Optical Interconnects for Computer Systems

Bhanu Jaiswal

University at Buffalo

Introduction

Nature of data traffic in a computer Converse to city traffic Ever increasing data transfer rate Very high data rates restricted by

fundamental limitations of current copper interconnects

Need for a long term solution

Interconnect Issues

In present computer systems, interconnections handled via parallel electrical busses

Interconnect performance does not increase comparably with the system performance Solutions – Increase performance of present EI – Use completely different physical medium

Problems with Electrical Interconnects

Physical Problems (at high frequencies) Cross-talk Signal Distortion Electromagnetic Interference Reflections High Power Consumption High Latency (RC Delay)

Why Optics ?

Successful long-haul telecommunication system based on fiber optics

Advantages: Capable to provide large bandwidths Free from electrical short-circuits Low-loss transmission at high frequencies Immune to electromagnetic interference Essentially no crosstalk between adjacent signals No impedance matching required

Evolution of Optical Interconnects – Current & Future possibilities

This approach to signal transfer is moving from longer-distance applications, such as linking separate computers, to joining chips within a computer

Basic Ingredients

SOURCEDETECTOR OPTICAL PATH

VCSEL

Edge-EmittingLaser

LED’s P-I-N Photodiodes

SML Detector

MQW P-I-N

Guided WaveFree-Space

World wide projects

Heriot Watt University – Optically Interconnected Computing (OIC) group– SPOEC Project

DaimlerChrysler, McGill University– Optical Backplanes

UC San Deigo– Optical Transpose Interconnect System

Target – Terabits/second

US based research

$70 million program run by US Defence Advanced Research Projects Agency

Companies in business– Primarion Corp. – Thinking inside the box– Agilent Technologies – Optical connecters

between computers– Lucent Technologies – Optical Crossbar switch

matrix

SPOEC Project

SPOEC System Layout

Test bed developed by the SPOEC project

Optical Backplanes Speed Data

In DaimlerChrysler's optical backplane, the beam from a laser diode passes through one set of lenses and reflects off a micromirror before reaching a polymer waveguide, then does the converse before arriving at a photodiode and changing back into an electrical signal. A prototype operates at 1 Gb/s.

Free-Space Interconnects Pack in Data Channels

An experimental module from the University of California, San Diego, just 2 cm high, connects stacks of CMOS chips. Each stack is topped with an optics chip [below center] consisting of 256 lasers (VCSELs) and photodiodes. Light from the VCSELs makes a vertical exit from one stack [below, left] and a vertical entry into the other. In between it is redirected via a diffraction grating, lenses, an alignment mirror [center], and another grating. Each of the device's 256 channels operates at 1 Gb/s.

Principal Challenges

Multi-disciplinary field Device Integration, Interfacing & Packaging

– Electronic components – Si CMOS based– Optoelectronic Components – III-V Compound

based– Optical components – MicroLens and

MicroMirrors based

Misalignment in FSOIs

Conclusions

Interconnect problem significant in ultra deep submicron designs

Performance of Electrical lnterconnects will saturate in a few years

OIs – very promising for future computers OIs do not aim to completely replace EIs

References

Linking with light - IEEE Spectrum http://www.spectrum.ieee.org/WEBONLY/publicfeature/aug02/opti.html

Optically Interconnected Computing Group

http://www.phy.hw.ac.uk/~phykjs/OIC/index.html

Optoelectronics-VLSI systemintegration Technological challenges

www.phy.hw.ac.uk/~phykjs/OIC/Projects/ SPOEC/MSEB2000/MSEB2000.pdf

Ref. follows

International Technology Roadmap for Semiconductors (ITRS), 2001

R. Havemann and J.A Hutchby, “High-Performance Interconnects: An integration Overview”, Proc. Of IEEE, Vol.89, May 2001

D.A.B Miller, “Physical reasons for optical interconnections”, Int. Journal of Optoelectronics 11, 1997, pp.155-168.

MEL-ARI: Optoelectronic interconnects for Integrated Circuits – Achievements 1996-2000