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

2. 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/ The Role of Light in High Speed Digital Design Esther Renner

3. 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/ The Role of Light in High Speed Digital Design Esther Renner

4. 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/ The Role of Light in High Speed Digital Design Esther Renner

5. 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/ The Role of Light in High Speed Digital Design Esther Renner

6. 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/ The Role of Light in High Speed Digital Design Esther Renner

7. 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/ The Role of Light in High Speed Digital Design Esther Renner

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

9. 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/ The Role of Light in High Speed Digital Design Esther Renner

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

11. 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/ The Role of Light in High Speed Digital Design Esther Renner