The characteristics of the multiple filaments formed by prefocused and freely propagating femtosecond laser pulses are investigated and compared. It is shown in our experiments that the diameter, length, stability, and interaction for the two cases can be quite different. The filaments formed by prefocused beam indicate dynamic spatial evolution with higher laser intensity and electron density. They have a typical diameter of $100\mu \mathrm{m}$ are of shorter length. In the free propagation case, the filaments exhibit interesting properties such as hundred-meter propagation distance and mm-size diameter. Moreover, only the interaction of the filaments with the energy background affects the evolution of the filaments. Filament-filament interactions such as the filament splitting and merging were not observed in this case.

• Received 29 March 2006

DOI:https://doi.org/10.1103/PhysRevE.74.066402