Roland Kersting
kerstr@rpi.edu
The Science of Information Technology
Computing with Light
•
the processing of signals
•
properties of light
•
building a photonic computer
•
future trends ?
Department of Physics, Applied Physics, and Astronomy
1
Roland Kersting
kerstr@rpi.edu
voltage
voltage
Signals in IT
(1)
(10)
(5)
(0)
(7)
(9)
(0)
time
binary system: 01100101
Department of Physics, Applied Physics, and Astronomy
time
not applicable
2
Roland Kersting
kerstr@rpi.edu
Making a Byte out of Bits
channel 1
channel 2
understanding:
computing problems can be
separated into processing of
single bits.
channel 3
channel 4
channel 5
channel 6
tools are:
• transport
• comparison
• storage
channel 7
channel 8
11000010 = 194
Department of Physics, Applied Physics, and Astronomy
3
Roland Kersting
kerstr@rpi.edu
Signal Processing in IT
transport of bits:
0 1 0 1 1 0
input
switching:
0 1 0 1 1 0
distance, connector
output
0 1 0 1 1 0
0 1 0 1 1 0
input 1
0 1 0 1 1 0
logic operation
switch
output
input 2
Department of Physics, Applied Physics, and Astronomy
4
Roland Kersting
kerstr@rpi.edu
What is a Bit ?
Fourier transform
1.0
Amplitude (arb. units)
0.8
Signal (arb. units)
one bit
in frequency-domain
0.05
one bit
in time-domain
0.6
0.4
0.2
0.04
0.03
0.02
0.01
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Time (arb. units)
Department of Physics, Applied Physics, and Astronomy
0.00
0
50
100
150
Frequency (arb. units)
5
Roland Kersting
kerstr@rpi.edu
The cut-off frequency
1.2
cut-off
frequency
1.0
0.04
Signal (arb. units)
Amplitude (arb. units)
0.05
0.03
0.02
cut-off
frequency
0.01
0.8
0.6
0.4
0.2
0.0
-0.2
0.00
0
20
40
60
80
100
120
140
Frequency (arb. units)
Department of Physics, Applied Physics, and Astronomy
0.2
0.4
0.6
0.8
Time (arb. units)
6
Roland Kersting
kerstr@rpi.edu
Electronics
transport of bits:
metal wire
cut-off= R / L
switching:
Gate
Source
p-type
S ilicon Wafer
cut-off = R*C
Drain
Oxide
n-type
Department of Physics, Applied Physics, and Astronomy
n-type
7
Roland Kersting
kerstr@rpi.edu
Cut-off frequency vs. clock frequency
0.05
1.2
cut-off
frequency
clock
0.04
Signal (arb. units)
Amplitude (arb. units)
1.0
0.03
0.02
cut-off
frequency
0.8
clock
0.6
clock
0.4
0.2
0.01
0.0
0.00
0
20
40
60
80
100
120
140
Frequency (arb. units)
Department of Physics, Applied Physics, and Astronomy
-0.2
0.2
0.4
0.6
0.8
Time (arb. units)
8
Roland Kersting
kerstr@rpi.edu
Clock Frequency (Hz)
Clock Frequency of Computers
10
13
10
12
10
11
10
10
physical limit
10
9
10
8
10
7
10
6
technological limit
PCs
after Malone (1995)
1970
1980
1990
2000
2010
2020
2030
Year
Department of Physics, Applied Physics, and Astronomy
9
Roland Kersting
kerstr@rpi.edu
The heat problem
Department of Physics, Applied Physics, and Astronomy
10
Roland Kersting
kerstr@rpi.edu
Clock Frequency (Hz)
Clock Frequency of Computers
10
13
10
12
10
11
10
10
physical limit
10
9
10
8
10
7
10
6
technological limit
PCs
after Malone (1995)
1970
1980
1990
2000
2010
2020
2030
Year
Department of Physics, Applied Physics, and Astronomy
11
Roland Kersting
kerstr@rpi.edu
Photonics
Idea: substitute electrical currents with light
metal wire
ons
r
t
c
ele
glass fiber
cut-off= R / L
cut-off = ?
( 30*108 Hz )
( 30*1012 Hz )
Department of Physics, Applied Physics, and Astronomy
12
Roland Kersting
kerstr@rpi.edu
Let’s build a photonic computer
information
modulator
bit stream
semiconductor

laser
modulator
photonic
switch
(AND)
bit stream
modulator
bit stream
information
clock
output to detector
Department of Physics, Applied Physics, and Astronomy
13
Roland Kersting
kerstr@rpi.edu
Semiconductor laser
Department of Physics, Applied Physics, and Astronomy
14
Roland Kersting
kerstr@rpi.edu
Output of a laser
rapidly oscillating electromagnetic field
1.0
 = 800 nm
0.8
1 fs = 10
s
= 0.000000000000001 s
–15
0.6
Field (arb. units)
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
0
2
4
6
8
Time (fs)
Department of Physics, Applied Physics, and Astronomy
15
Roland Kersting
kerstr@rpi.edu
Desired: short pulses and pulse trains
1.0
3
0.8
2
Signal (arb. units)
Field (arb. units)
0.6
1
0
0.4
0.2
0.0
-0.2
-0.4
-0.6
 = 800 nm
 = 30 fs
-0.8
-1.0
-1
0
20
40
60
80
100
120
Time (fs)
Department of Physics, Applied Physics, and Astronomy
0
50
100
150
200
250
Time (fs)
16
Roland Kersting
kerstr@rpi.edu
Let’s build a photonic computer
information
modulator
bit stream
semiconductor
laser
modulator
(AND)
bit stream

photonic
switch
modulator
bit stream
information
clock
output to detector
Department of Physics, Applied Physics, and Astronomy
17
Roland Kersting
kerstr@rpi.edu
Opto-electronic modulation
Search: Interface between optical & electrical pulses
Electro-optic modulators
• example liquid crystals:
• get dark when electrical bias is applied
• very slow
• Pockels-effect:
• index of refraction depends
on applied voltage
• very fast
Department of Physics, Applied Physics, and Astronomy
18
Roland Kersting
kerstr@rpi.edu
Using a Mach-Zehnder interferometer
t
lithium tantalate
Department of Physics, Applied Physics, and Astronomy
19
Roland Kersting
kerstr@rpi.edu
Constructive & destructive interference
Field (arb. units)
Field (arb. units)
branch 1
branch 2
constructive
interference
0
2
destructive
interference
4
6
8
Time (fs)
Department of Physics, Applied Physics, and Astronomy
0
2
4
6
8
Time (fs)
20
Roland Kersting
kerstr@rpi.edu
Integration of intensity modulators
material: lithiumniobate
Department of Physics, Applied Physics, and Astronomy
21
Roland Kersting
kerstr@rpi.edu
Let’s build a photonic computer
information

semiconductor
laser

modulator

bit stream
modulator
bit stream

photonic
switch
(AND)
modulator
bit stream
information
clock
output to detector
Department of Physics, Applied Physics, and Astronomy
22
Roland Kersting
kerstr@rpi.edu
All-optical switching
the problem:
light doesn’t interact with light
Department of Physics, Applied Physics, and Astronomy
23
Roland Kersting
kerstr@rpi.edu
Absorption saturation
idea: use matter (electrons) to mediate the light-light interaction
atom:
• electrons in orbits/states
• Pauli-rule: up to 2 electrons
per state are allowed
• transitions by light absorption
Department of Physics, Applied Physics, and Astronomy
24
Roland Kersting
kerstr@rpi.edu
empty
states
energy
Optical transition of electrons
saturated
absorption
filled states
a b s o r p tio n o f
a p h o to n
atom in
ground state
a t o m in
e x c ite d s ta te
Department of Physics, Applied Physics, and Astronomy
atom fully in
excited state
25
Roland Kersting
kerstr@rpi.edu
All-optical switching by saturated absorption
puls
e
A
#1
transmission
signal
C
B
e
puls
#2
Department of Physics, Applied Physics, and Astronomy
AND-gate:
A
B
C
0
0
0
1
0
1
0
1
1
0
0
1
26
Roland Kersting
kerstr@rpi.edu
energy
conduction
band
energy
Excitation of bulk semiconductors
electron
valence
band
thickness
Department of Physics, Applied Physics, and Astronomy
absorption
27
Roland Kersting
kerstr@rpi.edu
conduction
band
energy
energy
Better: semiconductor heterostructures
electron
state
hole state
valence
band
layer thickness
Department of Physics, Applied Physics, and Astronomy
absorption
28
Roland Kersting
kerstr@rpi.edu
AlGaAs-Switch
Department of Physics, Applied Physics, and Astronomy
29
Roland Kersting
kerstr@rpi.edu
We are done: a photonic computer (???)
information

semiconductor
laser

modulator

bit stream
modulator
bit stream


photonic
switch
(AND)
modulator
bit stream
information
clock
output to detector
Department of Physics, Applied Physics, and Astronomy
30
Roland Kersting
kerstr@rpi.edu
Input
Keep the information for some time
Solution: bistable devices
1
Output
Electronics: Flip-Flop
d
c
a
b
a
b
c
d
Time
d
Time31
Output
1
0
0
1
a
b
0
1
Input
0
Department of Physics, Applied Physics, and Astronomy
c
Roland Kersting
kerstr@rpi.edu
Layer Thickness
Department of Physics, Applied Physics, and Astronomy
apply voltage
Layer Thickness
Energy
Energy
Energy
The SEED (self-electro-optic effect device)
with photo
carriers
Layer Thickness
32
Roland Kersting
kerstr@rpi.edu
a p p ly vo lta g e
Absorption
E n e rg y
Photoinduced absorption
Laser
Energy
L a ye r T h ic kn e s s
with photo
carriers
Energy
Department of Physics, Applied Physics, and Astronomy
Layer Thickness
33
Roland Kersting
kerstr@rpi.edu
Demonstration of concepts
The first steps towards photonic computing:

efficient transfer of data by fibers
 rates up to 30 THz

switching times as fast as 100 fs

low switching energies
 close to switching energies in electronic

high repetition rates
 > 100 GHz
 factor 100 higher as in PCs
Department of Physics, Applied Physics, and Astronomy
34
Roland Kersting
kerstr@rpi.edu
Technological problems



interface electronics-optics
 usually slow (10 GHz)
 expensive ( ~ 100 US$)
micro integration
 devices of dimension 0.03 – 10 mm
 for parallel processing arrays of several cm
hybrid technologies
 expensive
 not acceptable
Department of Physics, Applied Physics, and Astronomy
35
Roland Kersting
kerstr@rpi.edu
The market

assume for 10 years:
 500 Mio Computers
 100 US$ for photonic components
50 billion US$

more important:
 relation between market potential
and risk:
50 billion US$
risk = ?
Department of Physics, Applied Physics, and Astronomy
36
Roland Kersting
kerstr@rpi.edu
Research at Rensselaer



optical on chip interconnects
fiber optical connects (Persans)
terahertz optoelectronics (Zhang, Shur, Kersting)
Department of Physics, Applied Physics, and Astronomy
37
Roland Kersting
kerstr@rpi.edu
The electromagnetic spectrum
1
k
H
z
1
M
H
z
1
G
H
z
1
T
H
z
1
P
H
z
r
f
e
q
u
e
n
c
y
m
i
t
e
1
m
s
1

s
1
n
s
1
p
s
1
s
f
HiFi
IT
v is ib le
lig h t
radio waves
Department of Physics, Applied Physics, and Astronomy
38
Roland Kersting
kerstr@rpi.edu
THz pulses
1.0
0.8
0.6
Field (arb. units)
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1
0
1
Time (ps)
2
3
Properties:
 THz pulses are information carrier
 measure the field
 very short light pulses possible
 propagate free space & on metal wires
 fibers are no longer necessary
 switching medium : semiconductors
 can be tailored for THz pulses
 no hybrid technologies
Department of Physics, Applied Physics, and Astronomy
39
Roland Kersting
kerstr@rpi.edu
Logic operations with THz pulses
phase shift
output C
input A
THz phase
modulator
input B
Department of Physics, Applied Physics, and Astronomy
A
B
C
0
0
1
1
0
1
0
1
0
0
0
1
40
Roland Kersting
kerstr@rpi.edu
THz semiconductor devices
Science fiction ?
our work:
THz modulator
• operating @ 3THz
Department of Physics, Applied Physics, and Astronomy
41
Roland Kersting
kerstr@rpi.edu
Terahertz differentiator
analog computer:
• calculates the first time-derivative
• operates at THz frequencies
input
THz pulse
silicon
substrate
output
THz pulse
d ~ 10 m
Electric Field (arb. units)
metallic
grating
3.0
x0.1
incident pulse
2.0
1.0
transmitted pulse
0.0
calculation
-1.0
-0.5
0.0
0.5
1.0
1.5
Time (ps)
Department of Physics, Applied Physics, and Astronomy
42