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Raving mad: Glow sticks are secret weapon in Facebook's 2.1Gbps laser internet drones

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At Facebook's F8 Developer Conference this year, Mark Zuckerberg revealed more details about his laser-firing drones that will encircle the world and relay Facebook, sorry, the internet to far-flung places, reaching potentially all seven billion of us.

The first solar-powered Aquila autonomous drones to do that job have now been built and are undergoing flight testing. However, Facebook's skunkworks squad has been quiet about the hush-hush laser communications system. Now they've published a paper [PDF] in the journal Optica detailing how it will work.

Lasers hold a lot of promise for this kind of data linkage, but have some problems too – chiefly the optical pickup to receive the signal. As lasers travel through the atmosphere they "bloom," spreading out and diffusing. This degrades the quality of the signal, reducing the maximum possible bandwidth of the system.

You could use equipment to tightly focus the beam to mitigate the bloom effect, but that's impractical on an ultra-light solar-powered drone. So instead the team have turned to fluorescent materials to catch the incoming light. Glow stick-looking fluorescent fibers collect and channel the received signal into a detector to convert it into electrical signals and ultimately network packets.

"We demonstrated the use of fluorescent optical fibers that absorb one color of light and emit another color," said Tobias Tiecke, who leads the research team.

"The optical fibers absorb light coming from any direction over a large area, and the emitted light travels inside the optical fiber, which funnels the light to a small, very fast photodetector."

The receiver uses plastic optical fibers coated in dye molecules that absorb blue light and emit green light two nanoseconds later. This light then shines through the fibers and is directed down to a photodetector in the base of the unit.

The test receptor is shaped like a light bulb, to give 360-degree coverage. In tests, the team was able to get data rates up to 2.1Gbps using a receptor area that was 126cm squared, but Tiecke said that rates of up to 10Gbps were theoretically possible.

"We achieved such high data rates using commercially available materials that are not designed for communications applications," he said. "We want to get other groups interested in developing materials that are tailored for communications applications."

In case you're wondering how many people around the world are going to look up at a drone and get blinded, there's no need to worry. The receptor's light-collecting ability means the laser beams used aren't powerful enough to damage retinas. ®

PS: You can read about Aquila's first test flight here by Jay Parikh, Facebook's global head of engineering and infrastructure.

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