Infrared Links Could Replace Wires in Data Centers

A team of engineers is proposing to eliminate most of the wires in data centers and instead use infrared free-space optics for communications.

An infrared laser beam is going into the receiver of the signaling system. Courtesy of Patrick Mansell, Penn State.

The Free-space optical Inter-Rack nEtwork with high FLexibilitY — known as Firefly — architecture is a joint project of Penn State, Stony Brook University and Carnegie Mellon University.

Firefly uses infrared lasers and receivers mounted on top of data center racks to transmit information. The laser modules are rapidly reconfigurable to acquire a target on any rack.

Mohsen Kavehrad, W. L. Weiss Chair Professor of Electrical Engineering at Penn State, said human interference is minimal because the racks are more than 6.5 feet high and most workers can walk between the rows of racks without breaking the laser beams.

"It uses a very inexpensive lens, we get a very narrow infrared beam with zero interference and no limit to the number of connections with high throughput," said Kavehrad.

While fiber-optic cabling and energy expenditure for idle servers are problems, throughput is more critical. When hundreds of cables merge into a few, the data transfer bottlenecks that result reduce the speed at which the data center can deliver information. A flexible, configurable system could be the fix, reducing bottlenecks and decreasing the number of servers needed.

The engineers have created a simplified, proof-of-concept system to show that their infrared laser can carry the signal and target the receiver. They are transmitting wavelength division multiplexed — multiple signals sent by different colored lights — bi-directional data streams each carrying data at a transmission rate of 10 Gbps from a Bit Error Rate (BER) test set. BER testing determines the number of errors in a signal caused by interference, noise, distortion or synchronization problems.

The receiver captures the infrared signal and directs it to the fiber-optic cable which sends the information to its final destination. Courtesy of Patrick Mansell, Penn State.

The proof of concept setup has the bidirectional signal wavelength division multiplexed with a one-way cable television signal. The total data stream goes from fiber-optic cable to the infrared laser, across the room to the receiver and shows the results on a TV and the BER test set. A hand breaking the laser beam shuts off the system, but when the hand is removed, the signal is rapidly reacquired.

The system uses microelectromechanical systems (MEMS) with tiny mirrors for rapid targeting and reconfiguring. These MEMS use tiny amounts of electricity from four directions to reposition the mirror that targets the receiver. The movement of the mirrors is so small it is undetectable, but the computer program quickly locates the receiver and then narrows the target to pinpoint accuracy. The laser beam can also be rapidly moved to target a different receiver.

Two different microelectromechanical devices containing micromirrors are used to position an infrared laser beam to target a receiver and send information. Courtesy of Patrick Mansell, Penn State.

Accurately targeting and sending a signal via infrared laser are only two of the hurdles the researchers need to pass before Firefly is operational. Once the signal arrives at the target it must seamlessly enter the fiber-optic cable. Controlling and managing the data distribution system in an unwired environment is also important.

"We are trying to come up with something reconfigurable using light instead of millimeter waves (radio frequency)," said Kavehrad. "We need to avoid overprovisioning and supply sufficient capacity to do the interconnect with minimal switches. We would like to get rid of the fiber optics altogether."

The National Science Foundation supported the Firefly project.