© 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