The Role of Light in High Speed Digital Design EECS 713 Esther Renner 11/30/17

Current use of fiber optics Long-houl, to metroplolitan area to fiber to the home Rack to rack interconnects over 10s of meters have become more common: typically they use 8 or 12 channel commercial parallel optical modules, operating at datarates up to 5 Gb/s/ch use multimode fiber ribbons Communication technology as a function of transmission distance [1] Focus on Chip-to-Chip technology 11/30/2017 The Role of Light in High Speed Digital Design Esther Renner

Why use light for HSD? Bottlenecks for electrical transmission: Limitation of I/O functions for each chip, bandwidth limitations Maximum clock rate is limited through propagation delay Crosstalk restricts spacing (capacitive and inductive coupling) Increasing power dissipation, losses (Skin effect), reflections Advantages of optical transmission: Low attenuation, low loss for optical channels Lower dispersion, crosstalk Sensitivity to electromagnetic interference (EMI) Increasing power dissipation for higher frequencies 11/30/2017 The Role of Light in High Speed Digital Design Esther Renner

Basic Optical Communication System Data Laser PD Modulator Signal Processing Components: Transmitter: Laser diode (Modulator) Waveguide, Fiber (glass, polymer) (Amplifier, for long distance transmission) Receiver: Photodiode 11/30/2017 The Role of Light in High Speed Digital Design Esther Renner

Technologies for Optical Chip-to-Chip Transmission Vertical-cavity surface emitting laser (VCSEL): Emit from surface, not from edge Can be placed on top of chip Same size as transistors GaAs, no direct integration in CMOS Different approaches: Discrete fiber links Electrical optical PCBs Free-space interconnects Lots of research has been done on this area Long distance telecommunications Made from gallium arsenide and can not be directly integrated with the CMOS silicium devices Free-space transmission [6] 11/30/2017 The Role of Light in High Speed Digital Design Esther Renner

Electrical Optical Circuit Board (EOCB) Hybrid carrier, contains electrical + optical interconnects Additional optical layer, multimode waveguide structures Works at 830 nm (infrared) Attenuation 0.3 dB/cm Optoelectronic devices (VCSEL, PIN) EOCB [4] 11/30/2017 The Role of Light in High Speed Digital Design Esther Renner

Polymer-Waveguide Board-Level Optical Interconnects CMOS-based optical transceivers & polymer waveguide on FR4 PCB VCSEL & PD arrays,16 channels, error-free operation up to 15 Gb/s 20%-80% rise time of RX channels is 32ps (+ LD driver and PD amplifier) 240 Gb/s aggregated bandwidth Eye starts closing but still significant margin available for error free operation FR4 circuit board with optical trace and eye diagram for RX channel at 15 Gb/s [2] 11/30/2017 The Role of Light in High Speed Digital Design Esther Renner

Power Comparison for Interchip Communication Compare board-level high-speed optical and electrical interconnects with similar properties Power dissipation [3] 11/30/2017 The Role of Light in High Speed Digital Design Esther Renner

Conclusion Project: Draw conclusion if light will be used to transmit data from one chip to another, compare different approaches and power consumption Optical technologies will be used for shorter and shorter distances – become more cost effective and applications more bandwidth hungry Trade-offs: Improvements in power, cost, density (space) and reliability Problems: simple packaging, coupling of light into waveguide, extra power consumption and delay time for optoelectronic conversion Copper interconnects will stay state of the art for chip-to-chip connections unless higher data rates are needed Lots of research Need more parts, LD, PD Multiplexers Not entirely persuaded that ever going to be used on board level 11/30/2017 The Role of Light in High Speed Digital Design Esther Renner

Thank you for your attention! Questions? 11/30/2017 The Role of Light in High Speed Digital Design Esther Renner

References [1] A. Alduino and M. Paniccia, “Wiring electronics with light“, Nature Photonics, Vol. 1, 2007 [2] Doany, Schow, Baks et. al., „160 Gb/s Bidirectional Polymer- Waveguide Board-Level Optical Interconnects Using CMOS- Based Transistors“, IEEE Transactions on Advanced Packaging, Vol. 32, 2009 [3] Cho, Kapur, Saraswat, „Power Comparison Between High- Speed Electrical and Optical Interconnects for Interchip Communication​“, Journal of Lightwave Technology, Vol. 22, 2004 [4] Schroeder, Bauer, Ebling, Scheel, “Polymer Optical Interconnects for PCB“, IEEE Photonics Polymers II, 2001 [5] Taubenblatt, Optical Interconnects for High-Performance Computing, Journal of Lightwave Technology, Vol. 30, 2011 [6] Savage, “Linking with Light“, IEEE Spectrum , 2002 11/30/2017 The Role of Light in High Speed Digital Design Esther Renner