FEATURE ARTICLE
Filaments of Light
Pulsed terawatt lasers create some surprising effects when shone through the air—including the channeling of light
Jérôme Kasparian
Stepping Out
The laser-research consortium that I coordinate was established in
1999, when French teams led by Jean-Pierre Wolf at the Laboratoire
de spectrométrie ionique et moléculaire (part of the
Université Claude Bernard Lyon I) and André Mysyrowicz
at Laboratoire d'optique appliquée (part of the École
polytechnique in Palaiseau) joined forces with German groups led by
Ludger Wöste at the Freie Universität Berlin and Roland
Sauerbrey at the Friedrich Schiller Universität in Jena,
Germany. Our aim was to create an experimental laser that could be
brought into the field to study how light filaments propagate over
greater distances than one can possibly arrange in the lab and to
develop ways to use them for probing the atmosphere. A laser of this
sort allows for the remote examination of gaseous or aerosol
pollutants released, say, from automobiles or industrial
installations. And it can be used to study the formation of water
droplets in clouds.
It was clear early on that pursuing such investigations demands
mobility, yet the high-power pulsed lasers then available took up
most of a room—not something one could easily pack up and
move. The solution was to install a laser of this type in a standard
20-foot-long freight container, which could be carried by truck (or
by ship) as needed anywhere in the world and operated even in
adverse weather conditions. We call this portable terawatt laser
system the "Teramobile."
The laser we use is quite sophisticated. It sends out short pulses
of infrared light (800-nanometer wavelength) 10 times per second.
Each pulse is only 70 femtoseconds (70 millionths of a nanosecond)
long when it exits the laser and carries 350 millijoules of energy.
The peak power works out to 5 terawatts (5 x 1012 watts).
My colleagues and I have been experimenting with this laser for
several years, working mostly on schemes for measuring the
composition of atmospheric trace gases as well as the abundance and
nature of aerosol particles.
Several optical techniques for probing such properties of the
atmosphere already exist, methods that go by such complicated names
as "Fourier-transform infrared spectroscopy,"
"differential optical absorption spectroscopy" and
"light detection and ranging" (lidar). The Teramobile
laser adds the possibility of carrying out such studies using one or
more white-light laser filaments instead of the usual sources of
light—ordinary (monochromatic) lasers or in some cases the
natural illumination that the Sun or Moon provides.
Although laser filaments do not suffer the diminution in intensity
that accompanies the spreading of a conventional laser beam, members
of the Teramobile team were concerned at the outset of our
investigations that these narrow channels of light might easily be
blocked by raindrops or atmospheric dust. So we carefully studied
the interaction of light filaments with such aerosol particles,
introducing droplets of various sizes into the light path. It turned
out that our worries were unjustified. We discovered that opaque
droplets as large as 100 micrometers in diameter do not obstruct the
propagation of a light filament, although they are about as large as
the filament itself. At the same time we were doing these studies,
See Leang Chin and his coworkers at the Université Laval in
Québec, found that a laser filament cannot be sent through a
hole, even one that is several times the diameter of the filament.
This counterintuitive result is explained by the fact that a
filament of light is not simply a tube through which all the photons
flow; rather, it reflects a dynamic balance within the much more
diffuse beam that surrounds it, something I like to call a
"photon bath," which acts as an energy reservoir feeding
the filament when it encounters an obstacle. Thus, blocking the
propagation of a filament in one place naturally spawns a new
filament elsewhere within the wider beam. Numerical simulations by
Jerome V. Moloney and his coworkers at the University of Arizona and
by Luc Bergé at Commissariat à l'energie atomique
(CEA, the French atomic energy agency) in Bruyères le
Châtel show this effect well.
Light filaments sent into the sky can thus traverse a cloud so
long as the accompanying photon bath makes it through. Small-scale
laboratory tests had suggested that laser filaments should be able
to pass through a typical cumulus or stratocumulus cloud without
being visibly affected. My colleagues and I found similar results
when we scaled up the experiment using the Teramobile beam and an
open cloud chamber producing a 10-meter-long cloud of 1-micrometer
droplets. Light filaments were visible exiting the fog, even for a
concentration of almost 100,000 droplets per cubic centimeter,
meaning that one filament must have hit an average of 2,000 droplets
for each meter it traveled.
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