Research On Preparation Of Low Absorption Infrared Coatings And Its Environmental Stability And Reliability

Light absorpting in infrared optical coatings has become an increasingly limiting issue in advanced infrared optical applications such as infrared detection and imaging systems,especially infrared high-energy laser systems.For instance,light absorpting losses can cause the thermal distortions of mirrors and beam splitters in infrared lasers,leading to degrade the beam quality.When the absorption is large enough,the laser systems cannot even excit laser beam.In general,the materials used to produce low-absorption infrared coatings have sulfides,fluorides,and other materials that perform the poor firmness and durability.However,the layer thicknesses in infrared coatings are generally more than ten microns,and are far thicker compared to those in typical optical coatings.Hence,the study in the low-absorption infrared optical coatings is of great significance.In this thesis,for low-absorption infrared optical coatings,the researchs has been done systematically for the measurement of thin film parameters,the optimization of coating process,the environmental stability,and so on.The obtained main results can be summarized as follows:1.The design method for low-absorption infrared coatings is provided.By using the potential transmittance approach,the absorption of infrared coatings is analyzed,and then the structures of high-reflection optical coatings and narrow-band filters in low absorption are obtained.On this basis,the absorption properties of high-reflection coatings with three different stack structure types are discussed.Moreover,a method for accurately measuring the extinction coefficient of optical thin films is proposed.The extinction coefficients of several film materials were measured,and can be accurate to the order of 10^-6.2.The refractive indices of YbF₃ coatings deposited at different bias voltages of Leybold advanced plasma source?APS?were measured by using the ellipsometry and full-spectrum fitting method.The measured results based on these methods are little different.Based on the Sellmeier dispersion model,the refractive index of YbF₃ layer in stacks was obtained by fitting the transmittance of the YbF₃/ZnS bilayer coatings in the range of 2.5?m-11?m.The results show that there are remarkable differences in the refractive indices of the YbF₃ layers in the stacks and exposed in the atmosphere.The refractive index of the YbF₃ layer in the atmosphere is abrupt because this layer adsorbs water vapor,while that of the YbF₃ layer in the coatings is no mutation.3.The influences of the substrate temperature and the APS bias voltage on the optical properties,stress,crystal structure,roughness and scattering of YbF₃ coatings were provided experimentally,which laid the foundation of the design and deposition of low absorption infrared coatings.4.The preparation process of low defect density infrared coatings was studied experimentally.The effects of deposition rate,substrate temperature,and deposition method on the defects of YbF₃,ZnS and ZnSe coatings are discussed.The preparation parameters in the less defect density cases are obtained.The defects and absorptions of multilayer coatings are analyzed.5.The water absorption of infrared coatings and the method of reducing water absorption are discussed.The effects of ZnS and HfO₂ protective layer technology on the water vapor protection and the environmental stability of coatings were studied experimentally.6.The low absorption thorium-free middle-wave reflection coatings of which the reflectances are greater than 99.99%and the absorption are less than 30 ppm have been produced successfully.Meanwhile,by optimizing the design of stack,combined with the correction and precise control of film layer thickness,the six-wavelength zinc sulfide window having the high transmission at six wavelengths from 0.5?m to 10?m have also been produced based on quartz crystal oscillator.