Light strings offer new techniques for remote sensing

17 Jun 2002

Narrow beams that propagate over several kilometres may be the key to improved remote sensing of air turbulence and pollutants. They may also help to redirect lightning strikes from sensitive installations such as power and telecommunications facilities.

These beams, known as light strings, are created with extremely short laser pulses. These pulses are amplified to peak powers of megawatts or gigawatts. The shortness of the pulses, about 100 femtoseconds, prevents the beam from ionizing the atoms in the surrounding air. Instead the pulses create an electrically charged air channel for the laser light to glide through.

A group led by Roland Sauerbrey at the Friedrich-Schiller-Universitat in Germany recently used light strings to produce a white light source higher than 10 kilometres in the sky. This experiment demonstrated the potential applications for remote sensing.

Laser induced lightning rods from light strings are also being studied, at the University of New Mexico in the US, by a group led by Jean-Claude Diels. Light strings were discovered four years ago by Diels and independently by researchers at the University of Michigan in the US.

Researchers are eager to learn more about the physics of light strings and how they might be controlled. Ewan Wright and Jerry Moloney, of the University of Arizona in the US, are carrying out theoretical studies of these beams with the aim of making strings that are long enough for commercial applications.

Wright says that potential applications include monitoring of wind shear and turbulence both at airports and from the air. Other uses might include detection of small quantities of various pollutants in the gases emitted from factories. These applications rely on interferometric techniques to detect characteristic phase differences in the light between the light string emitter and the point source of light that it creates. The most appropriate wavelength of light depends on the application. Wright says that ultraviolet pulses are best for laser induced lightning as they lead to a more efficient generation of electrons in the air that attract lightning. By contrast, infrared lasers are better for detection applications as they avoid strong optical losses caused by electron generation therefore allowing for longer light strings, ideally of the order of 4 or 5 kilometres.

Wright believes that these beams will not pose many safety risks because, although the pulses have high powers, they are very short so the resulting beam is low in energy. He says that the only possible danger is to the eyes as the wavelengths currently studied are short enough to cause damage. This would need to be investigated further before the technique was used in ground-based environments like an airport.

This research has attracted industrial interest. In addition to working with experimental researchers at the University of New Mexico, the team at the University of Arizona collaborates with the US company Lite Cycles. James Murray of Lite Cycles says that the company is interested in applications of light strings for LIDAR. He added that it is important to understand the fundamental physics of these beams first and he expects that the first practical systems will not be available for about 5 or 10 years, depending on the investment in this technology.

SH