© intec 2000 Reasons for parallel optical interconnects Roel Baets Ghent University - IMEC Department of Information Technology (INTEC)

© intec 2000

Reasons for parallel optical interconnects

Roel Baets

Ghent University - IMEC

Department of Information Technology (INTEC)

http://www.intec.ugent.be/IODate workshop, February 2004

Overview

• Introduction

• Electrical interconnects: the limitations

• Optical interconnects: the merits

• Optical interconnects: the challenges

• Conclusion

http://www.intec.ugent.be/IODate workshop, February 2004

Interconnect: what ?

Interconnects = transmission of information

http://www.intec.ugent.be/IODate workshop, February 2004

Optical interconnects

Optical interconnects is a success for telecommunication

long-distance (several km)

shorter distance (tens to hundreds meters): data-communications (LAN) system-level interconnects

(parallel optical datalinks)

And shorter distance is electrical ?

http://www.intec.ugent.be/IODate workshop, February 2004

Electrical connections (1)

Electrical tracks on PCB exhibit high loss

Solution pre-emphasis driver = higher-power dissipation

repeaters = higher power dissipation + more real estate

1m 8mil 50 stripguide with GETEK dielectric

http://www.intec.ugent.be/IODate workshop, February 2004

Electrical interconnects (2)

Electrical connectors are large

= a density problem

Electrical connector at best 2 Gbps/mm2

http://www.intec.ugent.be/IODate workshop, February 2004

Progress electrical interconnects

ITRS Roadmap 2003:

chip-to board for peripheral busses is 5 to 6 Gbps for differential pairs in 2008-2009

but limited to a small number of pins

http://www.intec.ugent.be/IODate workshop, February 2004

Optical interconnects !

Shorter-distance interconnects benefit from optical technologies !

A good reason for optical interconnects:

optics is better than electrical interconnects

in terms of

power dissipation is distance independent

data density: Gbps per mm2 is larger

transmission distance: loss in fibre is negligible and data rate independent

http://www.intec.ugent.be/IODate workshop, February 2004

Parallel optics: merits

Reduced power dissipation, especially for long-distance Typical power dissipation per link, for 2.5 Gbps, is 20-30mW

Larger data density due to 2-D parallelism ! Electrical backplane connector is limited to 50 Gbps/cm

Optical backplane connector allows >50 Gbps/mm2 , thus few Tbps/cm

100

1000

10000

100000

1000000

maximal bandwidth over 60cm backpanel

[Gbps]

optical -upper limit

optical - IOconnectors

electrical -upper limit

electrical -state-of-the-art

ATCA backpanel extrapolated to 12.5Gbps line rate

Assuming 250um pitch(smaller pitch is possible) B=B0A/L2

(D.A.B. Miller)

http://www.intec.ugent.be/IODate workshop, February 2004

Parallel optics: merits

Longer transmission distances optical loss is <1dB/m, loss electrical track on backplane is

>5dB (1m @ 2.5Gbps)

Smaller chip size opto driver and receiver circuit is comparable to (or even

smaller than) LVDS circuit (for given technology)

Simpler system design !! optical path replaces high-speed electrical tracks, thus simpler

packaging and PCBs

optics is scalable: same transceiver for intra-board, board-to-board AND system-to-system interconnects !

http://www.intec.ugent.be/IODate workshop, February 2004

Optical interconnects ?

So why is optics not yet inside your computer today ?

Optics is a new technology (30 years younger than electronics), components are available only recently

Optics integration requires different novel technologies, optics seems complex

Performance of electrical interconnects is acceptable for current applications

http://www.intec.ugent.be/IODate workshop, February 2004

Optics: where and when ?

ElectricalElectrical

According to different roadmaps, optical interconnects will be introduced in system around 2008:

Source: INTEL (2002)

http://www.intec.ugent.be/IODate workshop, February 2004

Interfacing optics to CMOS

Optical interconnect needs

ED: digital CMOS circuitry

EA: analog driver + receiver circuitry

OE: light sources (or modulators) and detectors

O: passive optical pathway (fiber, waveguides in board, free space)

Options:

EA+OE+interface to O in one package

in some applications: ED+EA+OE+O in one package

http://www.intec.ugent.be/IODate workshop, February 2004

Building OE on electronic ICs

Key challenges:

• integration of OE components on EA chipsyield

cost

• packaging of this chip to allow for interfacing to optical pathway

alignment issues

hermeticity issues

thermal issues

• integration of optical interconnect into the IC design methodology

http://www.intec.ugent.be/IODate workshop, February 2004

320Gbps160Gbps

80Gbps

40Gbps

On-chip optical access: roadmap

Degree of parallelism

1.25Gbps

2.5Gbps

3.125Gbps

5Gbps

10Gbps

CMOS technology

0.35um

0.18um

0.13um

90nm

4x8

8x8

2x8

x8

16

x16

12.5Gbps

2x1

6x1

6

65nm

4x4

16 32 64 128 256 512 #channels

Feasible today8

1x8

2x8

4x8

x8

640Gbps

1.2Tbps

2.5Tbps

Feasible with future IC

technologies

Fine-pitch optics

Line-rate(over backpanel !!!)

http://www.intec.ugent.be/IODate workshop, February 2004

Conclusions

The road ahead

Bridge the 30-years age gap with electrical interconnects

(extra) proof of reliability

Offer an integrated solution

Bring all components vendors together

Optimise performance of components to get an efficient and cost-effective link

Cooperate with the end-user