Thermal Damage Analysis And Simulation Of Laser-Induced Multilayer Films

Optical thin film elements play an important role in laser systems,and they are also prone to damage.The wide application of high energy laser puts forward higher requirements for the performance of thin film components.During laser irradiation,the temperature in the film increases,resulting in melting and gasification of the film.The uneven temperature distribution in the film layer will cause the film to deform,causing the film to break and fall off.Studying the influencing factors of optical film damage through theoretical analysis and simulation calculation is very important for optimizing the coating process and improving the performance and laser damage resistance of optical films.In this paper,based on the theory of heat conduction and thermoelastic mechanics,the temperature field and stress field model of laser induced multilayer film is established.According to the knowledge of mathematical physics,the analytical expression of temperature field and stress field of laser induced multilayer film is obtained.The simulation calculation is carried out by using Matlab software,and the laser damage resistance of multilayer films is studied by means of laser damage test system.The main conclusions are as follows:（1）Based on the interaction mechanism between laser and film,it is concluded that film damage is mainly affected by laser parameters and film properties.Laser energy,wavelength,spot radius,specific heat capacity,absorption coefficient and thermal expansion coefficient of film materials will all affect it.（2）The temperature field and stress field model of laser-induced multilayer films were established,and the analytical expressions of temperature field and stress field of multilayer films were derived.Ta₂O₅,Zn S and HfO₂ were selected as high refractive index materials,SiO₂ and Mg F₂ were selected as low refractive index materials,and G|HL|A and G|HLHL|A were used as the initial film system to obtain Ta₂O₅/SiO₂,Zn S/Mg F₂ and HfO₂/SiO₂ multilayer antireflection films.The analysis shows that the temperature and stress field distribution of the multilayer film is similar to that of the bilayer film.During laser irradiation,the temperature change in the thickness direction of the film is not obvious.The temperature of the center of the film is the highest,and the temperature decreases along the radius of the film.（3）The damage of the film is affected by the material properties and laser parameters.The temperature and stress values in different films are different.When the laser energy is 10m J and the spot radius is 4mm,the maximum temperatures of SiO₂ and Ta₂O₅ films in Ta₂O₅/SiO₂multilayers are 695°C and 95°C,respectively,which do not exceed the melting point of the material and no thermal damage occurs.The maximum tensile stress is 5MPa and 19MPa respectively,and the maximum compressive stress is 54MPa and 118MPa respectively,which do not exceed the fracture and compressive strength of the material,and no thermal stress damage occurs.Under these conditions,Zn S/Mg F₂ and HfO₂/SiO₂ multilayer films will not be damaged.By analyzing the temperature and stress fields of multi-layer films,it can be concluded that the temperature and stress values at the junction of adjacent films will jump,and the interface of films is prone to damage.When designing the film system to prepare the film,materials with small differences in thermal physical parameters should be selected to minimize the damage of the film and avoid shedding and separation between the films.（4）The laser damage resistance of multilayer films was tested by damage test system.The experimental results show that the smaller the wavelength,the higher the photon energy,and the lower the damage threshold of the multilayer film.By analyzing the damage morphology of multi-layer films under different laser energy,it can be concluded that the higher the laser energy,the more serious the film damage,and the film is prone to burst and spalling.The experimental results are consistent with the theoretical part,which proves the rationality of the theoretical model.In this paper,the influence factors of film damage are discussed by analyzing the temperature field and stress field of multilayer films after laser irradiation.The research results have a very important guiding role in optimizing the coating process,improving the laser damage resistance of the film and ensuring the normal operation of the laser system.