Analysis And Simulation Of Stress Field Effect Of Laser-Induced Optical Thin Films

Optical thin films are the most important part of high-energy laser systems and also the most vulnerable part.The two main reasons for damage of optical thin films induced by laser are thermal damage and thermal stress damage.Under the irradiation of laser,temperature changes between films and substrate or between film layers will lead to thermal damage such as melting and gasification and temperature difference will lead to thermal stress damage such as fracture and shedding,which seriously affect the operation of the system.The laser-induced damage process of optical thin films is studied in depth and the causes and mechanisms of film damage are analyzed.The theoretical analysis results and simulation results are combined to guide the coating experiment and optimize the coating process.It is of great significance to prepare films with high laser-induced damage threshold to prolong the service life of optical thin film components,to meet the needs of high-energy laser systems and to improve the performance of laser systems by theoretical analysis and experimental guidances.Based on the thermal absorption effect for laser energy absorbed by materials and the thermal stress damage mechanism in optical thin films induced by laser,the models of monolayer and multilayer films under laser irradiation are established.By solving heat conduction equations,thermoelasticity equations and equilibrium differential equations,analytical expressions of temperature field and thermal stress field are obtained.The temperature field and thermal stress field are calculated and simulated by Matlab software.Summarizing the simulation results and rules,combining with theoretical analysis,the following conclusions are drawn:1)The distributions of temperature field and thermal stress field in multilayer films are similar to those in monolayer films,which both decrease with the increase of radius and thickness of films.2)Temperature and thermal stress jump at the interface of the multilayer films.For MgF₂/ZnS bilayer film,when the laser output energy is 50 mJ,the interfacial temperature is650?and 1335?,and the radial thermal stress is-509MPa and-444MPa,and the circumferential thermal stress is-509MPa and-444MPa,and the axial thermal stress is-1114MPa and-972MPa,respectively.The fracture strength of MgF₂ single layer film is about300MPa and the fracture strength of ZnS single layer film is about 100MPa.Thermal stress damage may occur in the double layer films.3)For different film materials,the damage process and types are different:the highest temperature in DLC film is 696?,which is much lower than its melting point,but the radial thermal stress with 3132MPa and the axial thermal stress with 6917MPa exceed the fracture strength of 200-400MPa.Thermally induced stress damage may occur in the film.4)The damage location of the monolayer is near the surface of the films and the center of the laser beam.For multilayer films,besides the above two positions,the temperature jump and thermal stress jump at the interface of films are also the main reasons for the damage.5)The films with small absorption coefficient,large thermal diffusivity,small thermal expansion coefficient and little difference in thermal expansion coefficient of multilayer materials should be selected as far as possible or the protective coating should be added to reduce the energy absorption on the surface and beam centre to improve the LIDT of films.These conclusions provide a theoretical basis for the preparation of high LIDT optical thin films.In order to enrich the applicability of the model,the program interface of laser-induced model simulation and calculation is designed.The input and selection of laser parameters,thin film materials and simulation results are realized.The more abundant model calculations and simulations can be realized by modifying the relevant parameters and more accurate rules can be obtained to guide the coating experiment and improve the performance of optical thin film devices.