Computational Research On The Damage Characteristics Of Pulsed Laser To Optical Materials

In this thesis,the damage characteristics of two kinds of optical materials by pulse laser are studied by means of theoretical analysis and numerical simulation.The main contents include the discussion of laser damage criterion,the calculation of heating/ablation effect of repetitive pulse laser on semiconductor optical materials and the calculation of transmission damage of single pulse laser on dielectric optical materials.The former focuses on theoretical discussion,while the latter two focus on computational analysis.This thesis summarizes the research background and significance of laser-induced damage mechanism,as well as the current research progress.Several main damage theoretical models are described,including macroscopic mechanism of inclusion-initiated thermal explosion(external mechanism),microscopic mechanism of impact ionization and photoionization(intrinsic mechanism).The relationship between the inclusion-initiated thermal explosion and photoionization is also pointed out.The basic theory of laser damage to optical materials is expounded,and the contents of general concern in the research field are summarized:the relative contribution of impact ionization and photoionization;the influence of laser pulse shape(such as Gaussian pulse and rectangular pulse)on the damage results;the damage characteristics of femtosecond laser.The existing damage criteria,such as temperature,heat stress and electron density,are reviewed.In particular,according to the experimental results in the literature,it is proposed and analyzed that the concentration of broken bond can be used as a damage criterion.It is worth noting that temperature criterion and electron density criterion are adopted in the subsequent calculation and analysis.A physical model of "bulk absorption" of silicon and germanium for heating/ablating semiconductor materials by repetitive frequency pulse laser irradiation is established.The effect of heating/ablating is researched by computation and the influence of pulse structure and material thickness are investigated.According to the calculation results,the heating curve of the front surface of the material is toothed,similar to that of the metal;the back surface is stepped,showing the difference with the metal.Under the condition of the same average laser power,the shorter the single pulse width of the repetitive pulse laser,the faster the temperature rise of the material surface and the deeper the ablation.Compared with continuous laser,repetitive pulse laser has advantages in laser heating and ablation.For the transmission and damage process of dielectric optical materials by single pulse laser,based on one-dimensional calculation program,the calculation of SiO2 and Al2O3 materials is carried out,and the results of filamentous transmission and damage of the two materials are obtained,and the differences between Al2O3 and SiO2 materials are compared.An effective simplified physical model considering the effect of radial distribution of light intensity is proposed,and a calculation method for two-dimensional effect damage result considering arbitrary distribution of light intensity is obtained through the model.For the two cases of Gaussian and linear distribution of laser peak electric field intensity,the damage results and damage morphology considering two-dimensional effect are calculated.Finally,a summary and prospect are given.The main simplifications adopted in the calculation are combed out,and further possible improvements in the calculation and analysis are pointed out.