Optical Computing

18 Dec

Computer chips that transmit data with light instead of electricity consume much less power than conventional chips, but so far, they’ve remained laboratory curiosities. Professors Vladimir Stojanović and Rajeev Ram and their colleagues in MIT’s Research Laboratory of Electronics and Microsystems Technology Laboratory hope to change that, by designing optical chips that can be built using ordinary chip-manufacturing processes.Optical data transmission could solve what will soon be a pressing problem in chip design. As chips’ computational capacity increases, they need higher-bandwidth connections to send data to memory; otherwise, their added processing power is wasted. But sending more data over an electrical connection requires more power.

Optical chips use structures called waveguides to direct light, and researchers trying to add optical components to a silicon chip usually carve the waveguides out of a single crystal of silicon, Stojanović says. But waveguides made from single-crystal silicon require insulating layers above and below them, which standard chip-manufacturing processes like TI’s and Intel’s provide no way to deposit. They do, however, provide a way to deposit insulators above and below layers of polysilicon, which consists of tiny, distinct crystals of silicon clumped together and is typically used in the part of a transistor called the gate. So the MIT researchers built their waveguides from polysilicon instead.

So far, TI has produced two sets of prototypes for the MIT researchers, one using a process that can etch chip features as small as 65 nanometers, the other using a 32-nanometer process. To keep light from leaking out of the polysilicon waveguides, the researchers hollowed out the spaces under them when they got the chips back — the sole manufacturing step that wasn’t possible using TI’s in-house processes. But “that can probably be fixed more elegantly in the fabrication house if they see that by fixing that, we get all these benefits,” Watts says. “That’s a pretty minor modification, I think.”

To meet the bandwidth demands of next-generation chips, however, the waveguides will have to carry 128 different wavelengths of light, each encoded with its own data. So at the receiving end, the ring resonators provide a bank of filters to disentangle the incoming signals. On the prototype chips, the performance of the filter banks was “the most amazing result to us,” Stojanović says, “which kind of said that, okay, there’s still hope, and we should keep doing this.” The wavelength of light that the resonators filter is determined by the size of their rings, and no one — at either TI or MIT — could be sure that conventional manufacturing processes were precise enough to handle such tiny variations.

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