Research On The Damage Mechanism Of Thin Film Elements By Continuous Laser

Since its introduction in the 1960s,the laser has been widely used and rapidly developed in many fields due to its good monochromaticity,directionality and coherence.With the rise of the ultra-high power market in recent years,continuous lasers with high power,high precision and high efficiency have emerged and achieved excellent results in laser cutting,laser welding and laser inertial confinement fusion.As an important optical component in laser systems,the resistance of optical films to laser damage has been one of the weak links limiting the development of lasers towards high energy and high power.As the output power and energy of lasers continue to increase,even very weak absorption is sufficient to cause damage to thin film components,which places increasingly high demands on the optical properties and damage threshold of optical films.For continuous high power lasers,thermal damage due to photothermal absorption is the most important decisive factor limiting the damage threshold of the film.Under laser action,temperature changes between the film and the substrate as well as between the film layers can lead to damage such as thermal melting and vaporisation of the film;temperature differences can also lead to thermal deformation of the film and the resulting thermal stresses can cause damage such as fracture and peeling of the film.As mentioned above,it is important to study the resistance of thin film components to continuous laser damage.In this paper,monolayer films were prepared on fused silica substrates by electron-beam evaporation,and the preparation process of monolayer films was optimized according to the temperature rise of monolayer films under continuous laser light.The effects of the preparation process parameters,such as evaporation temperature and oxygen partial pressure,on the refractive indices of Hf O₂,Ta₂O₅ and Si O₂monolayers as well as the roughness of the film layers were investigated.Combined with the laser test system,the variation patterns of the temperature rise of the monolayers by different preparation processes were analyzed and the process was optimized.Based on the double effective interface method and single-layer film preparation process,the 1064 nm reflective film of Hf O₂ and Si O₂was designed and prepared by electron beam evaporation and ion beam assisted deposition,and the effect of half-wavelength Si O₂protective and buffer layers on the temperature rise of the reflective film under continuous laser light was investigated.The main factors contributing to the temperature rise of the reflectance-reducing film under continuous laser light were investigated.The results show that both the half-wavelength Si O₂protective layer and the buffer layer can improve the resistance to continuous laser damage,and that the half-wavelength Si O₂ buffer layer can reduce the sub-surface defects of the substrate and reduce the absorption of continuous laser light by the optical film,thus improving its resistance to continuous laser damage.Finally,the paper starts from the temperature field of the multilayer medium,combines the theory of thermal damage of thin films under the action of laser,simulates the temperature field of single-layer film and multilayer film with COMSOL software,solves the temperature rise of reflective film with different film materials and different film structure under continuous laser irradiation,uses the probe to measure the temperature of each film layer and substrate,the simulation results are compared with the experimental results.The results show that the temperature rise of the reflective film under continuous laser irradiation is mainly concentrated inside the film layer and then transferred to the surrounding area and the substrate,which has a high thermal conductivity to conduct heat quickly and finally reach thermal equilibrium.