Researchers send signals over 12,000km of fibre

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Photonics researchers at the University of California, San Diego claim they have successfully deciphered information after it travelled 12,000km through fibre optic cables equipped only with standard amplifiers. The work is said to effectively eliminate the need for regenerators along the fibre link.

"With fibre 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 without needing a repeater," said Nikola Alic, a Qualcomm Institute scientist and one of the leaders of the research.

The breakthrough relies on wideband 'frequency combs' developed by the researchers. These combs ensure that crosstalk between the information travelling through the fibre is predictable and therefore reversible at the receiving end.

"After increasing the power of the optical signals we sent by 20 times, we could still restore the original information when we used frequency combs at the outset," said Doctoral electrical engineering Eduardo Temprana, The frequency comb ensured the system did not accumulate random distortions that make it impossible to reassemble the original content.

"Crosstalk between communication channels within a fibre optic cable obeys fixed physical laws: it's not random. We now have a better understanding of the physics of crosstalk," said Professor Stojan Radic from UC San Diego. "Our approach conditions the information before it is sent, so the receiver is free of crosstalk caused by the Kerr effect."

The experiments involved three and five optical channels, which interacted with each other within silica fibre optic cables. The researchers believe their approach could be used in systems with far more channels.

Kerr Effect
The optical Kerr Effect occurs when light generated by a laser travels down a fibre, creating an electric field. This causes a variation in that fibre's index of refraction, bringing with it a range of nonlinear optical effects.