Ultraviolet communications has been on the US military’s agenda since the 1960s, but only recently has the Army been able to perform small-scale, short-range, nonline-of-sight UV radio experiments that could lead to a novel communications model.Facilitated by developments in semiconductor detectors and optical sources in the deep-UV or ultraviolet C band, the military is determined to stay ahead of the game by improving UV-C systems it can use for short-range broadcasting. Dr. Brian M. Sadler, a scientist at the Army Research Laboratory, has been working to develop this technology for unattended ground sensors. He also hopes to uncover the complex way in which ultraviolet light scatters throughout the atmosphere and to model the signal for use in UV communications.“My primary goal is to understand the basic science,” Sadler said.Over and outThe components of a UV test bed radio system include a transmitter consisting of seven ultraviolet LEDs with a radiated optical power of 0.3 mW. Each LED yields its own beam divergence gauged at 17°, which projects a cone-shaped beam toward the sky. A solid-state avalanche photodiode, or receiver, generates numerous electrons from a single photon. The device contains a solar-blind filter placed on a PerkinElmer Inc. photomultiplier tube that manages photon detection and counting. When propagated into the atmosphere, photons scatter into the receiver’s field of view, and the detector measures the optical power emitted by the LEDs.A transmitter equipped with an array of ultraviolet LEDs broadcasts a beam toward the sky at an optimal angle. A receiver, placed a short distance away and positioned at a similar angle, emits large amounts of electrons that detect and count scattered photons. Upon propagation into the atmosphere, the photons scatter into the receiver’s field of view, and the detector measures the optical power produced by the LEDs.An advantage to the technology is that the signal scatters, enabling the nonline-of-sight communication. In addition, the radios operate in the solar-blind portion of the UV-C band, where light emits at a wavelength of 200 to 280 nm. In this band, according to Sadler, when solar radiation propagates through the environment, it is strongly attenuated by the Earth’s atmosphere. This means that, as it gets closer to the ground, the amount of background noise radiation drops dramatically, and low-power communications link operation is possible. On the other hand, environmental elements such as oxygen, ozone and water can weaken or interrupt the communications broadcast, limiting usage to short-range applications.Early experimental versions of UV radios were not economical because the transmitters depended on lamp technology, and the receiver was a large and cumbersome photodetector based on a vacuum tube. “Back then, the technology they had was extremely bulky by today’s standards,” Sadler said. Now the equipment is smaller and less expensive.Depicted are line-of-sight (a) and nonline-of-sight (b) arrangements that differ in bandwidth sizes. Ultraviolet communication greatly relies on the transmitter’s beam position and the receiver’s field of view. As a result, researchers are refining the pointing apex angle by experimenting with supplementary equipment to enhance the UV-C signal.Understanding UV scatteringWhen UV waves spread throughout the atmosphere, they are strongly scattered into a variety of signal paths. Signal scattering is essential to UV systems operating in nonline-of-sight conditions, and the communications performance is highly dependent on the transmission beam pointing and the receiver’s field of view.Sadler and his collaborator, Zhengyuan Xu from the University of California, Riverside, are trying to understand the process of UV scattering. If successful, they could create a new approach to optical communications. “A lot of that comes down to the physics of transmission and the physics of the atmosphere,” Sadler said.Moreover, they are testing the UV-C radios by placing the receiver and transmitter at various distances apart, comparing their performance during day and night operations and observing efficiency during low-to-medium- and high-solar noise. The investigators also are experimenting with assorted transmitting and receiving equipment to refine the pointing apex angle, which can enhance the signal.“The goal of the research is to have models and equations to be able to predict performance,” Sadler said.