In the new method for capturing and concentrating light for free-space optical communication, reported in Optica, 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. [Image: Tobias Tiecke, Facebook Inc.]
Scientists have been trying to improve atmospheric (free-space) optical communications at all size scales, from long-range systems that would bring internet access to Earth’s remote regions to short-range mechanisms for delivering advertisements to the mobile devices of passers-by. One of the challenges at both scales has been the small diameters of the signal beam and light detector, resulting in the need for complex tracking schemes and precise aim.
Researchers at Facebook Inc.’s Connectivity Lab (USA) have built proof-of-concept “luminescent detectors” that use fluorescent fibers to concentrate light onto a small receiver with a fast response time (Optica, doi:10.1364/OPTICA.3.000787). The team achieved throughput of 2.1 gigabits per second (Gbps) with the setup, built from commercially available materials and using eye-safe light levels.
Doped fibers
At the heart of the Facebook group’s device are polystyrene optical fibers doped with wavelength-shifting fluorescent dyes. These so-called luminescent concentrators absorb light at 405 nm and re-emit it at 490 nm. In the researchers’ first experiment, they bundled the fibers together into a planar light-absorbing array with an active area of 126 cm2, guiding the light at one end through a small-area avalanche photodiode. To maximize the available bandwidth of 100 MHz, the team employed orthogonal frequency division multiplexing (OFDM).
In another proof-of-concept, the researchers, led by Facebook scientist Tobias Tiecke, designed an omnidirectional detector that looked roughly like a round, 50-mm-diameter cage made of the doped-plastic optical fibers (see image). The shape provides omnidirectional sensitivity.
Improving the setup
The scientists suggest several ways that this free-space optics scheme could be improved: for example, by using fibers doped with dyes that are photoluminescent in the near infrared, where eye-safety power limits are much higher. Introducing non-imaging optical components, such as a compound parabolic concentrator, would increase the efficiency of light collection. Finally, tailoring the fluorescent dyes to narrow their absorption and emission spectra would enable the future use of wavelength division multiplexing, which would be especially important in visible-light communications.