Theoretical Simulation And Experimental Study Of The Spectral Excitation And Acquisition System Of An Aerosol Laser-Induced Breakdown Spectroscopy System

Atmospheric aerosol pollution is an important environmental pollution,for the prevention and control of aerosol pollution,the detection of its elements is helpful to trace the source of aerosol pollution,and is an important support for the research and treatment of aerosol pollution.Laser-induced breakdown spectroscopy(LIBS)is a newly developed emission spectroscopy technique that uses focused lasers to excite a sample to produce a Laser-induced breakdown spectroscopy plasma,collecting emission spectra of plasma to obtain sample element information,this technique has the unique advantages of real-time and on-line in-situ analysis,and is suitable for the rapid analysis of aerosol elements,however,due to the low density and discrete distribution of aerosol particles in aerosol samples,there are some problems such as large spectral fluctuation and weak signal.From the point of view of aerosol spectral excitation and spectral acquisition system,this thesis simulated and analyzed the influence of system parameters on spectral excitation and spectral acquisition.By eliminating the spherical aberration to obtain more concentrated energy distribution and selecting the optimal spectral acquisition parameters and spectral acquisition components,the spectral fluctuation was reduced,the spectral intensity was increased,and the element detection capability of LIBS for aerosols was enhanced.The specific findings are as follows:(1)Aiming at the problem of large fluctuation of LIBS spectra of aerosols,it was proposed that for the samples with low laser absorptivity,such as aerosols,the fluctuation of LIBS spectra can be reduced by getting more concentrated energy distribution.Based on the ideal laser focusing theory and ZEMAX physical optics function,the focusing parameters which make the energy distribution more concentrated after laser focusing were obtained by simulation analysis.On this basis,the influence of aspherical spherical aberration lens on the evolution of plasma in the process of laser-induced breakdown of air was investigated,the effect of eliminating the more concentrated energy distribution caused by spherical aberration on the breakdown process of aerosols was verified,which increased the LIBS spectral intensity by more than 2 times and reduced the spectral fluctuation to 47.3%.(2)Aiming at the problem of weak spectral intensity of aerosols,a spectral acquisition simulation model was established,and the acquisition efficiency of aerosols was improved by optimizing the spectral acquisition parameters.Firstly,based on the ideal imaging formula,the ideal imaging spectrum acquisition model was established,and the influence of the spectrum acquisition parameters on the spectrum acquisition was analyzed,the selection method of the spectral acquisition parameters to reach the upper limit of the acquisition efficiency corresponding to the optical expansion of the system was discussed.Furthermore,based on ZEMAX non-sequential mode,the simulation model of actual spectrum acquisition was established.First,the parameters of the spectrometer were measured by experiments,and the transmission characteristics of the step multimode fiber were simulated.Finally,the acquisition characteristics of various spectral acquisition elements were analyzed and compared,the spectral acquisition efficiency of each wavelength can be enhanced by 1.95 times on average,and the difference of the total acquisition efficiency of each wavelength can be decreased significantly.To sum up,this thesis proposed a more concentrated energy distribution to reduce spectral fluctuation and a method to select the types and parameters of spectral acquisition elements,which improved the sensitivity and stability of LIBS for aerosol element detection,it is expected to be applied to similar low absorption samples and has important application prospects.