The advent of ultrashort
laser pulses and the discovery of self-guided ionized
filaments (by Braun et al.
in 1985) opened new perspectives in the domain of
weather modulation, especially through Teramobile, an
international project initiated jointly by the National
Center for Scientific Research
in France and the German
Research Foundation. Teramobile allows researchers to
use the first mobile terawatt
laser in the world for atmospheric studies. It enabled us
to conduct outdoor experiments of self-guided ionized filaments for real-scale
Kerr “lens”: n=n0 + n2 l(r)
distances up to a few kilometers from the laser source
by an adequate choice of the
laser parameters, and then
directed to any position in the
atmosphere by sweeping the
beam using a steering mirror.
A TW laser for field
Plasma + saturation of
Kerr effect: diverging “lens”
ultrashort laser pulses
Filamentation results from
the propagation of ultrashort
intense laser pulses in the air.
At high incident power (tens
of GW to TW) and intensi-ties, the air cannot be considered linear anymore. Rather,
it is a nonlinear medium
that allows the formation of
“self-guided filaments.” This
process is initiated by Kerr
self-focusing. At a high power,
the refractive index of the air varies with the incident laser
intensity: n(I ) = n0 + n2• I, where n2 ≈ 3 3 1019 cm2/ W is its
nonlinear refractive index, related to nonlinear polarizability.
Since the intensity profile of the beam is generally bell-shaped, the Kerr effect generates a refractive index profile that
behaves like a series of lenses with shorter and shorter focal
lengths. If the incident power exceeds a so-called critical power
(a few GW in air at a wavelength of 800 nm), the Kerr lens
overcomes diffraction and the beam energy tends to concentrate on its axis. The local intensity is then sufficient to ionize
the air up to an electron density of 1014 to 1015 cm3.
The negative contribution of the free electrons to the refractive index, as well as negative higher-order Kerr indices, balances the self-focusing. The resulting dynamical balance guides
light over distances that largely exceed the Rayleigh length, up
to hundreds of meters in the atmosphere. These ionized light
strings, which are electrically conducting, can be generated at
Colored conical emission is clearly visible around the filament (white center).
24 | OPN Optics & Photonics News