By exploiting quantum mechanics, researchers at the Naval Research Laboratory in Washington have used free-space optics to communicate with unmanned aerial vehicles. The technique promises to allow data transfer and the control of fist-size fliers or of larger vehicles in space. It also may provide power.Using quantum-well modulating retroreflectors, researchers have demonstrated free-space optical data links with unmanned aerial vehicles. The technique may be used to communicate with as well as to power vehicles on reconnaissance missions, such as this Predator, seen over the USS Carl Vinson aircraft carrier. Courtesy of the US Department of Defense.Key to the development, said G. Charmaine Gilbreath, an engineering scientist at the facility and a researcher on the project, is the quantum-well modulating retroreflector. Applying 10 to 15 V to the device changes its optical characteristics at a given wavelength from absorbing to reflecting. "It's like a tunable notch filter," she said. "It's fast, and it's robust. It's radiation-hard, not terribly sensitive to temperature, and we've got devices that are supporting on the order of 10 Mb/s."The modulating retroreflectors are constructed using molecular-beam epitaxy. Gilbreath and her co-workers deposit 100 layers -- each approximately 10 nm thick -- of a compound semiconductor material on top of a semiconductor wafer. The materials include gallium arsenide, aluminum gallium arsenide and indium gallium arsenide. Depending on the material and the construction of the quantum well, the layers create a sharp absorption edge at a given wavelength.An applied voltage shifts that edge, thus making absorbing layers transparent. Shining a laser beam at the right wavelength at such a retroreflector establishes a communication channel. The retroreflector modulates the returning beam, switching it on and off rapidly in response to an onboard signal.0.5 Mb/s over 35 mThe researchers mounted the retroreflectors on an unmanned aerial vehicle and found that they could achieve data rates in excess of 0.5 Mb/s over distances greater than 35 m under less than ideal weather conditions. With further development, the technique may find a place in unmanned aerial reconnaissance.Work on the technology continues, and space-borne applications are under consideration. Combining the retroreflectors with other technologies, Gilbreath said, may further enhance their performance. "You would couple it with photovoltaics and, if you're going at a low data rate, you could be self-powered by the laser."