20 Jul 2016
With data rates of more than 2Gbits/s, new approach in photodetection could simplify free-space optics.Facebook’s Connectivity Lab have demonstrated what they call “a conceptually new approach for detecting optical communication signals traveling through the air”.
The development team described the new technology, which could pave the way for fast optical wireless networks capable of delivering internet service to remote locations, in Optica.
Bridging the digital divide
Facebook’s Connectivity Lab develops technologies aimed at providing affordable internet services to the approximately 4 billion people in the world who cannot currently access it. “A large fraction of people don’t connect to the internet because the wireless communications infrastructure is not available were they live, mostly in very rural areas of the world,” said Tobias Tiecke, who leads the research team.
Light-based wireless communication, or free-space optics, offers a promising way to take the internet to areas where optical fibers and cell towers can be difficult to deploy cost-effectively. Using laser light to carry information across the atmosphere could offer high bandwidths and data capacity, but a key challenge is how to precisely point a laser beam at a remote detector.
The Facebook researchers developed a method for using fluorescent materials instead of traditional optics to collect light and concentrate it onto a small photodetector. They combine this light collector, which features 126cm2 of collecting surface, with existing telecommunications technology to achieve data transfer rates of more than 2Gbits/s.
“We demonstrated the use of fluorescent optical fibers that absorb one color and emit another,” said Tiecke. “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.”
A combination of optics and mechanical systems track the position of the detector and point it to the laser, but these approaches add significant complexity. The new light collector uses plastic optical fibers containing organic dye molecules that absorb blue light and emit green light. This 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,” said 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 achieving high speeds.”
The fast speeds are possible because the lapse between the blue light absorption and the green light emission is than 2ns. In addition, by incorporating a signal modulation method called orthogonal frequency division multiplexing, or OFDM, the researchers can transmit more than 2 Gbits/s despite the system’s bandwidth of 100 MHz. OFDM is a method of encoding digital data so that multiple data streams can be transmitted simultaneously. Although it 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.
”If materials were developed that could operate in the infrared part of the spectrum, 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 Gbit/s,” he added.
In the Optica paper, the researchers demonstrate a light-bulb shaped light collector made from a bundle of fluorescent optical fibers. Although many shapes are possible, the light-bulb shape offers a very large bandwidth and omnidirectional sensitivity. The researchers have also demonstrated that this geometry can gather light from an area as large as 126cm2, making it less sensitive to alignment.
The research team is planning to take 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.”