July 23, 1999 Contact Information: Ewan M. Wright, (520) 621-2406, ewan.wright@optics.Arizona.EDU
TUCSON, ARIZ. — Scientists are learning to shoot invisible, low-energy beams of laser light for many miles through the atmosphere.
These "atmospheric light guides" or "light strings" could be used to detect wind shear at airports, to find factories that are emitting deadly biological or chemical agents, or to create artificial stars as navigational aids. They also could be used to create "laser-induced" lightning rods.
This may sound like science fiction, but German scientists already have used light strings, which are no wider than a human hair, to produce a white light source higher than six miles in the sky. Their experiment demonstrated that light strings can greatly improve remote sensing techniques used to gather data on the chemical make-up of Earth’s upper atmosphere.
"The key is using high power but extremely short laser pulses," says Professor Ewan M. Wright, who holds a joint appointment in the Optical Sciences Center and physics department at the University of Arizona in Tucson. "If we use longer pulses the laser doesn’t propagate as a string; it just produces an electrical discharge that breaks down instantly and terminates the propagation. We call this "optical breakdown.’"
If the pulses are not short enough, the high-powered lasers that produce them rip the atoms of air apart and the laser light goes nowhere, Wright says. But if the pulses are ultra-short, they don’t have enough energy to pull too many electrons from the surrounding air molecules. Instead, they create a low-energy, electrically charged air channel in which laser light glides through the atmosphere. And the pulses are short — only a trillionth of a second, or 100 femtoseconds in science-talk.
The light string phenomenon was discovered in 1995 by a group of University of Michigan Researchers and Professor Jean-Claude Diels of the University of New Mexico. Diels now is advancing the state of the art in light string technology in collaboration with Wright and Jerry V. Moloney of UA’s Arizona Center for Mathematical Sciences.
Wright and Moloney are studying the detailed physics and mathematics of how light strings are generated so they can learn to make strings long enough for commercial applications. The challenge is to create laser pulses that make longer and longer light strings. This is computationally intensive, highly theoretical research that promises big payoffs.
For instance, their fundamental research is useful to James Murray of Lite Cycles, a Tucson company that is developing LADAR systems. (LADAR — Laser Detection and Ranging — is like radar, but with lasers.) Murray is interested in potential applications of light strings for cloud-penetrating LADAR. Clouds and fog scatter light, which is why they are opaque rather than transparent. But with LADAR, "there will still be light scattering, but we think we can bring it down by orders of magnitude so the laser for all intents and purposes can get through," Wright says. "This ability is unique for this phenomenon."
Among many other potential civilian and military applications: If light strings could reach 50 miles up to the layer of sodium atoms that envelopes the Earth, they could create artificial "guide stars" for navigators, and for astronomers using adaptive optics (a technique that reduces air shimmer for ground-based telescopes) to study celestial objects anywhere in the sky. Laser light strings could sweep airport runways to precisely locate any windshear in the way of planes ready for takeoff or could detect any dangerous air turbulence in aircraft flight paths.
The UA team ‘s basic research also fuels Diels’ interest in using light strings to draw lightning strikes away from power generating stations, radio communications networks and other equipment that is vulnerable to lightning damage.
"You can’t prevent lightning, but you can get it to strike one point rather than another," Wright says. "Think of it: The last place you want to be in a lightning storm is in the vicinity of a high metal pole, which is a great conductor and the easiest path for lightning to discharge to the ground." The atmospheric light guide — which is a string of ionized (electrically charged) air — is the ultimate tall metal pipe, he adds.
Diels began working with ultrafast pulsed lasers as a means of lightning control in 1990 and is co-owner of a patent on the laser-induced lightning technique. Lightning causes about half the power failures in areas prone to thunderstorms, costing U.S. utility companies as much as $1 billion annually in damaged equipment and lost revenue, Diels notes in the August 1997 issue of Scientific American. And lightning can disrupt navigational devices on commercial airliners or even on rockets launched into space, he adds.
Unraveling the fundamental physics of wave propagation and light strings is a major challenge for UA mathematicians and optical scientists. But the potential commercial applications are great – and that makes the work compelling, Wright says.
The Air Force Office of Scientific Research has expressed interest in funding their future research, he added.
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