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JANUARY 2021 I PCB007 MAGAZINE 89 typical high-performance com- puter (HPC) has a chip-mod- ule-board structure seen in Fig- ure 12a. The bandwidth limits are characterized in Figure 12b. The electrical packaging tech- nology is mature and there is not enough space or pins (in- cluding per-pin BW) to get past the "chokepoint" at the mod- ule-to-circuit board interface, which is only 12.6 Tb/s. What is needed is a more miniatur- ized structure proposed in Fig- ure 12c, using optical TX and Rx along with waveguides (OE Modules) and optical bussing on the PCBs. This raises the Figure 10: The [OPCB-12] Project finished test system with daughter cards (8x8 crosspoint switches), optical array connectors and mating optical connector in the multilayer electro-optical midplane. The closeup of the midplane shows the cut-outs for the optical connections and waveguide path. Loughborough University is developing and testing polymer waveguide fabrication meth- ods including: • Direct laser write • Laser ablation • Inkjet printing The fabrication process is done by Stevenage Circuits and assembled by Xyratex. This culminated with the building of a hybrid integrated optical and electrical inte connect- ed test system backplane incorporating multi- ple layers of copper tracks and polymer wave- guides to demonstrate bi-directional error-free interconnections using 10 Gb/s ethernet digital traffic, as seen in Figure 10. Terabus Project (IBM's Optocard) IBM Research has invested heavily over the years in optical printed circuit board technolo- gy based on multi-mode polymer waveguides. This is now referred to as Optocard. This re- search was partially funded by the U.S. Gov- ernment (DARPA) as the Terabus Program [4,9] . IBM believes this technology will be needed to provide the bandwidth for future server gen- erations (clouds), allowing highly integrat- ed electrical-optical links of waveguides, flex and fiber between systems, modules, boards, and chips. Agilent (withdrew in 2005), Em- core, HP Labs and Avago also participated. A Figure 11, OPCB-12 UK project.