If you think your internet is slow, spare a thought for the estimated 4 billion people on the planet who have no access at all. Rather than the costly process of building towers and burying cables, Facebook is taking the internet to the sky to get those people online, much like Google's Project Loon. After last year unveiling the drones that would beam data-laden lasers down to remote areas, the social networking giant's Connectivity Lab has now solved the problem of how those beams of light are received, achieving speeds of up to 2 Gbps.
Transmitting information wirelessly using light, or free-space optical communication, has the potential for very high bandwidths and data capacity. The problem is, it's a precise science at the best of times, and aiming a tiny laser at a tiny photodetector across vast distances is no easy task, especially when the laser is moving. Using mechanical systems to reorient the detector so the laser hits it is one solution, but it's not an elegant one.
Instead, the Connectivity Lab team designed a lightbulb-shaped light collector, made up of a bundle of fluorescent, plastic optical fibers. With a surface area of 126 sq cm (19.5 sq in), the unique shape of the device allows it to collect light from any angle, which it then concentrates onto a small photodetector. The fibers also contain organic dye molecules that absorb blue light but emit green.
"The fact that these fluorescent optical fibers emit a different color than they absorb makes it possible to increase the brightness of the light entering the system," says research lead, Tobias Tiecke. "This approach has been used in luminescent concentrators for solar light harvesting, where the speed of the color conversion doesn't matter. We showed that the same concept can be used for communication to circumvent pointing and tracking problems while accomplishing very high speeds."
The high speeds in question can reach up to 2 Gbps, which is even more impressive when you consider that the system only has a bandwidth of 100 MHz. The speeds are thanks to the use of orthogonal frequency division multiplexing (OFDM), which is a method of encoding data that allows multiple streams to be transmitted simultaneously. The fact that it takes less than two nanoseconds for the device to absorb blue light and emit green light helps as well.
The researchers say blistering speeds of up to 10 Gbps may be possible in future, if the system can be adapted to incorporate materials that work with infrared wavelengths.
"We achieved such high data rates using commercially available materials that are not designed for communications applications," says Tiecke. "We want to get other groups interested in developing materials that are tailored for communications applications. This is a very new system, and there is a lot of room for future development."
The research was published in the journal Optica.
Source: The Optical Society
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