilizing a
Fresnel or a volume hologram will, however, result in the loss of shift
invariance, which is an important advantage of the Fourier transform
hologram. By "shift invariance", it is meant that an object will be
recognized and reconstructed regardless of its position in the input
plane.
When the hologram 18 is irradiated by a complex wave front which is a
distorted version of the stored image, the hologram 18 may be used in
conjunction with the other components of the system, to match the
incomplete input image with the stored image on the hologram 18.
The distorted image is provided to the associative memory system by the
object input plane 12. Light from the object input plane 12 is directed at
the beam splitter 14. The beam splitter 14 redirects light from the object
input plane 12 to the Fourier transform lens 16. It will be noted that the
object input plane 12 is one focal length distance in front of the Fourier
transform lens 16. The hologram 18 is one focal length in back of the
Fourier transform lens 16. A composite product beam (which is a collection
of distorted reference beams, hereinafter referred to as "distorted
reference beam") is generated by the object wave front incident on the
hologram, and is provided through the correlation lens 32. The correlation
lens 32 is in back of the hologram 18.
The composite product or distorted reference beam is provided to the beam
splitter 20. This distorted reference light beam is then provided through
the alignment grid 33 to the photoconductor side 29 of the liquid crystal
light valve (LCLV) 28, which is one focal length in back of the
correlation lens 32. The image received by the LCLV 28 may be
characterized as the correlation of the stored image of the object, stored
within the hologram 18, and the distorted input image from the object
input plane 12.
In simplified mathematical terms, let
a=an object "a";
A=a Fourier transform of the object "a" stored within the hologram 18;
b=a reference "b", the Fourier transform of which, B, is also stored in the
hologram 18.
The quantities A and B are in general complex. Then, the amplitude
transmittance of the hologram 18 is proportional to the magnitude squared
of the sum of A and B or .vertline.A+B.vertline..sup.2.
If now a distorted input image a' is provided by way of the Fourier
transform lens 16 to the hologram 18, a transformed input image A' is
presented to the hologram 18. A Fourier transform hologram 18, when
arranged in a system as shown in FIG. 1, gives rise to the correlation of
the distorted image a' with the quantity a which is in turn convolved with
b.
It is well known that the convolution of two functions in the spatial
domain equals the product of the Fourier transforms of each function in
the spatial frequency domain. The symbol "*" is used herein to indicate
convolution, then, b * (a' * a) means b convolved with the quantity
consisting of the correlation (* *.circle.) of a' with a.
In the spatial frequency domain, the amplitude transmitted by the developed
hologram is proportional to the expression
A'.vertline.A+B.vertline..sup.2. If one were to expand this product, one
would derive the following expression:
A'(A.sup.2 +AB+B.sup.2 +BA).
Rearranging these terms, one would obtain the following expression:
A'(A.sup.2 +B.sup.2)+A'(BA)+A'(AB),
where A is the complex conjugate of A and B is the complex conjugate of B.
The first two terms of this expression, are respectively, the zero order
term and the order term and the -1 order term, neither of which are of
direct concern to this invention; however, the last term, A' (AB) is one
which is important to this invention. B is a tilted plane wave, and this
plane wave is derived from a reference b, which is a shifted delta
function. Therefore, in the spatial domain, the quantity b*(a' * a)
represents a shifted version of the correlation of the distorted input
image a' with the stored a. FIG. 1 shows that the above quantity is
present simultaneously at the liquid crystal (31) and photoconductor 29
sides of the liquid crystal light valve (LCLV) 28.
The above mathematical results are in keeping with what is experimentally
observed in optical systems. If a first object is placed one focal length
in front of a Fourier transform lens and a Fourier transform hologram of a
second object is placed in back of the same Fourier transform lens by one
focal length; then, a second lens is positioned one focal length behind
the Fourier transform hologram of the second object, a screen which is one
additional focal length behind the second lens will produce a correlated
and convolved image of the first object and the second object which is
stored in the Fourier transform hologram. If the stored image transform B
is the Fourier transform of a delta function b, (i.e. a tilted plane
wave), then the remaining terms of interest A' (A) in the frequency domain
correspond to the spatial domain correlation of input image a, with the
stored image a. These relationships of association arise intrinsically
when a Fourier transform hologram 18 is used within an associative memory
as shown in FIG. 1. The correlated images, as stated hereinbefore, are
provided by the beam splitter 20 to the photoconductor side 29 of the
liquid crystal light valve 28. This correlation image is then presented as
writing light to a spatial light modulator, which is a liquid crystal
light valve 28 in the preferred embodiment. The liquid crystal light valve
is an optical-to-optical image transducer that is capable of accepting a
low intensity input light image and converting it, in real time, to an
output image with light from another source. The device is designed so
that the input and output light beams are completely separated and
non-interacting. Other spatial light modulators such as magneto-optic
modulators or multiple quantum well heterostructure modulators may also be
used.
One of the significant aspects of the invention and system disclosed in
this application is the manner in which the correlated image provided to
the thresholded and conjugated liquid crystal light valve 28 is enhanced
by the feedback loop 21. A feedback arrangement of mirrors and lenses such
as that described by U. H. Gerlach et al., ("Single-Spatial Light
Modulator Bistable Optical Matrix Device Using Optical Feedback", Optical
Engineering, Volume 19, No. 4, July-August 1980, pp. 452-455) may be
suitably modified and used to feedback the modulated readout beam to the
hologram.
The LCLV is preferably operated in the forward slope mode and a positive
feedback loop is established using the feedback arrangement which images
the liquid crystal side 31 of LCLV 28 onto the photoconductor side 29 with
unity magnification and in registration. Lens 32, which is one focal
length away from both the mirror 92 and the LCLV photoconductor 29, images
the Fourier transform of the input on mirror 92 and the photoconductor 29.
The Fourier transform can be assumed to consist of a set of spots. Because
of the positive feedback loop, spots above the threshold level will latch
at the full intensity level of the readout light. These spots will be
output colinearly with the input. A large gain between the input 12 and
the output 34 is possible.
The phase of the Fourier transform of the distorted reference will be lost
in this configuration. However, it is not important in an associative
memory system application since each referenced beam reads out the
hologram 18 separately. Each of the superimposed holograms is incoherent
with respect to other recorded images. Thus, this invention can function
in the reference leg (FIG. 1 and 71 of FIG. 3) of the associative memory
as a high gain pseudo-conjugator with an adjustable threshold. The light
in the feedback loop 21 (FIG. 1) is polarized so the threshold level can
be adjusted by varying the polarizer 86 orientation.
The image presented to the photoconductor 29 side of LCLV 28 by the
correlation lens 32 and the beam splitter 20 has partially distorted
spurious light associated with this correlated image due to the input of
an imperfect real time image from the object input plane 12. In order to
remove distortions and provide a threshold value of amplitude intensity of
the optical signal to the system, the distorted correlation image incident
on photoconductor side of the LCLV 28 modulates a readout beam 36 incident
on the liquid crystal side 31. The modulated readout beam is fed back
through a feedback arrangement of mirrors and lenses. The modulated
readout beam proceeds from the polarizing beam splitter 30 as an amplitude
modulated signal onto mirror 80. This modulated readout beam diverges
until it reaches imaging lens 82. Imaging lens 82 causes this signal to
converge as the modulated readout beam is reflected off mirror 84. The
readout beam full converges at imaging point 57 where the image is
inverted and begins to diverge as it passes through adjustable polarizer
86. As the beam passes through adjustable polarizer 86, it is reflected
off the surface of mirror 88 and fully diverges onto imaging lens 90.
Imaging lens 90 then causes the iterating modulated readout beam to
converge onto point P.sub.0 of the mirror 92. It is important that the
mirror 92 be orthogonal to a line connecting the mirror 92 and the imaging
lens 90. The beam splitter 20 then receives the reflected image from point
P.sub.0 and passes that reflected image onto the hologram 18, by way of
the correlation lens 32, as well as back to converging point P.sub.1 for
another feedback iteration through loop 21. This enhanced beam then serves
to phase modulate a newly diverging beam which originates at point
P.sub.2. This phase modulated beam, when passed through the polarizing
beam splitter 30, amplitude modulates the new readout beam 36 and the
process of positive feedback through the iteration loop 21 is repeated
again. Each interaction assures yet a more enhanced and stable output
signal, which may be read at the output plane 34. The light intensity of
the correlation image presented to the photoconductive side 29 of the LCLV
28 which is above a threshold intensity will be enhanced by feedback,
whereas portions of the correlation image with an intensity below
threshold is eliminated or made negligible. In other words, if the image
on the hologram LCLV 28 is partially distorted, such as that of a circle
having flares or wings, these flares or wings at the outer portions of the
correlation are clipped, so that the adjustable thresholding provided by
the LCLV 28 results in a smooth, round correlation image. The liquid
crystal light valve (LCLV) 28 operates to modulate the phase of the
readout beam 36 according to the control provided by the writing light.
The polarizing beam splitter 30 directs the high intensity coherent
readout beam 36 to the LCLV 28 (liquid crystal side 31) and converts the
phase modulation of the reflected modulated readout beam into amplitude
modulation.
The polarizing beam splitter 30 provides half its signal back to the
hologram 18 along the feedback loop 21 to the hologram 18 shown in FIG. 1.
By operation of the liquid crystal light valve 28 (LCLV), a stronger
signal is provided to the hologram 18. This signal processing assures
sharper and clearer correlation of the image from the object input plane
12 with a stored image of the hologram 18, for viewing at the output plane
34.
The feedback loop can be repetitively performed until a stable thresholded
conjugated signal is obtained. This signal reads out the hologram. The
positive feedback mode enhances the nonlinear operating characteristics of
the LCLV which, in turn, improves the thresholding capability of the LCLV.
Furthermore, the inherent bistable characteristic results in a stable
output, that is the modulated readout beam continues even after the
original input to the photoconductor side switched off. Therefore, once
the input image is fed to the photoconductor side 29 of the LCLV and the
input image is "latched" onto, it can remain "latched" even if the input
image is thereafter turned "off", until the latched input image is
deliberately unlatched.
It will be noted that the optical conjugation and threshold effect achieved
in the preferred embodiment uses an LCLV to phase-modulate the readout
beam 36. Alternatively, phase-modulation of the readout beam 36 could be
achieved by use of a spatial light modulator (SLM) 28 such as a
Microchannel SLM or other electro-optical structure which is optically
addressable.
Thus, many variations of existing SLMs may be used in the system disclosed
herein to modulate the readout beam 36, and some of these variations of
the SLM would allow a reduction in the number and type of elements
essential to the functioning of the iteration loop 21.
An additional feature of the invention which is useful to the alignment
procedure necessary in the reference leg of the preferred embodiment of
this invention (all of FIG. 1 and the reference leg of FIG. 3) is shown at
FIGS. 2 (a) and FIG. 2 (b). FIG. 2 (b) shows a preferred grid 33 of FIG. 1
magnified. It will be noted that the grid 33 is a half tone mask which is
placed against the photo conductor side 29 (of FIG. 1) of the LCLV 28. It
will be noted that in the preferred embodiment of the grid 33, circular
apertures 43 are spaced in a uniformed matrix configuration to provide
proper alignment of the P.sub.1 input image to point P.sub.1 of the LCLV
28 (FIG. 1).
Alternatively, FIG. 2 (a) shows a grid 47 having a checker-board pattern of
square-shaped apertures 45 which are alternately spaced along the grid to
provide alignment for the input image to the LCLV of the reference leg.
Ordinarily, if the alignment between the image on the photoconductor side
29 and the liquid crystal side 31 of the LCLV 28 (FIG. 1) is not perfect,
or if the magnification is not unity, then threshold limited image is not
stable and may grow to fill the entire field of view. To avoid this
problem and to enhance the misalignment tolerance so that it may be
greatly increased, a grid such as 33 or 47 is used so as to confine
blooming to within each clear aperture, such as apertures 43 of FIG. 2 (b)
and 45 of FIG. 2 (a). Thus, it is possible to trade off space bandwidth
product for alignment ease by using larger period grids such as 47 and 33.
FIG. 3 shows an LCLV based optical associative memory system in a resonator
configuration. A distorted image is input at the input plane 42 to a beam
splitter 44. A portion of the image from the input plane 42 is imaged onto
the photoconductor side 74 of LCLV 100 of the feedback system. This
activates LCLV 100 which in turn modulates the phase of readout beam 81.
Polarizing beam splitter 76 converts this phase modulation into amplitude
modulation. The modulated readout beam is then directed through a system
of lenses and mirrors. The Fourier transform lens 46 receives the
modulated beam after the object leg iteration, and passes the enhanced
signal onto the hologram 48, where the correlation of the input image is
processed by a first iteration loop 71. A beam splitter 54 provides the
photoconductor side 61 of a liquid crystal light valve 62 with a convolved
and correlated image for further processing to eliminate losses and to
provide thresholding.
The LCLV 62 phase-modulates a readout beam 101 which is then
amplitude-modulated at the polarizing beam splitter 64. The output of the
polarizing beam splitter 64 is presented back to the beam splitter 54 and
the correlation lens 52, and passed by the mirror 50 back to the hologram
48. In the embodiment shown in FIG. 3, a resonator effect occurs because
rather than reading out an image at the LCLV 74, the enhanced output image
is now impressed upon the second iteration loop 73. The beam splitter 44
provides this enhanced image to the photoconductor side of LCLV 74. The
image on the photoconductor side of LCLV 74 then phase-modulates the
readout beam 81, which is projected through a polarizing beam splitter 76
and back to the Fourier transform lens 46 and the hologram 48. The
enhanced signal continues to loop back and forth between the first
iteration loop 71 and the second iteration loop 73 until the stable states
of the overall system of the object and reference pairs stored in the
hologram 48 are achieved.
The input image provided to this system by the input plane 42 is processed
through beam splitter 95 and onto imaging lens 93, where the distorted
input image is divided by the beam splitter 44. Beam splitter 44
simultaneously provides the input image to the liquid crystal light valve
(LCLV) 74 of the object leg. After passing through the hologram 48, the
mirror 50 provides the correlation lens 52 of the reference leg with a
composite image ready to be enhanced by the referenced leg iteration loops
71. The light provided to the beam splitter 54 by the correlation lens 52
is focused onto the photoconductor portion 61 of the LCLV 62 after passing
through the grid 33. This writing light serves to phase modulate the read
out beam 101 that is presented to the liquid crystal portion 63 of the
LCLV 62. The readout beam 101 is then amplitude modulated by the light
present on the photoconnector 61 side of the LCLV 62 and begins its
iteration around loop 71. The modulated readout beam is reflected off of
mirror 97, passing through imaging lens 101 and onto mirror 99, where it
is reflected through the adjustable polarizer 98 and reflected onto the
surface of mirror 96. Mirror 96 then provides the modulated beam to the
imaging lens 94 where the beam is provided back to the mirror 103. The
modulated signal at this point reflects off of mirror 103 and back down to
the beam splitter 54 where the light is then simultaneously presented back
to the grid 33 and photoconductor surface 61 of the LCLV 62 of the
reference leg and back through the correlation lens 52, so that the
modulated beam may be presented to the hologram 48 in an enhanced manner
for further processing in the object leg of the resonator configuration of
FIG. 3.
This enhanced referenced beam is then presented by means of the Fourier
transform lens 46 onto the LCLV 74 for writing a phase-modulation code
suitable for phase-modulating the readout beam 81 as it impinges upon the
polarizing beam splitter 76. Polarizing beam splitter 76 acts to amplitude
modulate the read out beam 81 in a manner which corresponds to the
phase-modulation occurring at the liquid crystal light valve due to the
light falling on the photo conductor surface of the liquid crystal light
valve 74 as enhanced by the referenced leg 71. The amplitude modulator
readout beam, modulated in accordance to the manner determined by the LCLV
74, and the polarizing beam splitter 76, is reflected off the surface of
mirror 37 and onto imaging lens 83. The light then iterates through the
loop to the object leg loop 73 in a manner similar to the journey of the
amplitude modulated readout beam of the reference leg 71. From mirror 85,
the amplitude modulated readout beam passes through the polarizer 87 where
its threshold may be adjusted and then onto mirror 89 for imaging by lens
91. The amplitude modulated signal passes to the beam splitter 44 where it
is carried through the imaging lens 93 and out for viewing at the output
plane 79 as well as back to the hologram 48 for another iteration through
the reference leg 91 so that the entire resonating cycle may commence
again.
Thus, a continually enhanced image iterates first through the referenced
leg 71 back to the hologram | | |
