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Computer giants embrace on-chip optics

27 Mar 2008

Competition is mounting in the race to use light rather than electronics to transfer information in next-generation supercomputers.

Ever since optical fibre revolutionized the performance of communications networks, computer scientists have been anticipating the leap in processing speeds that could be achieved with on-chip optical interconnects. Now, more than 30 years after the first commercial fibre link went live, engineers are tantalizingly close to creating on-chip optical networks that will power a new breed of ultrafast computers.

The last week alone has witnessed three significant new developments. Hot on the heels of its recent demonstration of an all-optical data bus, IBM revealed a silicon photonic switch based on cascaded microring resonators that can route data at speeds of 1 Tbit/s. The device, which measures just 40 x 12 µm, can switch up to nine channels at 40 Gbit/s per channel, and is capable of processing data with a switching speed of less than 2 ns.

Although small, the switch is slightly larger than the 9 x 9 µm silicon switch demonstrated by researchers at the University of St Andrews in Scotland earlier this year (see Photonic nanostructure yields tiny optical switch). But what makes IBM's switch different is that it operates over a broad range of wavelengths, while the bandwidth of the St Andrews device is limited to a few nanometres.

According to IBM, another critical feature of the new switch is that its operation is unaffected by the typical changes in temperature that can occur inside a computer processor. Error-free transmission is achieved over a spectral range of 2 nm, which is enough to accommodate a temperature drift of ±15 °C without degrading device performance (Nature Photonics doi:10.1038/nphoton.2008.31).

"This new development is a critical addition in the quest to build an on-chip optical network," said Yurii Vlasov, who heads IBM's research into silicon nanophotonics. "It looks like our vision for on-chip optical interconnects is becoming more and more realistic."

Meanwhile, NEC has worked with the Tokyo Institute of Technology to fabricate a prototype optical device that connects together the processing chips of a supercomputer. According to an article on Nikkei.net, the device – which has been developed specifically for a 10 petaflop supercomputer that the Japanese government hopes to build around 2010 – uses laser diodes to turn electrical signals into the optical domain, while data is transmitted between neighbouring chips using bundles of optical fibres.

US funds optical interconnect research
And last but very definitely not least, Sun Microsystems – along with several industrial and academic partners – has landed $44 million from DARPA (the Defense Advanced Research Projects Agency) to create a supercomputer that connects together low-cost silicon chips via on-chip optical networks. The project, which is due to run until 2013, forms part of DARPA's Ultraperformance Nanophotonic Intrachip Communication (UNIC) programme.

Sun's approach will be to construct arrays of low-cost chips linked together optically to create a single "macrochip". Although the details remain sketchy, it seems likely that Sun will exploit its "proximity communications" technology to develop some sort of optical proximity connection – in other words, the light will be coupled directly between the chips' edges, rather than via an optical fibre or waveguide.

"Optical communications could be a truly game-changing technology – an elegant way to continue impressive performance gains while completely changing the economics of large-scale silicon production," commented Greg Papadopoulos, Sun's CTO and head of R&D. Other project partners are Kotura, Luxtera, Stanford University and the University of California, San Diego.

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