Moving data around inside a computer means shoving it through wires, which have inherent bandwidth limitations and produce a lot of heat. Once that data hits a network, however, it often runs across optical hardware, which can send information long distances at high bandwidth without needing a dedicated nuclear reactor for power.
The contrast between the two methods has most companies thinking about ways of getting optical connections inside computers and, eventually, inside chips themselves. This poses a significant challenge. While it’s possible to use silicon to create light-handling features, the processes used to do so are incompatible with the CMOS techniques used to make circuitry. As a result, most efforts in this area have used separate chips: one for the processor, one for the optical interconnect.
Now, a research team has put together a single chip that handles both optical and electrical processing and uses an optical connection to its main memory. While the bandwidth remains low, the entire system was manufactured using standard CMOS processes. And it incorporates a small RISC processor that’s able to run standard text and graphical programs.
I need a laser!
As with many previous efforts, the one thing that isn’t on the chip is a laser; that’s separate hardware, with its output channeled into the chip. Once the light reaches the chip, however, everything that handles that light has to be made of silicon. This includes waveguides to direct the light to specific locations, modulators to chop it up into bits, and detectors that can register those bits.
In this case, the laser was a light source at a wavelength of 1,180 nanometers. That’s a frequency where silicon is transparent, but modifications can be used to change its properties. For instance, a silicon-germanium blend can act as a photodetector, absorbing photons and converting them into electrical pulses.
Putting together a mixed electronic and optical chip required figuring out a whole series of similar approaches to work around limitations. In locations where the silicon waveguides leaked light into surrounding materials, for instance, the surrounding materials had to be etched away. The efficiency of the modulators varied with temperature, which varied with the chip’s workload. So researchers created a feedback system that detected falling light levels and triggered a resistance heater to crank up the local temperature as needed.