Fiber Optic Innovation Could Hold Key To Bandwidth Breakthrough: A Twist On Data Transmission

 @rpalmerscience
on June 27 2013 3:40 PM

A new design for fiber-optic cables puts a literal twist on data transmissions, an innovation that could lead to Internet connections with dramatically bigger bandwidth capacity.

Cables might seem invisible in the age of Wi-Fi, but they’re still essential to communication. Even though your smartphone or laptop connects to the Internet wirelessly, it still relies on fiber optics to transmit your tweets and texts long distances. These fibers are, essentially, “light pipes.” The signals traveling down their lengths are ones and zeros at heart but are encoded in pulses of light. Since the 1990s, we’ve increased our bandwidth capacity by encoding data in different colors of light, but now that strategy by itself is starting to reach the limit of its usefulness.

“What happened is that we ran out of colors to add,” Boston University engineer Siddharth Ramachandran explained in a phone interview.

Ramachandran, along with colleagues from the University of Southern California, Tel Aviv University and Danish fiber optics company OFS-Fitel, describe a new dimension for fiber-optic transmission in a paper appearing in the journal Science this week. Their new design can transmit data through a 1 kilometer (.62 mile) long cable at a rate of 1.6 terabits per second -- the equivalent of eight Blu-Ray DVDs every second. Google Fiber, by comparison, is on the gigabit level, about 1,000 times less capacious.

The secret to the speed lies in how the researchers manipulated the laser signal to form an optical vortex (also known as an orbital angular momentum carrying beam, or OAM beam), where the light moves in a spiral, rather than a straight beam akin to a laser pointer. Different vortices of different colors of light, each carrying its own unique data stream, can all be packed together into a donut-shaped beam of light that travels along the cable. Optical devices can then filter out individual vortices at the other end of the signal.

“It’s similar to radio,” coauthor and USC engineer Alan Willner said in a phone interview. “You have all these different frequencies traveling in the air, but your radio can pull out one signal.”

Last year, in the journal Nature Photonics, Willner and his team had described a way to use these helical twists of light to transmit data. But in their experiments, the signal traveled through free space over a short distance -- not an easy model to translate into modern communication networks. Maintaining a swirling yet stable vortex of light in a fiber was thought to be improbable -- until now.

To keep the light spinning, the researchers strategically coated parts of the inside of the fiber with certain elements -- germanium and fluorine, in particular. Those elements alter the refractive index within the cable, changing the angle at which the laser signal is reflected. Think of it as similar to the rifling inside of a gun barrel, except that, while the grooves inside a gun create the spin of a bullet, the fiber’s design works to maintain an already existing spin.

More technical refinements, certainly, will need to be made before vortex-bearing fiber optics can replace the old models wholesale. But there is one arena in which the new kind of fiber could be tried out without digging up a lot of roads: server farms. Big tech companies like Facebook and Google manage their gargantuan stores of data with clusters of computers; OAM-carrying beams could be a way for those servers to talk to each other lickety-split.

Still, eventually a much bigger overhaul will have to be made, either with Ramachandran and Willner’s cable or with some other alternative. And in a couple of decades, transmission rates measured in terabits per second may not be fast enough.

“How can we get to exabits?” Ramachandran asked. (An exabit is equivalent to about a million terabits.) “That’s where we think we need to go.”

Making large-scale improvements to deal with bandwidth demand will take time and manpower. But that was equally true when we were on the cusp of a gigabit network years ago. Willner remembers a joke that Ivan Kaminow, a former colleague from his Bell Labs days, told when people fussed over the time it would take to upgrade: “How come God could create the world in six days? Because he didn’t have to worry about the embedded base.”

SOURCE: Bozinovic et al. “Terabit-Scale Orbital Angular Momentum Mode Division Multiplexing in Fibers.” Science 340: 1545-1548, 28 June 2013.

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