Propagation nonlinéaire des impulsions femtoseconde intenses à travers l’atmosphère

The propagation of intense femtosecond laser pulses in air leads to the self-focusing of the beam. This process is amplified until the local intensity reaches values for which photo-ionization of air molecules is possible. From the moment that this threshold is reached a competition between self-focusing in the neutral gas and defocusing from the ionized gas is established. This can lead to a stable regime of propagation characterized by the formation of a self-guided filament. We have observed such a self-guided propagation of intense femtosecond laser pulses in air for distances beyond 100 m.

 

Left: Self-guided propagation of a fs laser beam in air over a distance of 50 m [2]. Center: Picture of the conical emission from a femtosecond filament [1]. Right: filament of 1 m produced in laboratory by focusing the fs laser beam.

 

Different parameters of the self-guided filament, such as its diameter, energy, spectrum, and pulse duration have been measured. The emission of the filament has a conical component in the forward direction with multiple wavelengths. This emission is attributed to a Cerenkov effect, emission resulting from radiative losses of a dynamic waveguide.

 

À gauche : Emission conique d’un filament généré dans du Xénon par un faisceau ayant traversé une fente verticale. En filtrant la fréquence fondamentale on observe une émission conique très propre et large bande. À droite : Profil d’un faisceau laser femtoseconde TW en régime de multifilamentation après 50 m de propagation dans l’air.

 

References:

[1] Conical emission from self-guided femtosecond pulses in air.
Nibbering et al., Opt. Lett. 21, p. 62 (1996) [link]

[2] Anomalous long range propagation of femtosecond laser pulse through air: moving focus or pulse selfguiding?
Lange et al., Opt. Lett. 23, p. 120 (1998) [link]

[3] Range of plasma filaments created in air by a multiterawatt femtosecond laser.
Méchain et al., Opt. Commun. 247, p 171 (2005) [link]

[4] Femtosecond filamentation in transparent media.
A. Couairon and A. Mysyrowicz
Physics Reports, Vol. 441, p 47 (2007) [http]

 

 

ENSTA CNRS Ecole Polytechnique