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25 July 2016

Facebook Moves One Step Closer to Light-Based Wireless Communication

With a light bulb design, a team of researchers built a
detector that can be used for light-based wireless
communication.  
Credit: Facebook Connectivity Lab
Facebook Moves One Step Closer to Light-Based Wireless Communication
by Steph Yin, The New York Times, 
21 July 2016

The internet is often called the “World Wide Web,” but it’s not actually accessible to residents of a large portion of the world. Today, four billion people are offline, and 1.6 billion of them live in sparsely populated areas around the world.

In recent years, a race to solve that problem has emerged among big tech companies like Google, SpaceX and Facebook. Now, Facebook has published research on an unconventional solution: using light to wirelessly transmit internet signals. The work comes from a Facebook-led initiative called Internet.org, which, according to the initiative's website, has so far brought internet access to more than 25 million people.

Most internet signals today are transmitted at high rates through wired optical fiber networks — which require expensive infrastructure — or at lower rates through wireless radio frequencies, which are limited in bandwidth, subject to regulations and vulnerable to interception.

In a paper published Tuesday in Optica, researchers from Internet.org’sConnectivity Lab have outlined a new type of light detector that can be used for free-space optical communication, a communication technique that uses light to send data wirelessly.

“It’s a completely new design,” said Julian Cheng, a communications engineer at the University of British Columbia who was not involved in the study. With a device that’s simpler than traditional light receivers, he said, the Facebook team was able to achieve wireless data rates on the order of wired systems.

Free-space optical communication works by encoding communication signals in laser beams. Transmitters on the ground or in satellites shoot that light through the air to receivers that can decode the data. (To understand this on simple terms, think of encoding and sending information through morse code using a flashlight.)

One longstanding obstacle to free-space optical communication is a trade-off between speed and size. To increase the number of laser signals hitting a receiver, one can increase the size of the receiver. But doing so makes the receiver slower.

Instead, many free-space optical communication systems use smaller receivers with complex pointing and tracking systems. Because laser beams are narrow and travel in straight lines from point A to point B, these receivers have to continuously maneuver to catch laser beams head-on.

Imagine trying to water a small potted plant with a water gun from different angles, said Alan Willner, an optical communications scientist at the University of Southern California and president of the Optical Society, the professional society that published Internet.org’s paper. To maximize the amount of water you catch, you have to constantly move the pot around.

The Facebook researchers’ solution to this problem is a light detector that doesn’t need pointing and tracking, but still allows for fast transmission. To do this, they took advantage of fluorescence, the process of absorbing light and re-emitting it at a lower energy.

Facebook’s detector contains a spherical bundle of special fluorescent fibers. The bundle, somewhere between the size of a golf ball and tennis ball, is able to absorb blue laser light from any direction and re-emit it as green light. Because that green light is diffuse, it can then be funneled to a small receiver that converts the light back to data.

In our hypothetical example, imagine that instead of a water gun, you’re pointing a blow dart gun at a water balloon attached to a funnel over the potted plant. As soon as you hit the balloon, it pops and releases water. With the addition of the balloon, you’ve eliminated the need to move the pot around. You can shoot at the water balloon from any direction, and the plant will get watered.

Facebook’s new detector is able to achieve fast data rates of two gigabits per second — several orders of magnitude higher than those from radio frequencies — because light has a higher frequency than radio waves, and because the fluorescence process is fast. Free-space optical communication can also carry more information than radio communication, and is more secure because narrow laser beams are harder to intercept than wide radio waves.

Because of all these benefits, building on Facebook’s proof of concept holds tremendous potential not just for remote areas, but for meeting increased data demand all over the world, said Kamran Kiasaleh, an optical communications scientist at the University of Texas at Dallas who was not involved in the study.

The technology fits in with Facebook’s plans to beam internet access down from the skies using drones. With laser transmitters and arrays of these light detectors, drones could exchange data with one another and with ground stations. Indoors, these detectors could provide high data rates to mobile devices.

For now, the new light detector is still in early stages, said Tobias Tiecke, a communication systems scientist at Facebook and a co-author of the new paper. His team built their current device using off-the-shelf materials that are used for harvesting solar light.

“The performance of our system can still be increased much further by developing materials tailored for communication,” he said.

http://www.nytimes.com/2016/07/22/science/facebook-wireless-light-internet.html

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