Silicon photonics oil the works for high-speed switching
Optical interconnects are shaping up as the next big thing in high-speed switching, with viable chip to board and chip to chip solutions. When it comes to implementation however, the industry is at odds over whether to do it on the wafer or in the package.
"High-speed signalling at lower power requires that optics get closer to the ASIC than ever before. There's been a long history of optics displacing electrical links, from connecting networks to connecting server racks and chassis, and the next step will be connecting the chip to other chips on the board," says John Cunningham, a hardware architect at Oracle.
He will report on Oracle's recent work with DARPA and other partners on reduction in energy per bit with all-CMOS, silicon, photonic, chip-to-chip links in a talk at Semicon West in July.
Oracle's approach is to put the I/O engine next to the logic chips in multi-chip modules, so the electronic drivers and the photonic components can each be built separately with the best-in-class technology. Optical components are then attached in the same package as the processor by hybrid integration.
Recent progress in advanced packaging with micro bumps and module substrates eases much of this modular integration. Issues still remain with laser efficiency, with precisely aligning the optical fibres with the lasers and photonics, and with developing the industrial volume processes to do so.
"The next hurdle is to show that the technology is scalable, and that the cost is competitive with electrical solutions," Cunningham says. "This progression is on the industry roadmap to be developed over the next five years, but challenges still remain in developing all the building blocks and volume manufacturing processes."
Router taps optical links
Using a similar multi-chip module approach, Israeli start-up Compass-EOS has started shipping to some service providers routers with optical connections between the chips in the backplane. Shuki Benjamin, process engineering manager, says the company is now looking for a packaging specialist to ramp up production.
Compass-EOS's approach mounts conventional optical lasers and photo diodes directly on to the processor chip with a kind of flip-chip bump, and cuts a hole in the package substrate to let out the light. Analogue circuitry in the ASIC processor converts light to electrical signals and vice versa to get high bandwidth data in and out of the chip.
The arrays of lasers and photo diodes are aligned to a matching array of fibres in a fibre optic bundle to move the signals to and from the circuit board. The approach allows for direct optical links between multiple processers in the backplane of the company's routers, reducing system size and energy consumption.
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