The photonic switch

Large Array of Fast Photonic Switches Will Serve as “Traffic Cop” for Fiber-Based Data Centers

May 9, 2019
The array includes 50,000 switches, each capable of handling 240 light beams.

Engineers at the University of California, Berkeley have built a new photonic switch that can control the direction of light beams passing through optical fibers faster and more efficiently than ever. This optical “traffic cop” could one day revolutionize how information travels through data centers and high-performance supercomputers used for AI and other data-intensive applications.

The photonic switch consists of more than 50,000 microscopic light switches, each of which directs one of 240 tiny beams of light to either make a right turn when the switch is on, or to pass straight through when the switch is off. The 240-×-240 array of switches is etched into a silicon wafer and covers an area only slightly larger than a postage stamp.

“For the first time in a silicon switch, we are approaching the large switches people can only build using bulk optics,” says UC professor Ming Wu. “This switch is not only not only large, but it is 10,000 times faster, so it can switch data networks in ways not many people have thought about.”

The photonic switch is made using photolithography, in which each “light switch” structure is etched into a silicon wafer. Each light gray square on the wafer contains 6,400 of these switches.

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 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.

Data centers—where photos, videos, and documents are stored in the cloud—are composed of hundreds of thousands of servers that constantly send 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, says Wu.

“Electrical switches generate so much heat that even if we could cram more transistors onto a switch, the heat they generate would pose certain limits,” he says. “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.”

Each individual “light switch” is constructed like a microscopic highway overpass. When the switch is off, light passes straight through a lower channel (red lines). Turning the switch on lowers a tiny ramp, directing the light to an upper channel to make a right turn (blue lines). A second ramp lowers the light back down.

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

In the new switch from Berkley, beams of light travel through a crisscrossing array of nanometer-thin channels until they reach 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 deg. Another ramp lowers the light back into a perpendicular channel.

 “It’s literally like a freeway ramp,” Wu says. “All the light goes up, makes a 90-deg. turn, and then goes back down. This is an efficient process—more efficient than what everybody else is doing on silicon photonics. It is this mechanism that lets us make low-loss switches.”

The photonic switch is built with more than 50,000 microscopic “light switches” etched into a silicon wafer. Each light switch (small raised squares) directs one of 240 tiny beams of light to either make a right turn when the switch is on, or to pass straight through when the switch is off.

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

“Larger switches that use bulk optics are commercially available, but they are slow, so they are usable in networks you don’t change too frequently,” Wu said. “Now, computers work quickly, so if you want to keep up with the computer speed, you need much faster switch responses. Our switch is the same size, but much faster, so it will enable new functions in data center networks.”

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