Electro-optical Switch Transmits Data at Record-Low Temperatures

A silicon optical switch developed at Sandia National Laboratories is the first to transmit up to 10 gigabits per second of data at temperatures just a few degrees above absolute zero. The device, according to researchers there, could enable data transmission for next-generation superconducting computers that store and process data at cryogenic temperatures. Such supercomputers are still experimental, but could potentially offer computing speeds ten times faster than today’s computers while significantly decreasing power usage. The switch could also prove useful for transmitting data from instruments used in space, where power is limited and temperatures vary widely.

“Making electrical connections to systems operating at very cold temperatures is very challenging, but optics can offer a solution,” says Michael Gehl, lead researcher at Sandia. “Our tiny switch allows data to be transmitted out of the cold environment using light traveling through an optical fiber, rather than electricity.”

Engineers fabricated the device with standard techniques used to make CMOS computer chips, which means it can be integrated onto chips containing electronic components. Gehl’s team reported on its research in Optica , the Optical Society’s open-access journal for high impact research, describing the new silicon micro-disk modulator and showing how it transmits data in environments as cold as 4.8 K.

“This is one of the first examples of an active silicon optical device operating at such a low temperature,” says Gehl. “Our device could potentially revolutionize technologies that are limited by how fast you can send information in and out of a cold environment electrically.”

For low-temperature applications, optical methods provide several benefits over electrical data transmission because electrical wires conduct heat and often introduce heat into systems that need to stay cold. Optical fibers, on the other hand, transmit almost no heat, and a single optical fiber can also transmit more data at faster rates than an electrical wire, meaning that one fiber can do the job of many electrical connections.

The Sandia micro-disk modulator requires very little power to operate—around 1,000 times less power than today’s commercially available electro-optical switches—which also helps reduce the heat the device contributes to the cold environment.

To make the new device, the researchers fabricated a small silicon waveguide (used to transmit light waves) next to a silicon micro-disk only 3.5 microns in diameter. Light coming through the waveguide moves into the micro-disk and travels around the disk rather than passing straight through the waveguide. Adding impurities to the silicon micro-disk creates an electrical junction to which a voltage can be applied. The voltage changes the material’s properties in a way that stops the light from moving into the disk and allows it to instead pass through the waveguide. This means that the light signal turns off and on as the voltage switches on and off, providing a way to turn the ones and zeroes that make up electrical data into an optical signal.

Although other research groups have designed similar devices, Gehl and his colleagues are the first to optimize the amount of impurities used and the exact placement of those impurities to allow the micro-disk modulator to operate at low temperatures. And the approach could be used to make other electro-optical devices that work at low temperatures.

To test the micro-disk modulator, the researchers placed it inside a cryostat. The micro-disk modulator then converted an electrical signal sent into the cryostat to an optical signal. The researchers examined the optical signal coming out of the cryostat to measure how well it matched the incoming electrical data.

The researchers operated their device at room temperature, 100 K and 4.8 Kelvin with various data rates up to 10 gigabits per second. Although they observed a slight increase in errors at the highest data rate and lowest temperature, the error rate was still low enough for the device to be useful for transmitting data.

This work builds on years of effort to develop silicon photonic devices for optical communication and high performance computing applications, led by the Applied Photonics Microsystems group at Sandia. As a next step, the researchers want to demonstrate that the device works with data generated inside the low temperature environment, rather than only electrical signals coming from outside the cryostat.

Optica is an open-access, online-only journal dedicated to the rapid dissemination of high-impact peer-reviewed research across the entire spectrum of optics and photonics. Published monthly by The Optical Society (OSA), Optica provides a forum for pioneering research to be swiftly accessed by the international community, whether that research is theoretical or experimental, fundamental or applied.