Studies On Fluorescence And Lasing Phenomena Induced By Intense Femtosecond Laser In Flame And Air

Combustion diagnostics is of essential significance in understanding the complexity of reaction mechanisms and realizing high efficiency of energy conversion in combustion processes.In recent years,due to the non-invasive and real-time properties,laser-based spectroscopic techniques have been extensively employed for measuring a variety of combustion parameters,such as combustion species and their concentrations,temperature,pressure,and equivalent ratio.However,for most of the laser-based spectroscopic methods,resonant excitation is normally required,which makes that one species can be probed only at one time.Recently,a novel method called filament-induced nonlinear spectroscopy was proposed,which is based femtosecond laser filamentation phenomenon,and demonstrates the feasibility for probing multiple combustion intermediates using only one laser,and high potential in combustion diagnosis.On the other hand,one of the limitations in filament-induced nonlinear spectroscopy is the low signal-to-noise ratio,and thus developing various techniques of fluorescence amplification is of particular importance for the novel filament diagnostic technology.The thesis is devoted to exploring the application of filament-assisted spectroscopic techniques in the field of combustion diagnosis.The contents are listed as below:1.To deepen the understanding of the nonlinear interaction between filaments and combustion intermediates,we investigate the influence of laser pulse duration and polarization on the filament-induced fluorescence intensity in a flame.It is found that the“fingerprint”fluorescence intensities of all the combustion intermediates decrease as the laser pulse duration increases;and the fluorescence intensities also decrease as the driven laser polarization state changes from linear to circular.The variation trends of fluorescence are attributed to the effect of initial laser parameters on the merits of the filaments.2.To improve the signal-to-noise ratio of filament-induced fluorescence of the combustion intermediates,we further investigate filament-induced fluorescence enhancement techniques.We found that the polarization-steering laser pulse can induce the lasing behaviors?B²?_u⁺?v=0??X²?_g⁺?v''=0??of nitrogen molecular ions?N₂⁺?,which is enhanced by two orders of magnitude when compared to that by excitation of linear polarization light.By analyzing the sideway fluorescence and numerical simulation,the enhancement of lasing emissions of N₂⁺is ascribed to efficient population transfer from the ground state X²?_g⁺?v''=0?of N₂⁺to the intermediate state A²?_u in the polarization-modulated laser field,leading to the increase in the population inversion between the B²?_u⁺?v=0?and X²?_g⁺?v''=0?states.In summary,this thesis has been devoted to exploring the application of the filament-assisted spectroscopic techniques in the field of combustion diagnostics.We have investigated the fluorescence and lasing phenomena induced by ultra-strong femtosecond laser excitation in flames and air.The effects of the initial laser parameters such as pulse duration and polarization on the filament-induced fluorescence intensities in flames are studied,which provides new insights in understanding the interaction of filament and combustion field.In addition,we have proposed a viable manner to improve the intensity of filament-induced N₂⁺lasing,which offers the possibility in developing the novel filament-induced lasing phenomena in flames and paves the way for new-type laser spectroscopic techniques for combustion diagnostics.