Breakthrough could make fibre laying easier over long distance

Broadband fibre
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2 July 2015 | 0

One of the big problems with fibre has always been that the more power you add to a signal — to get the signal to travel further — the more distortion you get. That distortion degrades the quality of the data.

It is a problem because ideally you want fibre to travel long distances. It is cheaper to run one long cable than multiple pieces with added repeaters, as is the current process now. Scientists reckon they’ve got a solution, though.

Researchers at the University of California San Diego say they have been able to send data 12,000 kilometres along fibre without repeaters and still decipher the information at the end. They did use standard amplifiers, however.

Nevertheless, they say their experiment proves expensive regenerating repeaters can be eliminated. They accomplished this through distortion-reversing technology.

The photonics group published their findings in research journal Science.

Predictable crosstalk
It is a remarkably simple concept. The researchers reckon that the signal distortion, caused by crosstalk, is predictable. Therefore, it should be “reversible at the receiving end of the fibre,” the university says in an article on its web site.

Those radio-oriented scientists are working on self-interference cancellation in order to make better use of spectrum. The idea is that if you can second-guess how a transmission is affected by its own interference, you can cancel it and eliminate it.

Noise-cancelling headphone technology has used this basic concept for years. The headphone listens to unwanted noise and mimics it out of phase, thus cancelling it.

Wideband comb
In the case of the fibre crosstalk elimination, a wideband frequency comb reverses the crosstalk in the same optical fibre.

“Today’s fibre optic systems are a little like quicksand. With quicksand, the more you struggle, the faster you sink,” says Nikola Alic, another research scientist from the Qualcomm Institute and an author on the group’s Science paper.

With traditional “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,” Alic says.

The new findings, if implemented, might solve that and could eliminate the need for the electronic regenerators, also known as repeaters, placed periodically along the fibre link. That’s a major cost.

Crosstalk is the interference that occurs between channels in the cable. The signal transmitted in one circuit creates an effect in another — it spills over. The researchers say that they know what the effect is, so they can cancel it out and thus eliminate the distortion.

Pre-distorted waveforms
The photonics group is using physics to achieve the results. Pre-distorted waveforms are printed on frequency-comb-originating carriers at the beginning. The uncorrupted information is then restored at the output of the optical fibre.

“We are pre-empting the distortion effects that will happen in the optical fibre,” says Bill Kuo, a research scientist at the Qualcomm Institute. Kuo was responsible for the comb development and is an author on the group’s Science paper.

Cancelling tech
Cancelling technology is becoming more prevalent in general. Some scientists are experimenting with algorithms to cancel self-interference in radios.

In that case, it is cheaper processing power, which for the first time allows more complicated mathematical calculations and formulae to be run. Algorithms are becoming more accessible.



Patrick Nelson, IDG News Service

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