Demonstrating that low power optical chips can be built with no alteration to existing semiconductor manufacturing processes should make optical communication more attractive to the computer industry. But it also makes an already daunting engineering challenge even more difficult.
“You have to use new physics and new designs to figure out how you take ingredients and process recipes that are used to make transistors, and use those to make photodetectors, light modulators, waveguides, optical filters, and optical interfaces,” explained MIT professor of electrical engineering Rajeev Ram, referring to the optical components necessary to encode data onto different wavelengths of light, transmit it across a chip, and then decode it.
The new chip marks the next step in the evolution of fibre optic communication technology by integrating into a microprocessor the photonic interconnects, or I/O, needed to communicate with other chips.
"This is a milestone. It's the first processor that can use light to communicate with the external world," said Vladimir Stojanovic, an associate professor of electrical engineering and computer sciences at the University of California, Berkeley. "No other processor has the photonic I/O in the chip."
The chip has 850 optical components and 70million transistors, which, while less than the billion-odd transistors of a typical microprocessor, is enough to demonstrate all the functionality that a commercial optical chip would require. In tests, the researchers found that the performance of their transistors was virtually indistinguishable from that of all-electronic computing devices built in the same facility.
Today’s chips cannot keep logic circuits supplied with enough data to take advantage of their ever-increasing speed. Boosting the bandwidth of the electrical connections between logic and memory would require more power, and that would raise the chips’ operating temperatures to unsustainable levels.
Optical data connections are, in principle, much more energy efficient. And unlike electrical connections, their power requirements don’t increase dramatically with distance. So optical connections could link processors that were meters rather than micrometers apart, with little loss in performance.
The researchers’ chip was manufactured by GlobalFoundries, which uses a silicon-on-insulator process, where layers of silicon are insulated by layers of glass. The researchers build their waveguides — the optical components that guide light — atop a thin layer of glass on a silicon wafer. Then they etch away the silicon beneath them. The difference in refractive index between the silicon and the glass helps contain light travelling through the waveguides.
In an optoelectronic chip light signals have to be converted to electricity. But contact with metal interferes with optical data transmission. The researchers found a way to pattern metal onto the inner ring of a donut-shaped optical component called a ring resonator. The metal doesn’t interact with light travelling around the resonator’s outer ring, but when a voltage is applied to it, it can either modify the optical properties of the resonator or register changes in a data-carrying light signal, allowing it to translate back and forth between optical and electrical signals.
On the new chip, the researchers demonstrated light detectors built from these ring resonators that are so sensitive that they could get the energy cost of transmitting a bit of information down to about a picojoule, or one-tenth of what all-electronic chips require, even over very short distances.
The authors emphasised that these adaptations all worked within the parameters of existing microprocessor manufacturing systems, and that it will not be difficult to optimise the components to further improve their chip's performance.
