U.S. patent application number 13/255808 was filed with the patent office on 2012-02-09 for optical leaky integrate-and-fire neuron.
This patent application is currently assigned to LOCKHEED MARTIN CORPORATION. Invention is credited to Konstantin Kravtsov, Paul R. Prucnal, David Rosenbluth.
Application Number | 20120033966 13/255808 |
Document ID | / |
Family ID | 42728748 |
Filed Date | 2012-02-09 |
United States Patent
Application |
20120033966 |
Kind Code |
A1 |
Rosenbluth; David ; et
al. |
February 9, 2012 |
OPTICAL LEAKY INTEGRATE-AND-FIRE NEURON
Abstract
An optical system includes an optical integrator, a readout
mechanism, and an optical thresholder. The optical integrator is
configured to perform temporal integration of an optical input
signal having a first wavelength received at an input. The readout
mechanism is coupled to the optical integrator and provides optical
signals having a second wavelength to the optical integrator for
measuring a state of the optical integrator. The optical
thresholder is coupled to an output of the optical integrator and
is configured to receive a signal representing a temporal
integration of the optical input signal from the optical integrator
and produce an optical signal identifying if an amplitude of the
signal representing the temporal integration of the optical input
signal is above or below a threshold value.
Inventors: |
Rosenbluth; David;
(Swarthmore, PA) ; Prucnal; Paul R.; (Princeton,
NJ) ; Kravtsov; Konstantin; (Moscow, RU) |
Assignee: |
LOCKHEED MARTIN CORPORATION
Bethesda
MD
|
Family ID: |
42728748 |
Appl. No.: |
13/255808 |
Filed: |
March 10, 2010 |
PCT Filed: |
March 10, 2010 |
PCT NO: |
PCT/US10/26835 |
371 Date: |
October 26, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61158986 |
Mar 10, 2009 |
|
|
|
Current U.S.
Class: |
398/38 |
Current CPC
Class: |
H01S 5/509 20130101;
G06E 3/003 20130101 |
Class at
Publication: |
398/38 |
International
Class: |
H04B 17/00 20060101
H04B017/00 |
Claims
1. An optical system, comprising: an optical integrator for
performing temporal integration of an optical input signal having a
first wavelength received at an input; a readout mechanism coupled
to the optical integrator for providing optical signals having a
second wavelength to the optical integrator for measuring a state
of the optical integrator; and an optical thresholder coupled to an
output of the optical integrator for receiving a signal
representing a temporal integration of the optical input signal
from the optical integrator and producing an output identifying if
an amplitude of the signal representing the temporal integration of
the optical input signal is above or below a threshold value.
2. The optical system of claim 1, wherein the optical integrator
includes: a semiconductor optical amplifier (SOA) having a decaying
response function, the SOA configured to receive the optical
signals having the first and second wavelengths and first and
second amplitudes and provide the signal representing temporal
integration of the optical input signal in response, and an optical
filter coupled to an output of the SOA, the optical filter for
passing optical signals having the second wavelength and blocking
the optical signals having the first wavelength.
3. The optical system of claim 1, wherein the optical thresholder
includes a first polarization controller and a non-linear fiber
optical loop mirror.
4. The optical system of claim 1, further comprising: an optical
amplifier disposed between an output of the optical integrator and
an input of the optical thresholder for amplifying the signal
representing the temporal integration of the optical input signal
output from the optical integrator.
5. The optical system of claim 1, wherein the readout mechanism is
an optical pulse train generator providing an optical signal having
a lower energy than an energy of the optical input signal.
6. The optical system of claim 1, further comprising: a first
optical coupler having first and second inputs, the first optical
coupler for coupling a first optical input signal having the first
wavelength with a second optical signal having the first wavelength
into a single fiber to provide the optical input signal, the first
input of the first optical coupler for receiving the first optical
signal having the first wavelength, the second input of the first
optical coupler for receiving the second optical signal having the
second wavelength; and a second optical coupler having third and
fourth inputs and an output, the third input coupled to an output
of the first optical coupler, the fourth input for receiving the
optical signals having the second wavelength, the second optical
coupler for coupling the optical input signal having the first
wavelength with the optical signals having the second wavelength
into a single fiber coupled to an input of the optical
integrator.
7. The optical system of claim 6, further comprising: a first
variable attenuator for providing a weight to an optical signal;
and a first variable delay line for delaying an optical signal
coupled to the first variable attenuator, wherein one of the first
variable attenuator and the first variable delay line is coupled to
the first input of the first optical coupler.
8. The optical system of claim 7, further comprising: a second
variable attenuator; and a second variable delay line coupled to
the second variable attenuator, wherein one of the second variable
attenuator and the second variable delay line is coupled to the
second input of the first optical coupler.
9. An optical signal processing method, comprising: temporally
integrating a first optical signal having a first wavelength at an
optical integrator; determining if the temporally integrated
optical signal has an amplitude that is above a threshold at an
optical thresholder; and outputting an optical signal identifying
if the amplitude of the temporally integrated optical signal is
above or below the threshold.
10. The method of claim 9, further comprising: combining the first
optical signal having the first wavelength with an optical signal
having a second wavelength into a single fiber at a first optical
coupler, the first optical coupler having an output coupled to an
input of the optical integrator.
11. The method of claim 10, further comprising: combining a second
optical signal having the first wavelength with a third optical
signal having the first wavelength at second optical coupler to
provide the first optical signal having the first wavelength; and
outputting the first optical signal having the first wavelength to
the first optical coupler.
12. The method of claim 9, further comprising: increasing an
amplitude of the temporally integrated optical signal at an optical
amplifier prior to determining if the amplitude of the temporally
integrated optical signal is above the threshold level.
13. The method of claim 9, wherein the thresholder includes a
non-linear optical loop mirror and a tunable optical isolator.
14. The method of claim 11, further comprising: adding a weight to
at least one of the first and second optical signals having the
first wavelength prior to combining them.
15. The method of claim 11, further comprising: delaying at least
one of the first and second optical signals having the first
wavelength using a variable delay line prior to combining them.
16. An optical system, comprising: a semiconductor optical
amplifier(SOA) having a decaying response function for temporally
integrating an optical signal having a first wavelength received at
an input, the SOA configured to output an optical signal
identifying a state of the SOA response to receiving a signal
having a second wavelength from a readout device; an optical filter
coupled to an output of the SOA, the optical filter for passing the
optical signals having the second wavelength and blocking optical
signals having the first wavelength; and an optical thresholder for
receiving optical signals having the second wavelength identifying
the state of the SOA from the optical filter and providing an
optical signal identifying if an energy of the optical signals
having the second wavelength are above or below a threshold
value.
17. The optical system of claim 16, further comprising: an optical
amplifier disposed between the output of the optical filter and the
input of the optical thresholder, the optical amplifier for
amplifying the optical signals having the second wavelength
received from the optical filter.
18. The optical system of claim 16, wherein the optical thresholder
includes a non-linear optical loop mirror and a tunable optical
isolator.
19. The optical system of claim 16, wherein an amplitude of the
optical signal having the second wavelength is smaller than an
amplitude of the optical signal having the first wavelength.
20. The optical system of claim 16, further comprising: a first
optical coupler having first and second inputs for summing the
first optical signal having the first wavelength with a second
optical signal having the first wavelength, the first input of the
first optical coupler for receiving the first optical signal having
the first wavelength, the second input of the first optical coupler
for receiving the second optical signal having the first
wavelength; and a second optical coupler having third and fourth
inputs and an output, the third input coupled to an output of the
first optical coupler, the fourth input for receiving the signal
having the second wavelength, the output of the second optical
coupler coupled to an input of the SOA.
21. The optical system of claim 20, further comprising: a first
variable attenuator for providing a weight to an optical signal;
and a first variable delay line for delaying an optical signal
coupled to the first variable attenuator, wherein one of the first
variable attenuator and the first variable delay line is coupled to
the first input of the first optical coupler.
22. The optical system of claim 21, further comprising: a second
variable attenuator for delaying an optical signal; and a second
variable delay line for delaying an optical signal coupled to the
second variable attenuator, wherein one of the second variable
attenuator and the second variable delay line is coupled to the
second input of the first optical coupler.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 61/158,986, which was filed on Mar. 10, 2009, and
is incorporated by reference herein in its entirety.
FIELD OF DISCLOSURE
[0002] The disclosed circuit and method relate to optical circuits.
More specifically, the disclosed circuit and method relate to an
all-optical circuit for performing computations.
BACKGROUND
[0003] Signal processing continues to become more complex as data
transfer rates continue to increase. Conventionally, signal
processing is either performed by an analog system or by a digital
system. Analog systems may be implemented in compact circuits
making them a popular choice when circuit area is of significant
importance. However, one significant drawback of analog devices is
that they are highly susceptible to noise accumulation, which
limits the number of analog operations that can be applied to data,
and therefore the complexity of the computations that can be
practically implemented using only analog devices.
[0004] In contrast, digital systems are not as susceptible to noise
accumulation as are analog systems. However, the number of digital
devices needed to implement a computation rapidly increases with
the complexity of the computation performed.
[0005] Photonic devices provide the ability to process signals of
much higher bandwidth than is possible with electronic devices, but
they are larger and more expensive than electronic devices.
Practical implementation of complex high bandwidth processing
algorithms using photonic devices therefore requires an approach
that minimizes the number of devices needed without over
constraining the complexity of computations that can be
implemented.
[0006] Accordingly, a hybrid processing system and method that
combines the advantages of digital and analog systems is
desirable.
SUMMARY
[0007] An optical system is disclosed that includes an optical
integrator, a readout mechanism, and an optical thresholder. The
optical integrator is configured to perform temporal integration of
an optical input signal having a first wavelength received at an
input. The readout mechanism is coupled to the optical integrator
and provides optical signals having a second wavelength to the
optical integrator for measuring a state of the optical integrator.
The optical thresholder is coupled to an output of the optical
integrator and is configured to receive a signal representing a
temporal integration of the optical input signal from the optical
integrator and produce an optical signal identifying if an
amplitude of the signal representing the temporal integration of
the optical input signal is above or below a threshold value.
[0008] A signal processing method is also disclosed. The optical
signal processing method includes temporally integrating a first
optical signal having a first wavelength at an optical integrator,
determining if the temporally integrated optical signal has an
amplitude that is above a threshold at an optical thresholder, and
outputting an optical signal identifying if the amplitude of the
temporally integrated optical signal is above or below the
threshold.
[0009] Additionally, an optical system is disclosed including a
semiconductor optical amplifier (SOA), an optical filter, and an
optical thresholder. The SOA has a decaying response function for
temporally integrating an optical signal having a first wavelength
received at an input. The SOA is configured to output an optical
signal identifying a state of the SOA in response to receiving a
signal having a second wavelength from a readout device. The
optical filter is coupled to an output of the SOA and is configured
to pass the optical signals having the second wavelength and
blocking optical signals having the first wavelength. The optical
thresholder is configured to receive optical signals having the
second wavelength identifying the state of the SOA from the optical
filter and provide an optical signal identifying if an energy of
the optical signals having the second wavelength are above or below
a threshold value.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1A is a block diagram of one example of an optical
neuron.
[0011] FIG. 1B is a block diagram illustrating the integration and
thresholder blocks in accordance with the optical neuron
illustrated in FIG. 1A.
[0012] FIG. 2 is a block diagram of one example of a semiconductor
optical amplifier.
[0013] FIG. 3A illustrates one example of input pulses received at
an input of the optical integrator.
[0014] FIG. 3B illustrates one example of output pulses from the
optical integrator in response to the receiving the input pulses
illustrated in FIG. 3A.
[0015] FIG. 4A is an energy versus time graph showing the response
of a semiconductor optical amplifier that receives a series of
optical pulses.
[0016] FIG. 4B is an example oscilloscope trace of a plurality of
optical signals of a pulse train.
[0017] FIG. 5A illustrates one example of output pulses from an
optical thresholder having a threshold set between zero and one in
response to receiving the pulses illustrated in FIG. 3B.
[0018] FIG. 5B illustrates one example of output pulses from an
optical thresholder having a threshold set between one and two in
response to receiving the pulses illustrated in FIG. 3B.
DETAILED DESCRIPTION
[0019] The leaky integrate-and-fire ("LIF") neuron is one of the
most widely studied neuron models in computational neuroscience.
The spike processing performed by these computational elements is a
hybrid of analog and digital processing that exploits the
efficiency of analog computation while overcoming the problem of
noise accumulation suffered by analog systems. In spike processing,
information is encoded in the timing of spikes rather than in the
size or shape of the spikes. Accordingly, information is conveyed
by a spike being present or absent, much like in digital systems
how a bit is either a one or a zero. In contrast, traditional
neural network models perform purely analog computation in which
neuron inputs, intermediate results, and outputs are represented as
analog values.
[0020] From the standpoint of computability and complexity theory,
LIF neurons are powerful computational primitives capable of
simulating both Turing machines and traditional neural networks.
LIF models have a number, N, of inputs, .sigma..sub.i(t), where
i=1,2, . . . , N; an internal activation state, V.sub.m(t); and a
single output state, O(t). At rest, the internal state of the
neuron is actively maintained at a resting voltage, V.sub.rest.
Each input, .sigma..sub.t(t), of the neuron is a continuous time
series consisting of either spikes or continuous analog values.
These inputs are typically weighted by .omega..sub.i and delayed
.delta..sub.i, which may be mathematically represented as
.omega..sub.i.sigma..sub.i(t+.delta..sub.i). The delayed and
weighted input time series is spatially integrated through
pointwise summation in accordance with the following equation:
.SIGMA..sub.i=1.sup.N.omega..sub.i.sigma..sub.i(t+.delta..sub.i)
Eq. (1)
[0021] The activation state, or membrane voltage, V.sub.m(t), of
the neuron is an exponentially weighted temporal integration of the
spatially summed input time series. If the magnitude of the
temporally integrated signal exceeds a threshold value, then the
neuron outputs a spike, e.g., O(t)=1, if
V.sub.m(t)>V.sub.thresh. After a spike, there is a short period
of time, known as a refractory period, during which another spike
cannot be issued, e.g., if O(t)=1, then O(t+.DELTA.t)=0,
.DELTA.t.ltoreq.T.sub.refract. Accordingly, the output of the
neuron, O(t), consists of a continuous time series of spikes.
[0022] There are three primary influences that affect the magnitude
of the membrane voltage, V.sub.m(t), of an LIF neuron: (1) an
active pumping current, (2) current leakage, and (3) external
inputs generating time varying membrane conductance changes. Each
of these influences are part of the following differential equation
for approximating the membrane voltage over time:
V m ( t ) t = V rest .tau. m - V m ( t ) .tau. m + 1 c m V m ( t )
.sigma. ( t ) Eq . ( 2 ) ##EQU00001##
Where,
[0023] V m ( t ) t ##EQU00002##
represents the activation of the neuron;
V rest .tau. m ##EQU00003##
represents the active pumping current of the neuron;
V m ( t ) .tau. m ##EQU00004##
represents the leakage current of neuron; and
1 c m V m ( t ) .sigma. ( t ) ##EQU00005##
represents the external inputs to the neuron.
[0024] A direct correspondence has been discovered between the
equation governing temporal integration of LIF neurons set forth in
Equation 2 and the carrier density of a semiconductor optical
amplifier (SOA). The primary state variable for a SOA in this case
is the carrier density above transparency N'(t)=N(t)-N.sub.0, where
N(t) is the actual carrier density, and N.sub.0(t) is carrier
density at transparency. The integrative properties of the SOA are
determined by the carrier lifetime, .tau..sub.e; a mode confinement
factor, .left brkt-top.; a differential gain coefficient, .alpha.;
a photon energy, Ep; and the active SOA pumping current, I(t).
Spontaneous carrier decay tends to drive the carrier density, N',
of the SOA towards zero, and thus the active pumping current is
needed to counter the carrier decay to maintain a resting carrier
density of N'.sub.rest. The three contributions to the value of the
carrier density, N'(t), are a leakage term due to passive carrier
decay, a term for carrier density due to active optical pumping of
the SOA, and a term for carriers generated by external inputs. The
gain dynamics of a SOA when input pulse widths are much shorter,
e.g., two orders of magnitude shorter, than the carrier lifetime
may be described as follows:
N ' ( t ) t = N rest ' .tau. e - N ' ( t ) .tau. e + .GAMMA. a E p
N ' ( t ) I ( t ) Eq . ( 3 ) ##EQU00006##
[0025] Comparing Equation 2 with Equation 3 demonstrates a
remarkable similarity between the electrical model of membrane
voltage of an LIF neuron and the optical model of SOA carrier
density. The discovery of the correspondence between the Equations
1 and 2 has enabled the development of an all-optical
implementation of an LIF neuron, which can advantageously be used
as a computational primitive in large scale complex photonic
computational systems.
[0026] FIG. 1A is a block diagram of one example of an optical
implementation of an LIF neuron 100. As shown in FIG. 1A, the
optical LIF neuron 100 includes an optical coupler 106-1 configured
to receive N optical input signals. Each input to the optical
coupler 106-1 may include a variable attenuator 102-1:102-N and a
variable delay line 104-1:104-N. Accordingly, the variable
attenuators 102-1:102-N, variable delay lines 104-1:104-N, and
optical coupler 106-1 respectively provide the passive weighting,
delay, and the summation of inputs of the optical LIF neuron 100.
The optical coupler 106-1 is coupled to an integrator block 108
which is also coupled to a readout mechanism 112 for reading out
the state of the integrator block 108. The state of integrator
block 108 is output to a thresholder block 110 in the form of an
optical signal. Auxiliary devices such as supplementary electronics
and optical modulators may be included, but are not shown to
simplify the figure.
[0027] FIG. 1B is a more detailed schematic of the optical
integrator and thresholder 108, 110. As shown in FIG. 1B, the
optical integrator 108 includes a second optical coupler 106-2
having an output coupled to an input of SOA 114. The SOA 114 also
has an output coupled to an input of optical filter 116, and a
charge pumping circuit 118 is provided for pumping the SOA 114 with
electrons. An optional optical amplifier 120 is shown coupled
between an output of the integrator 108 (e.g., an output of the
optical filter 116) and an input of the thresholder 110.
[0028] The thresholder 110 includes a non-linear optical loop
mirror 122 formed by a non-linear doped fiber 124 and an optical
coupler 126. A plurality of optical components may be disposed
along non-linear fiber 124 for optical tuning. For example,
polarization controllers 128-1, 128-2 and a tunable isolator 130
may be disposed along the non-linear fiber 124.
[0029] Optical couplers 106 (e.g., optical couplers 106-1 and
106-2) may be any optical coupler configured to couple optical
signals of different wavelengths and amplitudes in separate fibers
into a single fiber. In one example, the optical coupler 106-1 is
an N:1 optical coupler configured to couple N optical input signals
into a single fiber, and optical coupler 106-2 is an optical
coupler configured to couple the optical input signals output from
optical coupler 106-1 with optical signals from a pulse train
provided by the optical readout mechanism 112 into a single fiber.
An example of a suitable fiber coupler 106-2 is a thermally tapered
and fused pair of single-mode fibers, with the cores of the fiber
pair coming into contact such that optical energy may be exchanged.
Multiport coupler 106-1 may be, for example, a tree of 2:1 couplers
as will be understood by one skilled in the art. The optical
signals of the sampling pulse train may have a wavelength
.lamda..sub.0, and the optical input signals may have one or more
wavelengths .lamda..sub.1, .lamda..sub.2, etc., which are different
from the wavelength of the pulse train. Additionally, the optical
input signals have amplitudes that are greater than the amplitudes
of the optical signals of the pulse train such that the optical
signals of the pulse train do not have a significant effect on the
cross-gain modulation (XGM) of the SOA 114 as described below. For
example, the amplitudes of the optical input signals may be ten
times the amplitude of the optical signals of the pulse train.
However, one skilled in the art will understand that the difference
between the amplitudes of the optical input pulses and the optical
signals of the pulse train may be increased or decreased.
[0030] Readout mechanism 112 (FIG. 1A) may be any device configured
to provide a signal for reading out a current state of SOA 114. For
example, readout mechanism 112 may be an optical pulse train
generator for providing optical signals having uniform wavelengths
and amplitudes such as, for example, a mode-locked ring fiber laser
(MLL) configured to provide pulses on the order of picoseconds.
[0031] One example of an SOA 114 is illustrated in FIG. 2. As shown
in FIG. 2, the SOA 114 includes a semiconductor substrate 200,
which may be a Group III-V compound substrate as will be understood
by one skilled in the art. Substrate 200 may be an n-type substrate
having an n-doped region 202 and a p-doped region 204. Metal layers
206 and 208 may be formed on a top and a bottom surface of the
substrate 200. As shown in FIGS. 1B and 2, the charge pumping
circuit 118 is coupled to SOA 114 and configured to restore the
gain of SOA 114 through population inversion after the gain of the
SOA 114 has been depleted. The charge pumping circuit 118 may be
implemented as an electrical circuit in which a current is supplied
to the substrate of the SOA 114, or the charge pumping circuit 118
may be implemented as an optical circuit in which light is used to
perform population inversion of the SOA 114.
[0032] Referring again to FIG. 1B, optical filter 116 may be a
short-pass, a long-pass, or a band-pass optical filter configured
to block the wavelengths of the optical input signals and pass
optical signals provided by the readout mechanism 112. Examples of
optical filter 116 include, but are not limited to, thin film
multi-layer dielectric filters, Bragg gratings, and arrayed
waveguide gratings.
[0033] The optical amplifier 120 may be any optical amplifier
configured to increase the amplitude of an optical signal. In one
arrangement, the optical amplifier 120 is an erbium-doped fiber
amplifier (EDFA). Another example of optical amplifier 120 is an
erbium-ytterbium-doped fiber amplifier, which may provide a higher
output power than an EDFA.
[0034] The non-linear fiber 124 of thresholder 110 may be a
GeO.sub.2-doped silica-based non-linear fiber. Other non-linear
fibers such as, for example, microstructured fibers, photonic
crystal fibers, and hi-oxide fibers, to name a few, may also be
implemented. Parasitic reflections may be suppressed for proper
thresholder operation as will be understood by one skilled in the
art.
[0035] Polarization controllers 128-1, 128-2 may use a controllable
bend of fiber to control the polarization of light and may be based
on a medium having a weak controllable birefringence. Tunable
isolator 130 may be any kind of optical isolator and may be based
on the Faraday effect and have a controllable leak in a backward
direction. Coupler 126 may be the same type of optical coupler as
couplers 106-1, 106-2 except that it may have an unequal coupling
ratio. Examples of coupling ratios include, but are not limited to,
an 80:20 ratio, a 90:10 ratio, to name a few.
[0036] An optional optical inverter 132 may be coupled to the
output of the thresholder 110 for inverting the thresholder output.
In some embodiments, the optical inverter 132 may be an optical
logic gate such as the one taught by Miyoshi et al. in Ultrafast
All-Optical Logic Gate Using a Nonlinear Optical Loop Mirror Based
Multi-Periodic Transfer Function, Optics Express, Vol. 16, Issue 4,
2570-2577 (2008), the entirety of which is incorporated by
reference herein. The optical logic gate may be configured to
output an optical signal having the same characteristics of the
optical input signals (e.g., wavelength and amplitude) in response
to the output of the thresholder 110. One skilled in the art will
understand that other optical devices may be used to invert the
optical signal output from the thresholder 110. For example, the
optical inverter 132 may include an SOA for inverting the optical
signal or an optical data format converter based on a tetrahertz
optical asymmetrical demultiplexer (TOAD) such as the one disclosed
in U.S. Pat. No. 6,448,913 issued to Prucnal at al., the entirety
of which is incorporated by reference herein.
[0037] With reference to FIGS. 1A and 1B, the operation of the LIF
optical neuron 100 is now described. One or more optical input
signals having a wavelength .lamda..sub.1 are coupled together at
the first optical coupler 106-1. The coupled optical input signals
having one or more wavelengths .lamda..sub.1, .lamda..sub.2, etc.
are coupled with optical signals of the pulse train having a
different wavelength .lamda..sub.0 provided by the readout
mechanism 112 at the second optical coupler 106-2. As described
above, the amplitude of the pulsed optical input signals may be ten
time greater than the amplitude of the optical signals of the pulse
train.
[0038] Optical coupler 106-2 outputs a multiple wavelength optical
signal to an input of the SOA 114. SOA 114 of the integration block
108 is pumped with electrons from a charge pump circuit 118, which
performs population inversion of the SOA 114. When a pulse from one
of the optical input signals is received at the SOA 114, the gain
of the SOA 114 is depleted due to the depletion of charge that
occurs due to XGM. The external pumping of the SOA 114 causes the
gain of the SOA 114 to gradually increase, but if another pulse is
received from the optical input signals, then the gain of the SOA
114 will again be depleted. The recovery time of the gain of the
SOA 114 is based on the carrier lifetime, T.sub.e, which functions
as the integration time constant of the integrator 108. Thus, the
smaller the carrier lifetime of the SOA 114 the faster the gain of
the SOA 114 recovers and less temporal integration of the input
signals is performed.
[0039] The integrated optical signal is output from the SOA 114 and
received at the optical filter 116. As described above, the optical
filter 116 may be tuned such that the optical filter 116 passes the
optical signals having a wavelength .lamda..sub.0 and the optical
input signals are blocked, reflected, or otherwise filtered
out.
[0040] The filtered optical signal is received at an input of the
optical amplifier 120, which may be an EDFA as described above. The
optical amplifier increases the amplitude of the filtered and
integrated optical signal and outputs the amplified optical signal
to the thresholder 110. The gain of the optical amplifier 120 may
be designed to provide sufficient amplification of the filtered
optical signal so that the amplitude of the signal falls within the
linear region of the non-linear fiber 124 of the thresholder
110.
[0041] Thresholder 110 outputs a signal identifying if the optical
signal received at the input is greater than or equal to a
predetermined threshold level. However, due to the utilization of
the SOA 114 with gain sampling, the output of the thresholder when
one of the optical input signals is a logic one, the thresholder
will output a logic zero.
[0042] As described above, an optical inverter 132 may be coupled
to the output of the thresholder 110 to invert the signal output
from the thresholder 110. For example, if the inverter 132 is an
optical logic gate such as the ones described by Miyoshi et al.,
the inverter may output an optical signal having a wavelength of
.lamda..sub.1 and an amplitude equal to the amplitude of the
optical input signal when the thresholder 110 outputs a logic zero.
The output of the optical inverter 132 may be fed into another
processing element as will be understood by one skilled in the art.
For example, the output of optical inverter 132 may be fed into
another thresholder to provide standardization of the pulses at the
neuron output according to the LIF neuron model.
[0043] An optical LIF neuron 100 as illustrated in FIGS. 1A and 18
and described above was designed and tested. The optical LIF neuron
was implemented with the N:1 optical coupler 106-1 having two
inputs, i.e., N=2. A supercontinuum generator with spectral slicing
was utilized to provide the optical signals for the pulse trains
for multiple wavelengths. The resulting width of optical signals of
the pulse train was about 3 ps at full-width half maximum (FWHM).
Mach-Zehnder optical modulators and a standard bit-error tester
were used for creating different pulse patterns required in
performed measurements. A master pulse source having a 1.25 GHz
mode-locked ring fiber laser was used to generate the optical input
signals. Five-bit patterns of `01100` with a delay of approximately
1 bit were input to each of the inputs of the optical coupler 106-1
for a net input of `01210` into the optical system 100. The optical
inputs were coupled with the optical signals of the pulse train
provided by the 1.25 GHz mode-locked ring fiber laser at an optical
coupler 116-2 to provide an input to the SOA 114 as illustrated in
FIG. 3A. SOA 114 was an Alcatel A1901SOA available from
Alcatel-Lucent of Murray Hill, N.J.
[0044] The gain dynamics of the tested SOA 114 are shown in FIG.
4A. The y-axis in FIG. 4A corresponds to the energy of the sampling
pulses, which is proportional to the SOA gain, N'(t). The "resting
potential", i.e. the maximum SOA gain when no control signal is
present, was approximately equal to 43 fj. FIG. 4B illustrates an
oscillogram of the control signal in the same time scale as the
time scale of FIG. 4A. As shown in FIGS. 4A and 4B, each pulse of
the optical input signal leads to a decrease of energy of sampling
pulses due to the XGM in the SOA 114. After a pulse depletes the
gain of the SOA 114, the power gradually increases while SOA gain
recovers. The input pulses create a saw-tooth SOA gain curve as
shown in FIG. 4A. The integration time constant, i.e. the SOA
carrier lifetime, T.sub.e, was able to be adjusted in the range of
approximately 100 to 300 ps by changing the SOA pump current
supplied by the pump circuit 118 from approximately 70 mA to
approximately 170 mA. In the example illustrated in FIGS. 4A and
4B, the carrier lifetime, T.sub.e, was approximately equal to 180
ps.
[0045] The output of the integrator 108 is illustrated in FIG. 3B.
As shown in FIG. 3B, the integrator block 108 outputs optical
pulses at three different levels, e.g., zero, one, and two, which
are the inverse of the received pulses. Accordingly, when both
optical input signals were logic ones, then the sum of the inputs
was a two, which the integration block 108 output as a zero.
[0046] The thresholder 110 was constructed using a non-linear fiber
based on a modified nonlinear optical loop mirror. The thresholder
110 included 10.5 m of a silica-based non-linear fiber 124
heavily-doped with GeO.sub.2 (preform 311). The fiber parameters
measured at .lamda.=1550 nm were a nonlinear coefficient 35
W.sup.-1 km.sup.-1; propagation losses of 36 dB/km; chromatic
dispersion -70 ps/nm km; and a refraction index difference,
.DELTA.n, of approximately 0.11. The idealized model of the
thresholder 110 predicted that the output power was proportional to
the cube of the input power, A measured transfer function had a
cubic dependence for some range of input powers with saturation at
a certain input level at which the nonlinear phase shift approaches
.pi.. The input power of the thresholder was controlled by
adjusting the gain of the optical amplifier 120, which was
implemented as an EDFA with a maximum output power of approximately
23 dBm.
[0047] The neuron 100 was able to discriminate between the lowest
and middle pulse energies, e.g., a zero or a one, or between the
middle and the highest pulse energies, e.g., a one and a two. Both
possibilities were experimentally demonstrated in the setup with
corresponding diagrams shown in FIGS. 5A and 5B. Also, two cases of
thresholding with the threshold below the smallest pulse energy and
above the highest were also realized, but are not shown.
Accordingly, the thresholder was capable of clear separation
between a pulse and no pulse at its input depending on the
threshold.
[0048] The experimentally demonstrated photonic LIF device was
shown to be operable using picosecond-width pulses and have an
integration time constant of 180 ps, which was adjustable within
the range of approximately 100 to 300 ps. Reconfiguration of device
parameters enables it to perform a wide variety of signal
processing and decision operations, its analog properties makes it
well-suited for efficient signal processing applications. The
digital properties of the optical LIF neuron make it possible to
implement complex computations without excessive noise
accumulation.
[0049] Although the systems and methods have been described in
terms of exemplary embodiments, they are not limited thereto.
Rather, the appended claims should be construed broadly, to include
other variants and embodiments of the systems and methods, which
may be made by those skilled in the art without departing from the
scope and range of equivalents of the systems and methods.
* * * * *