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Having described the invention, what is claimed as new and secured by
Letters Patent is
1. Memory apparatus for read/write storage and retrieval of digital data,
the apparatus comprising
optical storage means including a multidimensional holographic storage
medium for storing phase holographic images representative of the digital
data,
selected regions of said medium containing said holographic images being
independently addressable by an interrogating beam of light characterized
by selected spatial positions,
said holographic storage medium comprising a spectral hole burning material
having absorption regions independently addressable by an interrogating
beam of light having a selected frequency,
said spectral hole burning material comprising selectively bleached
absorption structures forming addressable frequency channels, and
said phase holograms being stored in index modulation regions near spectral
hole absorption edges.
2. Apparatus according to claim I, further comprising
frequency channeling means for providing controlled bleaching of said
spectral hole burning material to generate said bleached absorption
structure frequency channels in said spectral hole burning material.
3. Apparatus according to claim 2, wherein said frequency channeling means
comprises a narrow linewidth tunable laser.
4. Apparatus according to claim 2, further comprising
means for applying an external electric field of selected voltage to said
optical storage medium to induce Stark effect broadening of said spectral
holes, so that at a given frequency of interrogating light, a different
phase hologram can be stored for each of a plurality of voltages, whereby
multiple phase holograms can be stored for each frequency of interrogating
light.
5. Apparatus according to claim 4, further comprising Stark effect control
means for controlling voltage of the applied electric field to address
holographic images recorded at selected electric field strengths.
6. Apparatus according to claim 2, further comprising
means for applying a physical stress of selected magnitude to said optical
storage medium to induce broadening of said spectral holes, whereby at a
given frequency of interrogating light, a different phase hologram can be
stored for each of a plurality of stress field magnitudes, such that
multiple phase holograms can be stored for each frequency of interrogating
light.
7. Apparatus according to claim 6, further comprising physical stress
control means for controlling the magnitude of the physical stress applied
to said optical storage medium to address holographic images recorded at
selected stress field magnitudes.
8. Apparatus according to claim 6, wherein said means for applying physical
stress to said optical storage medium comprises at least one piezoelectric
element in contact with at least one surface of said optical storage
medium, said piezoelectric element being excitable by an applied
electrical signal to apply a selected physical stress to said optical
storage medium.
9. Apparatus according to claim 1, further comprising
read/write means for reading and writing holographic images into and out of
said medium,
said read/write means comprising a frequency agile laser for generating an
interrogating beam of laser light having at least one selected frequency.
10. Apparatus according to claim 9, wherein said read/write means further
comprises spatial light modulator means interposed between said frequency
agile laser and said medium, for providing selected spatial modulation of
said interrogating beam of laser light.
11. Apparatus according to claim 9, further comprising beam steering means,
in combination with said frequency agile laser, for steering said laser
beam in at least a first direction.
12. Apparatus according to claim 11, further comprising control means for
controlling said interrogating beam independently in frequency and spatial
position.
13. Apparatus according to claim 12, wherein said control means comprises a
microprocessor.
14. Apparatus according to claim 1, wherein said spectral hole burning
material comprises porphyrin tautomers in a polyethylene matrix.
15. Apparatus according to claim 1 wherein said spectral hole burning
material comprises organic dyes in organic polymer host materials.
16. Apparatus according to claim 1 wherein said spectral hole burning
material comprises organic dyes in inorganic host materials.
17. Apparatus according to claim 1 wherein said spectral hole burning
material comprises color centers in glass.
18. A method for storing digital data, the method comprising the steps of
configuring a multidimensional holographic storage medium from a spectral
hole burning material,
utilizing a laser to generate in the spectral hole burning material a set
of selectively bleached absorption structures, to form addressable
frequency channels, and utilizing a laser to write phase holograms
representative of the digital data in index modulation regions near
spectral hole absorption edges.
19. Optical interconnection apparatus for selectively interconnecting an
input plane and an output plane in accordance with selected
interconnection encodings, the optical interconnection apparatus
comprising
optical interconnection storage means including a multidimensional
holographic storage medium for storing phase holographic images
representative of the interconnection encodings,
selected regions of said medium containing said holographic images being
independently addressable by an interrogating beam of light characterized
by selected spatial positions,
said holographic storage medium comprising a spectral hole burning material
having absorption regions independently addressable by an interrogating
beam of light having a selected frequency,
said spectral hole burning material comprising selectively bleached
absorption structures forming addressable frequency channels, and
said phase holograms being stored in index modulation regions near spectral
hole absorption edges.
20. Apparatus according to claim 9, further comprising
feedback means for transmitting feedback signals from selected nodes in the
output plane to selected nodes in the input plane.
21. Optical computing apparatus for digital information processing, the
computing apparatus comprising
optical storage means including a multidimensional holographic storage
medium for storing phase holographic images representative of digital
data,
selected regions of said medium containing said holographic images being
independently addressable by an interrogating beam of light characterized
by selected spatial positions, said holographic storage medium comprising
a spectral hole burning material having absorption regions independently
addressable by an interrogating beam of light having a selected frequency,
said spectral hole burning material comprising selectively bleached
absorption structures forming addressable frequency channels,
said phase holograms being stored in index modulation regions near spectral
hole absorption edges. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates generally to optical memory techniques and devices,
and, more particularly, relates to optical data storage techniques and
devices utilizing holographic storage in volume media, in conjunction with
coherent writing and reading beams.
In recent years, a wide range of different optical media have been
considered or proposed for Providing high capacity data storage and
retrieval. The dimensionality of various media is one identifying
characteristic of optical storage systems. Two-dimensional media, such as
optical disk and microfiche storage devices, are common. Three-dimensional
memory media are also possible. Three-dimensional media include, for
example, volume holographic memories. Such media are discussed in Collier
et al., "Optical Holography" Academic Press, New York (1971), pp. 454-493,
incorporated herein by reference.
A second identifiable characteristic is the use of either holographic or
bit-oriented storage. Holographic storage is inherently parallel in
nature. Although schemes have been proposed for selective erasure of data
in holographic memories, selective erasure using bit-oriented storage is
conceptually simpler Holographic storage methods in which the reading or
writing radiation is incident on the entire memory medium are limited in
information capacity by the erasure of old data during reading operations
or the storage of new data. However, photon gating or electronic gating of
memory planes can solve these problems for both holographic and
bit-oriented storage. Holographic storage is less susceptible to dust and
media imperfections, but the same effect may be achieved using
bit-oriented storage through the use of coding schemes. Holography
provides a method of storing and accessing information stored throughout
the volume of a medium without the requirement of a complex optical system
to access individual planes in the medium.
It is known that multiple two-dimensional planes of data can be stored in a
volume holographic medium, and that these planes may be accessed
individually by introducing the reference beam into the medium at a
different angle for each individual plane of data. A volume medium
therefore has three spatial dimensions, corresponding to the
dimensionality of the information stored in a two-dimensional output array
multiplied by the number of independent reference beam directions in a
linear array of reference beams.
Certain conventional optical data storage systems, such as optical disk
memory, can provide large storage capacity In particular, 30,000 tracks
multiplied by 150,000 bits per track results in a capacity of
approximately 560 megabytes on a 12 centimeter optical disk. However, the
use of a single detector for readout provides only a serial data stream,
which limits the data transfer rate. The disadvantages of this memory
device also include difficulties caused by the dynamic focusing and
tracking problems associated with a moving disk and the latency or time
required to wait for the desired bits to rotate to the reading location.
In other optical data storage systems, cascaded orthogonal beam-steering
stages are used to access data Pages stored in a two-dimensional
holographic format. In this system, the memory medium is stationary,
eliminating the need for active tracking in the beamsteering system.
Because the data are stored holographically, no readout optics are
required, eliminating the need for a wide field of view high-resolution
lens. Parallel readout can be used to obtain an entire two-dimensional
array of bits from one beam position, allowing the use of a somewhat
slower beamsteering mechanism to be used than for an optical disk, while
still maintaining the same data transfer rate. A millisecond deflection
time provides a possible data rate of 10.sup.9 bits per second, which
exceeds the data transfer rates of current detector arrays.
However, two-dimensional holographic memory requires a high spatial
frequency response memory medium, and is characterized by limited storage
capacity and excessive size, because the information is spread out over a
two-dimensional area. The storage capacity of two-dimensional holographic
memory is also limited by the resolution of the medium. Assuming that an
array of 1000 by 1000 bits can be stored in a 1 centimeter by 1 centimeter
hologram, a 10 centimeter by 10 centimeter memory plane can contain 100
holograms with 10.sup.6 bits per hologram or 10.sup.8 bits in total. Since
each bit is approximately 10 micrometers in size at the detector array,
the optical system must have an optical configuration of approximately
f/20 for a 0.5 micrometer reading wavelength. Thus, the detector must be
approximately 20 centimeters from the memory plane. Since none of the
pages can be directly on-axis, the pages at the far side of the 10
centimeter by 10 centimeter array must have an angle of approximately
60.degree. between the illumination and the signal beam, corresponding to
a hologram fringe spacing of one wavelength, or 0.5 micrometers. Storage
of more holograms in a single memory plane would require even greater
spatial resolution.
In view of the constraints discussed above, there has long been a need for
high capacity optical data storage techniques and devices that eliminate
the requirements for mechanical translation or rotation of a storage
medium and read/write element --with its associated latency and tracking
problems --while providing compact, high density data storage.
High capacity multi-dimensional optical data storage systems are disclosed
in related U.S. patent application Ser. No. 236,604. The storage devices
disclosed therein utilize a four-dimensional optical storage medium,
having three spatial dimensions and one wavelength-dependent dimension.
Photochemical spectral hole burning materials (SHBs) are employed to store
holograms at multiple wavelengths. While the data storage systems
discussed in U.S. patent application Ser. No. 236,604 offer substantial
improvements in storage density over conventional storage systems, even
greater resistance to erasure and data density may be required in future
optical computing applications.
It is accordingly an object of the invention to provide improved high
capacity optical data storage methods and systems.
It is another object of the invention to provide such systems affording
high density data storage.
A further object of the invention is to provide optical data storage
methods and systems having high access speeds, in which both the storage
medium and the read/write element are substantially stationary.
It is another object of the invention to provide methods and devices
adapted for use in high speed optical computing interconnection systems.
Other general and specific objects of the invention will in part be obvious
and will in part appear hereinafter.
SUMMARY OF THE INVENTION
The foregoing objects are attained by the invention, which provides methods
and apparatus for storage and retrieval of digital data. In one aspect of
the invention, a multidimensional holographic storage medium is provided
for storing phase holographic images representative of the digital data.
Selected frequency or spatial regions of the medium are independently
addressable by a beam of light.
The storage medium is a spectral hole burning (SHB) material having
absorption regions independently addressable by selected frequencies of
light, and including selectively bleached absorption structures forming
frequency channels. Frequency channeling elements, including a narrow
linewidth tunable laser, provide controlled bleaching of the SHB material
to generate these frequency channels. The SHB material Can include
porphyrin tautomers in a polyethylene matrix, organic dyes in organic
polymer host materials, organic dyes in inorganic host materials, or color
centers in glass.
Read/write elements, including a frequency agile laser for generating an
interrogating beam of laser light having at least one selected frequency,
enable reading and writing of holographic images into and out of the
storage medium. Beam steering elements steer the laser beam in at least a
first direction, and a spatial light modulator interposed between the
frequency agile laser and the medium provides selected spatial modulation
of the interrogating beam of laser light. The beam steering elements and
frequency agile laser can be controlled by a microprocessor and associated
control elements, to vary frequency and spatial position of the
interrogating beam.
The invention also provides a method for storing digital data, including
the steps of configuring a multidimensional holographic storage medium
from a spectral hole burning material, utilizing a laser to generate in
the spectral hole burning material a set of selectively bleached
absorption structures to form addressable frequency channels, and
utilizing a laser to write phase holograms representative of the digital
data in index modulation regions near spectral hole absorption edges.
A further aspect of the invention includes optical interconnection
apparatus for selectively interconnecting an input optical plane and an
output optical plane in accordance with selected interconnection
encodings. The optical interconnection apparatus includes a
multidimensional holographic storage medium for storing phase holographic
images representative of the interconnection encodings. Selected regions
of the medium containing the holographic images are independently
addressable by an interrogating beam of light, the position and frequency
of which can be controlled. The SHB material contains selectively bleached
absorption structures forming addressable frequency channels, and the
interconnection-specifying holograms are stored in index modulation
regions near spectral hole absorption edges.
In this aspect of the invention, feedback elements can be employed for
transmitting feedback signals from selected nodes in the output plane to
selected nodes in the input plane.
The invention can be practiced in optical computing systems for digital
information processing, utilizing a multidimensional holographic storage
medium for storing phase holographic images representative of digital
data. The medium includes a spectral hole burning (SHB) material having
absorption regions independently addressable by an interrogating beam of
light having a selected frequency and position. In accord with the
invention, the SHB material contains selectively bleached absorption
structures forming addressable frequency channels.
In a further aspect of the invention, an external electric field of
selected voltage can be applied to the storage medium to induce Stark
effect broadening of the spectral holes, so that at a given frequency of
light, a different phase hologram can be stored for each of a plurality of
voltages. Multiple phase holograms can thus be stored at each given
frequency of light. The voltage of the applied electric field can be
varied, thereby utilizing the Stark effect to address holographic images
recorded at selected electric field strengths. The Stark effect
accordingly provides another "dimension" of information storage within the
holographic medium.
Similar broadening can also be induced by applying a physical stress of
selected magnitude to the optical storage medium This stress alters the
local environment of each recording location in the medium, changing the
frequency of response of each recording location, so that at a given
frequency of interrogating light, a different phase hologram can be stored
for each of a plurality of electric field magnitudes. Consequently,
multiple phase holograms can be stored for each frequency of interrogating
light.
By controlling the magnitude of the physical stress applied to the optical
storage medium, the system can address holographic images recorded at
different stress field magnitudes. In one aspect of the invention,
physical stress is applied to the optical storage medium by at least one
piezoelectric element in contact with at least one surface of the optical
storage medium. The piezoelectric element can be excited by an applied
electrical signal.
The invention will next be described in connection with certain illustrated
embodiments; however, it should be clear to those skilled in the art that
various modifications, additions and subtractions can be made without
departing from the spirit or scope of the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the nature and objects of the invention,
reference should be made to the following detailed description and the
accompanying drawings, in which:
FIG. 1 is a schematic diagram depicting a four dimensional memory device
constructed in accordance with the disclosure of related U.S. patent
application Ser. No. 236,604.
FIG. 2 is a schematic diagram depicting an optical computing
interconnection apparatus constructed in accordance with the disclosure of
related U.S. patent application Ser. No. 236,604, utilizing a four
dimensional memory for providing selected interconnections between the
input plane and the output plane.
FIG. 3 is a graph of absorption versus wavelength, schematically
illustrating the principle of spectral hole burning.
FIG. 4 is a graph illustrating the principle of phase hologram writing with
regard to the relationship between real and imaginary refractive indices
for an SHB material.
FIG. 5 is a graph depicting the principle of frequency channelling with
regard t adjacent absorption structures.
FIG. 6 is a graph depicting the principle of Stark effect broadening.
FIG. 7 is a schematic diagram depicting an embodiment of the invention
utilizing an applied electric field to generate rearrangement of the
spectral hole profile to enhance the recording capacity.
FIG. 8 is a schematic diagram depicting a further embodiment of the
invention, utilizing piezoelectric elements to generate rearrangement of
the spectral hole Profile by applying physical stress.
DESCRIPTION OF ILLUSTRATED EMBODIMENTS
FIG. 1 is a schematic diagram depicting a four dimensional memory device
constructed in accordance with the disclosure of related U.S patent
application No. 236,604. The optical data storage apparatus 100 shown in
FIG. 1 uses a conventional tunable laser source 101 of coherent radiation
for optically writing holographic data representations in an optical
storage or memory medium 112 and optically reading holographic data
representations from the memory medium 112. The reading and writing of
holographic data is known in the art, and is discussed in R.J. Collier et
al., Optical Holography, Academic Press, New York (1971), pp 454-493,
incorporated herein by reference.
Unlike conventional holographic storage media, however, memory medium 112,
as disclosed in related U.S. patent application Ser. No. 236,604, is a
four-dimensional optical storage medium, having three spatial dimensions
and one wavelength-dependent dimension. Photochemical spectral hole
burning materials (SHBs) can be employed in the illustrated system to
store holograms at multiple wavelengths. It is known in the art that
absorption sites in the volume of an SHB material are affected by the
local polymer environment to absorb photons at slightly different
wavelengths. At very low temperatures, each absorption site is unaffected
by thermal phonons, so the spectral width of the absorption is very
narrow. Since the volume of an absorption site is very small, much less
than a cubic wavelength, many absorption sites are available in each
region which would normally store one sample of the hologram fringe
pattern, and multiple fringe patterns can be stored using different
optical wavelengths.
Optical memory media with inhomogeneously broadened spectral absorption
permit multiple bits to be stored in any physical wavelength-sized region.
The number of bits is proportional to the ratio of the inhomogeneous
linewidth of the absorption spectrum to the homogeneous linewidth. For
example, in SHB materials this ratio can be as high as 10.sup.3 to
10.sup.7. SHB materials are discussed in greater detail in W.E. Moerner,
ed., Persistent Spectral Hole-Burning: Science and Applications,
Springer-Verlag, N.Y. (1988), incorporated herein by reference; and A.R.
Gutierrez et al., "Multiple Photochemical Hole Burning in Organic Glasses
and Polymers: Spectroscopy and Storage Aspects," IBM Vol. 26 J. Res.
Develop. p. 198 (1982), incorporated herein by reference.
As disclosed in related U.S. patent application Ser. No. 236,604, multiple
quantum well dot structures can also be utilized to provide a
four-dimensional memory medium. It is known in the art that the wave
function of a conduction electron trapped in a cubic well or dot structure
defines a series of sharp resonances. These resonances correspond to
preferred photon energies for absorbed photons to "bump" an electron into
the conduction band. By changing the well dimensions, the wavelength of
absorption can be changed. The dimensions of a quantum dot can be made
smaller than a wavelength, so that in each region that would normally
store one hologram fringe sample, multiple fringe pattern samples can be
stored, corresponding to the number of quantum dots of different
dimensions within that region. Quantum dot materials are discussed in
greater detail in H. Temkin et al., "Low-Temperature Photoluminescence
from InGaAs/InP Quantum Wires and Boxes," 50 App. Phys. Lett. 413-415,
incorporated herein by reference; and Wei-Yu Wu et al., "Effect of Size
Non-Uniformity on the Absorption Spectrum of a Semiconductor Quantum Dot
System," 51 Applied Physics Letters 710 (1987), incorporated herein by
reference.
Referring again to FIG. 1, the laser beam generated by the tunable laser
source 101 is divided into two mutually coherent beams of radiation by the
beamsplitter 102. The first of these two beams is directed by mirrors 104,
105 into a conventional beam deflection stage 106, in which the beam can
be deflected, for example, vertically (i.e., in the plane of the drawing)
to provide a reference beam 109. The deflection stage 106 preferably
provides selected deflection in response to externally applied control
signals.
Those skilled in the art will understand that the beam can be controlled in
several different ways. For example, utilizing conventional means, the
laser wavelength spectrum can be tuned, the laser beam direction can be
varied, or the laser wavefront curvature can be changed. Each of these
laser beam control methods provides a means to access information stored
in the optical medium. Techniques for producing tunable lasers are well
known, as are means for producing lasers with a "comb" spectrum. A large
number of means for steering a beam of laser radiation exist, including
moving mirrors, acousto-optic deflectors, electro-optic deflectors, and
photorefractive deflectors.
The reference beam 109 is directed into the four-dimensional memory medium
112, where the beam is used as the reference beam during the writing of
information in a holographic format, and as the reconstruction beam during
the reading of data stored in a holographic format.
The second beam derived from the tunable laser 101 by means of the
beamsplitter 102 is directed by mirror 103 through an external shutter 107
to a data-specifying plane 108. The data-specifying plane 108 includes a
conventional spatially-variable transparency which carries a pattern of
data in the form of a two-dimensional array of bright or dark spots. This
pattern is impressed upon, and carried by, the beam passing through the
data-specifying plane 108. The open or closed state of shutter 107
controls execution of the alternate writing and reading functions of the
memory apparatus 100.
During writing, the shutter 107 is open, permitting the beam from mirror
103 to reach the data-specifying plane 108. The input data beam 111, which
carries the input data specified by data-specifying plane 108, passes
through collimator 110 to the memory medium 112 where the input data beam
111 is combined with the reference beam 109. The reference beam 109 and
the input data beam 111 are coherent, and form an interference pattern
throughout the volume of the memory medium 112. This interference pattern,
when recorded in the memory medium at a specific wavelength, can be used
to reconstruct the data Pattern impressed on the data beam 111.
During the reading process, the shutter 107 is closed, and only the
radiation in the reference beam 109 reaches the memory medium 112. The
memory medium 112 transmits diffracted output radiation which is
collimated by lens 113 to produce an output data beam 114. In accordance
with the invention, when a large number of different patterns are stored
in the memory medium 112, the pattern of interest can be selected by
controlling the direction and wavelength of the reference beam 109 to be
exactly equal to the direction and wavelength used to record the desired
data. When this equality is satisfied, the beam 114 carrying the output
data diffracted from the memory medium 112 will project the desired array
of output data onto the output plane 115. The output plane 115 may be, for
example, a two-dimensional CCD array which produces electrical signals
responsive to the light incident upon its surface. Alternatively, the
output plane 115 may be a spatial light modulating optical computing
element.
An important feature of the invention is the large data capacity which is
achieved due to the large number of independent reference beams that can
be generated by varying the angle and wavelength of the reference beam.
Extremely large storage capacity can be achieved through the use of
angular and spectral multiplexing of data in a thick volume medium. For
example, the memory medium can contain multiple holograms, each
representing an array of 1000.times.1000 bits. By varying the reference
beam angle, as many as 500 holograms can be stored at each wavelength.
Using 50,000 different wavelengths to store data in the medium, a total of
2.5.times.10.sup.13, bits can be stored in a 1 centimeter cube. By
utilizing applied electric or stress fields to rearrange the spectral hole
profile, providing a smooth profile for further recording, an additional
enhancement of up to 100 can be achieved resulting in a storage capacity
of up to 2.5.times.10.sup.15 bits/cm.
In addition to the extremely large capacity provided by the
four-dimensional memory apparatus, four-dimensional memory is
fundamentally necessary for certain important applications. Neural network
research, for example, has investigated the means for connecting two fully
populated data planes in a selectable fashion. The selectable connection
of two fully-populated two-dimensional planes has been an important goal
of recent optical computing research. The ability to realize a completely
selectable interconnection device has important implications for neural
network application to such important problems as vision preprocessing,
pattern and speech recognition, and the solution of difficult mathematical
problems which can be expressed in matrix form.
It has been shown that due to duplication of volume gratings stored in the
memory medium, selectable connections cannot be achieved between two
fully-populated two dimensional planes Instead, a solution has been
proposed which limits the number of populated elements in the input and
output planes to K.sup.3/2 where K is proportional to S/lambda, with S
being the dimension of one side of the input plane or interconnection
medium, and lambda being the wavelength used to illuminate the input
plane. The required K.sup.3 interconnections can be stored in a volume of
size proportional to K wavelengths on a side.
By utilizing an optical memory medium with four independent dimensions
available for storage of interconnections, K.sup.4 connections can be
stored, sufficient to interconnect two Planes each containing K.sup.2
elements. These K.sup.2 elements can be placed so as to completely fill
the input and output planes.
The four dimensional optical memory apparatus of FIG. 1 can be used to
implement one stage of a neural network which connects two 2-dimensional
planes; such a network can be used, for example, in speech or vision
preprocessing, or in the solution of complicated optimization problems
which are difficult to perform on current digital computers.
FIG. 2 depicts a four-dimensional interconnection apparatus 200, as
disclosed in related U.S. patent application Ser. No. 236,604. The
interconnection apparatus 200 operates in a manner similar to that of the
system illustrated in FIG. 1, utilizing a tunable laser source 201 of
coherent radiation for executing optical interconnection operations for
the interconnection of designated active sites of one memory element
(referred to as the input) with the designated active sites of another
memory element (referred to as the output). These operations include
optically writing data in the four-dimensional interconnection medium 212
and optically reading data from the medium.
The beam generated by the tunable laser source 201 is divided into two
mutually coherent beams of radiation by the beamsplitter 202. The first of
these two beams is directed by mirrors 204, 205 through an input
connection-specifying plane 206. The input connection-specifying plane 206
includes a conventional spatially-variable transparency which carries a
pattern of data in the form of a two-dimensional array of bright or dark
spots. The pattern carried on the input connection-specifying plane
specifies designated active input locations which are to be connected to
designated active output locations. This pattern is impressed upon the
beam from mirror 205 to generate an input state beam 217.
The second beam derived from the tunable laser by means of the beamsplitter
is directed using mirror 203 to an output connection-specifying plane 208
controlled by spatially variable shutter 207. Output connection-specifying
plane 208 includes a spatially-variable transparency carrying a pattern of
data in the form of a two-dimensional array of bright or dark spots. This
pattern is impressed on the beam from mirror 203 to transmit an output
state beam 218 specifying the designated active output locations. The
spatially-variable shutter 207 controls whether the "interconnection"
(reading) or "learning" (writing) function of the network apparatus 200
will be executed.
During the learning operation, the shutter 207 is opened at a sequence of
locations forming a set of vertical stripes, permitting the output state
beam 218 to reach the interconnection medium 212 through lens 209, where
it is combined with the input state beam 217. The two beams are coherent,
and form an interference pattern throughout the volume of the
four-dimensional interconnection medium 212. In accordance with the
invention, a different wavelength is used for each vertical stripe to
prevent crosstalk between the patterns stored in association with
different vertical columns. Each different wavelength is directed to the
proper vertical stripe by a grating 220 and a cylindrical lens 222.
Those skilled in the art will understand that this interference pattern,
when recorded in the interconnection medium 212 at a specific wavelength,
can be used to connect the data activity at a series of input locations to
an selectable set of output locations or states arranged in communication
with the two-dimensional output Plane 216. In particular, the selected
interference pattern stored in interconnection medium 212 is transmitted
through collimator 213, and the resulting output beam 214 is projected
onto output plane 216, which may be, for example, a two-dimensional CCD
array.
During the interconnection operation, the shutter 207 is closed, and only
the radiation in the input state beam 217 reaches the interconnection
medium 212. Those skilled in the art will appreciate that when many
different patterns have been stored in the interconnection medium 212, the
medium of interest can be used to execute selectable connections between
input states and output states by simultaneously illuminating all input
states with all the wavelengths used during the learning process. Each
input state can be connected to any output state, but only the
contributions at the output at the wavelength used to record connections
to that state are of interest. A spectral filter array 215, positioned in
the path of output beam 214 can be utilized for filtering the beam
directed at output plane 216, to select the proper contributions for each
state.
SPECTRAL HOLE BURNING
Understanding of the invention is facilitated by further explanation of
spectral hole burning, illustrated on a molecular level by FIG. 3. FIG. 3
depicts the relationship between absorption and wavelength for a sample
having guest molecules embedded in a solid host matrix, such as a rigid
glass or polymer film, including the spectral profile of the overall
absorption band for all sites (301); the absorption lines of individual
sites (302); removal of one site by hole burning (303); and a hole dip
(304) caused by spectral hole burning.
Within the sample, the degree of guest-host interaction for a molecule
embedded in the solid matrix depends upon the environment surrounding each
molecule. A potentially infinite number of different surroundings exist
around each embedded molecule. This leads to broad absorption bands with
approximately Gaussian profiles. The inhomogeneously broadened absorption
bands shown in FIG. 3 thus represen | | |
