IBM’s switch, which is described in a recent paper in Nature Photonics, is made of connected, resonating rings etched into silicon. The rings are only 200 nanometers tall–much smaller than the dimensions of optical fibers that normally carry light. When the switch is turned on, electrons are sent to a specific ring. These electrons change the way that the ring resonates, which effectively blocks light from passing through. The light bounces off the resonator and is reflected in another direction.
The design is unique for a number of reasons, explains Green. First, the switch does not filter the light based on its wavelength, unlike switches used in telecommunications networks that need to route specific types of light to specific destinations. And the more wavelengths of light that are let through an on-chip network, the more bandwidth is available.
A second distinguishing characteristic, Green notes, is that IBM’s switch is able to withstand a variation of about 30 °C, which is crucial to ensuring that the network is reliable. Within any given microprocessor, says Green, hot spots move around on the surface of the chip as a function of number crunching. If these optical interconnects are distributed all over the surface, he says, engineers need to make sure that the hot spots don’t change the properties of the devices, so that data can make it to each end of the chip unaltered. The temperature resilience of the switch, Green says, is due, in part, to allowing multiple wavelengths of light through. As the switch changes temperature, it also changes properties, which causes some wavelengths of light to be blocked. But since the switch was designed to route a broad spectrum, it can still function in an environment with a variable temperature.
Green says that it could be five to ten years before this switch finds its way into a commercial machine. IBM has already made ultrasmall optical silicon modulators, but, he says, it will take years to integrate the modulator, the switch, and other components with chip electronics.
Indeed, the promise of silicon photonics produces a new challenge: how to redesign a computer to communicate with light instead of with electrons. “How do you design an interconnected network that really exploits the optics?” asks Bergman. “You can’t follow the network design rules of electronics,” she says. “There are many things that are going to evolve dramatically as we go forward.”