Monday, December 9, 2013

Ten times more throughput on optic fibers

December 9, 2013
A graphical illustration of light signals sent through an optic fiber for 10x-enhanced data throughput (credit: Jamani Caillet/EPFL)
EPFL scientists have shown how to achieve a dramatic increase in the capacity of optical fibers by reducing the amount of space required between the pulses of light that transport data.
Optical fibers carry data in the form of pulses of light over distances of thousands of miles at high speeds. But their capacity is limited, because the pulses of light need to be lined up one after the other in the fiber with a minimum distance between them so the signals don’t interfere with each other.
EPFL researchers have come up with a method for fitting pulses together within the fibers, thereby reducing the space between pulses. Their approach, published in Nature Communications (open access), opens the door to a ten-fold increase in throughput in telecommunications systems.
Extending exponential growth of fiber-optic capacity
“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,” says Camille Brès, of the Photonics Systems Laboratory (PHOSL). “But for the last few years we’ve reached a bottleneck, and scientists all over the world are trying to break through.”
There have been several different approaches to the problem of supplying more throughput to respond to growing consumer demand, but they often require changes to the fibers themselves. That would entail pulling out and replacing the existing infrastructure.
The problem with this system is that the volume of data transmitted at one time can’t be increased. If the pulses get too close together, they no longer deliver the data reliably.
The EPFL team took a different approach:  they noticed that changes in the shape of the pulses could limit the interference. Instead of replacing the entire optical fiber network, only the transmitters would need to be changed.
Their breakthrough is based on a method that can produce what are known as “Nyquist sinc pulses” almost perfectly. “These pulses have a shape that’s more pointed, making it possible to fit them together,” says Brès. “There is of course some interference, but not at the locations where we actually read the data.” The EPFL team used a simple laser and modulator to generate a pulse that is more than 99% perfect.
Simple lasers are generally made up essentially of just one optical frequency, with a very narrow spectrum. However, a laser can be subtly modulated (using a device called a modulator) so that it has other frequencies. The result is a pulse with a larger spectrum. The problem is that the pulse’s main frequency generally still tends to be stronger than the others. This means the spectrum won’t have the rectangular shape needed. For that, each frequency in the pulse needs to be of the same intensity.
So the team made a series of subtle adjustments based on a concept known as a “frequency comb” and succeeded in generating pulses with almost perfectly rectangular spectrum. This constitutes a breakthrough, the researchers say, since the team has succeeded in producing the long-sought-after Nyquist sinc pulses.
The technology is already mature, as well as 100% optic and relatively cheap. In addition, it appears that it could fit on a simple chip.

Abstract of Nature Communications paper
Sinc-shaped Nyquist pulses possess a rectangular spectrum, enabling data to be encoded in a minimum spectral bandwidth and satisfying by essence the Nyquist criterion of zero inter-symbol interference (ISI). This property makes them very attractive for communication systems since data transmission rates can be maximized while the bandwidth usage is minimized. However, most of the pulse-shaping methods reported so far have remained rather complex and none has led to ideal sinc pulses. Here a method to produce sinc-shaped Nyquist pulses of very high quality is proposed based on the direct synthesis of a rectangular-shaped and phase-locked frequency comb. The method is highly flexible and can be easily integrated in communication systems, potentially offering a substantial increase in data transmission rates. Further, the high quality and wide tunability of the reported sinc-shaped pulses can also bring benefits to many other fields, such as microwave photonics, light storage and all-optical sampling.

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