Fig. 1. Experimental setup for two collinear laser beams propagating in opposite directions

Fig. 2. Comparison of fluorescence signals induced by collinear counter-propagating filaments and single laser filament. (a) Fluorescence spectra in which triangle represents peak value of fluorescence line; (b) intensity of filament induced fluorescence signal versus laser energy

Fig. 3. Attenuation time of filament induced fluorescence signals with different wavelengths. (a) 337 nm; (b) 427 nm; (c) 481 nm

Fig. 4. Fluorescence signal intensity excited by CPF versus delay time. (a) Intensity of fluorescence signal versus delay time at 350-900 nm; (b) R at characteristic peaks of N III 400.85 nm (rectangle) and N II 500.19 nm (dot) versus delay time

Fig. 5. Fluorescence spectra when solution mass fraction is 2000×10^-6. (a) KCl solution; (b) Na₂SO₄ solution; (c) MgSO₄ solution

Fig. 6. Relationship between solution mass fraction and fluorescence intensity. (a) KCl solution; (b) Na₂SO₄ solution; (c) MgSO₄ solution