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Task 2 Optical Interconnects

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Title: Task 2 Optical Interconnects


1
Task 2 Optical Interconnects
  • David Miller, Stanford

2
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)

3
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

4
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

5
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

6
Systems
7
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
8
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
9
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)
10
Integration Techniques
11
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
12
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)
13
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
14
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)
15
Optics
16
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
17
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
18
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.
19
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
20
Short Pulse Sources
21
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
22
Novel Laser, Detector, Modulator and Isolator
Devices
23
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

24
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
25
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
26
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
27
Quantum Dots
28
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
29
Photonic Nanostructures
30
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.
31
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.
32
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.

33
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

34
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
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