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.
In addition to saving power, they could make computers much faster. “If you just focus on the processor itself, you maybe get a 4x advantage with photonics,” Stojanović says. “But if you focus on the whole connectivity problem, we’re talking 10, 20x improvements in system performance.”
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.
MIT researchers have demonstrated that they can put large numbers of working optical components and electronics on the same chip. This winter they expect to be able to control the optics directly with the electronics.
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.”
The MIT researchers’ design uses light provided by an off-chip laser. But in addition to guiding the beam, the chip has to be able to load information onto it and pull information off of it. Both procedures use ring resonators, tiny rings of silicon carved into the chip that pull light of a particular frequency out of the waveguide. Rapidly activating and deactivating the resonators effectively turns the light signal on and off, and bursts of light and the gaps between them can represent the ones and zeroes of digital information.
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.