Prototype optoelectronic microprocessors that computes electronically but uses light to move information. Image courtesy of Glen Asakawa"Any data intensive computing application can benefit from this technology"Rajeev Ram
Prototype optoelectronic microprocessors that can compute electronically but use light to move information have been designed by scientists at MIT, the University of California at Berkeley and the University of Colorado in the US. The chips consist of 850 optical components and 70 million transistors, substantially less than the billion-odd transistors of a standard microprocessor but sufficient to show the necessary functionality of a commercial optical chip. The technology could find uses beyond computing and communications, such as in imaging, sensing and even quantum information applications.
The system uses on-chip photonic devices to directly communicate with other chips using light. Such optical communication could significantly lower the power consumption of microchips to help them keep up with the continuous increase in computing power. The new chip is based on a silicon-on-insulator process so that layers of silicon are insulated by layers of glass, while the team built waveguides on top of a thin layer of glass on a silicon wafer, before etching away the silicon underneath, with the difference in refractive index between the silicon and the glass containing light traveling through the waveguides.
To integrate electronics and photonics on this scale, they used a ‘zero-change’ approach to the integration of photonics so, rather than developing a custom process to allow the fabrication of photonics, they designed optical devices using a standard microelectronics foundry process. As reported in Nature [Sun et al. Nature (2015) DOI: 10.1038/nature16454], the new operational microprocessors could lead to optoelectronic chips being produced on a commercial basis. As team leader Rajeev Ram said, “Any data intensive computing application can benefit from this technology”.
For optoelectronic chips, light signals have to be converted to electricity somewhere in the process, while the contact with metal interferes with optical data transmission. Here, they showed how to pattern metal onto the inner ring of a ring resonator. While the metal does not interact with light traveling around the outer ring, upon a voltage being applied to, it either modifies the optical properties of the resonator or register changes in a data-carrying light signal, allowing it to move between optical and electrical signals. Light detectors developed from these ring resonators are sensitive enough to lower the energy cost of transmitting information down to around a tenth of that of all-electronic chips, even over short distances.
The research helps resolve questions about whether photonics can function in the challenging thermal and electrical environment of a microprocessor, while next generation systems using such devices could lead to computers with unprecedented energy efficiency and speed.