Design methodology development for VCSEL-based optical interconnects.

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Design methodology Design methodology developmentdevelopment

for VCSEL-based guided-wavefor VCSEL-based guided-waveoptical interconnectsoptical interconnects

Michiel De Wilde & Olivier RitsGhent University, Belgium

IMEC

Design methodology development for VCSEL-based optical interconnects.

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OverviewOverview

• Optical interconnect– rationale & structure

• Optical interconnect design space– exploration & optimization

• Simulations to extract system-level properties

Design methodology development for VCSEL-based optical interconnects.

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Optical interconnect rationale Optical interconnect rationale (1)(1)

– Denser inter-chip interconnect requirement– Rising clock frequencies

(source: Intel)

Moore’s Law

Design methodology development for VCSEL-based optical interconnects.

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Optical interconnect rationale Optical interconnect rationale (2)(2)

Wire capacitance + resistance & skin effect limit the electrical interconnect bandwidth B

sbits10 217

LA

B

A

ground plane

signal wire

(Miller-Ozaktas)

wire length = L

Inter-wire spacing area

Design methodology development for VCSEL-based optical interconnects.

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Optical interconnect rationale Optical interconnect rationale (3)(3)

• Problematic interconnectbetween ICs and at the IC access level– Industrial packet routers– Some parallel and distributed processing systems

• Use of optics: a solution on physical grounds– No electromagnetic interference problems– Almost distance & frequency independent losses

• Optical I/O integration with CMOS solvesinterconnect problems at the IC access level

Design methodology development for VCSEL-based optical interconnects.

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VCSEL-based parallel optical I/OVCSEL-based parallel optical I/O

VCSELsCMOS substrate (top side visible)

Photodiodes

Package

Connector

Solder balls

Fiber bundle

PCB

Design methodology development for VCSEL-based optical interconnects.

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Fields involvedFields involvedVCSELtechnology

Photodetectortechnology

Optics-CMOShybridization

Connectorization

Waveguide routing technology

Optical waveguides

IC packagingClock re-synccircuitry

Driver & receivercircuitry

TX

Design methodology development for VCSEL-based optical interconnects.

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Complex design spaceComplex design space• Several approaches are possible• Some continuously valued parameters too

– λ, operating currents, numerical aperture, physical dimensions…

• Decisions in one field may affect other fields

(not to scale)

Increase of numerical aperture of fiberbettercoupling

worsecoupling

lessbendlosses

Design methodology development for VCSEL-based optical interconnects.

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Design optimizationDesign optimization

• Systematic way of making choices= design methodology1. Designer states constraints2. Tool suggests good solutions meeting

constraints

• Important system-level property categories– Technological feasibility

– Performance (timing/power characteristics)

– Reliability

– Implementation cost

Design methodology development for VCSEL-based optical interconnects.

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Design methodology Design methodology developmentdevelopment

VCSEL drive current

Photodetector sensitivity

Fiber numerical aperture

Interface technology…

Product and parameter choices

Power dissipationLink latency

Link reliabilityLink skew

Implementation cost

System-level properties

…

STAGE 1predict

STAGE 2Construct multi-objective solution

(e.g.) link bit error rate

(e.g.) total power dissipation

Infeasibledesign region

Sub-optimaldesigns

Pareto-optimaldesigns

STAGE 3target

Design methodology development for VCSEL-based optical interconnects.

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Estimating Estimating timing/power/reliabilitytiming/power/reliability

• Issues for direct estimation(e.g. from tabular data)– Non-linear interactions between different fields

(electrical, optical, thermal)– Impact of noise and process variations

• Interconnect simulation– Simulator– Simulator models– Stimuli– Calculation of properties from simulation

results

Design methodology development for VCSEL-based optical interconnects.

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Simulator modelsSimulator models

VCSEL model

Photodetectormodel

Clock re-syncmodel

VCSELdriver model

Photodetectorreceiver model

Optical path model

Design methodology development for VCSEL-based optical interconnects.

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Simulator choiceSimulator choice

• Device-level simulators– OptiWave, RSoft, WinLase, …– Too detailed (some parameters are IP)– Too slow for this purpose (finite-element

methods)

• Circuit-level simulators– SPICE, Verilog-AMS, VHDL-AMS, …– More concise parameter set possible– Faster (only integration over time)

Design methodology development for VCSEL-based optical interconnects.

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Example: photodiode modelExample: photodiode model

module pin_photodiode(in,anode,cathode); input in; inout anode, cathode; power in; electrical anode, cathode; parameter real Cdep=0, Cbo=0, Rbas=0, Resp=0, Id=0; parameter real pole=-1/(Cdep*Rbas); parameter real laplace_coeff_0=Cdep+Cbo; parameter real laplace_coeff_1=Cdep*Cbo*Rbas; charge rc; analog begin I(cathode,anode) <+ laplace_zp(Resp*Pwr(in)+Id,{},{pole,0}); Q(rc) <+ laplace_np(V(cathode,anode),{laplace_coeff_0,laplace_coeff_1},{pole,0}); endendmodule

•Terminals•Model parameters•Equations describing internal state and outputs

Design methodology development for VCSEL-based optical interconnects.

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Driver/receiver modelDriver/receiver model

• Normal analog electrical circuits• IP protection: no real circuit provided• Alternative: parameterised flowchart• Validation: measurements

on non-hybridized CMOS

Receiver flowchart

Transimpedance preamplifier

Postamplifier

Decision circuit

Equalizer

Limiting amplifier

Photocurrentinput

Digital output

Design methodology development for VCSEL-based optical interconnects.

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VCSEL modelVCSEL model• Nonlinear differential equation system• Jungo, et.al.: VISTAS software package

equations without spatial integration

• Difficult parameterization• Validation: measurements on non-hybridized

VCSELs

(source: M.X. Jungo)

Design methodology development for VCSEL-based optical interconnects.

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Fiber-based optical pathFiber-based optical path• Abstraction of dispersion (short distance)• Coupling coefficients for losses & crosstalk

macrobendlosses

absorption

Connectorlosses & crosstalk

VCSEL-fiber coupling losses

VCSEL-fiber crosstalk

(not to scale)

Fiber-photodetector crosstalk

Fiber-photodetector coupling losses

Design methodology development for VCSEL-based optical interconnects.

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Fiber bend lossesFiber bend losses• Bend losses can be approximated using a combination of

raytracing results (H. Lambrecht, et.al.)• Validation through optical path measurements

90° bendX axis: NA fiberY axis: bending radiusBlue color = high losses

(source: H. Lambrecht)

Design methodology development for VCSEL-based optical interconnects.

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Simulation illustrationSimulation illustration

(exaggerated VCSEL model parameters)

(inverted output)

Design methodology development for VCSEL-based optical interconnects.

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Simulation setupSimulation setup

• Interconnection signals– Digital pseudorandom– Design-specific

• Parameters– Process corners– Monte-Carlo

generated• + inter-device

correlation

• Noise– asynchronous– synchronous– mesochronous

• Predominant noise: substrate noiseat the receiver preamplifier– amplify some few µA

of photocurrent

Design methodology development for VCSEL-based optical interconnects.

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ConclusionConclusion

• Illustrated optical interconnect– rationale & structure

• Discussed optical interconnect design space– exploration & optimization

• Simulations to extract system-level properties– Explained approach– Discussed models– Future work

Design methodology development for VCSEL-based optical interconnects.

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AcknowledgementsAcknowledgements

• IST Interconnect by Optics project partners

• Hannes Lambrecht (Ghent University, IMEC-INTEC)– Fiber bend losses modelling

• Marc Jungo– VISTAS VCSEL modelling project

• Fund for Scientific Research – Flanders (Belgium) (F.W.O.)– Research assistantship