ATMOSPHERIC RESEARCH: Dancing filaments may fuel light strings

Scientists are learning to shoot infrared low-energy beams of laser light for many kilometers through the atmosphere. Since the discovery of such light strings (also called guides) in 1995 by researchers at the University of Michigan (Ann Arbor, MI) and the University of New Mexico (Albuquerque, NM), a variety of possible applications for them have been explored—ranging from wind-shear detection to laser-induced lightning rods.

A recent European research effort, for example, directed a terawatt-peak-power, 200-fs laser pulse at 790 nm to propagate about 12 km vertically into the atmosphere (see Laser Focus World, Nov. 1999, p. 36). The goal of the white-light pulsed lidar experiment was spectral analysis of trace gases in the atmosphere.

Isosurfaces show simulated details of the distribution of collapsing light filaments within a single pulse of a light string at propagation distances (z) of 9, 9.9, 10.8, and 11.7 m.

According to researchers Ewan Wright and Jerry Moloney at the University of Arizona (Tucson, AZ), the key to expanding the practical application of light-guide technology will lie in the ability to produce laser pulses that make longer light strings. If, for instance, directed strings could travel some 80 km (50 miles) up to the layer of sodium atoms that envelopes the earth, they could create artificial guide stars for navigators and astronomers using adaptive optics to study celestial objects. Wright and Moloney are studying the detailed physics and mathematics of light-guide generation and working to develop a simulation-based model of how the light pulses travel long distances—a process they believe may involve a turbulent flow of light filaments that continuously feed off themselves (see figure).

"The key to the light-guide phenomena is the use of high-power, but extremely short, laser pulses," said Wright. "If pulses are too long, 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-power lasers that produce them rip the atoms of air apart, and the laser light goes nowhere."

If the pulses are ultrashort, though—say 100 fs—they create a low-energy electrically charged air channel for the laser light to glide through the atmosphere. So far, the researchers have carried out several simulations that attempt to expose the physics underlying long-distance wave propagation in air. They reportedly have produced direct numerical evidence that a robust, optically turbulent, femtosecond light guide provides the physical basis for long-distance light propagation in air.

In essence, according to Wright and Moloney, when the initial laser beam enters the channel of propagation, it breaks up into filaments. As the "beam" propagates, these filaments must feed off each other, dynamically interacting in a turbulent wave. This continuous feeding frenzy is necessary if the filaments are to retain enough power for beams to move long distances into the atmosphere, even as the air pressure drops off.

Paula M. Noaker