Largest, Fastest Array of Microscopic ‘Traffic Cops’ for Optical Communications

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Data centers — where our photos, videos and documents saved in the cloud are stored — are composed of hundreds of thousands of servers that are constantly sending information back and forth. Electrical switches act as traffic cops, making sure that information sent from one server reaches the target server and doesn’t get lost along the way.

But as data transfer rates continue to grow, we are reaching the limits of what electrical switches can handle, Wu said.

“Electrical switches generate so much heat, so even though we could cram more transistors onto a switch, the heat they generate is starting to pose certain limits,” he said. “Industry expects to continue the trend for maybe two more generations and, after that, something more fundamental has to change. Some people are thinking optics can help.”

Server networks could instead be connected by optical fibers, with photonic switches acting as the traffic cops, Wu said. Photonic switches require very little power and don’t generate any heat, so they don’t face the same limitations as electrical switches. However, current photonic switches cannot accommodate as many connections and also are plagued by signal loss — essentially “dimming” the light as it passes through the switch — which makes it hard to read the encoded data once it reaches its destination.

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 uses a technique called photolithography to etch the switching structures into silicon wafers. The researchers can currently make structures in a 240-by-240 array — 240 light inputs and 240 light outputs — with limited light loss, making it the largest silicon-based switch ever reported. They are working on perfecting their manufacturing technique to create even bigger switches.

“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,” Wu said. “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.”

Co-lead authors on the paper are Tae Joon Seok of the Gwangju Institute of Science and Technology and Kyungmok Kwon, a postdoctoral researcher and Bakar Innovation Fellow at UC Berkeley. Other co-authors are Johannes Henriksson and Jianheng Luo of UC Berkeley.

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