Study On Absorption Front Model Of Laser Induced Damage In Optical Materials

At present,intense laser induced damage of optical materials has been extensively and deeply studied,but it is still difficult to quantitatively describe the formation process of initial damage.At this stage,the absorption front model proposed by Lawrence livermore national experiment is a good model for quantitative description of initial damage.The previous impurity-induced damage model can only analyze the relationship between the damage threshold and the laser parameters,while the absorption front model can analyze the relationship between the impurity-induced damage pit size and the laser parameters.Although the absorption front model can analyze the relationship between the size of damage pits of optical materials and laser parameters,and represent the latest progress in the research on the damage mechanism of optical materials,the model itself has some shortcomings.An important limitation of the model is that the existing absorption front model does not involve the process of absorbing laser field energy by the impurity defect and its heating process,but only assumes that the impurity has a very high temperature at the beginning,so it cannot truly reflect the process of damage generation.In our paper,we modified the absorbing front model adding the absorption process of impurities on the laser field into the absorbing front model.The modified model can more realistically represent the process of strong laser-induced damage in optical materials.The main research contents are as follows:(1)An absorption front model containing an impurity defect absorption term is established.We use the Mie scattering theory to describe the absorption of the laser by inclusions,couple the absorption term to the heat conduction equation of the absorption front,establish a modified absorption front model containing the inclusion term by using the time domain difference method.(2)The temperature field distribution and its evolution described by the absorption front model under different energy gaps and wavelengths are numerically simulated.research shows:(I)The speed of absorption front propagation decreases as the band gap width increases;(II)At different wavelengths,the higher the absorption coefficient,the faster the absorption front propagation.(3)Using the modified absorption front model,the influence of the interfacebetween the impurity and the substrate on the temperature field was studied.We conducted a systematic numerical simulation of the different conditions of the contact interface.The results show that the interface thermal resistance has a strong influence on the temperature rise of inclusions(the higher the thermal resistance,the higher the temperature of the inclusions).When there is obvious thermal resistance,the impurities can be heated up to the high temperature of more than 10,000 K observed in the experiment.If it is ideal contact(no thermal resistance),it will not reach the temperature of the damaged area observed in the experiment.(4)We performed numerical simulations of the absorption front caused by different inclusion sizes in fused silica materials.In the numerical simulation process,we comprehensively consider the influence of temperature on material parameters and the thermal excitation of free electrons.The results show that when the impurity size is0.216 times the laser wavelength,the elevated temperature is the highest,and the propagation velocity of the absorption front is also the fastest(ie,the initial damage pit is the largest).Our study predicted the size of inclusions most likely to cause damage.