Ewan M. Wright
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
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
"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.