  The femtosecond laser filamentation is the result of the dynamic interplay
between plasma self-focusing and defocusing generated by the multiphoton/tunnel
ionization of air molecules. This equilibrium allows the filament to stably
propagate over long distances at high power densities, making it a promising
tool for remote sensing in chemical and biological applications, detection of
air pollution, and lightning control, which has attracted wide attention.
Laser-induced filamentation is a highly nonlinear process, and initial
conditions such as changes in polarization state can affect the fluorescence
emission of N2 molecules excited by the filament in air. In this article, we
analyze in detail the effect of polarization state changes on the 337 nm
fluorescence signal excited by the filament at a distance of 30 m. It was found
that the fluorescence signal intensity of the linear polarization state was
higher than that of the circular polarization state. Through analysis of these
phenomena, we explore the mechanism of plasma fluorescence emission during the
long-range filamentation process of femtosecond laser, including the electron
collision model and the polarization effect on the critical power for
filamentation. These findings are important for the understanding of the
stimulated radiation from filaments and may find applications in remote sensing
of electric field and THz radiation.
