U.S. patent application number 15/666668 was filed with the patent office on 2019-02-07 for integrated optical circuit for holographic information processing.
The applicant listed for this patent is INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to Folkert Horst.
Application Number | 20190041796 15/666668 |
Document ID | / |
Family ID | 65200317 |
Filed Date | 2019-02-07 |
![](/patent/app/20190041796/US20190041796A1-20190207-D00000.png)
![](/patent/app/20190041796/US20190041796A1-20190207-D00001.png)
![](/patent/app/20190041796/US20190041796A1-20190207-D00002.png)
United States Patent
Application |
20190041796 |
Kind Code |
A1 |
Horst; Folkert |
February 7, 2019 |
INTEGRATED OPTICAL CIRCUIT FOR HOLOGRAPHIC INFORMATION
PROCESSING
Abstract
An integrated optical circuit for holographic information
processing is disclosed. The optical circuit comprises a
photorefractive medium and two transmitter arrays. The transmitter
arrays are adapted for locally changing the refractive index of the
photorefractive medium for holographic encoding of the information
in a working plane of the photorefractive medium by transmitting
light via optical paths into the photorefractive medium such that
an interference pattern is generated in the working plane. The
optical paths and the working plane are arranged in a single
optical plane.
Inventors: |
Horst; Folkert; (Wettingen,
CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
INTERNATIONAL BUSINESS MACHINES CORPORATION |
Armonk |
NY |
US |
|
|
Family ID: |
65200317 |
Appl. No.: |
15/666668 |
Filed: |
August 2, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03H 1/0005 20130101;
G03H 1/0402 20130101; G03H 1/02 20130101; G06N 3/0635 20130101;
G03H 2223/16 20130101; G03H 1/0465 20130101; G03H 1/28 20130101;
G02F 1/0126 20130101; G03H 1/2286 20130101; G03H 2001/026 20130101;
G11C 13/044 20130101; G03H 2222/34 20130101; G03H 2227/02 20130101;
G03H 2260/36 20130101; G03H 1/265 20130101; G03H 1/024 20130101;
G03H 2260/54 20130101; G06N 3/084 20130101; G03H 1/10 20130101 |
International
Class: |
G03H 1/02 20060101
G03H001/02; G03H 1/04 20060101 G03H001/04; G02F 1/01 20060101
G02F001/01; G03H 1/00 20060101 G03H001/00; G03H 1/10 20060101
G03H001/10; G11C 13/04 20060101 G11C013/04; G06N 3/063 20060101
G06N003/063 |
Claims
1. An integrated optical circuit for holographic information
processing, the integrated optical circuit comprising a
photorefractive medium and two transmitter arrays, the two
transmitter arrays being adapted for locally changing a refractive
index of the photorefractive medium for holographic encoding of the
holographic information in a working plane of the photorefractive
medium by transmitting light via optical paths into the
photorefractive medium such that an interference pattern is
generated in the working plane, the optical paths and the working
plane being arranged in a single optical plane, wherein each of the
two transmitter arrays comprise an electrical input and an array of
output waveguides, the array of output waveguides being adapted for
transmitting the light into the photorefractive medium, each of the
two transmitter arrays being adapted for modulating the light
transmitted through the array of output waveguides based on an
electrical modulation signal received through the electrical input
in order to encode the holographic information.
2. (canceled)
3. The integrated optical circuit of claim 1, being adapted for
encoding the holographic information by simultaneously generating
the interference pattern for at least two different combinations of
an output waveguide of a first one of the two transmitter arrays
and an output waveguide of a second one of the two transmitter
arrays.
4. The integrated optical circuit of claim 1, wherein at least one
of the two transmitter arrays comprising a binary tree of passive
power splitters, the binary tree being adapted for distributing the
light into the array of output waveguides, the at least one of the
two transmitter arrays further comprising an optical modulator for
at least part of the array of output waveguides, the optical
modulator being adapted for performing the light modulation.
5. The integrated optical circuit of claim 1, wherein at least one
of the two transmitter arrays comprising a binary tree of
electrically controllable power splitters, the binary tree being
adapted for distributing the light into the array of output
waveguides, the electrically controllable power splitters being
adapted for performing the light modulation.
6. The integrated optical circuit of claim 1, wherein the
modulation of the light comprises either being amplitude modulation
and/or phase modulation.
7. The integrated optical circuit of claim 1, further comprising a
propagation region adapted for coupling the light received from the
two transmitter arrays into the photorefractive medium.
8. The integrated optical circuit of claim 7, wherein the
propagation region being confined perpendicular to the single
optical plane by cladding layers having a lower refractive index
than the propagation region.
9. The integrated optical circuit of claim 7, wherein the
propagation region comprising a first set of optical elements
arranged in the optical paths and optically coupling the
photorefractive medium to at least one of the two transmitter
arrays.
10. The integrated optical circuit of claim 9, wherein the first
set of optical elements being adapted for transforming a light wave
emerging from the at least one of the two transmitter arrays into a
plane light wave incident to the photorefractive medium.
11. The integrated optical circuit of claim 1, further comprising a
light input adapted for transmitting the light into the integrated
optical circuit, each of the two transmitter arrays being optically
coupled to the light input and being adapted for transmitting the
light received from the optical input into the photorefractive
medium.
12. The integrated optical circuit of claim 1, further comprising
two receiver arrays, each of the two receiver arrays comprising an
electrical output and being adapted for measuring optical power
received from the photorefractive medium, generating an electrical
signal encoding the measured optical power, and outputting the
electrical signal through the electrical output, the integrated
optical circuit being adapted for controlling the holographic
encoding using the electrical output as feedback.
13. The integrated optical circuit of claim 12, wherein each of the
two receiver arrays further comprises a plurality of input
waveguides, each of the two receiver arrays being adapted for
independently measuring optical power received from the
photorefractive medium by at least a receiving portion of the
plurality of input waveguides, the electrical signal encoding the
measured optical power and an identifying information for each of
the receiving portion of the plurality of input waveguides.
14. The integrated optical circuit of claim 12, wherein each of the
two receiver arrays being optically coupled to the light input and
being further adapted for measuring a phase difference between the
light received from the photorefractive medium and the light
received from the light input, the electrical signal further
encoding the measured phase difference.
15. The integrated optical circuit of claim 12, wherein a
propagation region comprises a second set of optical elements, the
second set of optical elements being arranged in the optical paths
and optically coupling the photorefractive medium to at least one
of the two receiver arrays.
16. The integrated optical circuit of claim 15, wherein the second
set of optical elements being adapted for focusing a plane light
wave emerging from the photorefractive medium onto at least one of
the two receiver arrays.
17. A method for holographic information processing, the method
comprising: providing an integrated optical circuit comprising a
photorefractive medium and two transmitter arrays; and changing,
locally, a refractive index of the photorefractive medium for
holographic encoding of information in a working plane of the
photorefractive medium by transmitting light from the two
transmitter arrays via optical paths into the photorefractive
medium such that an interference pattern is generated in the
working plane, the optical paths and the working plane being
arranged in a single optical plane, wherein each of the two
transmitter arrays comprise an electrical input and an array of
output waveguides, the array of output waveguides being adapted for
transmitting the light into the photorefractive medium, each of the
two transmitter arrays being adapted for modulating the light
transmitted through the array of output waveguides based on an
electrical modulation signal received through the electrical input
in order to encode the holographic information.
18. A method for holographic information processing, the method
comprising: providing an integrated optical circuit comprising a
photorefractive medium, two transmitter arrays, and two receiver
arrays, the photorefractive medium comprising a refractive index
pattern holographically encoding holographic information; and
decoding the holographic information by measuring optical power of
a light pulse received by the two receiver arrays, the light pulse
being directed through the two transmitter arrays into the
photorefractive medium such that it passes the refractive index
pattern.
19. The method of claim 18, wherein the holographic information
comprises an amplitude information and a phase information, each of
the two transmitter arrays being adapted for applying a phase
modulation to the light pulse, the decoding comprising: selecting a
constant first phase difference between the two transmitter arrays
for the phase modulation; measuring the optical power of a first
light pulse received by the two receiver arrays, the first light
pulse being directed through the two transmitter arrays into the
photorefractive medium such that it passes the refractive index
pattern, the two transmitter arrays being set to the constant first
phase difference; selecting a constant second phase difference
between the two transmitter arrays for the phase modulation, the
constant second phase difference differing from the constant first
phase difference by between 45 and 135 degrees; measuring the
optical power of a second light pulse received by the two receiver
arrays, the second light pulse being directed through the two
transmitter arrays into the photorefractive medium such that it
passes the refractive index pattern, the two transmitter arrays
being set to the constant second phase difference; and decoding the
amplitude information and the phase information from the optical
power measured for the first light pulse and the second light
pulse.
Description
BACKGROUND
[0001] The present invention relates to integrated optics, and more
specifically, to integrated holographic information processing.
[0002] Training of Deep Neural Nets (DNNs) is a computationally
demanding task. A large fraction of the computational workload is
spent on evaluating signal propagation through the synaptic network
that connects the consecutive levels of neurons in the neural net.
This synaptic network connects each neuron in a next layer of M
neurons to all N neurons in the previous layer, where the strength
of each "synaptic" connection is set by a weighting factor. The
total input signal to a next-level neuron is given by the weighted
sum of output signals from all active neurons in the previous
level. Depending on whether this sum reaches a certain threshold
level, the neuron will fire or not.
[0003] The computational cost of the synaptic processing step for
all neurons comprises M.times.N memory accesses to the weight
values, M.times.N multiplications and M.times.N additions.
[0004] To train the neural network for performing a specific task,
all synaptic weight values have to be optimized. This is performed
in a training algorithm in which the response of the neural network
to an input signal is compared with the desired output. Then the
synaptic weights are updated in an iterative procedure to minimize
the difference between the output and the desired output. The
well-known Back Propagation training process is an established
method to determine the synaptic weight factors. One iteration of
the backpropagation algorithm requires 3.times.M.times.N memory
reads, 3.times.M.times.N multiplications, 3.times.M.times.N
additions and M.times.N memory stores.
[0005] It has been shown that speed and power consumption of the
weight optimization processing steps (evaluation and update) can be
improved massively by performing these steps using analog
computation on a dedicated hardware array of M.times.N nodes where
each node locally stores and processes its corresponding weight
value. This method overcomes the memory access bottleneck and
allows a parallel optimization of all synaptic weights in one
connection layer.
[0006] Synaptic weight processing using holographic weight storage
in photorefractive materials has been demonstrated in free-space
optic setups. These setups typically use high-power lasers as light
sources, liquid crystal light valves as optical modulators, CCD
cameras as detectors and mm to cm scale photorefractive crystals as
storage media. However, these systems are hampered by the large
size and low stability of the bulk optics and the low speed of the
optical modulators and detectors.
[0007] In "Holographic interconnections in photorefractive
waveguides", Applied Optics, vol. 30, pp. 2324-2333, of 1991, the
authors David D. Brady and Demetri Psaltis proposed a partially
integrated vector matrix multiplier. In this proposal, the matrix
coefficients are stored in a photorefractive planar waveguide and
the light beams for matrix evaluation are applied and extracted
using integrated optics input and output waveguides. The
holographic gratings are written using out-of-plane beams, free
space optics and a spatial light modulator (liquid crystal light
valve) to encode the information.
SUMMARY
[0008] It is an objective of the present invention to provide for a
holographic information processing device and methods. Embodiments
of the invention are given in the dependent claims. Embodiments of
the present invention can be freely combined with each other if
they are not mutually exclusive.
[0009] In one aspect, the invention relates to an integrated
optical circuit for holographic information processing, the optical
circuit comprising a photorefractive medium and two transmitter
arrays, the transmitter arrays being adapted for locally changing
the refractive index of the photorefractive medium for holographic
encoding of the information in a working plane of the
photorefractive medium by transmitting light via optical paths into
the photorefractive medium such that an interference pattern is
generated in the working plane, the optical paths and the working
plane being arranged in a single optical plane.
[0010] In another aspect, the invention relates to a method for
holographic information processing, the method comprising: [0011]
providing an integrated optical circuit comprising a
photorefractive medium and two transmitter arrays; and [0012]
changing, locally, a refractive index of the photorefractive medium
for holographic encoding of information in a working plane of the
photorefractive medium by transmitting light from the two
transmitter arrays via optical paths into the photorefractive
medium such that an interference pattern is generated in the
working plane, the optical paths and the working plane being
arranged in a single optical plane.
[0013] In yet another aspect, the invention relates to a method for
holographic information processing, the method comprising: [0014]
providing an integrated optical circuit comprising a
photorefractive medium, two transmitter arrays, and two receiver
arrays, the photorefractive medium comprising a refractive index
pattern holographically encoding holographic information; and
[0015] decoding the holographic information by measuring optical
power of a light pulse received by the two receiver arrays, the
light pulse being directed through the two transmitter arrays into
the photorefractive medium such that it passes the refractive index
pattern.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0016] In the following, embodiments of the invention are explained
in greater detail, by way of example only, making reference to the
drawings.
[0017] FIG. 1 depicts a schematic cut through an exemplary
integrated optical circuit;
[0018] FIG. 2 is a block diagram representing an exemplary method
for holographic information processing.
DETAILED DESCRIPTION
[0019] Driven by telecommunication technology, considerable
advances have been made in integrated optical devices during the
last two decades. Integrated optical devices are known to deliver
improvements in power efficiency and processing speed compared to
traditional bulk-optical setups.
[0020] Embodiments of the invention may have key advantages which
arise from their implementation as an integrated optical device.
Compared to known implementations using bulk optics, the integrated
optical circuit according, to embodiments of the invention, may
feature a higher mechanical and optical stability due to a strong
decrease in the number of degrees of freedom. Moreover, the
integrated optical circuit may allow for an encapsulation of light,
and therefore the photorefractive effect may be achieved with a
lower amount of optical power. Another advantage may be the
availability of non-absorptive and/or high speed optical devices
for integrated optics, such as optical modulators and tunable power
splitters. The integrated optical circuit according to embodiments
of the invention is clearly characterized and discernable from the
state of the art in that the optical parts (e.g. transmitter
arrays, mirrors) for interacting with the photorefractive medium
(including writing the information to the photorefractive medium by
generating the interference pattern) are arranged in a single plane
(i.e. the optical plane as defined below).
[0021] Possible advantages of embodiments of the invention will be
outlined in more detail in the remaining description. Examples
given in the description are meant solely for the purpose of
illustration, but not as a limitation, as alternatives will be
known or apparent to a person of ordinary skill in the art for each
example given. Furthermore, descriptions of embodiments given
herein will assume the optical plane to be horizontal, solely for
the purpose of simplification of the description, as embodiments of
the invention are not restricted to be manufactured or used in a
horizontal orientation.
[0022] The integrated optical circuit disclosed herein may
advantageously deploy integrated optics for holographic information
storage and processing. Various technologies and/or technology
nodes are available for implementing an integrated optical circuit
according to embodiments of the invention. Usually, the structural
dimensions and operating wavelengths of the integrated optical
circuit will be determined by the chosen integration technology. A
commonly used, but not restrictive example is silicon photonics,
which allows for manufacturing integrated optical devices from
silicon as the optical medium. Silicon is transparent in the range
of about 1.3-2.0 .mu.m. Hence, typical structural dimensions are in
the .mu.m to mm range, with overall device dimensions in the mm to
cm range. Manufacturing the integrated optical circuit in silicon
photonics may be especially advantageous because a wide range of
material manufacturing techniques is available for silicon
micro-structuring from silicon electronics manufacturing.
[0023] According to embodiments, the photorefractive medium is
formed as a wave guiding layer. Compared to a bulk optical setup, a
photorefractive wave guiding layer, which may also be characterized
as a slab waveguide or a thin layer, may beneficially allow for
causing the photorefractive effect with less optical energy than
for a thick optical crystal adapted for incorporating a volume
hologram. In an example, a wave guiding layer of 1 mm.times.1
mm.times.1 .mu.m requires an optical power reduced by a factor of
1000 compared to a photorefractive volume crystal of 1 mm.sup.3 to
create an identical interference pattern for holographic encoding
of the information.
[0024] The photorefractive medium may be formed by any
photorefractive material which is suitable for manufacturing on the
selected integration technology. A non-exhaustive selection of
photorefractive materials includes barium titanate (BaTiO.sub.3),
lithium niobate (LiNbO.sub.3), vanadium doped zinc telluride
(ZnTe:V), an organic photorefractive material, a photorefractive
photopolymer, or a photorefractive semiconductor including a III/V
semiconductor such as GaAs.
[0025] The integrated optical circuit may further comprise a light
source. Any light source used for holographic encoding of the
information in the photorefractive medium should be capable of
generating coherent light in order to create the interference
pattern in the working plane of the photorefractive medium. An
integratable coherent light source may be a laser diode, which may
be implemented as a discrete device or integrated in the
transmitter arrays. Preferably, the light is produced by a single
coherent light source and then distributed to the transmitter
arrays, e.g. using a sequence of optical waveguides and power
splitters.
[0026] In an example, a coherent light source external to the
integrated optical circuit is utilized for generating the light for
performing the holographic information processing. According to an
embodiment, the optical circuit further comprises a photorefractive
medium and two transmitter arrays, the transmitter arrays being
adapted for locally changing the refractive index of the
photorefractive medium for holographic encoding of the information
in a working plane of the photorefractive medium by transmitting
light via optical paths into the photorefractive medium such that
an interference pattern is generated in the working plane, the
optical paths and the working plane being arranged in a single
optical plane. A light input may allow for using a light source
which overcomes limitations of integratable coherent light sources,
e.g. regarding optical output power, wavelength, and/or dimensions.
The light input may be implemented as a suitable optical window
providing an optical access to the transmitter arrays or optical
waveguides optically coupled to the transmitter arrays as described
above. The light input may provide further optical functionality,
such as a lens or an increased aperture which may simplify the
coupling of an external light beam into the optical circuit.
[0027] A transmitter array can be any device capable of providing
an array of point sources which are adapted for locally changing
the refractive index of the photorefractive medium for holographic
encoding of the information in a working plane of the
photorefractive medium by transmitting light via optical paths into
the photorefractive medium such that an interference pattern is
generated in the working plane. The array of point sources is
preferably one-dimensional, such that the optical paths are
provided in a single optical plane within the working plane of the
photorefractive medium. More specifically, the point sources are
individually controllable, allowing for creating a custom
one-dimensional light pattern. Controlling a point source may
include switching the point source on or off, and/or modulating the
light generated by the point source in terms of amplitude
modulation, phase modulation, frequency modulation, and the
like.
[0028] A point source may be a discrete device capable of
generating the light, or an open end of an essentially
one-dimensional optical waveguide. According to an embodiment, each
of the transmitter arrays comprises an electrical input and an
array of output waveguides, the output waveguides being adapted for
transmitting the light into the photorefractive medium, each of the
transmitter arrays being adapted for modulating the light
transmitted through each of the output waveguides based on an
electrical modulation signal received through the electrical input
in order to encode the information. Using an array may have the
advantage that each of the output waveguides may be used to couple
a beam of light into the photorefractive medium from a slightly
different angle. This way, multiple instances of information may be
holographically stored in the photorefractive medium by invoking
the photorefractive effect by creating a unique interference
pattern for each combination of an output waveguide of a first one
of the transmitter arrays and an output waveguide of the second one
of the transmitter arrays. Particularly, the photorefractive medium
may thus be used to store two-dimensional information such as
entries of a matrix.
[0029] According to an embodiment, the integrated optical circuit
is adapted for encoding the information by simultaneously
generating the interference pattern for at least two different
combinations of an output waveguide of a first one of the
transmitter arrays and an output waveguide of a second one of the
transmitter arrays. Providing the transmitter arrays with arrays of
output waveguides may enable a simultaneous programming of the
photorefractive medium with information supplied by more than one
pair of output waveguides.
[0030] In environments deploying the integrated optical circuit
with an external source, the transmitter arrays may be adapted for
distributing the optical power from the external source to the
array of output waveguides in several different ways. According to
an embodiment, at least one of the transmitter arrays comprises a
binary tree of passive power splitters, the tree being adapted for
distributing the light into the output waveguides, the at least one
transmitter array further comprising an optical modulator for at
least part of the output waveguides, the optical modulator being
adapted for performing the light modulation. This may allow for
individual control of the output waveguides by providing a
homogeneous light distribution to the output waveguides which is
then modulated by a dedicated array of modulators or a suitable
multichannel modulator.
[0031] According to an embodiment, at least one of the transmitter
arrays comprises a binary tree of electrically controllable power
splitters, the tree being adapted for distributing the light into
the output waveguides, the electrically controllable power
splitters being adapted for performing the light modulation being
an alternative way of individually controlling the output
waveguides, embodiments using a binary tree of electrically
controllable power splitters may yield the further advantage of
essentially lossless, non-absorptive distribution of optical power
to the output waveguides. While the modulation characteristics,
such as intensity or phase, can be set directly for each output
waveguide with embodiments comprising a binary tree of passive
power splitters, the modulation characteristics must be calculated
for each output waveguide in a binary tree of electrically
controllable power splitters as the product of all modulation
characteristics sets along the respective path through the binary
tree, as the modulation characteristics of each level of the three
are determined by the modulation characteristics set on the
previous tree level.
[0032] According to an embodiment, the modulation is amplitude
modulation and/or phase modulation. In particular, the transmitter
arrays may provide both modulation functionalities at the same
time. Implementations comprising a binary tree of power splitters
may be of the active or passive type for both modulation types, or
alternatively, use one of the two different tree types for each
type of modulation. More precisely, a binary tree of passive power
splitters in terms of amplitude or intensity modulation may be
followed by an intensity modulator, such as a spatial light
modulator, to perform the amplitude modulation, while each node of
the binary tree of passive power splitters comprises an
electrically controllable phase modulator (e.g. a fiber optical
phase modulator) for performing the phase modulation as an active
binary tree, or vice versa. On the other hand, the binary tree may
be electrically controllable for amplitude modulation and phase
modulation on each node, or it may be a fully passive binary tree
of passive power splitters, followed by a sequence of a phase
modulator and an amplitude modulator for each of the output
waveguides.
[0033] It is understood by a person of ordinary skill in the art
that the transmitter arrays may comprise further optical
functionality, such as an amplifier, to enable further
customization of the light to be provided to the photorefractive
medium.
[0034] Exemplary embodiments of the integrated optical circuit use
silicon optics, a photorefractive medium implemented as a waveguide
slab, and transmitter arrays comprising a one-dimensional array of
between 100 and 2000 optical waveguides as the output waveguides.
This corresponds to an envisioned information density up to an
order of 10.sup.6 units of information per sq. mm.
[0035] The integrated optical circuit may also be adapted for
receiving light transmitted through the photorefractive medium.
According to an embodiment, the circuit further comprises two
receiver arrays, each of the receiver arrays comprising an
electrical output and being adapted for measuring optical power
received from the photorefractive medium, generating an electrical
signal encoding the measured power, and outputting the electrical
signal through the electrical output, the optical circuit being
adapted for controlling the holographic encoding using the
electrical output as feedback. A receiver array comprises a
plurality of means for converting optical power into an electrical
quantity such as voltage or current, for instance, a plurality of
photodiodes. The converting elements are preferably arranged in a
one-dimensional array to enable spatially resolved detection of
light transmitted through the photorefractive medium. The two
receiver arrays may be arranged on different sides with respect to
the photorefractive medium. In an example, the receiver arrays are
arranged in a 90.degree. orientation around the photorefractive
medium so as to detect a beam of light incident to the
photorefractive medium from an optical path opposite to one of the
two receiver arrays once in transition (using the transmitter array
in opposition to the incident optical path), and concurrently in
reflection (using the receiver array arranged in the 90.degree.
position).
[0036] According to an embodiment, each of the receiver arrays
further comprises a plurality of input waveguides, each of the
receiver arrays being adapted for independently measuring optical
power received from the photorefractive medium by at least a
receiving portion of the input waveguides, the electrical signal
encoding the measured optical power and an identifying information
for each of the receiving input waveguides. The input waveguides
may be implemented as discussed above for the output waveguides
which may be used for the transmitter arrays according to
embodiments of the invention. Input waveguides are preferably
interfacing the opto-electrical converters (such as photodiodes)
and the optical path connecting the photorefractive medium with the
respective receiver array and may therefore decrease the
converter's sensitivity to erroneous signals such as light signals
originating from different sources than the photorefractive medium
or cross talk between neighboring converting elements.
[0037] The receiver arrays may also be adapted for performing phase
sensitive detection of the light received from the photorefractive
medium. According to an embodiment, each of the receiver arrays is
optically coupled to the light input and is further adapted for
measuring the phase difference between the light received from the
photorefractive medium and the light received from the light input,
the electrical signal further encoding the measured phase
difference. A phase information, which may be holographically
encoded in the photorefractive medium using phase modulation as
discussed above, may be beneficial for storing an additional
informational degree of freedom. The integrated optical circuit may
therefore be used to write and read multipolar information to/from
the photorefractive medium. A simple example of multipolar data is
positive and negative numbers being encoded holographically using
two different phase angles, e.g. 0.degree. representing a positive
number, and 180.degree. representing a negative number. The concept
can, however, be generalized to more than two information
polarities, which may be encoded holographically by a corresponding
number of modulated phase angles. A phase sensitive receiver array
may use a phase sensitive optical element (for example a hybrid
coupler) on each channel to compare the phase of the light received
from the photorefractive medium with the phase of the reference
beam received from the light input or the coherent local light
source. However, it is also possible to restore holographically
encoded phase information with a phase insensitive detector, as
will be discussed further below.
[0038] The electrical signal generated by each of the receiver
arrays may encode the previously holographically stored
information, particularly including the amplitude information, the
phase information, and the identifying information, in various
different ways known or apparent to a person skilled in the art.
Non-exhaustive examples of electrical signals encoding such
information include a serial signal, a parallel signal, a direct
current (DC) signal, an alternating current (AC) signal, a digital
signal, and combinations thereof. In a more specific example, each
channel representing one of the input waveguides is mapped to one
channel of a parallel signal and encodes previously holographically
stored pieces of amplitude and phase information in the amplitude
and phase of an AC signal transmitted through the given parallel
channel.
[0039] The transmitter arrays may be arranged relative to the
photorefractive medium such that the optical paths along which the
light propagates from the transmitter array into the
photorefractive medium span a certain distance. The light may be
coupled out into the optical paths spanning that distance such that
it may be processed further before being coupled into the
photorefractive medium. According to an embodiment of the
invention, the optical circuit further comprises a propagation
region adapted for coupling the light received from the transmitter
arrays into the photorefractive medium. The propagation region is a
region of the integrated optical circuit where the light can
propagate between the transmitter arrays and the photorefractive
medium without being confined by optical guiding structures such as
the output waveguides or the input waveguides. Allowing the light
to travel through this propagation region may have the advantage
that a particular beam of light coupled out from the transmitter
array can expand within the optical plane, or horizontally, such
that its width spans a major part of the overall width of the
photorefractive medium as it is coupled into the photorefractive
medium. This may yield a higher precision and reliability for
restoring the information thus holographically stored.
[0040] The optical paths and the working plane are arranged in a
single optical plane. This optical plane is a theoretical reference
plane in which all optical paths involved in the holographic
encoding and/or decoding of the information are disposed such that
they stand at least one dimension comprised by the optical plane.
In particular, the optical plane spans a finite length of the
output waveguides and the input waveguides such that each open end
of the output and input waveguides lies within the optical plane
and the optical paths along which the light propagates to and from
the photorefractive medium lie within the optical plane.
[0041] This planar optical alignment may be further supported by
implementing the propagation region such that the light is
vertically guided towards the photorefractive medium. According to
an embodiment of the invention, the propagation region is confined
perpendicular to the optical plane by cladding layers having a
lower refractive index than the propagation region. This may be
beneficial to provide a stable and essentially lossless optical
coupling between the propagation region and the transmitter arrays.
Analogously, the cladding layers may also be used to guide the
light vertically through another propagation region spanning a part
of the integrated optical circuit between the photorefractive
medium and the receiver arrays to gain the same advantage also for
reading out the holographically stored information.
[0042] Preferably, the propagation region is filled with the core
material of the optical waveguides (e.g. the input waveguides
and/or the output waveguides) such that the light encounters no
change in refractive index when coupled between the optical
waveguides and the propagation region. Analogously, the cladding
layers may be made of the same material as the cladding used for
guiding the light through the optical waveguides. However, using a
different cladding material may be more advantageous for the
particular needs of a given implementation as long as its
refractive index allows for guiding the light through the
propagation region by total reflection.
[0043] The propagation region may be used to implement optical
devices in order to exert additional control on the light
propagating through the propagation region. According to an
embodiment, the propagation region comprises a first set of optical
elements arranged in the optical paths and optically coupling the
photorefractive medium to at least one of the transmitter arrays.
This may also be of use to control a beam of light between the
photorefractive medium and the receiver arrays. According to an
embodiment of the invention, the propagation region comprises a
second set of optical elements, the second set being arranged in
the optical paths and optically coupling the photorefractive medium
to at least one of the receiver arrays.
[0044] Numerous optical elements such as mirrors or lenses may be
formed in the photorefractive region. For example, the mirror may
be formed by creating a slot of a suitable geometry in the material
filling the propagation region. This may be accomplished, for
instance, using an etch process within the geometry needed to
implement a mirror of the desired shape. The cavity thus formed in
the material filling out the propagation region creates an optical
surface from which the light travelling through the propagation
region may be reflected by total reflection if the orientation of
the cavity has been chosen appropriately. For example, a plane
mirror can be formed by etching a straight slot into the material
filling the propagation region in an orientation which crosses the
optical path pursued by a wave, e.g. emanating from a transmitter
array in an angle exceeding the critical angle of total internal
reflection. Plane mirrors may be beneficial for folding the optical
paths in order to achieve a more complex setup of the integrated
optical circuit.
[0045] Special optical elements such as curved mirrors may be
formed in the propagation region to provide means for focusing
and/or defocusing light traveling through the propagation region.
According to an embodiment of the invention, the first set of
optical elements is adapted for transforming a light wave emerging
from at least one of the transmitter arrays into a plane light wave
incident to the photorefractive medium. Coupling a plane wave into
the photorefractive medium may minimize reflective losses at the
interface between the propagation region and the photorefractive
medium and enable a controlled, straight propagation of the light
through the photorefractive medium.
[0046] According to an embodiment, the photorefractive medium is
confined in the optical plane by plane coupling surfaces of
perpendicular orientation with respect to the optical plane, the
coupling surfaces being adapted for coupling the light in and out
of the photorefractive medium. This may advantageously facilitate
an essentially lossless coupling of planar light waves in and out
of the photorefractive medium. Furthermore, planar light waves are
collimated to a width which defines an interaction region between
the two light beams incident to the photorefractive medium. A
planar light wave having travelled through the photorefractive
medium may exit the photorefractive medium again through one of the
plane coupling surfaces and continue to propagate through the
propagation region interfacing the photorefractive medium and the
receiver arrays still as a planar wave.
[0047] According to an embodiment of the invention, the second set
of optical elements is adapted for focusing a plane light wave
emerging from the photorefractive medium onto at least one of the
receiver arrays. In the same vein as the first set of optical
elements discussed above, the second set of optical elements may
comprise, e.g. a curved mirror in order to focus a plane wave
emerging the photorefractive medium, e.g. onto the input waveguides
of one of the receiver arrays. Altogether, the first and second
sets of optical elements may advantageously provide the capability
to handle the light for holographic information processing such
that it is precisely focused at the transmitter and receiver
arrays, while travelling straightly and essentially losslessly
through the photorefractive medium.
[0048] The integrated optical circuit may form an advantageous
combination of the geometrical shape of the photorefractive medium
and the arrangement of the transmitter and receiver arrays around
the photorefractive medium. According to an embodiment of the
invention, the photorefractive medium has a square cross-section
parallel to the optical plane, and the receiver arrays and the
transmitter arrays are arranged around the photorefractive medium
in relative angular distances of 90.degree. to each other and in
the sequence transmitter array, transmitter array, receiver array,
receiver array in a constant sense of rotation around the
photorefractive medium, the optical path optically coupling each of
the transmitter arrays with one coupling surface of the
photorefractive medium, the optical paths crossing the respective
coupling surface in perpendicular direction, each of the receiver
arrays being optically coupled to one further coupling surface of
the photorefractive medium by further optical paths, the further
optical paths crossing the further coupling surfaces in
perpendicular direction.
[0049] In general, the topology of optical paths in the setup is
governed by the combination of photorefractive material and
polarization of light chosen for a particular setup. With GaAs as
photorefractive material, the 90.degree. layout may work with light
polarized perpendicular to the optical plane, but this choice of
polarization is less commonly used for Silicon photonics circuits.
With light polarized in the optical plane, another relative angle
may have to be used. In the following discussion of FIG. 1, the
prospective suitability of the setup for performing subsequent or
simultaneous reading and writing operations for holographic
encoding of the information in the photorefractive medium is
demonstrated with the example of a 90.degree. layout.
[0050] FIG. 1 shows a schematic cut through the optical plane of an
exemplary integrated optical circuit 100 incorporating various
embodiments of the invention. The photorefractive medium 104 is
placed at the center of optical circuit 100. It comprises a square
cross-section parallel to the optical plane, exhibiting four plane
coupling surfaces to the propagation region 110 surrounding the
photorefractive medium 104. Two transmitter arrays 102 are located
in two neighboring corners of the square optical circuit 100. Each
transmitter array 102 comprises an array of output waveguides 101
arranged in the optical plane, and an electrical input 103. The
transmitter arrays 102 are receiving light from an external
coherent light source (not depicted) coupling light into the
integrated optical circuit 100 through light input 108.
[0051] Two detector arrays 106 are located in the two neighboring
corners opposite to the neighboring corners hosting the transmitter
arrays 102. Each detector array 106 comprises an electrical output
107. The positions of electrical inputs 103 and electrical outputs
107 are indicated by thick arrows. The light from the external
light source is distributed from light input 108 towards
transmitter arrays 102 and reference inputs of detector arrays 106
through a series of optical waveguides and beam splitters. To
illustrate the functionality of the optical setup, the transmitter
arrays 102 are depicted as distributing the light they are
receiving from the light input 108 towards a single output
waveguide such that the course of the light following the optical
path towards the photorefractive medium 104 through propagation
region 110 is clearly visible.
[0052] The transmitter arrays 102 are oriented such that the light
starts to travel through the propagation region 110 towards a
respectively neighboring corner of optical circuit 100. The light
beams diverge and are deflected towards a central axis of setup 100
by flat mirrors 112 etched into the propagation region 110. Close
to the photorefractive medium 104, each light beam has the same
width as the interaction region of the photorefractive medium 104,
is transformed into a beam of plane waves and deflected into
photorefractive medium 104 by a curved mirror 114 etched into the
medium filling the propagation region in a position centered on the
respective central axis of setup 100 crossed by the respective
light beam. The light beams transmitted by transmitter arrays 102
enter the photorefractive medium 104 through two separate ones of
the plane coupling surfaces 90.degree. apart from each other.
[0053] In the photorefractive medium 104, the two plane light waves
superpose with each other, forming a regular interference pattern
over the whole area of the photorefractive medium 104. The optical
power coupled into photorefractive medium 104 is chosen such that
the photorefractive effect is invoked in the photorefractive medium
104 such that the spatial modulation of the refractive index of the
photorefractive medium 104, which is referred to as a refractive
index grating, is formed by the superposing beams of light. Due to
the different positions of the point sources along the array, each
source generates a plane wave under a slightly different angle in
the interaction region.
[0054] A grating created this way may holographically encode the
information to be stored in the photorefractive medium 104 in
several parameters which can be set using the modulation techniques
discussed above. All gratings, for all point source combinations
from the two transmitter arrays, overlap over the whole
photorefractive area. An amplitude or intensity modulation may
result in a change of reflectivity of the grating. A phase
modulation of one of the superposed light beams may result in a
lateral shift of the grating about a fraction of the grating
constant. The frequency modulation may result in a change of line
density of the grating.
[0055] The information encoded by a particular refractive index
grating may be read out using the light beam or light pulse sent
through one of the output waveguides which were used to create the
respective refractive index grating. Light crossing the refractive
index grating in the photorefractive medium 104 will be partially
transmitted through the grating, preserving its propagation
direction, and partially reflected away from its original
propagation direction. In the readout phase, when one of the point
sources is activated, due to the selectivity of the gratings, only
the gratings that were written using exactly this point source will
constructively reflect a part of the light. All the other gratings
will not interact with light from this point source.
[0056] As a peculiarity of the depicted square-shaped
photorefractive medium, in conjunction with the suitable
arrangement of the transmitters and the first set of optical
elements comprising the two plane mirrors and the two curved
mirrors mentioned above, the readout beam and the refractive index
grating are oriented such that the transmitted parts of the readout
beam is coupled out of the photorefractive medium 104 into
propagation region 110 through a third one of the plane coupling
surfaces, while the reflected part of the readout light beam is
coupled into the propagation region through the fourth plane
coupling surface of the square-shaped photorefractive medium. The
second set of optical elements also comprises two curved mirrors
114 and two flat mirrors 112, the curved mirrors 114 focusing each
of the emerging partial readout light beams onto the aperture of
one of the receiver arrays, and each pair of the curved mirror 114
and the flat mirror 112 folding one of the beams about 90.degree.
to yield the more complex design of the integrated optical circuit.
The two receiver arrays may then be used to perform a phase
sensitive detection of the intensity distribution encoded by the
refractive index grating at the position in the photorefractive
medium selected for transmission of the readout beam. The receiver
arrays 106 convert the information retrieved from the refractive
index grating into electrical signals which are then put out
through electrical outputs 107.
[0057] This way, the setup depicted in FIG. 1 may be used for
subsequent and/or simultaneous encoding or decoding of multiple
instances of information using the photorefractive medium and a
vectorial combination of the output waveguides transmitting
modulated light beams or light pulses into the photorefractive
medium. The electrical signals presented at the electrical outputs
107 may be used for further processing, including feedback to the
electrical inputs 103. Depending on the kind of processing applied
to the output generated by the integrated optical circuit, an
integrated optical circuit like the exemplary optical circuit
depicted in FIG. 1 may therefore be used to perform a closed loop
of reading and writing operations which may eventually result in a
convergence of the information holographically stored on the
photorefractive medium. This procedure may be useful to perform a
training operation in a deep neural network (DNN).
[0058] The methods for holographic information processing according
to aspects of the invention as specified in the summary may yield
the same advantages regarding energy efficiency, computational
speed, and optical stability as outlined above for the integrated
optical circuit as they provide and use the same for
holographically encoding or, respectively, decoding the information
in the working plane of the photorefractive medium.
[0059] As was noted above, the integrated optical circuit may be
used for phase sensitive decoding of the information stored in the
photorefractive medium even if the receiver arrays are not adapted
for phase sensitive detection, e.g. utilizing suitable hybrid
couplers.
[0060] According to an embodiment, the information comprises an
amplitude information and a phase information, each of the
transmitter arrays being adapted for applying a phase modulation to
the transmitted light, the decoding comprising: [0061] selecting a
constant first phase difference between the two transmitter arrays
for the phase modulation; [0062] measuring the optical power of a
first light pulse received by the two receiver arrays, the first
light pulse being directed through the two transmitter arrays into
the photorefractive medium such that it passes the refractive index
pattern, the two transmitter arrays being set to the constant first
phase difference; [0063] selecting a constant second phase
difference between the two transmitter arrays for the phase
modulation, the constant second phase difference differing from the
constant first phase difference by between 45 and 135 degrees;
[0064] measuring the optical power of a second light pulse received
by the two receiver arrays, the second light pulse being directed
through the two transmitter arrays into the photorefractive medium
such that it passes the refractive index pattern, the two
transmitter arrays being set to the constant second phase
difference; and [0065] decoding the amplitude information and the
phase information from the optical power measured for the first
light pulse and the second light pulse.
[0066] The advantage of enabling phase sensitive decoding with
phase insensitive receiver arrays can be illustrated with reference
to FIG. 2, which depicts a flow diagram illustrating the method 200
according to the aforementioned embodiment for the special case
that the first phase difference is set to 0.degree. and the second
phase difference is set to 90.degree.. At block 202, phase
modulation is switched off for both transmitter arrays. A light
pulse distributed to and transmitted through the two transmitter
arrays will thus feature a phase difference of 0.degree. between
the two portions of the light beam transmitted through each of the
transmitter arrays. This is also the case for the first light
pulse, which is transmitted through both transmitter arrays
simultaneously at block 204. The first light pulse is coupled into
the photorefractive medium simultaneously through two different
coupling surfaces of the photorefractive medium.
[0067] The first light pulse is transmitted and reflected by the
refractive index pattern (e.g. one or more refractive index
gratings) representing the stored information. The two portions of
the first light pulse thus influenced by the refractive index
pattern then leave the photorefractive medium through two further
different coupling surfaces and continues to propagate towards the
two receiver arrays, e.g. directed by the second set of optical
elements. The receiver arrays measure the optical power
distribution of the first light pulse transmitted through each of
the branches.
[0068] At block 206, the phase modulation is set to 90.degree. for
one transmitter array, such that a phase difference of 90.degree.
is created between the two transmitter arrays. Afterwards,
analogously to block 204, at block 208 the second light pulse is
transmitted through the transmitter arrays set to the phase
difference of 90.degree., interacts again with the refractive index
pattern representing the stored information in the photorefractive
medium, and is directed to the receiver arrays where its optical
power distribution is measured. Preferably, the intensity of the
first light pulse is selected low enough such that the influence of
the first light pulse on the information holographically stored in
the photorefractive medium is minimized.
[0069] At block 210, the optical power measured for each channel of
the two receiver arrays for the first light pulse is interpreted as
a Cartesian x-component, and analogously, each optical power
measured for the second light pulse is interpreted as a Cartesian
y-component. The two Cartesian components thus independently
measured with a relative phase difference of 90.degree. may then be
subject to a coordinate transformation into polar coordinates,
resulting in the desired pair of an amplitude and a phase retrieved
by each of the respective channels from the holographically stored
information in the photorefractive medium. This way, the phase
information stored in the refractive index pattern is restored by
two subsequent measurements performed with a setup which is not
supporting a direct optical phase observation.
[0070] In an exemplary usage scenario, the integrated optical
circuit is deployed as a synaptic weight storage in a neuromorphic
computing system. In holographic synaptic weight storage, the input
signals from N input neurons are encoded onto N plane waves that
propagate under different angles in the photorefractive storage
material. These plane waves represent the input columns of the
storage array. The output rows of the array are represented by
another set of M plane waves that propagate under another set of
different angles. Weighted coupling of light from an input to an
output plane wave, corresponding to synaptic coupling from an input
to an output neuron, is provided by diffraction on a refractive
index grating, written in the photorefractive medium. For each of
the M.times.N synaptic weight elements there exists a unique
coupling grating, characterized by a unique combination of grating
orientation and grating period. The amount of light coupled by the
grating, the synaptic weight, is determined by the refractive index
modulation depth of the grating which can be adjusted.
[0071] For the evaluation step, the amplitudes of the N optical
input plane waves are set (modulated) according to the signals from
the N source neurons. Light is then diffracted toward the different
output plane waves, proportional to the strength of the
corresponding diffraction grating (the multiply operation). All
signals that are diffracted into one output plane wave are focused
onto one of the transmitter arrays where they add (coherently), and
are converted to an electrical signal that is sent to the
corresponding output neuron for further processing.
[0072] A diffraction grating for a certain synapse can be formed
and modified by providing two plane light waves, under the row and
column angles corresponding to the addressed synapse. These waves
interfere and the corresponding optical interference pattern writes
a refractive index pattern through the photorefractive effect. This
process maps directly to the update step required for the
Backpropagation algorithm for training of Deep Neural Networks.
[0073] More generally, in view of its property of connecting a pair
of one-dimensional inputs with a pair of one-dimensional outputs
via a two-dimensional data storage, the integrated optical circuit
may be used as an optical calculator for performing mathematical
matrix and vector operations. In analogy to the synaptic weights
mentioned above, the information stored in the photorefractive
medium may represent numerical entries of a matrix, while the
modulation characteristics of light transmitted through a given
output waveguide may represent the numerical value of a
corresponding vector entry. This way, a matrix--vector
multiplication may be performed by transmitting a light pattern
modulated with the vector entries into the photorefractive
medium.
[0074] The invention can also be described by the following
features:
[0075] An integrated optical circuit for holographic information
processing, the integrated optical circuit including a
photorefractive medium and two transmitter arrays, the two
transmitter arrays being adapted for locally changing a refractive
index of the photorefractive medium for holographic encoding of the
holographic information in a working plane of the photorefractive
medium by transmitting light via optical paths into the
photorefractive medium such that an interference pattern is
generated in the working plane, the optical paths and the working
plane being arranged in a single optical plane.
[0076] The integrated optical circuit of feature 1, wherein each of
the two transmitter arrays include an electrical input and an array
of output waveguides, the array of output waveguides being adapted
for transmitting the light into the photorefractive medium, each of
the two transmitter arrays being adapted for modulating the light
transmitted through the array of output waveguides based on an
electrical modulation signal received through the electrical input
in order to encode the holographic information.
[0077] The integrated optical circuit of feature 2, being adapted
for encoding the holographic information by simultaneously
generating the interference pattern for at least two different
combinations of an output waveguide of a first one of the two
transmitter arrays and an output waveguide of a second one of the
two transmitter arrays.
[0078] The integrated optical circuit of feature 2, wherein at
least one of the two transmitter arrays including a binary tree of
passive power splitters, the binary tree being adapted for
distributing the light into the array of output waveguides, the at
least one of the two transmitter arrays further comprising an
optical modulator for at least part of the array of output
waveguides, the optical modulator being adapted for performing the
light modulation.
[0079] The integrated optical circuit of feature 2, wherein at
least one of the two transmitter arrays including a binary tree of
electrically controllable power splitters, the binary tree being
adapted for distributing the light into the array of output
waveguides, the electrically controllable power splitters being
adapted for performing the light modulation.
[0080] The integrated optical circuit of feature 2, wherein the
modulation of the light includes either being amplitude modulation
and/or phase modulation.
[0081] The integrated optical circuit of feature 1, further
including a propagation region adapted for coupling the light
received from the two transmitter arrays into the photorefractive
medium.
[0082] The integrated optical circuit of feature 7, wherein the
propagation region being confined perpendicular to the single
optical plane by cladding layers having a lower refractive index
than the propagation region.
[0083] The integrated optical circuit of feature 7, wherein the
propagation region including a first set of optical elements
arranged in the optical paths and optically coupling the
photorefractive medium to at least one of the two transmitter
arrays.
[0084] The integrated optical circuit of feature 9, wherein the
first set of optical elements being adapted for transforming a
light wave emerging from the at least one of the two transmitter
arrays into a plane light wave incident to the photorefractive
medium.
[0085] The integrated optical circuit of feature 1, further
including a light input adapted for transmitting the light into the
integrated optical circuit, each of the two transmitter arrays
being optically coupled to the light input and being adapted for
transmitting the light received from the optical input into the
photorefractive medium.
[0086] The integrated optical circuit of feature 1, further
including two receiver arrays, each of the two receiver arrays
including an electrical output and being adapted for measuring
optical power received from the photorefractive medium, generating
an electrical signal encoding the measured optical power, and
outputting the electrical signal through the electrical output, the
integrated optical circuit being adapted for controlling the
holographic encoding using the electrical output as feedback.
[0087] The integrated optical circuit of feature 12, wherein each
of the two receiver arrays further includes a plurality of input
waveguides, each of the two receiver arrays being adapted for
independently measuring optical power received from the
photorefractive medium by at least a receiving portion of the
plurality of input waveguides, the electrical signal encoding the
measured optical power and an identifying information for each of
the receiving portion of the plurality of input waveguides.
[0088] The integrated optical circuit of feature 12, wherein each
of the two receiver arrays being optically coupled to the light
input and being further adapted for measuring a phase difference
between the light received from the photorefractive medium and the
light received from the light input, the electrical signal further
encoding the measured phase difference.
[0089] The integrated optical circuit of feature 12, wherein a
propagation region includes a second set of optical elements, the
second set of optical elements being arranged in the optical paths
and optically coupling the photorefractive medium to at least one
of the two receiver arrays.
[0090] The integrated optical circuit of feature 15, wherein the
second set of optical elements being adapted for focusing a plane
light wave emerging from the photorefractive medium onto at least
one of the two receiver arrays.
[0091] A method for holographic information processing, the method
including: [0092] providing an integrated optical circuit
comprising a photorefractive medium and two transmitter arrays; and
[0093] changing, locally, a refractive index of the photorefractive
medium for holographic encoding of information in a working plane
of the photorefractive medium by transmitting light from the two
transmitter arrays via optical paths into the photorefractive
medium such that an interference pattern is generated in the
working plane, the optical paths and the working plane being
arranged in a single optical plane.
[0094] A method for holographic information processing, the method
including: [0095] providing an integrated optical circuit
comprising a photorefractive medium, two transmitter arrays, and
two receiver arrays, the photorefractive medium comprising a
refractive index pattern holographically encoding holographic
information; and [0096] decoding the holographic information by
measuring optical power of a light pulse received by the two
receiver arrays, the light pulse being directed through the two
transmitter arrays into the photorefractive medium such that it
passes the refractive index pattern.
[0097] The method of feature 18, wherein the holographic
information includes an amplitude information and a phase
information, each of the two transmitter arrays being adapted for
applying a phase modulation to the light pulse, the decoding
including: [0098] selecting a constant first phase difference
between the two transmitter arrays for the phase modulation; [0099]
measuring the optical power of a first light pulse received by the
two receiver arrays, the first light pulse being directed through
the two transmitter arrays into the photorefractive medium such
that it passes the refractive index pattern, the two transmitter
arrays being set to the constant first phase difference; [0100]
selecting a constant second phase difference between the two
transmitter arrays for the phase modulation, the constant second
phase difference differing from the constant first phase difference
by between 45 and 135 degrees; [0101] measuring the optical power
of a second light pulse received by the two receiver arrays, the
second light pulse being directed through the two transmitter
arrays into the photorefractive medium such that it passes the
refractive index pattern, the two transmitter arrays being set to
the constant second phase difference; and [0102] decoding the
amplitude information and the phase information from the optical
power measured for the first light pulse and the second light
pulse.
* * * * *