Research On High-performance Optical Thin Films For High-power Laser Systems

High-power laser systems have now become one of the commanding heights of national defense strategy,cutting-edge technology and emerging industries,and play a major role in national security and national economic construction.Large-diameter high-performance reflective films,high-precision beam splitting films and high-performance laser beam combiners are the three key components that control the light propagation.With the breakthrough of high-power solid-state laser technology and the improvement of output power,laser systems have put forward more stringent requirements for the comprehensive performance of these three types of thin-film devices.First,thin-film devices need to have high spectral efficiency to ensure high-efficiency transmission of laser beams;secondly,thin-film devices must have high laser damage threshold to ensure long-term stable operation of the system;finally,thin film devices also need to maintain a low temperature rise during service to ensure that the beam quality of the laser beam does not deteriorate.There are still some cognitive deficiencies and limitations in the current research to solve the problem that three devices cannot simultaneously achieve high spectral efficiency,low temperature rise and high damage threshold.Specifically,for large-diameter high-performance reflective films and high-precision beam splitting films,there is currently a lack of a multifunctional high-refractive-index film that can simultaneously achieve low absorption,low scattering,high threshold and low temperature rise characteristics.For high-performance laser beam combiner thin-film devices,there is currently a lack of a thin film design structure that can take into account high fabrication feasibility and low standing wave electric field strength.In order to solve these key problems,this paper will mainly carry out research work in three aspects:1.The effects of SiO₂ doping content and annealing temperature on the microstructure and optical properties of IAD（ion-beam assisted deposition）-Hf_1-xSi_xO₂mixed films were studied.The relationship between the crystallographic structure,crystallinity,grain size,surface morphology and surface roughness of Hf_1-xSi_xO₂ mixed films was explored,and the relationship between film structural defects,stoichiometric imbalance defects and film absorption was clarified.On this basis,the absorption and crystallization of IAD-HfO₂ are suppressed,and the optimal preparation process for large-diameter high-performance reflective films is obtained.Finally,the IAD-Hf_0.70Si_0.30O₂ mixed film was used to prepare a high-performance reflective film with high spectral efficiency,low temperature rise and high damage threshold:the reflectivity was higher than 99.98%;under the high-power continuous laser irradiation of 130 k W/cm²,the temperature rise is only 4.9 ℃;under the condition of high concentration carbon pollution,its damage threshold is higher than that of EBE（electron beam evaporation）-Hf O₂/Si O₂ reflective film,and the temperature rise is lower than that of IAD-Ta₂O₅/Si O₂ reflective film.2.The effects of the thickness of the sublayer Hf O₂ and the annealing temperature on the microstructure and optical properties of the IBS（ion beam sputtering）-Hf_1-xSi_xO₂laminated film were studied.The relationship between the hole defects,crystallographic structure,crystallinity,grain size,surface morphology and surface roughness of Hf_1-xSi_xO₂ laminated films was explored,and the relationship between film structural defects,stoichiometric imbalance defects and film absorption was clarified.On this basis,the peculiar hole defects of the IBS-Hf O₂ film are eliminated,the absorption and crystallization of IAD-Hf O₂ are suppressed,and the optimal preparation process of the high-precision beam splitting film was obtained.Finally,a high-performance beam splitting film with high spectral efficiency,low temperature rise and high damage threshold was prepared by using the 19 layers of Hf_1-xSi_xO₂ laminated film:the reflectance at 1050-1080 nm is higher than 99.968%,the transmittance at 1030 nm is97.1%,and the average transmittance at 600-900 nm is 98.3%;under the high-power continuous laser irradiation of 130 k W/cm²,the temperature rise is only 5 ℃;under the condition of high concentration of carbon pollution,the temperature rise is lower than that of IBS-Hf O₂/Si O₂ and IBS-Ta₂O₅/Si O₂ mirrors so that it is expected to have a higher anti-pollution damage threshold.3.A design structure of the depolarization beam combiner film with high fabrication feasibility and low standing wave electric field strength is proposed.A 45-degree depolarization beam combiner with high spectral efficiency,high damage threshold and low temperature rise was fabricated using this structure.The beam combiner has a transmittance of 98.57%at 1080 nm and a reflectivity of 99.52%at1064 nm,with good spectral efficiency;under 130 k W/cm² high-energy continuous laser irradiation,its temperature rise is only 14 ℃,and no damage occurs.