An interpretation is proposed for the dynamics of light filaments formed when a femtosecond laser pulse propagates in air. The pulse evolves as a set of coupled nonlinear oscillators forming a coherent structure over several Rayleigh lengths. The plasma generated by photoionization is numerically shown to be the single saturation mechanism of the beam self-focusing. Other physical processes such as group velocity dispersion and the quintic susceptibility ${\chi }^{\mathrm{}\left(5\right)\mathrm{}}$ for the polarization promote different propagation regimes. From theoretical expressions of the oscillation period for the spatial profile of the pulse, we show that the electron density in a femtosecond filament may be estimated.

• Received 26 February 2003

DOI:https://doi.org/10.1103/PhysRevA.68.015801