New Detector for Free-Space Optical Communication

The fluorescent optical fibers absorb blue light coming from any direction over a large area and emit green light that travels inside the optical fiber until it reaches a very fast photodetector (Source: T. Tiecke, Facebook Inc.)

The fluorescent optical fibers absorb blue light coming from any direction over a large area and emit green light that travels inside the optical fiber until it reaches a very fast photodetector (Source: T. Tiecke, Facebook Inc.)

Today’s high-speed wired communi­cation networks use lasers to carry information through optical fibers, but wireless networks are currently based on radio frequencies or microwaves. In an advance that could one day make light-based wireless communi­cations ubiquitous, researchers from Facebook Inc.’s Connec­tivity Lab have demonstrated a concep­tually new approach for detecting optical communi­cation signals traveling through the air. The team developed a technology, which could pave the way for fast optical wireless networks capable of delivering internet service to far-flung places.

The researchers develope 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, also called free-space optical communi­cations, offers a promising way to bring the internet to areas where optical fibers and cell towers can be challen­ging to deploy in a cost-effective way. Using laser light to carry infor­mation across the atmo­sphere can poten­tially offer very high band­widths and data capacity, but one of the primary challenges has been how to precisely point a very small laser beam carrying the data at a tiny light detector that is some distance away.

In the new study, researchers demonstrate a method for using fluorescent materials instead of traditional optics to collect light and concentrate it onto a small photodetector. They 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 gigabits-per-second (Gbps). “We demonstrated the use of fluorescent optical fibers that absorb one color of light and emit another color,” 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 high-speed free-space optical network requires very fast detectors to receive the laser light carrying infor­mation. But speed must be balanced against size; although larger detectors make an easier target to hit with a beam of laser light that’s traveling through the air, increasing the size of a detector makes it slower. A combi­nation of optics and mechanical systems can be used to track the position of the detector and point it to the laser, but these approaches add quite a bit of 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 accom­plishing very high speeds.”

The fast speeds are possible because less than 2 nano­seconds lapse between the blue light absorption and the green light emission. In addition, by incorporating orthogonal frequency division multiplexing, or 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. 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 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, Tiecke said. 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, 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.

“Our detector absorbs the same amount of power and gets the same communi­cation signal through inde­pendently of the alignment,” said Tiecke. In addition to working with partners to develop new materials, the research 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.” (Source: OSA)

Reference: T. Peyronel et al.: Luminescent detector for free-space optical communication, Optica 3, 787; DOI: 10.1364/OPTICA.3.000787

Link: Facebook Inc., Connectivity Lab, Menlo Park, California, USA

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