A team of UC Berkeley researchers has reported fabricating the “largest integrated photonic switch ever.” Their achievement comes at a time when there are many R&D efforts seeking ways to improve and expand the use of optical interconnect on-chip and at the system level.
“For the first time in a silicon switch, we are approaching the large switches that people can only build using bulk optics,” said Ming Wu, professor of electrical engineering and computer sciences at UC Berkeley and senior author of the paper (Wafer-scale silicon photonic switches beyond die size limit), which appeared online April 11 in the journal Optica.
“Our switches are not only large, but they are 10,000 times faster, so we can switch data networks in interesting ways that not many people have thought about,” said Wu in an article posted on the UC Berkeley web site.
Here’s an excerpt from the paper’s abstract: “[W]e propose to use wafer-scale integration to overcome the die size limit. As a proof of concept demonstration, we fabricated a 240×240 switch by lithographically stitching a 3×3 array of identical 80×80 switch blocks across reticle boundaries. Stitching loss is substantially reduced (0.004 dB) by tapering the waveguide width to 10μm. The fabricated switch on a 4 cm×4 cm chip exhibits a maximum on-chip loss of 9.8 dB, an ON/OFF ratio of 70 dB, and switching times of less than 400ns. To our knowledge, this is the largest integrated photonic switch ever reported.”
Wu noted in UC Berkeley article, “Larger switches that use bulk optics are commercially available, but they are very slow, so they are usable in a network that you don’t change too frequently. Now, computers work very fast, so if you want to keep up with the computer speed, you need much faster switch response. Our switch is the same size, but much faster, so it will enable new functions in data center networks.”
Currently, the only photonic switches that can control hundreds of light beams at once are built with mirrors or lenses that must be physically turned to switch the direction of light. Each turn takes about one-tenth of a second to complete, which is eons compared to electronic data transfer rates. The new photonic switch is built using tiny integrated silicon structures that can switch on and off in a fraction of a microsecond, approaching the speed necessary for use in high-speed data networks.
In the new photonic switch, beams of light travel through a crisscrossing array of nanometer-thin channels until they reach these individual light switches, each of which is built like a microscopic freeway overpass. When the switch is off, the light travels straight through the channel. Applying a voltage turns the switch on, lowering a ramp that directs the light into a higher channel, which turns it 90 degrees. Another ramp lowers the light back into a perpendicular channel.
“It’s literally like a freeway ramp,” Wu said. “All of the light goes up, makes a 90-degree turn and then goes back down. And this is a very efficient process, more efficient than what everybody else is doing on silicon photonics. It is this mechanism that allows us to make lower-loss switches.”
The team used photolithography to etch the switching structures into silicon wafers. While the researchers can currently make structures in a 240-by-240 array, they are working on perfecting their manufacturing technique to create even bigger switches.