Winter/Spring 2008

SCIENTISTS AT the Georgia Tech Research Institute (GTRI) are laying the foundation for techniques that could do for ground-based imaging what the Hubble Space Telescope did for astronomy.

photo by Sheree Colestock GTRI researcher Dave Roberts examines output from a lidar system being used to measure atmospheric turbulence.

Optical turbulence is the distortion of light caused by its passage through the atmosphere. The phenomenon causes stars to twinkle and a desert horizon to shimmer – and makes accurate, detailed ground-based observation of distant objects all but impossible.

With a laser radar (lidar) developed at GTRI, “We can point that system in any direction in the sky and measure the strength of the turbulence effect,” says Gary G. Gimmestad, GTRI’s Glen Robinson Chair in Electro-Optics and senior faculty leader in remote sensing technology. “That has never been accomplished before.”

The three-year Department of Defense-sponsored project represents a crucial step toward controlling the effects of optical turbulence, itself a separate, fast-growing field called adaptive optics. But first, turbulence “must be measured and characterized and monitored,” Gimmestad notes.

The eventual development of algorithms or other techniques to compensate for optical turbulence could provide Earth-based telescopes with improved clarity and dramatically boost the quality of all kinds of long-distance imaging.

“Any kind of imaging you do on the ground is going to be affected by it,” says Gimmestad. “With surveillance imaging, you tend to get waviness in the images. Certainly looking at space objects – stars, planets or whatever – your image quality is really degraded by turbulence.”

Optical turbulence also inhibits long-range “free-space” laser applications; that is, laser light moving through the air rather than through a medium such as fiber optic cable.

One potential free-space laser application would facilitate high data-rate communication between a ground station and aircraft, particularly the unmanned aerial vehicles used for reconnaissance in military and natural disaster situations.

Another possibility attracting interest in scientific circles is the use of lasers to transfer power. Specifically, powerful ground-based lasers could recharge satellite batteries when their beams are trained upon a photovoltaic panel installed on the side of an orbiting satellite.

“The big problem is that turbulence is worse by the ground, right where your transmitter is, and it tends to spread the laser beam and make it wander all over the place,” creating bright and dim spots on the receiving panel rather than the requisite uniform intense light, explains Gimmestad. Compensating for that “spread” at either the transmission point, the receiving point or both, could not only have an enormous impact on satellites, but also open the door to a number of laser-operated tasks.

Free-space optical communication, according to Gimmestad, is an existing technology that would be substantially enhanced if the problem of atmospheric interference can be solved. A commercially available system consisting of laser transmitters, modulators, encoders and receivers is typically used to set up a communications link between two buildings in an urban setting, where digging up streets to install fiber optic cable is neither practical nor cost effective. But the range of these systems is limited to a half-mile at best.

“At some point when you get enough optical turbulence, the whole thing becomes totally inoperative,” he says. “So again, characterizing the level of turbulence out there becomes an issue.”

It’s work that requires a lot of room. Light energy must be measured at numerous points along the entire path of the laser, from transmitter to target, to gauge how much of it is lost or scattered at certain points along the way.

— Gary Goettling