Study On Thermal Absorption Of Thin Film Coatings Irradiated By Infrared CW Laser

The optical components irradiated by high-power continuous wave (CW) laser may be damaged in short time because of the thermal damage, leading to the malfunction of laser devices. As the most basic components of optical systems, the optical thin film coatings are very important but weak elements and easily damaged. The damage of the optical thin film coatings by laser irradiation limits the development of high-power and high- energy lasers. Thus, it is of great importance to improve the damage threshold of optical coatings. The thermal absorption of optical thin film coatings is one of the main reasons for thermal damage. It not only affects the optical quality of thin film, but also causes heat deposit in thin film. Especially for high-power laser, even the very weak absorption in thin film is high enough to result in grave damage. Thus, it is necessary to measure the absorption of the optical thin film coatings accurately, fast and in-situ.In this thesis, by using the surface thermal lensing technique, the weak absorption of different optical thin films, including the single-layer ZnS and YbF3 dielectric thin films with different thickness, as well as the different multilayer thin films YbF3/ZnS deposited on Si, and multilayer anti-reflectivity thin films YbF3/ZnS deposited on CaFi window, were measured. The experimental results of the laser irradiation of 3.8μm and 2.8μm were compared and analyzed. Furthermore, the temperature field and thermal stress distribution of the thin film coatings irradiated by 3.7μm laser were studied.main work is as follows:1. The basic theory of the temperature field of optical thin films was built. The temperature field distribution of thin films with high reflectivity irradiated by 3.8μm DF laser was simulated numerically and analyzed theoretically by using alternating direction-implicit technique.2. Based on the thermal conduction equations and the thermal elastic equations, and using the finite element analysis method, the 3-D model for the temperature distributions and thermal stress distributions of optical coating substrates were built up by MSC.Marc software. The temperature distributions and thermal stress distributions of optical coating substrate irradiated by Gaussian beams and annular Gaussian beams were studied.3. By using the surface thermal lensing technique, the experimental setup was built up to measure the weak absorption of optical thin films. The weak absorption of different optical thin films irradiated by 3.8μm and 2.8μm laser was measured. The experimental results of 3.8μm and 2.8μm were compared and analyzed. The experimental results have shown that the lowest absorption at 3.8μm measured by our experimental system is 4.57×10-4.The results obtained in this thesis would be of both theoretical significance and practical interest in the fabrication of thin films, and would be useful for the research of weak absorption, the thermal stress and the thermal damage of optical thin films.