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Today's optical fibre systems have twice the theoretical capacity currently attributed to them, according to research from the University of Tel Aviv.

The study, published on Arxiv, is part of the widespread academic interest in studying the channel capacity of fibre optic systems. Over long distances, and particular where wavelength division multiplexing is used, fibre systems run into constraints caused by noise, and channel capacity falls as distance increases.

An accurate prediction of systems' theoretical maximum capacity is of keen practical interest to system owners and engineers, since it feeds into investment decisions such as “do we start designing a new submarine cable system, or will we be able to get more capacity out of the old fibre in five years?” As the researchers note, the world's hunger for capacity is growing so fast that “the latest capacity estimates are being rapidly approached by the rates of commercial communication systems.”

WDM is a particular problem because of a non-linear effect called cross-phase modulation, or XPM, one of the greatest sources of noise in WDM systems. As the paper puts it, nonlinear effects in the optical fibre “generates complicated distortions of the transmitted optical waveforms.”

In a perfect world, an optical signal on wavelength A would not affect the transmission on wavelength B, but in XPM, the distortion that each wavelength experiences shows up as noise on the other wavelength and eats into its capacity.

The group, comprising Ronen Dar, Mark Shtaif and Meir Feder of the University of Tel Aviv, suggests that the noise itself can be used to improve the capacity of the systems. What they've found is that the XPM shows “strong temporal correlations” that can be used to create filters to eliminate it.

By taking advantage of those temporal correlations to analyse the channel noise, they say, the peak capacity of each channel on a WDM system can be increased by 1 bit/s per Hertz, “equivalent to doubling the SNR [signal-to-noise ratio – editor] in a linear system.

“Equivalently, we showed that the length of a system can be doubled for a given transmission rate,” the paper concludes.

Putting the paper's extensive mathematics into practise would, The Register assumes, mean designing end-systems that take advantage of this effect. ®

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