Filamentation and supercontinuum generation during the propagation of powerful ultrashort laser pulses in optical media (white light laser)

The fundamental physical mechanism responsible for the self-focussing, fila mentation, supercontinuum generation and conical emission of a powerful ult rashort laser pulse in a transparent optical medium is reviewed. The propag ation can be described by the model of moving focus modified by the defocus sing effect of the self-induced plasma through multiphoton interaction with the medium. Spatial and temporal self-phase modulation in both the neutral Kerr medium and the plasma transform the pulse into a chirped (elongated) and strongly deformed pulse both temporally and spatially. The manifestatio n of the deformation is supercontinuum generation and conical emission. A n ew phenomenon of refocussing was observed. It is due to the diffraction of the trailing part of the pulse by the plasma that results in a ring structu re of positive index changes surrounding the plasma column. This ring struc ture refocuses the pulse partially. The measured coherence lengths of the v arious frequencies components of the supercontinuum are independent of the optical media used and are essentially equal to that of the pump laser puls e when compared to an incoherent white light source. We thus justify that s uch a deformed pulse with a very broad spectrum could be called a chirped w hite light laser pulse.