Long-pulse Laser In Optical Dielectric Films Heat-stress Injury Research

In this paper, a theoretical model of long-pulse laser leading to thermal stress damageof dielectric film in film/substrate systems is developed. Based on this theoretical model,transient distributions of temperature field and thermal stress field are simulated by usingthe finite element method(FEM). Furthermore, thermal stress damage mechanism ofdielectric film by long-pulse laser is analyzed.In accordance with classic thermal conduction equation and thermal stress equation,this paper analyzed transient temperature field and thermal stress field in film/substratesystems under long-pulse laser radiation provided dielectric film and substrate are isotropic,and material's thermal physical properties are unchangeable.An optimized finite element is developed to simulate transient temperature filed infilm/substrate systems based on a well understanding of the influences of element size andintegration time step on the solution stability. Numerical results indicate: Great temperaturegradient in dielectric film and substrate exists in radial directions, while small temperaturegradient exists in axial directions; radial direction temperature gradient is more great andcentral temperature is higher as power density of laser is higher.Transient thermal stress field is simulated in film/substrate systems by long-pulselaser. Numerical simulation results demonstrate that thermal stress damage is critical andcircumferential stress plays a main role in damage mechanism of dielectric film. ForAL₂O₃/K9 system, the damage under long-pulse laser radiation starts from the substrate.Furthermore, it is found that the laser spot rim is a high stress area where damage startsfirst.The results in this paper may provide theoretical base for theories of thin film thermalstress damage and numerical simulation research under laser radiation, as well aslong-pulse laser technology and widening its application scope.