Facebook's Connectivity Lab uses light collector to achieve data rates of more than 2 Gbps

Researchers from Facebook's Connectivity Lab said they've demonstrated a conceptually new approach for detecting optical communication signals traveling through the air – and it could have implications for delivering internet service to far-flung places around the world.

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Researchers transmitted
more than 2 Gbps using
OFDM. (Source: Facebook)

The team described the new technology in Optica, The Optical Society's journal for high impact research. Tobias Tiecke, who leads the research team, said the technology is optimized for areas where people live far apart from one another.

So, just how does it work? It involves a method for using fluorescent materials instead of traditional optics to collect light and concentrate it onto a small photodetector. The research team combined this light collector, which features 126 square centimeters of surface that can collect light from any direction, with existing telecommunications technology to achieve data rates of more than 2 Gbps.

"We demonstrated the use of fluorescent optical fibers that absorb one color of light and emit another color," Tiecke explained in a press release. "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."

They're able to achieve fast speeds because fewer than 2 nanoseconds lapse between the blue light absorption and the green light emission. In addition, by incorporating orthogonal frequency division multiplexing (OFDM), the researchers transmitted more than 2 Gbps despite the system's bandwidth of 100 MHz. OFDM is a method of encoding digital data so that multiple data streams can be transmitted at once. It's commonly used for wired and wireless communication, but it isn't typically used with laser communication.

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

The new light collector uses plastic optical fibers containing organic dye molecules that absorb blue light and emit green light. Such a setup replaces the classical optics and motion platform typically required to point the light to the collection area.

"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," Tiecke said. "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."

If materials were developed that operate in the infrared part of the spectrum, which would be invisible to people, and were even faster than the blue/green light system, the new approach could theoretically allow free-space optical data rates of more than 10 Gbps, according to Tiecke.

In the Optica paper, the researchers demonstrated a light-bulb shaped light collector made from a bundle of fluorescent optical fibers. The light-bulb shape offers a large bandwidth and omnidirectional sensitivity, which means it would work with mobile devices that move around with respect to the transmitter. The researchers also demonstrated that this geometry can gather light from an area as large as 126 square centimeters, making it less sensitive to alignment. 

Besides working with partners to develop new materials, the research team plans to move the technology out of the lab by developing a prototype that could be tested in a real-world situation. "We are investigating the feasibility of a commercial product," said Tiecke. "This is a very new system, and there is a lot of room for future development."

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