“Overview of short -reach optical interconnects: from ... · -2-1 0 1 Frequency (GHz) Modulator...

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“Overview of short -reach optical interconnects: from VCSELs to silicon nanophotonics”

Ashok V. Krishnamoorthy ( Jack Cunningham)

Hardware Architect

Sun Labs, Oracle

Physical Sciences Center, San Diego, CA

Acknowledgements:

J. Cunningham, R. Ho, X. Zheng,

J. Lexau, H. Thacker, J. Yao, Y.

Luo, G. Li, I. Shubin, F. Liu, D.

Patil, K. Raj, and J. Mitchell

M. Asghari

T. Pinguet

This work was supported in part by DARPA under contract NBCH30390002. The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense


Introduction

> Definitions

> Penetration of optics into communication systems

Fibers, connectors, and module packaging

> Optical product segmentation

> Some examples of systems using optical interconnects

Optics to the package/chip

Link energy efficiency metric and goals

Silicon photonics and WDM

Overview of recent optical component results

Brief introduction to the macrochip

Outline


Optical Transceivers

> Integrated modules incorporating optical laser transmitters and photodiode receivers. These modules convert physical signals from electrical to optical and vice-versa in a network and couple the optical signals into (and out of) optical fiber. Transceivers have serial electrical interfaces on the host board.

Parallel Optical Transceivers, Modules, Interconnects or “Parallel Optics”

> Integrated optical laser transmitter and receivers incorporating multiple signaling channels in a single housing, each channel having a separate serial electrical interface to the host board. Typical values are 12 channels, although higher numbers (24, 36) have been developed. Parallel optical modules typically utilize an array of VCSELs and detectors to transmit and receive optical signals traveling in multi-mode fibers over a distance of up to 300m.

VCSEL

> Is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor laser which emit in-plane from surfaces formed by cleaved facets. VCSELs are today the most-efficient, lowest-cost, and most widely used laser source for interconnects.

WDM

> Wavelength Division Multiplexing. Enables multiple data streams of varying wavelengths (“colors”) to be combined into a single fiber, significantly increasing the overall capacity of the fiber and of the connector. There are two types of WDM architectures: Coarse Wavelength Division Multiplexing (CWDM), typically handling up to 8 wavelengths, and Dense Wavelength Division Multiplexing (DWDM), supporting up to 160 wavelengths.


Price evolution of optical linksJ. Cunningham et al., SPIE Photonics West, Jan 2006

Approaching ~$1/Gbps

Consumer application could further reduce price

1994 1996 1998 2000 2002 2004 2006 2008 2010 20120

1

2

3

Time (yr)

100

101

102

Cost

per

Gbp

s ($

)

Data Rate ( x 10 Gbps)

Module Price ($k)

Cost per Gbps ($)


Optics in communications

10,000km

1000km

100km

10km

1km

100m

10m

1m

10cm

Metro, access, cross -campus

Across central office, data centers

To the box/rack

To the board‘active cables”

$10,000

$3,000

$1,000

$300

$100

$30

$10

$3

1980 1985 1990 1995 2000 2005 2010 2015

Trans -oceanic

Cross -countrySM, CWDM, FSO

SM or MM, Serial or Parallel

SM, DWDM

Multi -mode, Parallel

SM, WDM

1Mbps 10Mbps 100Mbps 1Gbps 10Gbps 100Gbps 1Tbps

Link

Dis

tanc

e

Transceiver Cost (per G

bps)

Year of Introduction

Link Bandwidth

Num

ber of Links per system

1

10

100

1,000

10,000

To the package/chip

100 Gbps * meter

Krishnamoorthy, Optoelectronics Letters, May 2006


Optical link market segments

670nm,

850nm,

1300nm

LED

850nm VCSEL

(MM)

1.3um DFB (SM)1.3um Fabry Perot

(SM)

155Mbps 622Mbps 2.5Gbps 10Gbps 40Gbps

300m

2km

10km

40m

50m

Aggregate Data Rate

80m1.55um DFB (SM)

1.55um DFB

Ext.Mod.

(SM)

850nm ParallelVCSEL (Ribbon)

Re

ach

120Gbps

10m Optical Active Cables

VCSELs(MM)

120Gbps

850nm Parallel VCSEL (Ribbon)


Silicon photonic interconnects

670nm,

850nm,

1300nm

LED

850nm VCSEL

(MM)

1.3um DFB (SM)1.3um Fabry Perot

(SM)

155Mbps 622Mbps 2.5Gbps 10Gbps 40Gbps

300m

2km

10km

40m

50m

Aggregate Data Rate

80m1.55um DFB (SM)

1.55um DFB

Ext.Mod.

(SM)

Re

ach

120Gbps

10m

120Gbps

Silicon

Photonics.

(SM)

Optical Active Cables


I/O for the world’s largest IB switchGen 2: Up to 648 QDR Infiniband (40Gbps) ports [Gen 1: 3,456 SDR ports]

First 12x QDR cable developed by Merge Optics> Very high panel density requirement for Sun/Oracle QDR switch

> CXP active optical cable with three 4x10Gb/s (120Gbps per direction)

> Over 50Tbps front side I/O => Areal connection density > 1.7Tbps/sq. in

~6.8 Petabits/s of data bandwidth deployed> Over 28,000 air-cooled VCSEL-based active cables installed

> Over 500km of these active optical cables into datacenters

http://www.oracle.com/us/products/servers-storage/networking/infiniband/031556.htm

11 U


I/O for IBM’s P7 -IH computing system

12 drawers, 8 MCMs per drawer, 4 P7 chips per MCM, 8 cores per P7

http://www.avagonow.com/Newsletters/PDFs/IEEE-MitchFields-march-021610.pdf

2 U

Over 35Tbps of optical I/O per drawer

Each drawer can be configured as 256-way SMP

Water-cooled VCSEL modules for drawer I/O

> areal connection density of 1.2Tbps/sq. inch

A. Benner et al., paper OTuH1, OFC 2010


VCSEL w avelength: 850nm(other w ork at 980nm)

70°C

VCSELs and detectors on CMOS

Areal density: demonstrated over 37Gbps/sq. mm (24Tbps/sq. in)

Many independent R&D efforts, e.g.

> Bell Labs - late ‘90s

> AraLight/Xanoptix - 2002

> Agilent – 2004

> IBM - 2009

VCSEL Pads

Detectors

VCSELs

144 µm

C. Cook et al., IEEE JSTQE, March/April 2003

Krishnamoorthy et al., IEEE PTL, August 2000

C. Schow et al., OSA/IEEE JLT, April 2009

B. Lemoff et al., OSA/IEEE JLT, September 2004

J. Trezza et al., IEEE Commun. Mag., Feb 2003


Krishnamoorthy et al., IEEE JSTQE Spec. Issue on Green Photonics, to appear

GbE switch with VCSELs & detectors


Fiber Bundle Front View (facing bundle)

• Hexagonal closepack (tightest geometry)

• Multimode 50micron-core fiber

• Terminated to MTP connectors at other end

• One optical channel per fiber (ultimately

limits density)

497 512

1 16

Multimode fiber bundle array


32


0.01

0.1

1

10

100

1000

0.001 0.01 0.1 1 10 100 1000

Link

Ene

rgy

Effi

cien

cy (

pJ/b

it/m

)

Link Distance (m)

VCSEL links today

Electrical links today

Link energy efficiency vs distance


0.01

0.1

1

10

100

1000

0.001 0.01 0.1 1 10 100 1000

Link

Ene

rgy

Effi

cien

cy (

pJ/b

it/m

)

Link Distance (m)

Active Optical Cables

Si Target

System Target: <<1 mW per Gigabit/s per meter

Electrical links today

Improving link energy efficiency

100fJ/bit (Core)

500fJ/bit (Core - L3 cache & MM)

2pJ/bit (Core –Distant Memory)



Optics to the chip: CMOS photonics

10G modulators WDM optical filters

10G receivers

Flip-chip laser

Ceramic package

Fiber cable

C. Gunn, IEEE Micro, March/April 2003


Introduction to CMOS photonics

Buried Oxide

OWELLP OWELLN NMODPMODPMDH NMDH

P+ N+FOX FOX

Modulator Transistor Waveguide Grating

Contacts

Silicon

Multi-layer Metal Backend

Standard silicon process with SOI wafers (e.g. Luxtera)

High index contrast => sub-micron structures => fast, compact devices

Proven CMOS-compatible germanium waveguide detectors

C. Gunn, IEEE Micro, March/April 2003


Optical proximity communication (OPxC)

OPxC enables seamless multi-chip optical interconnects

> Various approaches

- Grating couplers, reflecting mirrors, ball lens in pit…

> High performance

- High bandwidth density (potentially > 32Tbps/mm2)

- Passive coupling (no conversion pwr)

- Performance limited by transceivers

OPxC demonstration

> Reflecting mirror OPxC

- 3 chips with 2 OPxC hops

> Promising optical performance

- Passive alignment with etch pits and balls

- Broad band coupling, >100nm

- <4dB insertion loss per coupling interface

- Negligible power penalty at receiver for 10Gbps transmission

10G Transmission of OPxC

1.00E-141.00E-131.00E-121.00E-111.00E-101.00E-091.00E-081.00E-071.00E-061.00E-051.00E-041.00E-03

-23 -22 -21 -20 -19 -18

Power at Receiver (dBm)

Log

BE

R

Tx/Rx Back-to-Back

Opxc Double Hop

Broad band operation

1400nm 1600nm

8.0

12.0

16.0

20.0

24.0

OPxC optical

Zheng et al, Optics Express, September 2008,

Krishnamoorthy et al., IEEE Journal of Quantum Elec., July 2009


Carrier depletion ring modulator

• Carrier depletion ring for low power, high speed modulation

• Free-scale 130nm SOI CMOS process

• Relatively low Q design (<105)

• >15GHz small-signal bandwidth with 1V reverse bias

• Stable large-signal operation (no feedback control or dynamic tuning)

0 5 10 15 20

-7

-6

-5

-4

-3

-2

-1

0

1

Frequency (GHz)

Mod

ulat

or E

O re

spon

se (d

Be)

1 V

0.1 1

0.1

1

bandwidth

Dep

letio

n le

ngth

(µ

m),

Γ, lo

ss (

cm-1)

Doping concentration x10 18cm-3

depletion length

confinement factor

breakdown voltageloss

Q=104

10-5 10-4 10-3∆n/n Refractive index

1

Bre

akdo

wn

volta

ge (

V), B

andw

idth

(x10

GH

z)

X. Zheng et al., Optics Express, Feb 2010


BER better than 10-15 with clocked digital Tx using ring modulator

BER< 10-15

Performance Summary:

• 5Gbps, digitally clocked

• 2V, 1.95mW or 395fJ/bit

• ER 3dB; IL 6dB

• Error free transmission for over 1.5 peta bits of data

• Better than 10-15 BER

400fJ/bit all -CMOS Tx (circuits + device)

X. Zheng et al., Optics Express, Feb 2010


Tuning out resonance imperfections

Krishnamoorthy et al., Proceedings of the IEEE, July 2009


25µm ring modulator w/ integrated heater

• Ring radius = 25 µm, FSR = 3.9 nm

• Total working wavelength range > 150 nm

• Heating power: 11.5 mW/nm, or 45 mW per FSR tuning

• 12.5 Gbps is achieved with a Vpp = 3 V, RE>6dB, limited by BERT

• Modulation energy/power of 200 fJ/bit or 2.5 mW

• No performance degradation over one FSR tuning

(a) n++p++ npSi

Metal Contact Ti heater

oxide

Bus waveguide

p contact

to RF pads

Ti

heater

to heater

pads

n contact

P. Dong et al., IEEE Summer Topical Meeting on Optics in Data Centers, July 2010

Heating power = 0 mW12.5 Gbps, λ: 1551.56 nm

Heating power = 56 mW12.5 Gbps, λ : 1556 nm


• Ring radius = 5 µm, FSR = 19 nm

• Heating efficiency: 2.4 mW/nm, or 46 mW per FSR tuning

• 12.5 Gbps is achieved with a Vpp = 3 V, 6dB ER

• Modulation energy of 40 fJ/bit or 0.5 mW

• no detrimental effects found ver ~93 ºC range

bus waveguide

p contact

to RF pads

Ti heater

to heater pads

n contact

(a) n++p++ npSi

Metal Contact Metal ContactTi heater

oxide

Heating power = 0 mW

12.5 Gbps, l: 1550 nm

Heating power = 18 mW

12.5 Gbps, λ: 1558 nm

P. Dong et al., Optics Express, May 2010

5µm ring modulator w/ integrated heater


Efficient, tunable CMOS rings

Schematic cross-section.

0 mW

0.7 mW2.7 mW

0 mW

0.7 mW2.7 mW

0 mW

0.7 mW2.7 mW

J. Cunningham et al., IEEE Summer Top. Meet. on Optics in Data Centers, July 2010

• Add/drop tunable filters

• 0.13 µ m SOI 6 metal CMOS process

• Dual heater stages with integrated waveguide heating

• Integrated back-side etch pit

3.9 mW per 2π phase shift


690fJ/bit all -CMOS Rx (circuits + device)

X. Zheng et al., Optics Express, January 2010


• CMOS integrated germanium photodetector

• 5Gbps, digitally clocked TIA-based receiver

• -18.9 dBm sensitivity at 10-12 BER

with 0.7A/W responsivity, >10GHz BW, and

<20fF detector capacitance

• BER measured below 10-14

• 3.45mW or 690fJ/bit


High- responsivity photodetectors► Kotura Ge photodetectors

• Butt-coupling between SOI and Ge waveguides enables short device lengths

(~ 10 µm)

• => Capacitance a few fF, device not RC-time limited)

• Horizontal p-i-n junction design enables compatibility with larger Si waveguides

• Narrow Ge WG width (0.65 µm) minimizes transit-time limitation (speed > 40 GHz)

SEM Cross-section

D. Feng et al., Applied Physics Lett, December 2009


• Responsivity of 1.1 A/W @ 1550 nm

• Dark current of 0.24 µ A @ -0.5 V, 1.3 µ A @ -1 V

• Bandwidth > 32 GHz

108

109

1010

-9

-6

-3

0

Res

pons

e(dB

)

-0.0V: 3dB BW 17.5GHz-1.0V: 3dB BW 32.6GHz-3.0V: 3dB BW 36.8GHz


“Macrochip” logical & physical views

Krishnamoorthy et al., Proceedings of the IEEE, July 2009R. Ho et al., IEEE Communications Mag., July/August 2010


Interlayer link components

-55-50-45-40-35-30-25-20-15-10-50

1530 1532 1534 1536 1538 1540 1542 1544 1546 1548 1550

Wavelength (nm)

Loss

(dB)

-12.7dB, -13.8dB, -13.7dB, -12.8dB

AR coated

AR coated

AR coated

AR coated

~12µm

~3µm~2mm

Mode Transformer

z

x

y

Echelle Grating (< 0.2 mm 2)Etched Waveguide Facet

z

x

y54.7º mirror

12 x 2.5 mm 2

Mirror Coupler

D. Lee et al., IEEE Summer Topical Meeting on Optics in Data Centers, July 2010


Sacrificial layer etch, spring lift-off

and Au-plating

Micro-spring interconnects

+

Co-integrate both technologies

Rematable power, ground, & alignment

Package to test rematability

I. Shubin et al., IEEE ECTC, May 2009

http://www.parc.com/�


0.1

1

10

100

2006 2008 2010 2012 2014

Industry Trend

DARPA UNIC Goals

Optical interconnect energy roadmapLi

nk E

nerg

y (p

J/bi

t)

Year

Krishnamoorthy et al., Proceedings of the IEEE, July 2009


REFERENCES:

X. Zheng et al.,Optics Express, October 2008

Krishnamoorthy et al., IEEE JQE, April 2009

I. Shubin et al., Proc. ECTC, May 2009

Krishnamoorthy et al., Proc. of the IEEE, July 2009

R. Ho et al., IEEE ASSCC, November 2009

D. Feng et al., Applied Physics Lett., December 2009

X. Zheng et al.,Optics Express, January 2010

X. Zheng et al.,Optics Express, February 2010

J. Cunningham et al., IEEE Photon. Summer Top. OND, TuD3.4,July 2010

P. Dong et al., IEEE Photonics Summer Top. OND, MD2.3,July 2010

D. Lee et al., IEEE Photonics Summer Top. OND, TuD3.3,July 2010

R. Ho et al., IEEE Communications Mag., July/August 2010


• Demonstration of passively-aligned multi-chip, multi-channel optical proximity communication

• Integration of ball-in-pit alignment with CMOS

• Record low-power silicon photonic link components > 320fJ/bit photonic Tx @ 5Gbps(w/ Kotura ring & custom driver)

> 690fJ/bit photonic Rx @ 5Gbps (w/ Luxtera Ge PD & custom receiver)

> 3.9mW FSR tunable mux/demux (w/ Luxtera ring & backside etch pit)

> 1.1A responsivity, 0.24µ A dark current large-core Ge detector (Kotura)

> Areal density of ~730Gbps/sq. mm based on WDM link components

• Record efficiency SOI passive components > Thin silicon routing waveguide with 0.27dB/cm loss (Kotura)

> 1x2 splitter with 0.1dB excess loss (Kotura)

• Demonstration of rematable power/gnd & chip alignment

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