The high clamping laser intensity inside the filament can ionize molecules and fragment them into plasma because of multiphoton ionization or tunneling ionization. Filamentation occurs in solids, liquids, and gases. The dynamic energy exchange between the filament core and background energy reservoir, as well as the dynamic balance of self-focusing and defocusing propagate filaments over hundreds of kilometers. Nevertheless, filaments can also overcome complex atmospheric environments. The self-healing and replenishment from background energy reservoir and the generation of acoustic waves make filaments penetrate fog and clouds. The nonlinear effect of filaments can restrain the beam wander induced by turbulence. These unique properties make femtosecond laser filamentation applicable to remotely detecting atmospheric pollution, such as gases, aerosols, metals, and biological matter.
The lateral spatial distribution of fluorescence induced by the femtosecond laser filamentation is significance for measuring the parameters of laser intensity and plasma inside the filament, studying the physical process, and controlling the filament. It is a non-invasive and in‐situ measurement method. The backward fluorescence distribution is useful for filament-based Lidar to promote the remote signal intensity and signal-to-noise ratio. Forward and backward air lasers are ideal light sources for remote sensing. Studying the far-field spatial distribution characteristics, such as the divergence angle and directivity, is important. This paper reviews the research progress of the spatial distribution of fluorescence induced by the femtosecond filamentation from lateral, backward, and forward spatial orientations.
The laser polarization, repetition rate (Fig. 2), and the molecule alignment (Fig. 3) affect the lateral distribution of fluorescence, in addition to laser energy, chirp, and external focusing condition. Measuring the lateral distribution of fluorescence is a noninvasive and in‐situ filament visualization method (Fig. 1); it measures the laser intensity (Fig. 5), plasma density, and temperature (Fig. 6) inside the filament. It is a simpler and more sensitive method than measuring electrical conductivity, acoustic waves, and other pump-probe methods. Different physical phenomena have been discovered by measuring the lateral distribution, such as anti-correlated plasma density and THz pulse generation during two-color laser filamentation (Fig. 4). Controlling the filament is important for different filament-based applications, including controlling the laser intensity and plasma density inside the filament, controlling the spatial position and filament length, and organizing multiple filaments. Spatiotemporal phase modulation is used to enhance and elongate the filament (Fig. 8). Different spatial phase modulation methods, such as axicon, deformable mirror, phase plate, and beam ellipticity, are used to control the generation of multiple filaments (Fig. 9).
An amplified spontaneous emission inside the filament has been observed for different compositions, such as N2, O, CN, OH, and NH (Fig. 10). Thus, the amplified spontaneous emission (ASE) phenomenon has gained considerable attention for air laser applications; furthermore, the backward spatial distribution and divergence angle are important remote sensing characteristics (Fig. 11). Backward N2 lasers are generated with an energy conversion efficiency of 0.5% and a small divergence angle of 1.6 mrad (Fig. 11) by electron impact excitation. Different pump-probe methods have been proposed to generate forward air lasers, including self-generated harmonic waves, white light, and external probe beams. The forward spatial distribution is also analyzed with an annular profile (Fig. 15). The forward divergence angle is sensitive to the gas pressure and external focus length.
For filament characterization and control, understanding the physical process inside the filament, and generating air lasers, the spatial distribution of fluorescence induced by filaments is reviewed from lateral, backward, and forward spatial orientations. Challenges remain with the application of filamentation-based remote sensing. First, the physical mechanism of the interaction between filaments and molecules, dust, and aerosols, and the mechanism of fluorescence emissions still need to be studied. The mechanisms of gain and amplification inside the filament are also important. Second, the influence of complex atmosphere conditions on the spatial distribution is still unclear, including the turbulence, pressure, and temperature distribution in the atmosphere and atmosphere scattering and absorption. Third, controlling long-distance filaments and promoting remote signal intensity and the signal-to-noise detection ratio are also practically important.
