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Free-space optics bridges the gap

05 Feb 2010

Researchers pursue variations on the theme in free-space optical communications

News reports in recent weeks have been describing how Taliban insurgents in Afghanistan and Pakistan have intercepted the RF transmissions of Allied "drones" in war zones. In several cases, captured insurgents' computers have contained videos taken by the drones, showing the locations of Allied troops and their targets.

Concern over security breaches in RF links, like the ones exploited by the insurgents, motivated a group of students and faculty at the University of North Dakota (Grand Forks, ND) to develop a laser-based optical link between an unmanned aircraft system (UAS) and the ground.

Link security is one concern, explained North Dakota researcher Mario Czarnomski, but for civilian applications at home, avoiding the over-crowded RF spectrum provides another motivation.

Moreover, an optical data link has a higher bandwidth than an RF link, and it requires no licence from the US Federal Communications Commission – a big advantage from a practical perspective.

The North Dakota engineers mounted a modulated 808 nm, 200 mW diode laser on a two-axis gimballed pointing system in a custom-made UAS with a 12-foot wingspan. The entire transmitter package weighed less than 25 pounds and occupied a little under two cubic feet.

An on-board control algorithm oriented the laser to point at the receiver, using differential GPS and inertial-measurement information from the aircraft's autopilot.

The receiver on the ground comprised 16 photodiodes arranged in an square array measuring 4.5 cm on an edge. To shield it from sunlight and external noise, the engineers positioned an 808 nm notch filter in front of it.

But as of last week, when the work was reported at the Photonics West meeting in San Francisco, CA, the North Dakota group had been unsuccessful in getting the laser to point steadily at the receiver.

"Everything worked in the lab," Czarnomski lamented, but in the field something – the researchers aren't sure what – went wrong. Czarnomski says that the team is confident of finding the glitch and, after demonstrating an aircraft-to-ground link, the researchers plan to develop an optical link between two aircraft.

All at sea

More sophisticated optical links were described by William Rabinovich of the Naval Research Laboratory (Washington, DC). NRL has a 10-mile test bed stretching across Chesapeake Bay, with an active, manned station on one side and a mostly passive, unmanned station on the other. The lab uses the installation to test both direct links and modulated-retroreflector links.

The direct links, intended for communication between large warships, operate with a diode-laser transmitter at either end, transmitting data at hundreds of megabits per second over distances of approximately 10 miles. In a recent shipboard test, Rabinovich said, the link "worked every time we brought it up".

The modulated-retroreflector links, on the other hand, are intended for communication between a mother ship and a smaller craft. They transmit at several megabits, over distances of several kilometres.

The advantage of these links is that the weight and power consumption are all at one end – on the mother ship. The beam is directed from the mother ship to a simple retroreflector on the smaller craft, where a modulator embeds data in the beam returned to the mother ship.

Both the retroreflector links and the direct links have been successfully demonstrated in several tests, and Rabinovich hopes the day will soon come when the Navy begins installing operational laser links on its vessels. But, he noted wryly, there is an inherent hesitancy to replace RF links, which have served the navy well for decades.

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