Filament propagation of laser pulses in the atmosphere was observed in 1994 by Gérard Mourou and his team at University of Michigan. The balance between the self-focusing refraction and self-attenuating diffraction by ionization and rarefaction of a laser beam of terawatt intensities, created by chirped pulse amplification, in the atmosphere creates "filaments" which act as waveguides for the beam thus preventing divergence. If a light filament drops below the intensity needed for this dynamic balance, called modulation instability, it can merge with another filament and continue propagating without broadening as with all earlier means of sending light. The filaments, having made a plasma, turn the narrowband laser pulse into a broadband pulse having a wholly new set of applications.
