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 SOURCE DETECTOR OPTICAL PATH VCSEL Edge-Emitting Laser 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 Linking with light - IEEE Spectrum Optically Interconnected Computing Group Optoelectronics-VLSI system integration Technological challenges 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 MEL-ARI: Optoelectronic interconnects for Integrated Circuits – Achievements