E. Coli

(L-R) Eduardo Temprana, ECE Ph.D. student and first author on the Science paper and co-author Nikola Alic.

Faster Fiber Optics

On June 26, 2015 the New York Times ran an article with the headline “An Advance May Double the Capabilities of Fiber Optics.” The story described Jacobs School research pub-lished in the journal Science. The next month, the same team of UC San Diego electrical en-gineers doubled those capabilities yet again. This work has the potential to significantly increase the data transmission rates and ener-gy efficiency of the fiber optic cables and sys-tems that we all rely on for the internet as well as our cable, wireless and landline networks.

“Today’s fiber optic systems are a little like quicksand,” said Nikola Alic, the project lead from electrical engineering professor Stojan Radic’s lab. “With quicksand, the more you struggle, the faster you sink. With fiber optics, after a certain point, the more power you add to the signal, the more distortion you get, in effect preventing a longer reach.”

The electrical engineers making these advances have figured out how to bypass “the quicksand effect” which has effectively capped how much power you can give to a fiber optic signal and still decipher it on the other end. This power limit caps the distance

information can travel in fiber optic cables.

“Our approach removes this power lim-it, which in turn extends how far signals can travel in optical fiber without needing a re-peater,” said Alic, who earned his electrical en-gineering Ph.D. at UC San Diego in 2006.

Repeaters, also known as electronic re-generators, are expensive and power-hungry systems that extend the reach of fiber optic signals.

In lab experiments at UC San Diego’s Qualcomm Institute, the electrical engineers successfully deciphered information after it travelled a record-breaking 12,000 kilometers through fiber optic cables with standard am-plifiers and no repeaters.

The breakthrough relies on wideband “frequency combs” developed in Radic’s Photonics Systems Group. The frequency combs described in their 2015 Science paper ensure that the signal distortions — called crosstalk —t hat arise between bundled streams of information travelling long distanc-es through the optical fiber are predictable, and therefore, reversible at the receiving end of the fiber.

“Crosstalk between communication chan-nels within a fiber optic cable obeys fixed physical laws. It’s not random. We now have a better understanding of the physics of the crosstalk,” explained Radic. This approach pre-compensates for the crosstalk that occurs between the multiple communication chan-nels within the same optical fiber.

"After increasing the power of the optical signals we sent by 20 fold, we could still re-store the original information when we used frequency combs at the outset,” said electrical engineering Ph.D. student Eduardo Temprana, the first author on the Science paper.

According to Alic, this work could be im-plemented in real-world fiber optics systems in just a few years. The last big challenge is to get the pre-compensation step running in real time. Right now, it’s being done offline.

Google supported this work through a research grant and Sumitomo Electric Industries provided fibers used in the ex-periments. Stojan Radic was recently named the Charles Lee Powell Professor in Wireless Communications at UC San Diego.

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