Etude de la filamentation d'impulsions laser femtosecondes dans l'air.

摘要

When propagating through the atmosphere intense femtosecond laser pulses organize into intense self-guided light pulses (filament). In turn, such self-guided pulses generate in their wake extended thin plasma column. My PhD thesis work has been dedicated to the study of this non-linear propagation, to its control over long distances propagation and to its applications to atmospheric sciences. Filamentation allows multi-terawatt femtosecond laser pulses to propagate over very long distances, reaching hundred meters. Using appropriate temporal chirps, the onset of the filament as well as its length can be controlled. A maximum ionization distance of 300 m was found. Shorter pulse duration favoured the amount of broadband continuum, while longer negative chirps led to the appearance of bright light channels (I~1012 W m-2) at 2350 m. We also show that it is possible to organize regular filamentation patterns by imposing either strong field gradients or phase distortions into the input beam profile. In order to achieve a control of the filamentation process over long distances through the atmosphere, the properties of filaments have been studied in details as a function of the pressure. The results show that fs filamentation can occur at low pressures (0.2≤p≤1 atm), which corresponds to altitudes up to ~11 km. Moreover, we have demonstrated that triggering and guiding high-voltage discharges thanks to plasma strings generated by femtosecond filamentation is possible over gaps up to 4,50 m, even under rain with an efficiency comparable to that observed in dry air. These results are particularly promising for a laser lightning rod.