Optical Fiber Throughput Boosted Tenfold

A simple, innovative solution reduces the amount of space required between the pulses of light that transport data through an optical fiber, dramatically increasing their capacity.

Optical fiber can carry data as light pulses over thousands of miles at amazing speeds, but their capacity is limited because the pulses need to be spaced apart so they don’t interfere with each other. This leaves unused empty space inside the fiber.

Illustration of square-shaped light signals sent through an optic fiber for 10×-enhanced data throughput. Courtesy of Jamani Caillet/EPFL.

Ecole Polytechnique Fédérale de Lausanne’s (EPFL) Camille Brès of the Photonics Systems Laboratory and Luc Thévenaz of the Fiber Optics Group devised a method for fitting pulses together, reducing the space between them inside the fiber. Their approach makes it possible to use all the capacity in an optical fiber, opening the door to a tenfold increase in throughput in telecommunications systems.

“Since it appeared in the 1970s, the data capacity of fiber optics has increased by a factor of ten every four years, driven by a constant stream of new technologies,” Brès said. “But for the last few years we’ve reached a bottleneck, and scientists all over the world are trying to break through.”

Several different approaches to the problem have emerged, but they often require changes to the fibers themselves. That would mean existing systems would have to be pulled out and replaced. The EPFL team took a different approach, looking at the fundamental issue of how to process the light itself. By discovering how best to generate the pulses that carry the digital data, they could avoid the need to replace entire optical fiber networks — only the transmitters would need to be changed.

The EPFL team noticed that changes in the shape of the pulses could limit the interference. Their breakthrough is based on a method that can produce “Nyquist sinc pulses” almost perfectly.

“These pulses have a shape that’s more pointed, making it possible to fit them together, a little bit like the pieces of a jigsaw puzzle lock together,” Brès said. “There is of course some interference, but not at the locations where we actually read the data.”

The idea of putting pulses together like a puzzle to boost fiber throughput isn’t new. However, the “puzzle” had never been “solved” before: despite attempts using sophisticated and costly infrastructures, nobody had managed to make it work accurately enough — until now. The EPFL team used a simple laser and modulator to generate a pulse that is more than 99 percent perfect.

EPFL scientists Luc Thévenaz (left) and Camille Brès. Courtesy of Alain Herzog/EPFL.

The shape of a pulse is determined by its spectrum. In order to be able to generate the “jigsaw puzzle,” the spectrum needed to be rectangular. This means that all the frequencies in the pulse need to be of the same intensity. Brès and Thévenaz had this in mind when modulating their lasers.

Through a series of subtle adjustments using a frequency comb, the team generated pulses with an almost perfectly rectangular spectrum — the long-sought-after Nyquist sinc pulses.

The technology is already mature, 100 percent optic, relatively cheap and could fit on a chip.

“It almost seems too good to be true,” Thévenaz said.

The work was published in Nature Communications. (doi:10.1038/ncomms3898)

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