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Experts see bumpy ride to silicon photonics

Posted: 02 Feb 2009 ?? ?Print Version ?Bookmark and Share

Keywords:silicon photonics? optical interconnect? multicore processor?

The computer industry is set on having optical interconnects for its future multicore processors, but just how it will get them remains unclear. That was the conclusion of a panel of experts from Hewlett-Packard, IBM, Intel, MIT and Sun at Photonics West.

Several factors are driving the need for silicon photonics in computing. However, experts agreed there is no clear light source to drive on-chip optics, and the technology needs to get to significantly lower heat, power and cost to be viable.

"We need a byte [of bandwidth] per Flop [of performance], said Ray Beausoleil, an optics researcher at HP Labs. In a keynote he said the computer industry sees on-chip optics as a way to maintain a relatively flat programming model for tomorrow's massively multicore processors.

"We found we had to maintain that byte-per-flop metric," agreed Jeffrey Kash who manages an optical research group at IBM, referring to the company's work on a massively parallel supercomputer it is developing as part of a government research program.

"The perception is the need for bandwidth is the reason for making the shift to optics, but it's also the freedom to repartition the computer architecture to get access to more memory and new abilities to cool the system," said Mario Paniccia, director of Intel's silicon photonics lab. "It's not just the next speed bump," he added.

The panelists agreed that no clear silicon light sources are on the horizon.

"Teams are working on silicon lasers in many companies, but even if the technology was working today it would still be five years from commercialization," said Paniccia. "Many of us our hopeful, but [it's hard to find] the milliwatt efficiencies we are looking for with a decent conversion rate on infrared wavelengths for transparency," he added.

Kash of IBM noted Intel has taken an interesting hybrid approach using III-V materials for the gain function in ways that could be packaged to be compatible with silicon photonics.

"Silicon lasers are a ridiculous concept if you want to build something real," said Eugene Fitzgerald, a professor of materials engineering at Massachusetts Institute of Technology. He said a practical solution "will be III-V devices on-board, and then you have to find an inexpensive platform to get the heat out."

"We strongly prefer not to have the light source on the chipor even in the data centerbecause we don't want to cool it," said Ashok Krishnamoorthy, a distinguished engineer in optics at Sun Microsystems. "I'd rather have light pumped in from outside if I could."

"I'd prefer to get [the light source] away from other heat sources so we could cool it separately," agreed Kash. IBM researchers have found on-chip optics "need temperature stability within a degree which is not practical, so we need to make these systems more temperature tolerant," he added.

No clear migration path
Another issue is a lack of a clear technology transition. Initially, vertical-cavity surface-emitting lasers (VCSELs) will be used to deliver optical interconnects in computers "but it is clear VCSELs will not scale," said Beausoleil, noting the industry will need to transition to dense wavelength division multiplexing (DWDM) to deliver multiple links per channel efficiently on chip.

"How do we migrate from VCSELs to DWDM," asked Kash.

"There has to be a migration path, and certainly we don't see it today," he said. At the same time, "there's got to be some smaller markets that we can address on the way to the big vision" of silicon photonics in PC processors, he added.

Another key challenge is reducing costs of silicon photonics, as much as 70 percent of which are now in packaging and testing of optical components. "Testing has to take ten seconds and that's orders of magnitude changes from what we have today," Paniccia said.

The economic downturn might push out the transition to silicon photonics as companies cut back on long term research projects. In his keynote, Beausoleil said the capability needs to be in place in time for chips in 2017.

The economic reality will push out some of the technology," said Paniccia. If you can squeeze the sponge on the current generation technologythat's the default."

"It's like squeezing an orange, and in two or three years it will be like squeezing the peel," said Beausoleil, extending the metaphor. "The physical limits of wires are real and performance is already rolling off, so within a few years a brick wall will appear on the horizon," he added.

"I tell my team to focus on first downs and stay in the game," said Paniccia. "We are just at the beginning of this next generation of technology, but once we get there we will have built out an enormous amount of infrastructure, so if we are successful optics becomes a technology enabler like wirelessit goes everywhere," he said.

- Rick Merritt
EE Times

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