FEATURE ARTICLE
Filaments of Light
Pulsed terawatt lasers create some surprising effects when shone through the air—including the channeling of light
Jérôme Kasparian
Up, Up and Away
To determine whether filaments of light could indeed penetrate high
into the atmosphere, the scientists on the Teramobile project did
the obvious: We tried it. After directing the Teramobile laser
vertically upward, we studied the beam from the ground using the
2-meter-diameter astronomical telescope at Thüringer
Landessternwarte in Tautenburg. Because the laser was located some
distance from the telescope, we were able to obtain side-on images
of the beam by virtue of Rayleigh scattering (the scattering of
light off air molecules, which among other things causes the sunlit
sky to appear blue). The pictures we took also revealed the pattern
cast by the beam when it impinged on the bottom of clouds or layers
of diffuse haze. These experiments, which were carried out in 2002,
demonstrated for the first time an ability to bring light to a tight
focus as far as 2 kilometers away from the laser source, at which
point distinct filaments can propagate for hundred of meters. And
although the reach of these high-intensity filaments is currently
limited to such distances, the Teramobile laser is able to throw
diffuse white light as high as 18 kilometers—that is, well
into the stratosphere.
Having such a far reach holds great promise for probing the physical
and chemical makeup of the atmosphere. Investigators have long
applied lasers for this purpose, often using one or more refinements
to the basic lidar technique, whereby a pulsed laser is directed
into the air, and the backscattered light is measured as a function
of time. Performing these measurements with a temporal resolution
of, say, between one and ten nanoseconds provides a depth resolution
of a few meters or less. Such observations, which are often obtained
while sweeping the beam from side to side, allow for the
construction of three-dimensional maps of atmospheric aerosols or
trace gases.
Currently, the most popular way to detect such gases (often
pollutants) remotely is a technique called DIAL, shorthand for
"DIfferential Absorption Lidar." The strategy is to
compare the lidar signals obtained at two slightly different
wavelengths, one being set exactly to an absorption line in the
spectrum of the pollutant under scrutiny. Seeing a diminution in the
amount of light returned at that wavelength but not at a slightly
different wavelength attests to the presence of the targeted trace
gas and rules out the possibility that something more mundane (say,
clouds or haze) had obscured the light scattered back toward the
observation station.
The problem with the DIAL method is that it can only be used to map
trace gases that exhibit a narrow absorption line that is free of
interference from the absorption spectra of other atmospheric
components. This requirement limits its application severely. Worse,
the need to tune the laser wavelength exactly to the absorption line
makes it impossible to measure more than one pollutant at a time.
And it makes DIAL blind to the presence of an unanticipated
pollutant. Using the Teramobile laser or its equivalent for lidar
should provide a better way to probe the sky, because the telescope
can then gather light containing many wavelengths, not just one or
two, and the resulting absorption spectra would reveal a wealth of
information about the air this light passed through.
A similar tactic could one day be applied, for example, to
characterize the nucleation of water droplets and their subsequent
maturation in clouds. Measurements of droplet growth and density
could allow meteorologists to forecast when rain or snow will form,
or this information could be used to determine how much of the
sunlight falling on a given cloud reflects back into space. Why use
a ground-based laser for such investigations? Droplet nucleation and
growth take place over just of a few tens of minutes, so making the
required measurements from research aircraft is generally too
expensive to consider, and weather-balloon soundings are typically
too infrequent to provide helpful observations. Optical
remote-sensing techniques are clearly the most straightforward
avenue for conducting such research, and the capabilities of the
Teramobile laser in its white-light lidar mode are quite promising
in this regard.
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