Fig. 1. Suppressing formation of multiple filaments by focusing with conical lens^[29]

Fig. 2. Controlling characteristics of filaments by telescope system^[93]. (a) Experimental setup; (b) fluorescence signals at different positions

Fig. 3. Spectra of supercontinuous radiation under different initial chirps^[97]

Fig. 4. Modulation effects of laser system parameters on optical filament^[104]. (a) Modulation of energy deposition by geometric focusing parameters; modulation of optical filament by (b) annular beam radius and (c) beam waist width

Fig. 5. Optical filaments (first row) and supercontinuum spectra (second row) generated by ring Gaussian beam and Gaussian beam^[105]. (a)(b) ring Gaussian beam; (c)(d) Gaussian beam

Fig. 6. Intensity spectra of annular Gaussian beam^[106]. (a) Spectrum evolutions under different focal lengths of convex lenses; (b) spectrum evolutions under different spatial chirp coefficients

Fig. 7. Generating optical filaments using non-diffracted Bessel beam. (a) Schematic of shaping femtosecond laser pulse envelope^[111]; (b) plasma density diagram of three pulses with different energy values^[64]

Fig. 8. Generating filaments using femtosecond laser ^[112]. (a) Experimental setup; (b) spot distributions before and after modulation by hollow quartz diaphragm; (c) extending length of filament using quartz diaphragm

Fig. 9. Generating filaments using femtosecond ring Gaussian beam in air ^[119]. (a) Spatial plasma density versus propagation distance; (b) evolution of energy fluence distribution of beam; temporal distribution of on-axis intensity as a function of propagation distance for (c) ring Gaussian beam and (d) Gaussian beam

Fig. 10. Cross section diagrams of laser intensity on axis^[121]. (a) Single Gaussian filament; (b) dressed filament

Fig. 11. Controlling filament formation by external compensating energy^[122]. (a) Experimental setup; (b) initial intensity profile of Gaussian and ring Gaussian beams; (c) 11-fold extension of filament length with aid of dressing beam

Fig. 12. Extending filament length by double pulse technique^[123]. (a) Experimental setup; (b) measurement results of electric conductivity of optical filament (first column) and THz emission signal (second column) where first row denotes pulses A and B are launched separately, and second row denotes collinear transmission between two pulses with time delay of 100 fs (square) and 0 fs (triangle)

Fig. 13. Evolution diagrams of two-color filament^[129]. Generating plasma filaments by (a) sum of basic Gaussian beam (R) and primary annular beam (B1); (b) generating plasma filaments by sum of basic Gaussian beam, primary annular beam, and secondary annular beam (B2); (c) generating plasma filaments by 800 nm or 400 nm Gaussian beam that propagates alone with total energy of 2.2 mJ

Fig. 14. Filament-based white-light LIDAR^[6]. (a) Schematic of LIDAR experimental setup; (b) evolution of echo signal intensity at three wavelenths; (c) high resolution atmospheric absorption spectra at altitude of 4.5 km

Fig. 15. Spaceborne filament for atmospheric remote sensing^[81]. (a) Schematic; (b) numerical simulation results of spaceborne filaments propagating from Earth orbit at altitude of 400 km toward ground

Fig. 16. Filament characteristics under two varying pressure conditions^[137]. (a) Distribution of filament energy fluence; (b) energy versus propagation distance; (c)(d) evolutions of supercontinuum spectra at focal length of 1.2 m

Fig. 17. Propagation of femtosecond laser pulse from Earth orbit at altitude of 400 km toward ground^[138]. (a) Beam radius versus altitude; (b) evolutions of peak laser intensity (left) and peak plasma density (right)