<s>
An	O
optical	B-Algorithm
neural	I-Algorithm
network	I-Algorithm
is	O
a	O
physical	O
implementation	O
of	O
an	O
artificial	B-Architecture
neural	I-Architecture
network	I-Architecture
with	O
optical	O
components	O
.	O
</s>
<s>
Early	O
optical	B-Algorithm
neural	I-Algorithm
networks	I-Algorithm
used	O
a	O
photorefractive	O
Volume	B-Algorithm
hologram	I-Algorithm
to	O
interconnect	O
arrays	O
of	O
input	O
neurons	O
to	O
arrays	O
of	O
output	O
with	O
synaptic	O
weights	O
in	O
proportion	O
to	O
the	O
multiplexed	O
hologram	O
's	O
strength	O
.	O
</s>
<s>
Volume	B-Algorithm
holograms	I-Algorithm
were	O
further	O
multiplexed	O
using	O
spectral	O
hole	O
burning	O
to	O
add	O
one	O
dimension	O
of	O
wavelength	O
to	O
space	O
to	O
achieve	O
four	O
dimensional	O
interconnects	O
of	O
two	O
dimensional	O
arrays	O
of	O
neural	O
inputs	O
and	O
outputs	O
.	O
</s>
<s>
Some	O
artificial	B-Architecture
neural	I-Architecture
networks	I-Architecture
that	O
have	O
been	O
implemented	O
as	O
optical	B-Algorithm
neural	I-Algorithm
networks	I-Algorithm
include	O
the	O
Hopfield	B-Algorithm
neural	I-Algorithm
network	I-Algorithm
and	O
the	O
Kohonen	B-Algorithm
self-organizing	B-Algorithm
map	I-Algorithm
with	O
liquid	B-Device
crystal	I-Device
spatial	O
light	O
modulators	O
Optical	B-Algorithm
neural	I-Algorithm
networks	I-Algorithm
can	O
also	O
be	O
based	O
on	O
the	O
principles	O
of	O
neuromorphic	O
engineering	O
,	O
creating	O
neuromorphic	O
photonic	O
systems	O
.	O
</s>
<s>
Typically	O
,	O
these	O
systems	O
encode	O
information	O
in	O
the	O
networks	O
using	O
spikes	O
,	O
mimicking	O
the	O
functionality	O
of	O
spiking	B-Algorithm
neural	I-Algorithm
networks	I-Algorithm
in	O
optical	O
and	O
photonic	O
hardware	O
.	O
</s>
<s>
Photonic	O
devices	O
that	O
have	O
demonstrated	O
neuromorphic	O
functionalities	O
include	O
(	O
among	O
others	O
)	O
vertical-cavity	O
surface-emitting	O
lasers	O
,	O
integrated	O
photonic	O
modulators	O
,	O
optoelectronic	O
systems	O
based	O
on	O
superconducting	O
Josephson	O
junctions	O
or	O
systems	O
based	O
on	O
resonant	B-Algorithm
tunnelling	I-Algorithm
diodes	I-Algorithm
.	O
</s>
<s>
Biological	B-General_Concept
neural	I-General_Concept
networks	I-General_Concept
function	O
on	O
an	O
electrochemical	O
basis	O
,	O
while	O
optical	B-Algorithm
neural	I-Algorithm
networks	I-Algorithm
use	O
electromagnetic	O
waves	O
.	O
</s>
<s>
Optical	O
interfaces	O
to	O
biological	B-General_Concept
neural	I-General_Concept
networks	I-General_Concept
can	O
be	O
created	O
with	O
optogenetics	O
,	O
but	O
is	O
not	O
the	O
same	O
as	O
an	O
optical	B-Algorithm
neural	I-Algorithm
networks	I-Algorithm
.	O
</s>
<s>
In	O
biological	B-General_Concept
neural	I-General_Concept
networks	I-General_Concept
there	O
exist	O
a	O
lot	O
of	O
different	O
mechanisms	O
for	O
dynamically	O
changing	O
the	O
state	O
of	O
the	O
neurons	O
,	O
these	O
include	O
short-term	O
and	O
long-term	O
synaptic	O
plasticity	O
.	O
</s>
<s>
Properties	O
that	O
might	O
be	O
desirable	O
in	O
photonic	O
materials	O
for	O
optical	B-Algorithm
neural	I-Algorithm
networks	I-Algorithm
include	O
the	O
ability	O
to	O
change	O
their	O
efficiency	O
of	O
transmitting	O
light	O
,	O
based	O
on	O
the	O
intensity	O
of	O
incoming	O
light	O
.	O
</s>
<s>
Optical	O
computing	O
has	O
emerged	O
as	O
a	O
potential	O
alternative	O
to	O
GPU	O
acceleration	O
for	O
modern	O
neural	B-General_Concept
networks	I-General_Concept
,	O
particularly	O
considering	O
the	O
looming	O
obsolescence	O
of	O
Moore	O
's	O
Law	O
.	O
</s>
<s>
Consequently	O
,	O
optical	B-Algorithm
neural	I-Algorithm
networks	I-Algorithm
have	O
garnered	O
increased	O
attention	O
in	O
the	O
research	O
community	O
.	O
</s>
<s>
Presently	O
,	O
two	O
primary	O
methods	O
of	O
optical	O
neural	B-Architecture
computing	I-Architecture
are	O
under	O
research	O
:	O
silicon	O
photonics-based	O
and	O
free-space	O
optics	O
.	O
</s>
<s>
Unfortunately	O
,	O
modern	O
neural	B-General_Concept
networks	I-General_Concept
are	O
not	O
designed	O
for	O
the	O
4F	O
systems	O
,	O
as	O
they	O
were	O
primarily	O
developed	O
during	O
the	O
CPU/GPU	O
era	O
.	O
</s>
<s>
In	O
2007	O
there	O
was	O
one	O
model	O
of	O
Optical	B-Algorithm
Neural	I-Algorithm
Network	I-Algorithm
:	O
the	O
Programmable	O
Optical	O
Array/Analogic	O
Computer	O
(	O
POAC	O
)	O
.	O
</s>
