Task 2 Optical Interconnects

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Task 2 Optical Interconnects

• David Miller, Stanford

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Contributors

• Continuing contributors
• Bob Dutton (Stanford)
• James Castracane (Albany)
• Gene Fitzgerald (MIT)
• Clif Fonstad (MIT)
• Tom Gaylord (GIT)
• Jim Harris (Stanford)
• Mark Horowitz (Stanford)
• Kim Kimerling (MIT)
• David Miller (Stanford) (task leader)
• Serge Oktyabrsky (Albany)
• Krishna Saraswat (Stanford)

• New contributors
• Osama Aboelfotoh (NCSU)
• George Barbastathis (MIT)
• Joe Campbell (Texas)
• Connie Chang-Hasnain (Berkeley)
• Larry Coldren (UCSB)
• Peter Delfyett (U. Central Florida)
• Shanhui Fan (Stanford)
• Michal Lipson (Cornell)
• Elias Towe (Carnegie-Mellon)
• Jelena Vuckovic (Stanford)

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Key elements of program

• Driving applications
• Chip-to-chip and possible on-chip interconnects
• High density, high aggregate bit rate, high speed
  
• Interface to network-like interconnects
• Directly interface, e.g., wavelength-division-mult
  iplexed optical fiber connections to silicon
  chips
• Clock injection
• Need systems and devices that
• will run at at least 10 GHz, compatible with
  mainstream CMOS, with headroom for higher speeds,
  with low electrical and optical powers
• Drives optoelectronic device work for lower
  voltages, higher speeds, lower detector
  capacitance
• will be manufacturable, and packagable with
  silicon electronics
• Drives work on integration technology, novel
  optics schemes

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Key elements of program

• Understand and exploit emerging optical science
  and technology
• Nanophotonics and quantum dots
• Many possibilities for smaller, higher
  performance
• optical devices (e.g., wavelength splitters,
  waveguides, filters, subwavelength focusing
  devices)
• optoelectronic devices (e.g., lasers, modulators)
• Note this is enabled by the development of
  sub-100nm lithography for silicon
• Short pulse optics
• Precise timing injection
• short pulse laser is high-Q, low jitter,
  high-repetition rate oscillator
• clock injection, signal jitter removal
• Reduced latency, lower power interconnects
• Frequency comb laser source for
  wavelength-division multiplexed interconnects

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Specific research themes

• Systems
• analyze and test proposed optical features and
  devices, find most important features of optics
• Integration techniques
• ultimate manufacturability, improved performance
• Optics
• need compact, manufacturable optics
• Short pulse sources
• exploit short pulse feature (unique to optics)
• Novel laser, modulator, and detector devices
• high-speed, low voltage, tolerant, high-yield,
  integrable devices
• Quantum dots
• possible low threshold, high speed lasers
• Photonic nanostructures
• radical opportunities in emerging field, for both
  passive optics and active optoelectronics

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Systems
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Optical Clocking of Digital Circuit in the Blue
Miller, Stanford
100 nm thick silicon detectors in SOI 17 quantum
efficiency at 420 nm, 3 fF capacitance
See also Drego, Boning, Fonstad (MIT) chip for
optopill attachment of InGaAs/InP detectors for
clock injection
RMS Jitter 4.5 ps
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Best Jitter Measurement - lt 1 ps rms
Optical short pulse pair injection onto diode
pairs

• Optical injection allows very low jitter
  multiphase clocks for links, with precise control
  of clock phase timing
• 931 fs measured rms jitter
• External wire-bonded GaAs/AlGaAs photodetectors,
  830 nm excitation, 150 fs optical pulses, 400
  microWatts per detector, 80 MHz
• Measurement likely limited by scope triggering
  jitter

Miller, Horowitz, Stanford
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Power Comparison E/O Interconnects

• Prototype of Electrical and Optical Transceiver
  System
• Advanced technology (90nm Intel Process)
• Reasonable performance comparison between
  electricaland optical interconnects
• Give insight of impact each design/system
  parameters on performance

Saraswat (Stanford), Ian Young (Intel)
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Integration Techniques
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Integrated Optical Receivers CMOS Electronics
and Ge PIN Photodiode
Successful fabrication of Ge PIN photodiodes on
Si substrate Dark current Id 1 mA _at_10V
diameter 24mm Responsivity 0.57A/W _at_2V and l
1.3 mm Bandwidth 8 GHz _at_ 10V Future
plans Improve and optimize Ge PIN and adjacent
CMOS devices Develop compatible process
technology
Campbell, UT Austin
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High Reliability and Visible Lasers on Si
GaAs on Si
Fitzgerald, MIT

• Purpose
• CMOS-fab compatible lasers and detectors
• Inter-chip optical interconnects
• Visible lasers allow option of use of Si detector
  technology
• Progress in Reliability of lasers on Si
• Record reduction in threading dislocation density
  (now lt106 cm-2)
• Carrier lifetime indistinguishable from GaAs
  substrate comparison
• Currently processing devices on new material
• Progress with Yellow-Green Emission on Si
• Yellow-green quantum well luminescence achieved
  in InGaP/GaP
• Currently working on yellow laser on GaP
• Will begin working on integration on Si using
  SiGe or GaP/Si
• Progress in CMOS-compatible optical wafer
• Principles proven in epitaxy
• Working on first III-V material embedded in Si
  wafer

New rt7x105 cm-2
Previous
InGaP
GaP (is lattice-matched to Si, or InGaP is
lattice-matched to SiGe/Si)
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Integration of III-V Functionality on Si
Fonstad, MIT
Assembly and post-assembly process development
for OptoPill Integration
Heterostructure pill picked and placed in recess
Pill soldered to Cu pad (w. Au-Sn)
Ohmic contact ring patterned on pill bonded in
recess
BCB applied to replanarize wafer
THE GOAL InGaAs/InP P-i-N detectors on Si-CMOS
for optical clock and signal distribution
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Novel Integration Technology Oxidation Lift-off

VCSEL Bonded to Si via Oxidation Lift-off

• Goal Scalable technology for heterogeneous
  integration of III-V components on a Si
  electronics
• Approaches
• Polymer bonding (last year demonstration)
• Oxidation lift-off method. Demonstrated
• Bonded VCSEL performance
• Bonding, components separation and formation of
  oxide aperture within a single step

Optical top view
Series resistance of a bonded VCSEL 100 W
Threshold current 8 mA for the 24x24 mm2
devices
Oktyabrsky, Albany
FIB cross-sections
Electro-luminescent spectra
I-V and P-I characteristics
Oktyabrsky (UAlbany)
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Optics
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Easily alignable array optical interconnects and
3D diffractive optical structures
1. Folding
38 µm
FoldingPattern optical elements
FoldedMembranes are folded on the wafer
Aligned Novel hinge pins ensurelt 2 um alignment

• Accomplishments
• Vertical alignment through crystal symmetry
• (anisotropic etching)
• Controlled stress-based folding in bilayer
  structures
• Simulation GUI with kinematic model for origami

10 min
20 min
Controlling curvature through stress and etch
timing
Barbastathis, MIT
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Active Diffractive MOEMS Si and Polymer-based
Arrays
Objectives

• Establish diffractive MOEMS as a viable
• solution for a reconfigurable I/O approach
• to optical interconnects
• Achieve a low actuation voltage array to
• reconfigure a massively parallel
• interconnect architecture
• Develop a technology demonstrator for
• MOEMS-based system
• Verify system-level optical/mechanical
• performance to set foundation for inclusion
• in interconnect methods

Polymer MOEMS Prototypes
No Voltage
Displacement of Polymer MOEMS Surface (Required
Movement at 5-15 V Actuation)
Castracane, Albany
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Output from Waveguide to Volume Grating Coupler
to Polymer Pillars to Air
Polymer Pillar
Polymer Pillars on Gratings
Pillars Located at Vertical Lines
Normalized Intensity (arb. units)
Distance, x ( microns)
Gaylord, Georgia Tech.
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Optical Interconnects in Silicon Miniboard using
Lightwires and Thin-Film Isolators.

• Suitable for intrachip 2-3 cm long optical
  interconnects.
• Require further loss reduction for gt10 cm
  interconnects.

Optical Image
Chang-Hasnain, Berkeley
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Short Pulse Sources
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Generation of 100 GHz Clock
Tones Separated by FSR of Filter
WDM Clock Multiplication Distribution
Delfyett, UCF
Low Noise Modelocked Diode Laser
Curved Reverse Mesa Ridge Waveguide Modelocked
Oscillator Chip
N Channels _at_ Nx Clock Rate 100 GHz Clock
EDFA
Optical Electrical
WDM Hyperfine Demultiplexer
6.25 GHz Primary Clock Rate N 16
See also Harris, Miller Progress towards
integrated vertical modelocked laser c.w. laser
demonstrated
Mode Profile
Intensity Autocorrelation
Output Spectrum
100 GHz Clock
200mA
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Novel Laser, Detector, Modulator and Isolator
Devices
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High-Efficiency, High-Speed VCSELs with Lateral
Carrier Confinement
Coldren, UCSB

• Goal High efficiency, high-speed VCSEL operating
  at low power
• Tapered oxide aperture can eliminate optical loss
• Lateral Carrier Confinement can prevent carriers
  diffusing away from active region
• Results
• Novel Quantum Well Intermixing (QWI) process
  shows a selective 70nm (90 meV) bandedge shift
• QWI-VCSEL of 1 mm diameter shows 50 reduction in
  threshold and no reduction in quantum efficiency.
  
• Plans
• Characterize new low parasitic VCSEL structure
  for high modulation bandwidth
• Incorporate QWI into the new structure to realize
  high-efficiency, high-speed operation for optical
  interconnects

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GaInNAs Edge-emitting lasers and VCSELs
Harris, Stanford

• Nitride-Arsenide alloys can emit at long
  wavelengths
• GaAs-based technologty allows VCSELs to be
  fabricated
• long-wavelength (low bandgap) is CMOS compatible
• High performance edge-emitting lasers at 1.5 mm
• 15 peak wallplug efficiency
• Single mode grating lasers demonstrated
• Vertical-cavity surface-emitting lasers (VCSELs)
  at 1.46 mm
• Pulsed operation at 10oC
• Substantial improvement expected with improved
  fabrication

Harris group
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Si Electro-Optic Modulator based on Resonant
Cavity
Resonance Condition K (?rK/neff) 2pR
Lipson, Cornell
450nm
200nm
12µm
Q12800. Measured modulation gtthan 50
High-index contrast of cavity waveguide maximizes
interaction of the optical mode with the core of
transmission medium. Speed expected 1GHz
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Long-wavelength optical modulator for high-speed,
low-power, low-voltage, for array integration
with CMOS
Input output relative alignment insensitive to
position of device

• To connect optical networks directly to silicon
  CMOS,
• need optical output device with
• 1 V drive
• Easy to align
• Array fabrication
• Telecommunications wavelengths (1.5 microns)
• Potentially high speed
• Solution
• Avoid waveguide
• Use shallow angle for long interaction length,
  and weak cavity
• Use 3 bounce optical design for positional
  alignment tolerance
• Performance
• 1 V drive, 10 nm bandwidth, 30 microns alignment
  tolerance, array fabrication

Input
Output
Miller, Stanford
Contrast ratio vs. wavelength for 1 V drive
Tolerance to misalignment
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Quantum Dots
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Development of Hi F - Hi T Quantum Dot VCSEL
7xQD

• Goal III-V VCSELs with
• modulation frequencies 20-40 GHz
• operation temperatures gt100oC
• operation voltage swing lt1 V

3xQD

• Approaches
• Shape-engineered QD medium to trade photon
  density for differential gain, reduce non-linear
  effects, increase saturation gain, reduce
  inhomogeneous broadening, increase reliability
• Resonant tunnel injection to increase with fast
  carrier capture times and low loss of carriers
• Design to reduce cavity losses, and parasitics

7xQD medium for 1.15µm VCSEL max. in-plane
gain, 31 cm-1
Threshold current density vs .Temperature for QD
and QW edge-emitting lasers demonstrated
unsurpassed T0
Oktyabrsky, Albany
See also InGaAs/GaAs quantum dots for 1.3 microns
VCSELs (Towe, CMU)
Room temperature PL intensity vs. implantation
dose QDs show 2 orders higher defect tolerance
than QWs
Lasing spectrum of QD all-epitaxial VCSEL lasing
demonstrated
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Photonic Nanostructures
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Novel Polymer Structures for Optical Sources and
Detectors
Aboelfotoh, Kolbas, NC State

• Ordered and oriented polymer arrays are generated
  by encapsulation into hexagonally arrayed
  channels of nanoporous SiO2/Si structures.
• These nanoporous SiO2/Si structures are created
  using prepared anodic aluminum oxide (AAO) as a
  stencil etching mask (see Fig. 1). The control of
  the conjugation length and conformation of
  individual polymer chains is essential in
  controlling their optical emission efficiency.
• Poly(thiophenes) and poly(pphenylene vinylenes)
  (PPVs) are selected as the active luminescent
  media.
• This approach is fully compatible with silicon
  CMOS technology.

(c)
AAO used as a mask to pattern nanoporous
structures on a SiO2/Si substrate (a) side view
with AAO mask (b) side view after removal of AAO
and (c) top view of AAO mask and patterned
substrate.
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A new method for sensitivity analysis of photonic
devices

• Objective Calculate

(T response function, s design vector)
Dutton, Fan, Stanford

• Direct approach (DA)

New approach (AVM)

• 1. Finite-difference frequency-domain (FDFD)
  discretization
• 2. Adjoint variable method (AVM)
• 3. Perturbation theory techniques for geometrical
  parameter variations

Validation Excellent agreement between AVM and
DA in high-resolution grid.
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Theory of Mode-Locked of Monolithic Laser Diodes
Incorporating Photonic Crystals
Dutton, Fan, Stanford

• Mode-locked lasers
• Ultra-short pulse-train, ideal light source for
  WDM/TDM applications
• Challenge fundamental limit on the device size
  (millimeters at 10GHz repetition frequency)
• Incorporation of photonic crystals
• Coupled Resonator Optical Waveguide (CROW)
  structure, monolithic, compact
• slow-light effect results in dramatic device
  size reduction
• vertical-cavity array configuration suitable for
  high-power and parallel applications
• Numerical Results
• demonstration and verification of device
  operational principle
• device size reduction from 5mm to 150micrometer _at_
  7.5GHz repetition freq.

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High-speed photonic crystal lasers based on a
novel combined Si-InP structure
Vuckovic, Stanford
Idea 2D coupled photonic crystal (PC)
microcavity arrays in SOI bonding of active
material on top for higher power, high speed PC
lasers

• Status
• Fabricated structures in SOI
• Experimentally demonstrated band diagram
• InP wafer designed and
• grown
• Wafer bonding and InP processing in process

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Examples of Innovative Claims and Associated
Research Progress

• easily alignable array optical modulators
  compatible with optical network wavelengths and
  CMOS voltages
• Demonstrated successful device
• high-speed photonic crystal lasers based on a
  novel combined silicon-InP structure
• Coupled nanocavity structures fabricated and
  characterized
• very short (e.g., 20 microns) silicon modulators
  and optical couplers
• Very small silicon modulator demonstrated
• high-repetition rate (multi-GHz) optical clock
  distribution
• Sub-picosecond clock injection demonstrated (at
  low repetition rate)
• demonstration of very-high repetition rate
  (multiple 10s of GHz) semiconductor lasers and
  very high-frequency  optical oscillators
• 100 GHz pulse train demonstrated
• variation-aware optical and circuit analysis
  techniques
• New theoretical technique (adjunct variable
  method) demonstrated
• high reliability and visible lasers on Si
• Record reduction in threading dislocation density
• integrated photodetectors with lt 10fF capacitance
• 3 fF integrated detector demonstrated