Reference Guide on Optical Interconnects For High Performance Compute (HPC) Systems Prepared by: Nathan Harff, James Kruchowski, Mark Nelson, Brian Shamblin, and Vladimir Sokolov Special Purpose Processor Development Group Mayo Clinic Rochester, MN 55905 Phone: (507)284-4056 October 2009 Mayo-R-09-39-R1 Table of Contents Table of Contents................................................................................................................. i List of Figures ................................................................................................................... vii List of Tables ................................................................................................................... xii Acknowledgements........................................................................................................... xv 1 Basic Theory........................................................................................................1-1 1.1 Generic Electro-Optic Link......................................................................1-1 1.2 Guided Light............................................................................................1-2 1.2.1 Light Transmission in Free Space Versus an Optical Waveguide1-2 1.2.2 Types of Optical Waveguides......................................................1-2 1.2.3 Single-Mode and Multimode Optical Fibers ...............................1-4 1.2.4 Attenuation in Optical Fibers.......................................................1-4 1.2.5 Dispersion in Optical Fibers ........................................................1-5 1.3 Semiconductor Light Sources..................................................................1-6 1.3.1 Light Emission From a Solid .......................................................1-6 1.3.2 Energy Levels and Energy Band Diagrams in Semiconductors ..1-7 1.3.3 Diodes and Light Emitting Diodes (LEDs) .................................1-8 1.3.4 III-V Versus Silicon...................................................................1-10 1.3.5 Lasers .........................................................................................1-10 1.3.6 Quantum Wells..........................................................................1-11 1.3.7 Coupling Light to Waveguides ..................................................1-12 1.4 Imparting Information on Light.............................................................1-12 1.4.1 Modulation Schemes..................................................................1-12 1.4.2 Extinction Ratio and Optical Modulation Amplitude................1-16 1.4.3 Modulation Approaches.............................................................1-17 1.5 Light Detection......................................................................................1-17 1.5.1 Role of Photodetectors...............................................................1-17 1.5.2 Photon Absorption.....................................................................1-18 1.5.3 Photodiodes................................................................................1-19 1.5.4 PIN Diodes.................................................................................1-19 1.5.5 Avalanche Photodiodes (APDs)................................................1-20 i 1.5.6 Metal-Semiconductor-Metal (MSM) Photodiodes ....................1-21 1.5.7 Transimpedance Amplifiers (TIAs)...........................................1-21 1.6 Multiplexing/Demultiplexing ................................................................1-22 1.6.1 Time Division Multiplexing (TDM)..........................................1-22 1.6.2 Wavelength Division Multiplexing (WDM)..............................1-22 1.6.3 Mux/Demux Components..........................................................1-24 1.6.4 Star Coupler...............................................................................1-24 1.6.5 Array Waveguide Grating (AWG).............................................1-25 1.7 Optical Switching...................................................................................1-26 1.8 References..............................................................................................1-28 2 Fiber Types and Other Transmission Media........................................................2-1 2.1 Optical Fibers...........................................................................................2-1 2.2 Polymer Fibers and Optical Waveguides.................................................2-2 2.3 Optical Waveguides for Planar Lightwave Circuits (PLC) .....................2-4 2.4 References................................................................................................2-7 3 Light Source Options ...........................................................................................3-1 3.1 Light Emitting Diodes (LEDs).................................................................3-1 3.2 Lasers .......................................................................................................3-1 3.2.1 Fabry-Perot (FP) Lasers...............................................................3-1 3.2.2 Distributed Feedback (DFB) Laser..............................................3-5 3.2.3 Distributed Bragg Reflector (DBR) Laser ...................................3-5 3.2.4 Vertical-Cavity Surface-Emitting Laser (VCSEL)......................3-7 3.3 Modulators ...............................................................................................3-8 3.4 Optical Amplifiers.................................................................................3-10 3.4.1 Erbium Doped Fiber Amplifier (EDFA)....................................3-10 3.4.2 Semiconductor Optical Amplifier (SOA) ..................................3-11 3.5 References..............................................................................................3-13 4 Connectors and Interconnection Hardware..........................................................4-1 4.1 Standard Commercial Single-Way Connectors .......................................4-1 4.2 Small Form Factor Connectors ................................................................4-2 4.3 Blindmate Fiber Connector......................................................................4-3 ii 4.4 Ribbon and Parallel Fiber Connectors .....................................................4-4 4.5 Connector Ferrule Shapes and Polishes...................................................4-4 4.6 Abbreviation Meanings for Optical Connectors......................................4-5 4.7 References................................................................................................4-6 5 Analysis of Tradeoffs Between Optics and Copper.............................................5-1 5.1 Performance Parameters ..........................................................................5-1 5.2 Major Regimes of Operation for Optical Interconnects ..........................5-9 5.3 A Nearly Complete Electrical-to-Optical Crossover Example: High Speed Communication Links in Enterprise Data Centers......................5-11 5.4 Economic and Other Factors to Consider When Assessing “Optical Versus Copper”......................................................................................5-12 5.5 References..............................................................................................5-13 6 Existing Optical Technology ...............................................................................6-1 6.1 Optic Fiber and Waveguides....................................................................6-1 6.1.1 Single-Mode Fiber (SMF) ...........................................................6-1 6.1.2 Multimode Fiber (MMF) .............................................................6-2 6.1.3 Plastic Optical Fiber (POF)..........................................................6-3 6.1.4 Specialty Fiber.............................................................................6-3 6.1.5 Planar Waveguide Technologies..................................................6-3 6.1.6 Fiber Optic Appliqués..................................................................6-5 6.2 Light Sources...........................................................................................6-5 6.3 Modulators ...............................................................................................6-7 6.4 Active Optical Cables ..............................................................................6-8 6.5 Optical Amplifiers...................................................................................6-9 6.6 Photodiodes (PDs).................................................................................6-10 6.7 Connectors .............................................................................................6-12 6.8 Array Waveguide Gratings (AWGs) .....................................................6-12 6.9 References..............................................................................................6-14 7 Emerging Optical Technology.............................................................................7-1 7.1 Components .............................................................................................7-1 iii 7.1.1 Miniaturization and Monolithic Integration of Optical Components (Nanophotonics)...........................................................................7-1 7.1.2 Advanced Vertical Cavity Surface Emitting Lasers (VCSELs) ..7-4 7.2 Subsystem Design Strategies...................................................................7-6