Data processing system utilizing a holographic optical element

The data processing system makes use of a holographic optical element comprised of a number of facets, each facet containing a grating pattern for diffractively redistributing input light so that output light from a facet or a combination of facets represents encoded mathematical data.

CROSS REFERENCE TO RELATED APPLICATION 
Reference is made to my copending application, filed on July 31, 1980, Ser. 
No. 174,156, for "MULTI-FACETED HOLOGRAPHIC OPTICAL ELEMENT AND METHODS OF 
MAKING AND USING SAME". 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to data processing systems, and pertains 
more particularly to an optical system of this type utilizing a 
holographic optical element for splitting and redistributing input energy 
into output energy representative of certain mathematical data. 
2. Description of the Prior Art 
There are many mathematical operations in which an array of output data is 
obtained from multiplying an array of input data by a set of fixed 
numerical values arranged in a matrix. An example of such a vector-matrix 
multiplication operation would be, for instance, the performance of a 
discrete Fourier transform on an input vector. Here, an N element input 
vector would be multiplied by an M.times.N matrix to obtain an M element 
output vector. 
Such a multiplication process can, of course, be implemented by a digital 
computer. Present digital computers, however, carry out operations 
serially so that if the vector and matrix are large, several minutes of 
computation might be required. Optical systems, on the other hand, can 
perform the requisite multiplications in parallel so that the entire 
operation can be completed in the nanosecond to microsecond time range. 
The problem in optical computing is centered around the redistribution of 
the input light signals so as to provide appropriate output signals. One 
prior art redistribution method known to me employs fiber optics. Here, 
one bundle of optical fibers is illuminated by a light source whose 
intensity represents the value of one of the N numbers in the input 
vector. The fiber bundle is then divided into M sub-bundles with the 
number of fibers per sub-bundle being proportional to the values in one 
column of the matrix. The output ends of the sub-bundles are then located 
so that each sub-bundle illuminates one of M output detectors. The process 
is repeated for the other N inputs, each of which sends appropriately 
proportioned sub-bundles to the M output detectors. 
The fiber method is limited because the number of fibers per bundle is 
relatively small (.apprxeq.400) so that the relative light intensity 
redistribution cannot be performed very accurately. The system is also 
difficult to replicate because each of the many fibers must be 
individually connected. Obviously, where vast amounts of data are to be 
processed, the proper connecting of numerous optical fibers proves to be 
very costly. 
SUMMARY OF THE INVENTION 
The optical computing problem, as dealt with in this application, is 
reduced to finding a suitable method to take the light from N inputs and 
redistribute it with proper weighting to M outputs. 
A general object of my invention is to provide a data processing system 
utilizing a relatively low cost holographical optical element for the 
light redistribution. 
Another object is to achieve tremendous speed increases in the processing 
of mathematical data. 
Yet another object of my invention is to provide a system for processing 
data that is quite versatile, rendered so by virtue of the inclusion 
therein of a readily constructed holographic optical element utilizing 
individual gratings contained in numerous facets. 
Briefly, a holographic optical element when used in the practicing of my 
invention splits and redistributes the input light energy according to a 
given mathematical algorithm. In this regard, the holographic optical 
element constitutes a photographic film that has been divided into 
numerous facets. The facets themselves are also subdivided so that each 
sub-facet diffracts light into different areas or regions as far as an 
output plane is concerned. At the output plane is placed a bank of light 
detectors, so that whatever light energy strikes the plane due to the 
diffraction that takes place within the holographic optical element is 
representative of encoded mathematical output data. Light diffracted from 
a number of different facets can be directed to a given output area. In 
this latter situation, the sum of the intensities that are detected in the 
output plane will constitute the transformed mathematical data.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In this description, it will be assumed that it is desired to implement an 
optical computing system which will multiply an N element input vector by 
an M.times.N matrix to produce M output signals, this being a rather 
general problem. 
In FIG. 1, my system has been denoted generally by the reference numeral 
10. Playing a very important role in the practicing of my invention is a 
holographic optical element indicated in its entirety by the reference 
numeral 12. Actually, the element 12 is a transformation hologram which 
splits and distributes light, all as will be more fully explained 
hereinafter. At this time, though, it is to be perceived that the element 
12 is comprised of a number of volume facets 14 which in this example are 
arranged in horizontal and vertical rows. The width of a facet 14 is on 
the order of a centimeter or so and the number that have been depicted is 
only representative of a far greater number (N) of such facets. 
At this stage it will be well to refer briefly to FIG. 2. In FIG. 2, only 
the facet 14 in the upper left-hand corner of the holographic optical 
element 12 is depicted. It is subdivided into M sub-facets in order to 
redistribute the input light in accordance with the predetermined 
mathematical operation (e.g., matrix multiplication) that is desired. Some 
of these sub-facets have been indicated by the reference characters 14a, 
14b, 14c, 14d, 14e, 14f, 14g, 14h, etc. 
It will be explained at this point that the input light that is directed 
onto the hologram or holographic optical element 12 consists of N beams 
with various intensities which represent the input data. The input light 
beams or wavefronts have been denoted generally by the reference numeral 
16 in FIG. 1. The light 16 is produced from an electronic signal by 
energizing any or all of N light emitting diodes or it can be achieved 
optically by passing an enlarged laser beam through a film whose 
transmittance is proportional to the N input data values. For the sake of 
illustration, four LEDs 18, 20, 22 and 24 have been depicted, each being 
associated with one of the four facets 14 shown in the top row. A lens 26, 
as pictured in FIG. 2, is positioned between each LED 18-24 and the 
particular facet 14 with which it is positionally associated. It must be 
borne in mind, though, that a relatively large number of facets 14 are 
incorporated into the transformation hologram or element 12 and that there 
is one such LED 18-24 and one lens 26 for each of the N facets 14. 
Considering only the upper left-hand facet 14 of FIG. 1, which facet 14 has 
been enlarged in FIG. 2, it can be seen that in this instance the light 
impinging on this particular facet from the LED 18, the input light being 
in the form of the beam 16a, is diffracted by the facet 14 and is split 
into M parts or angularly separated beams, several of which are 
illustrated and labeled 28a, 28b, and 28c. The diffraction results from 
the specific grating patterns that have been recorded in the facet 14, and 
more specifically by the individual grating components contained in 
sub-facets 14a-14h, etc. As illustrated in FIG. 2, the gratings within the 
facets 14a-14h deflect portions of the input light 16a to produce the 
several angularly related beams 28a-28c which will impinge or strike a 
target at different spatial locations in output plane 30 which can be a 
film or an array of detectors. If we assume the plane 30 to contain a 
detection array, then when the several beams 28a-28c strike or impinge 
upon the detectors, the light signals provided thereby are detected. The 
output locations thereof correspond to positions in the M element output 
data vector. 
Owing to the rather small scale of FIG. 1, the upper left-hand facet 14 has 
been enlarged in FIG. 2, as already explained. Recapitulating, it is only 
necessary to recognize that the particular facet 14 that is now being 
examined is actually divided or composed of M sub-facets in order to 
redistribute the input light from one source in accordance with the 
appropriate mathematical weighting function. Stated somewhat differently, 
each sub-facet diffracts light into a different area or at a different 
location as far as the output plane 30 is concerned. Since the plane 30 
has been somewhat arbitrarily considered to constitute a detector array, 
then, as already explained, the several points or areas labeled 30a, 30b, 
and 30c represent photo-diodes and are where the angularly related beams 
28a, 28b, and 28c are detected. 
Having presented the foregoing description, the manner in which my system 
10 functions and the benefits to be derived from a practicing of my 
invention should be readily understood. Nonetheless, a brief operational 
description should make it clear as to the various advantages that are 
achievable. It should be understood that my system performs the general 
mathematical operation of multiplying an N element input vector by an 
M.times.N element matrix to arrive at an M element output. The values of 
the N inputs are represented by the intensities of N light emitting diodes 
such as the LED 18. Each LED illuminates one of the N facets 14 of the 
redistribution hologram 12. Each of the N facets is further subdivided 
into M sub-facets which serve to diffract a fraction of the input light in 
a given facet into the various M output detectors located in plane 30. The 
desired fraction of light to be diffracted by each sub-facet is given by 
the values of the matrix elements in the M.times.N matrix. The fraction is 
implemented by area modulating the sub-facets as shown in FIG. 2. That is, 
a larger sub-facet diffracts a larger fraction of the input optical power 
to a given output detector. Of course, it is the direction of the output 
beams, such as those denoted by the numerals 28a-28c that determine the 
output locations or areas. As shown in FIG. 1, a large lens 24, which is 
located immediately after hologram 12 and whose focal length is equal to 
the distance between the lens 24 and the output plane 30, serves to focus 
the beams 28a, 28b, 28c (and other such output beams that have not been 
illustrated) onto the small detectors 30a, 30b, 30c (and other such 
detectors that have not been referred to) located in plane 30. 
Each of the M detectors will, in general, receive light from one sub-facet 
in each of the N facets. The total amount of light received by each of the 
M output detectors then represents the M output values in the M element 
output vector. 
Inasmuch as the manner of making or constructing facets is fully described 
in my hereinbefore identified copending application, it is not believed 
necessary to present a detailed description at this time as to the 
techniques utilized in the forming of the facets and sub-facets. However, 
it will be of assistance to point out that each sub-facet 14a-14h etc., is 
exposed to a coherent object beam and a coherent reference beam, these 
beams successively impinging upon the photographic film at each surface 
each of a particular sub-facet. The various sub-facets and facets 
collectively constitute the holographic optical element 12 used in the 
practicing of my invention. 
Several techniques are possible to record the variable area sub-facets. The 
first technique is to use an exposure mask (whose purpose is more fully 
described in my copending application) in which the aperture size can be 
varied under computer control. Thus various size sub-facets can be 
recorded by exposure through the open area of the mask. Alternately, a 
mask with fixed open area A.sub.o corresponding to the size of the 
smallest desired sub-facet can be used. Larger sub-facets are then 
constructed by recording several of these small areas A.sub.o next to each 
other such that they all contain identical gratings to diffract light to 
the same output location. 
As already mentioned in my copending application, the advantage of using 
the interferometric recording process to construct the redistribution 
hologram 12 is that the hologram can be recorded in a volume phase film 
such as dichromated gelatin. Such holograms can have nearly 100% 
diffraction efficiency which leads to high signal-to-noise ratios for 
optical computing systems. Such holograms can also be easily replicated by 
optical means. 
An alternate technique, that is, to use computer generated holograms for 
the redistribution hologram 12 is also within the scope of the invention. 
Computer generated holograms would have one advantage over 
interferometrically recorded volume holograms in that the sub-facet areas 
could be much more easily varied and controlled. Thin, phase, square wave 
gratings, such as those recorded in photoresist, would be most suitable 
for this application because their diffraction efficiency can reach 40%.