Friday, December 22, 2006

Tiny Device Stores Light

IBM researchers have fabricated a silicon device that's a significant advance in making practical optical interconnects.

By Prachi Patel-Predd

An SEM image of an optical-delay line that has up to 100 microrings, all connected to a common silicon nanowire. The optical buffer can store up to 10 optical bits. Credit: IBM

By forcing light to circle multiple times through ring-shaped structures carved into silicon, researchers at IBM have been able to delay the flow of light on a microchip. Being able to delay light is crucial for high-performance, ultrafast optical computers of the future that will process information using light and electrical signals.

It's easy to store electronic data in computer memory; light is harder to control. The new silicon device, described in this week's issue of Nature Photonics, is ten times smaller than those made in the past. It also works much better at high data speeds. "This work is approximately a factor of ten over the best achieved with [ring-shaped devices] so far," says Keren Bergman, an electrical-engineering professor at Columbia University.

Storing light on silicon is key for electronic-optical hybrid computers that researchers believe will be available a decade from now. In these computers, devices will compute using electrons but will move data to other devices and components using light--avoiding the use of copper wires or interconnects that tend to heat up at high computer frequencies.

But the optical interconnects would have to be laid out in an intelligent network, just as the copper wires on today's chips are. To transfer data packets efficiently between devices, the copper network on a chip has nodes where many interconnects converge. If a processor is sending data to a logic circuit, the data travels from node to node until it gets to the logic circuit. Each node in the network reads and processes the data packet to route it correctly to the next node. While the node makes a routing decision, it temporarily stores the data in electronic memory. To process and route data at the nodes of an optical-interconnect network, one would need to store, or delay, light so the node can make the routing decision.

Yurii Vlasov and his colleagues at IBM's T.J. Watson Research Center delay light on a silicon microchip by circulating it 60 to 70 times through ring-shaped structures, called resonators. The researchers make these resonators on a thin silicon layer mounted on an insulting silicon-oxide layer. They etch parallel trenches into the silicon that reach down to the oxide. The raised portion between the trenches acts like a silicon wire that shuttles light.

The researchers employ the same silicon wafers and techniques that are used to fabricate microprocessors at IBM. This makes it easy to "think of combining optical circuitry with electrical circuitry on the same chip," Vlasov says.