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UCSD team breaks power, distance barriers for optical comms

08 Jul 2015

Frequency comb ensures crosstalk between multiple comms channels within the same fiber is reversible.

Photonics researchers at the University of California, San Diego (UCSD) have increased the maximum power — and therefore distance — at which optical signals can be sent through optical fibers. They say that this advance has the potential to increase the data transmission rates for the fiber optic cables that serve as the backbone of the internet, and cable, wireless and landline networks. The research was recently published in Science.

“Today’s fiber-optic systems are a little like quicksand. 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. Our approach removes this power limit, which in turn extends how far signals can travel in optical fiber without needing a repeater,” said Nikola Alic, a research scientist from the Qualcomm Institute, the corresponding author on the Science paper and a principal of the experimental effort.

In lab experiments, the UCSD researchers successfully deciphered information after it had travelled a record-breaking 12,000 km through fiber optic cables with standard amplifiers and no repeaters.

The team says that the new findings, “effectively eliminate the need for electronic regenerators placed periodically along a fiber link”. The electronic regeneration in modern lightwave transmission that carries between 80 to 200 channels also dictates the cost and, more importantly, prevents the construction of a transparent optical network. As a result, eliminating periodic electronic regeneration will drastically change the economy of the network infrastructure, ultimately leading to cheaper and more efficient transmission of information.

Predictable crosstalk

The breakthrough in this study relies on wideband “frequency combs” that the researchers developed. The frequency comb ensures that the signal distortions —“crosstalk” — that arise between bundled streams of information travelling through the optical fiber are predictable, and therefore, reversible at the receiving end of the fiber.

“Crosstalk between communication channels 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. In this study, we present a method for leveraging the crosstalk to remove the power barrier for optical fiber,” said Stojan Radic, a professor in the Department of Electrical and Computer Engineering and the senior author on the Science paper. “Our approach conditions the information before it is even sent, so the receiver is free of crosstalk caused by the Kerr effect.”

The photonics experiments were performed at UCSD’s Qualcomm Institute by researchers from the Photonics Systems Group led by Radic. The electrical engineers used their frequency comb to synchronize the frequency variations of the different streams of optical information, called the “optical carriers” propagating through an optical fiber. This approach compensates in advance for the crosstalk that occurs between the multiple communication channels within the same optical fiber.

Power boost

“After increasing the power of the optical signals we sent by 20-fold, we could still restore the original information when we used frequency combs at the outset,” said Eduardo Temprana, first author on the paper and a UCSD electrical engineering PhD student. “The frequency comb ensured that the system did not accumulate the random distortions that make it impossible to reassemble the original content at the receiver.”

The laboratory experiments involved setups with both three and five optical channels, which interact with each other within the silica fiber optic cables. The researchers note that this approach could be used in systems with far more communication channels. Most of today’s fiber optic cables include more than 32 of these channels, which all interact with one another.

In the Science paper, the researchers describe their frequency referencing approach to pre-compensate for nonlinear effects that occur between communication channels within the fiber optic cable. The information is initially pre-distorted in a predictable and reversible way when it is sent through the optical fiber. With the frequency comb, the information can be unscrambled and fully restored at the receiving end of the optical fiber.

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