Back in 2014 Facebook bought UK-based Ascenta to develop and build the unmanned aircraft that would provide the access from 60,000 ft in the air, but hasn’t been able to say how these craft would communicate with each other or with the ground. Now researchers at the Connectivity Lab have shown a new approach that would work over long distances providing over 2Gbit/s of data.
Instead of using optics to focus the light, the researchers have used plastic fibres doped with a fluorescent organic dye as the receptor. This can detect the light coming in from the free space link and convert it to a different frequency within the fibre. This signal can then be captured by a traditional high speed photomultiplier.
This light collector features 126 cm2 of surface that can collect light from any direction, and, crucially, doesn’t need to be steered. This provides a lightweight, simpler receiver.
“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 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,” he 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.”
The team found a light-bulb shaped light collector made from a bundle of the fluorescent optical fibers offered a large bandwidth and omnidirectional sensitivity, which means it would work with other aircraft and also with links to the ground. “Our detector absorbs the same amount of power and gets the same communication signal through independently of the alignment,” said Tiecke.
The fast speeds are possible because less than 2 nanoseconds lapse between the current absorption of blue light and the emission of green light in the fiber. Using orthogonal frequency division multiplexing (OFDM) modulation, the researchers transmitted more than 2 Gbit/s despite the system’s bandwidth of 100 MHz. While OFDM is commonly used for wired and wireless communication, it is not typically used with laser communication.
“We achieved such high data rates using commercially available materials that are not designed for communications applications,” said Tiecke. “We want to get other groups interested in developing materials that are tailored for communications applications.”
Using materials that work in the infrared spectrum, the new approach could theoretically allow free-space optical data rates of more than 10 Gbit/s, he said. In addition to working with partners to develop new materials for infrared operation, the team is also planning to move this 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.”
The work is published in Optica, an open source research journal: A Luminescent Detector for Free-Space Optical Communication