Thermal Deformation Of Coated Mirrors Irradiated By High Energy Laser

Study on thermal deformation of coated mirrors irradiated by high energy laser plays an important role in improving the performance of high energy laser systems. In this thesis, two issues are investigated. One is the scaling law of high reflecting mirrors'thermal deformation. The other is laser polarization's effects on thermal deformation of high reflecting mirrors. The main contents of this dissertation are summarized as follows:Firstly, the absorption of optical coatings under laser irradiation is studied, which is the base of investigating thermal deformation of coated mirrors. Based on standing wave field, optical coatings'absorption can be calculated. Therefore, the standing wave field and computing formula of absorption in optical coatings are derived. Considering the normal incidence, the temperature field and standing wave field are simulated using our own computing codes, in order to investigate the influence of interface in optical coatings. And a compensation process of reflection-induced retardance is proposed in this thesis, which successfully reduces the surface electric field of graded-index high-reflectance coatings.Secondly, the scaling law of high reflecting mirrors'thermal deformation is studied. Based on the quasi-static hypothesis and thermal-mechanical uncoupling approximation, the control equations about thermal deformation of mirrors are introduced. Considering the common postulations and the boundary conditions, both results are obtained from the equation analysis. First, the scaling law of high reflecting mirror's thermal deformation does exist under laser irradiation. Second, displacement, temperature rise, stress and strain change linearly with the increase of laser power density for the same model. Then, the numerical simulation is carried out with ANSYS and the results are consistent with the theory. The conclusions lay the foundation for using scaled small model to investigate the thermal deformation of large-size mirror under laser irradiation, and provide specific guidelines for designing the model and transforming experimental results to the prototype.Thirdly, laser polarization's effects on thermal deformation of high reflecting mirrors are studied. Under different states of laser polarization, high-reflectance coatings'absorption is calculated, and the results show that with an incidence angle of 45 degrees, the absorption of p-polarized light exceeds that of s-polarized light. An experimental scheme is designed, and the thermal deformations of high reflecting mirrors irradiated by s-polarized light and p-polarized light are measured. Then, a numerical calculation is performed with ANSYS, and the results consist with the experimental results well. These results reveal that, under the same condition, the thermal deformation of mirrors irradiated by p-polarized light is apparently greater than that of s-polarized light. This study may provide a new concept for beam propagating and steering in high energy laser systems.