Investigation On The Pulse Width Effect Of Femtosecond Laser Filamentation In Combustion Fields

Combustion diagnosis provides an experimental basis for understanding the complex chemical reaction process and improving the efficiency of energy conversion during combustion.The invention of various laser spectral diagnostic techniques,which has the advantages of real-time,noninvasive,high spatial and temporal resolution,has created a new situation for combustion diagnosis.In this paper,we verified the feasibility of combustion diagnostics by using femtosecond laser filamentation induced nonlinear spectroscopy.When effects of Kerr self-focusing and plasma defocusing get balance,femtosecond laser will produce a plasma channel,which is popularly called filament.The intensity inside the filament is high enough to induce atomic ionizations or moleculars ionization and fragmentation,which can cause characteristic fluorescence emission.Therefore,the femtosecond filamentation method shows unique abilities and advantages in monitoring various intermediate products simultaneously only by one laser source.However,the pulse width effect on filament-induced fluorescence in combustion fields is unknown.In this paper,we focused on the investigation on the influence of pulse width on filamentation properties in combustion fields.Firstly,the fluorescence spectrum of several intermediate products is obtained by femtosecond filamentation in alcohol-air flame.Then we observed the influence of pulse width on the fluorescence spectrum of flame induced by filament.In our experiment,the femtosecond laser with the wavelength of 800 nm was focused into an array alcohol lamp flame,the fluorescence induced by filament was collected at the right angle of the laser direction.And we investigated the flame fluorescence intensity dependence on pulse width,which can be controlled by adjusting the motor position in the compressor.As a result,the peak intensities of the signals in the spectra decrease with the increase of pulse width.Then,we observed the influence of pulse width on the critical power of filamentation in an alcohol flame.By measuring the focal shift of the filament,the critical power of filamentation in an alcohol-air flame is determined.And we also measured the critical power of filamentation in the alcohol flame dependence on the pulse width.We found that the critical power gradually decreases with the increase of pulse width in combustion fields,which is similar to the condition in air.