A fully functioning optoelectronic microprocessor has been produced in a joint venture by engineers at MIT, Berkeley and the University of Colorado. The new chip computes electronically, uses light to move the information and can be produced using current manufacturing infrastructure.
The sensitivity of the chip’s light detectors could drop the energy cost of transmitting data down to roughly a picojoule, roughly one-tenth of what all-electronic chips need to operate, even over short distances.
The chip consists of 850 optical components and 70 million transistors. Everyday microprocessors need around a billion transistors to operate, but the chip still had enough transistors to demonstrate its functionality.
In a comparison of overall performance, the team found that the transistors on the optical microprocessor were virtually identical to those in traditional electronics.
A silicon-on-insulator procedure was used to manufacture the chip. This process layers silicon with insulating glass.
Waveguides, the optical components that guide the light, were built on top of a silicon wafer and separated by a thin layer of glass. Once in place, the researchers slowly cut away at the silicon underneath. The light is contained within the waveguide due to the different refractive indexes between the light and silicon.
Traditionally, data is sent to and from logic circuits via electricity. To take advantage of how quickly these logic circuits work, they need to be supplied with more data. Supplying more data to the circuits requires more power to increase the bandwidth of the electrical connections. This raises the operating temperatures to levels that could damage the electronics.
One energy efficient solution is optical data connections. As the distance between transistors (and concordantly bandwidth) increases, the power requirements for optical data connections scale much slower than traditional electronics. Processors that are meters apart could be linked, with minimal loss in performance and increase of power consumption.
An ongoing problem with transistors is that they still need to conduct electrical power which can limit optical transmission. This electrical conductivity requires free charge carriers. Unfortunately, these carriers have a tendency to absorb light particles.
Another problem with optical data transmission occurs when the optical signal is converted into an electrical one. This requires light to contact the metal in the processor, which disrupts data transmission. To avoid this issue, the researchers fashioned an optical component called a ring resonator. The metal is patterned onto a doughnut shape within an optical component.
When the ring comes into contact with electricity it either modifies the optical properties of the resonator or it registers changes in a data-carrying light signal. Ultimately, this lets it go back and forth between optical and electrical signals without interfering with the data transmission.
For more information about this process, visit MIT’s webpage.