Ultraviolet Radios Beam to Life

Secret military communication scheme from the 1960s is finally practical

Image: istockphoto

Bat-Signal

Ultraviolet transmitters beam their signals into the sky [green]. Receivers look to the scattered UV light [gray].

The U.S. military has been chasing ultraviolet (UV) communication for decades. Now researchers say radios that communicate using UV light are finally within reach. Working with the Army Research Lab (ARL) in Adelphi, Md., these researchers are mapping out the steps needed to commercialize UV radios. They’ve reached the last piece of the puzzle: untangling the poorly understood, extraordinarily complex way ultraviolet light scatters. If they can do that, they will have unlocked the secret to a new form of non-line-of-sight communication.

Proposed UV radios communicate in the so-called solar blind portion of the UV-C band--light having wavelengths from 200 to 280 nanometers--which, unlike the sun’s UV-A and UV-B rays, is almost completely blotted out by the atmosphere. Near Earth’s ­surface, even a strong UV-C ­signal would die off within a few kilometers, as individual ­photons are picked off one by one by oxygen, ozone, and water molecules. But that attenuation also makes UVâ¿¿C radiation ideal for short-range wireless links, such as in unattended ground sensors. The U.S. military is interested in such short-range communications because they can’t be intercepted or jammed outside their intended range. What’s more, within its ­limited range the UV-C band has an inherently high signal-to-noise ratio, enabling the use of very-low-power transmitters, according to ARL scientist Brian Sadler.

In contrast with other ­optical schemes, which rely on the ­transmitter sending a signal more or less directly to the receiver, a UV system can take advantage of the signal scattering in the atmosphere. The transmitter beams a modulated signal into the sky in the shape of a cone (think of the Bat-Signal). The receiver is trained on the sky as well, at an overlapping angle. That positioning makes it ideal for sensors in dense urban environments where line-of-sight communication doesn’t work. ”This goes around corners, through forests, anyplace you can get light,” says Russell Dupuis, an electro-optics professor at Georgia Tech.

Early UV-C radio prototypes were far too clunky to make it out of the lab--the transmitter was a massive laser, and the receiver was a bulky vacuum-tube-based photo­detector. But thanks to ­materials-science advances, the new ­transmitters are tiny, commercially available UV LEDs. The receivers have also shrunk to tiny, solid-state avalanche photodiodes--devices in which a single photon produces an avalanche of electrons. With currently available devices, and under typical operating conditions, a low-power UV-C system with the right overlap could transmit roughly 100 kilobits per ­second at 10 meters, dropping to less than 10 kb/s at 100 meters, still more than enough for good digital audio.

A major remaining challenge is modeling the behavior of the signal as it scatters randomly in the sky.

”It’s not like there’s a mirror up there,” Sadler says. At the University of California, Riverside, he and electrical engineering professor Zhengyuan Xu experimented with different transmission sources and receivers to characterize the angle at which the transmitter beam and the receiver’s field of view should cross. ”It’s not just about getting the overlap volume as large as possible,” Xu adds. ”When it’s narrower, the signal is sometimes more enhanced.”

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