Research On The Fabrication Of Mid-infrared Dielectric Coatings For Laser System

Mid-infrared laser system are widly used in state of art weapons, remate sensing and atmosphere detection. Different kinds of multilayer coatings, including high reflectance mirrors, beam splitters, polarizers, are required in infrared laser systems. The current laser technology for applications appeals for high output energy density, compact solid state and small size, which bright great challenges for the development of mid-infrared optical coatings. Fine optical property, longlife stabilities and high laser energy resistance are critical for the coatings used in these laser systems. Especially, laser-induced damages of thin films are a limiting factor for high energy laser applications. Multi-band optical properties are desired of coatings applied in resonant optical parametric oscillators employed in optical-pumped midinfrared lasers and infrared spectrum system. At mean time, the choices of coating materials are limited in florides, sulfide and slenide which are commonly called soft matericals with characteristic of poor durability and optical stabilities. The research in this thesis foucuses on the interaction between laser and mid-infrared coating, including intrinsic propertery and defect influence, optimum deposition parameters of soft coating material, as well as design and fabrication of multi-band thin-films such as input mirror, output mirror and beam spliters which are key components used in infrared laser and spectral imaging equipments. The detailed contents of this thesis are as followed:1. As laser induced absorption in coatings are a significant factor of coating breakdown, a developed thermal model for calculating temperature distribution is presented to analyze thermal process between incident laser and mid-infrared coatings. Gaussian angular spectrum method, which is first proposed to investigate negative refraction phenomenon, is employed to simulate laser energy distribution in the coating. Comapared with simplified plane wave theroy, this method is more consistent with practical conditions and provide a simpler way to understand the effect of laser size, incident angle and polarization to the local heating of coatings.2. Electric field modeling of nodular defects is performed to investigate the interaction between defective multilayer coatings and Gaussian profile laser beam. Light intensity is significantly enhanced as large as 6 times in areas adjacent to the defect boundary as well as within the defects. Different geometry of defects irradiated by a range of 0 to 45 deg incident laser is analyzed. Nodules with large but shallow seed or in the case of 40 deg p polarization irradiation tend to have the greatest enhancement effect.3.. Fabricate and characterize single layers of different infrared coating materials to explore optimum deposition parameters for multi-layer infrared coating device. The influence of depositon temperature and deposition rate on the refractive index and water absorption of YbF₃, as well as the fundamental absorption edge of ZnSe coatings are analyzed. Moreover, we also measured the dependence between deposition rate and defect density of YbF₃ and we try to minimize material sputtering during EB depositon processure.4. Measure and analyze the rasidul stress of infrared coating materials. We introduce a stress analysis system to characterize the stress components and propertities of YbF₃ thin films. Since the material choice and deposition conditions are critical for reducing total stress of multi-layser thin films, mechanical stress and the microstructure of zinc selenide (ZnSe) and YbF₃ single layers deposited by electron beam evaporation at various substrate temperatures and deposition rates have been studied. The results obtained demonstrate that the ZnSe samples present compressive stress, while YbF₃ samples present tensile stress, and the stress of ZnSe samples is maximum at substrate temperature of 200℃. When the temperature increases above 200℃, the mechanical stress decreases with temperature. So it is possible to minimize the total stess of ZnSe/YbF3 stacks through optimizing deposition parameters. Finaly, the optimum parameters are discripted.5. Design and fabricate mirrors and AR coating for OPO laser system and two kinds of beam spliters used in infrared spectral imaging systerm. We discuss the coating design procesure and measure the reflectance and transmittance of corresponding band. For the OPO mirrors, the half-wavelength hole phenomenon, which lowers the transmittance of pump light, appears in the input mirrors, as the transmittion band is located at the half wavelength of the reflection band. Materials' dispersion and control errors of coating thickness are main reasons that are responsible for half-wavelength hole phenomenon. We eliminates this effect by accurately control the coating thickness and utilizing the refractive index of coating materials obtained through single layer tests, which can reduce the differences between design results and measured results. The transmittance of input mirrors at 2μm is above 96%.6. Laser induced damage threshold testing facility working at mid-infrared region is set up and used to measure LIDT of YbF₃ and ZnS single layers. Coating defects and particulates are the origin of damage and the main factor that decreases the LIDT.