Rents Rule: Applied to Optics

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Rents Rule Applied to Optics
WDM is the path upward
Bandwidth per wavelength saturates
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What Do The Data Rates MeanThe Information Types
Raw data uncompressed Voice (telephony) 64
kbit/sec 4 pages/sec Color Image (830 x 620, 24
bits) 12 Mbit 780 pages Color Image (1280 x 1024,
24 bits) 32 Mbit 2000 pages Color Video (830 x
620, 24 bits) 360 Mbit/sec 24,000
pages/sec Compressed Voice 10/1 Image 20/1
Video 40/1 Voice (telephony) 6 kbit/sec 0.4
pages/sec Color Image (830 x 620, 24 bits) 0.6
Mbit 40 pages Color Image (1280 x 1024, 24
bits) 1.6 Mbit 100 pages Color Video (1280 x
1024, 24 bits) 9 Mbit/sec 600 pages/sec
Assume 8 bits/letter, 8 letters/word, 200
words/page
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What Do The Data Rates MeanNetwork Types
Network Data Rate Pages/sec Pages/Books/sec Modem
56 kbit/sec 4 pages/sec 4 pages/sec Standard
Ethernet 10 Mbit/sec 650 pages/sec 2
books/sec Fast Ethernet 100 Mbit/sec 6,500
pages/sec 20 books/sec Standard Optical 1
Gbit/sec 65,000 pages/sec 200 books/sec WDM
Optical 50 Gbit/sec 3 Mpages/sec 10
Kbooks/sec Today- WDM 1 Tbit/sec 65
Mpages/sec 200 Kbooks/sec Future ?? ?? Lots of
authors
Assumes 8 bits/letter, 8 letters/word, 200
words/page, approx 300 pages/book
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Optical WDM Network Elements
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EE471 Topics

• Representative physics topics
• Basic semiconductor crystal physics
• Equilibrium electrons/holes densities
• Basic equations of electronic devices
• Photon generation/absorption
• Basic equations for photonic devices
• Representative microfabrication topics
• Microfabrication technologies
• Microfabricated device structures
• Scaling to smaller dimensions
• Representative application areas
• Optoelectronics communications
• Microelectronics VLSI
• High speed RF for wireless

Donor
Acceptor
Si
6
Indirect gap (e.g. silicon)
Electron and hole arrive at same place at same
time. Producing a photon? No mass left. No
carriers left.
e
k
h
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The Diode
P-Type(Charge neutral)
N-Type(Charge neutral)
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The Diode
DepletionRegion
P-Type(Charge neutral)
N-Type(Charge neutral)
A-
D
Dnp0
pn
Dnp0
np
Zero bias
D np
D pn
Excess electrons
Excess holes
Forward bias
np
pn
Reverse bias
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The Diode
P-Type(Charge neutral)
A-
Silicon Electronics III-V
Electronics Photonics
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The Diode
D np np0 exp(qV/kT) - 1
P-Type (Neutral Charge)
Depletion layer
Excess Charge changes with diode voltage

• III-V LED
• Current increases exponentially with forward
  voltage
• Light output increases similarly

• Silicon diode
• Current increases exponentially with forward
  voltage

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The Diode
DepletionRegion
P 1015
N 1017
Most of action is on this side.
For LED place higher density side close to
surface for surface emission.
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More Realistic View of Diode
Our View Until Now Horizontal
P
N
Excess minority carriers
Light emitted through spontaneous recombination -
all directions
Vertical Structure on Substrate
ImplantedP
ImplantedN
Depletion Layer
P Substrate
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More Realistic Representation of LED Structure
Implanted P-Type Region
Emerging LED Light
Excess minority carriers
P
Depletion Layer
Implanted N-Type Region
Light emitted in all directions through e-h
recombination
P-Type Substrate
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Amplification of Light in Recombination Area
Stimulated emission of a photon by e-h
recombination (gain)
Starting photon
Implanted P-Type Region
Region of Excess minority carriers
Depletion Layer
Implanted N-Type Region
Partially transparent mirror
Fully reflecting mirror
Fully reflecting mirror
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Use of grating filters to remove filter light
Grating acts as a filter in wavelength domain
Periodic dielectric constant
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Wavelength Tuning for WDMExternal Mirror Control
Deformable mirror (e.g., MEMS)
Laser
Mirror
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Confining the Excess Carrier Density with Bandgap
Enginering
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Confining the Light with Refractive Index
Engineering
Refractive Index Engineering Choose the index
of refraction of the center layer to have a
higher index of refraction than the surrounding
layers. The result is a built-in waveguide
confining the light to the active region
Index of refraction
Light guided in high index region
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Traditional laser diodes are edge-emitters,
with the light exiting each laser perpendicular
to the direction of the layer structure. The
laser cavity is formed by cleaving the end faces
of the diodes. Vertical cavity surface emitting
lasers (VCSELs) avoid many of the issues
associated with edge-emitters. VCSELs are
designed with mirrors at the top and bottom,
surrounding a thin active region. Because the
active layer is thin, the optical cross-section
is small, and the mirrors must be highly
reflective (above 90 as opposed to the 30 of
the cleaved faces in edge emitters). Early
designs included metallic mirrors, but to reduce
threshold current, the top and bottom mirrors are
now typically composed of alternating layers of
semiconductor materials of different indices of
refraction, creating a distributed Bragg
reflector (DBR) with reflectivity typically
around 99.
A typical 850-nm VCSEL will have a bottom DBR
constructed from a few tens of layers of AlGaAs,
then a few GaAs quantum wells, a layer of AlGaAs
with 97-98 Al, and a final couple dozen layers
of AlGaAs for the top Bragg reflector. The
ability to easily create arrays of transmitters
is another advantage for VCSEL technology.
Fabricating VCSEL transmitters, photodiode
receivers, and their associated send and receive
electronics opens up the possibilities for
unprecedented integration of telecommunications
capabilities.
From Research Device Magazine, 2007 Vertical
Cavity Lasers Push into the Future
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IBM, Armonk, N.Y., presented results from a
monolithically integrated 16-channel optical
transceiver. The chip contained a 4 x 4 array of
985-nm VCSELs. The VCSELs were built on a 250 µm
x 350 µm pitch, with a 62.5 µm offset between
rows. The arrangement is designed to accommodate
optical coupling to a set of arrayed waveguides.
Each of the channels, integrated with
transceiver electronics, is capable of modulation
at speeds of better than 10 Gb/sec. Together, the
integrated package demonstrated transmission of
optical data at rates greater than 160 Gb/sec.
Not only is the single-channel data rate
unprecedented, but the key figures of merit for
power dissipation (15.6 mW/Gb/s) and density (9.4
Gb/mm2) are also unprecedented. IBM is developing
this transceiver as part of a Defense Advanced
Research Projects Agency (DARPA)-sponsored
chip-to-chip program designed to speed up
communications between supercomputers
From Research Device Magazine, 2007 Vertical
Cavity Lasers Push into the Future
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Semiconductor Lasers Wavelength Range
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Diode Detectors
DepletionRegion
Reverse bias
P-Type(Charge neutral)
N-Type(Charge neutral)
Electric Field
Hole density
Electron density
E
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APD Diode Detectors
Reverse bias
P-Type(Charge neutral)
N-Type(Charge neutral)
DepletionRegion
Under sufficiently high electric fields, the
electron acquires sufficient energy to ionize an
atom, leading to an avalanche effect. Rather
than contributing a single electron hole pair to
the output current, each photon contributes
multiple carriers to the output current,
substantially increasing the output current for a
given optical power input. Photodetectors
operating in this mode are Avalanche
PhotoDetectors (APDs).
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PIN Diode Detectors
PN diode
How can the active area (the depletion layer in
the standard PN diode above) be increased
substantially? Add an insulator-like material
beteen P- and N-type regions. Intrinsic
semiconductor is an insulator but still allows
photon absorption by emission of an e-h pair.
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Optical Detectors
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Optical Detectors
Example 32 x 32 array of InGaAs/InP
photodetectors flip-bonded to silicon
preamplifier array. C.-M. Ry et al., SPIE Proc.,
vol 3952, p. 106, 2000. Arizona State
University. Conceptual illustration below.
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Optical Detectors (SiGe to reach 1300 nm)
Q. Wang, et al, Si-based resonant cavity-enhanced
photodetector, Opt. Eng, vol. 40(7), pp.
1192-1194, 2001. Chinese Academy of Sciences.
SiGe/Si multiple quantum well (MQW)
photodetectors are being explored for
cointegration of detectors with microelectronics
for 1300 - 1550 nm wavelengths. Silicon alone
absorbs only at much lower wavelengths.
Al contacts
i-Si
n-Si substrate
Separation by IMplanted Oxygen (SIMOX) - bottom
mirror
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Optical Devices Ternaries and Quaternaries
Structures for realizing contemporary
semiconductor lasers and detectors are highly
advanced, relying on fabrication techniques
(e.g., epitaxy) capable of creating very thin
layers of semiconductor and capable of creating
complex physical structures
Source Laser Focus World
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Optical Filters
Filters Passive Fabry-Perot based on
interferometry.Bragg grating based on
diffractionPrism based on refraction (filter is
space domain)Mach-ZenderThin filmDielectricAco
usto-optic filters
Adapted from Stamatos V. Kartolopoulos, Elastic
bandwidth, IEEE Circuits Devices Magazine, vol
18 (1), pp. 8-13, 2002.
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Grating for Demultplexing/Multiplexing
For reflected and pass-through gratings, the
angular separation between wavelengths for a
given order m (known as the angular dispersion D
is given on the right, where m is an integer, d
is the grating constant, and b is the angle of
diffraction.
Adapted from Stamatos V. Kartolopoulos, Elastic
bandwidth, IEEE Circuits Devices Magazine, vol
18 (1), pp. 8-13, 2002.
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Optical Filters
Grating Filter
Spatial filteringof wavelengths
Bragg Grating
Red reflections combine constructively
Green reflections combine destructively
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Semiconductor Optical Amplifierfor Wavelength
Conversion
V
Input signal l 1
Constant beam l 2
Output signal l 2
Semiconductor Optical Amplifier
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Multistage SwitchExample with internal blocking
There are many layouts, including non-blocking
variants
a0, uppera1, lower
b0, upperb1, lower
c0, upperc1, lower
abc
000
000
001
001
010
010
011
011
100
100
B
101
101
110
110
A
111
111
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In 0
Non-BlockingSwitch
In 1
In 2
N x N switchN2 elements
In 3
In 4
In 5
In 6
In 7
Out 0
Out 1
Out 2
Out 3
Out 4
Out 5
Out 6
Out 7
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Add/Drop for WDM Systemusing 1 x 2 switches and
1 x 4 switches
Added Wavelength
Dropped Wavelength
Switch
Switch
1 x 4
1 x 4
WDMdemultiplexor
WDMdemultiplexor
1 x 2
1 x 2
WDMOutput
WDMInput
1 x 2
1 x 2
1 x 2
1 x 2
1 x 2
1 x 2
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2-D, 8 x 8 optical cross-connect MEMS mirror
array switch
Micromirrors move in and out of optical path,
either allowing the beam to pass unimpeded or
deflecting the beam to the output port.
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Other MEMS Components for Optical Systems
Jan 00, Laser Focus World. Dave Bishop
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3-D, 256 x 256 optical cross-connect MEMS mirror
array switch
Shown is a single element of the MEMS switch,
with the rotating mirror able to deflect the
light beam over a 2-D area.
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MEMS Micromirror Switch(Lucent Bell Labs - In
Use)
Example of advanced micromirror switch array
(100x100) from Lucent Bell Laboratories. These
micromirror arrays illustrate the sophistication
of contemporary MEMS technologies.
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New applications driving development of new
technologies (e.g., plastic light sources
for flexible/disposable displays) New
technologies enabling entry into new
applications (e.g., Microelectromechanical
systems - MEMS - enabling new medical diagnostics)
The basic principles underlying semiconductor
device operation and microstructures extend to
the new technologies. BE PREPARED FOR A NEW ERA
OF ADVANCED SYSTEMS