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318 So l i d - S t at e La s e r s Ultrafast Solid-State Lasers 319

12.6 Applications

12.6.1 Filaments
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When focused into air, terawatt-level (10 W) femtosecond laser
pulses can, under the right circumstances, generate a tightly focused
filament that can propagate, without diffraction, over extended
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distances. These self-trapped filaments are formed by the balance of
high-intensity, self-focusing of light with the generation ionization of
the air, which defocuses the light. The result is a filament that keeps
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light focused at high intensity (> 10 W/cm ) over extended (> 100 m)
propagation lengths.
Because this light, when incident on a solid target, is intense
enough to cause ablation, filamentation has attracted recent attention
for military applications. Although a single filament is not sufficient to
directly cause disabling damage to an enemy missile or aircraft, a large
number of co-propagating filaments could cause significant damage
in a way that is exceedingly difficult to protect against, because no
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material can sustain greater than 10 W/cm without damage. This
can prepare the target’s surface for efficient absorption of high-energy,
longer-duration pulses that might otherwise simply be reflected with-
out harm. The disruption of optical and imaging sensors is another
obvious potential application. Furthermore, the target composition
could be determined by a “remote” version of laser-induced break-
down spectroscopy (LIBS). Finally, emission of an electromagnetic
pulse from the laser-matter interaction may also provide opportuni-
ties for disruption of sensors and electronic systems.
Another use of these high-intensity filaments is as a backlight
source for measuring atmospheric composition. This possibility has
been demonstrated, in dramatic fashion, in Europe, where a terawatt
(TW) laser system built into a cargo container, called the “teramobile”
(www.teramobile.org), has been used for a variety of atmospheric
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studies. These studies were made possible by the findings that a
white-light filament can be generated in the upper atmosphere at alti-
tudes up to 20 km and that the white light generated preferentially
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scatters in the backward direction. These characteristics essentially
provide a multispectral “lightbulb” source that can be placed any-
where within the range of the laser, giving simultaneous spectral and
light detection and ranging (lidar) information, while also making it
possible to measure atmospheric absorption and identify pollutants
and contaminants, such as atmospheric aerosols.
12.6.2 Precision Machining with Minimum
Collateral Damage
In recent years, micromachining with femtosecond lasers has received
considerable attention from researchers because the dynamics of