Optical Materials And Thin-film Laser Damage Mechanism Of Theoretical Research

This thesis is concerned with the laser-induced thermal and mechanical damage mechanisms of optical materials and thin films. Based on continuum theory, various laser-induced thermal and mechanical damage mechanisms are studied, including thermal phase-change, thermal deformation, thermal stress crack, inclusion-induced damage and multilayer films damage, etc.Firstly, absorptive characteristic of optical materials and the definition of laser-induced damage are reviewed. Main morphologies of laser-induced damage and their micro-mechanisms are summarized.A 1-D implicit finite difference program is coded to simulate heat conduction and phase change of optical materials under laser irradiation, with the latent of phase change being equivalently added to the heat capacity in a region around the phase change temperature. Velocities and depths of phase change and ablation are computed under different laser intensity and pulse duration.Using a finite element program, functional damage due to thermal deformation and crack resulted from thermal stress of several optical materials, such as MgF₂, sapphire and SiO₂, are numerically simulated, and the decisive parameters are discussed.From the damage morphologies observed in experiments and applications, a theoretical model of inclusion-induced damage is proposed, from which two main damage mechanisms of different optical materials: thermal stress and the pressure resulted from boiling of the inclusion, are identified. The effect of inclusion's size is studied and a most hazardous size is found to exist, which results in the lowest laser damage threshold.Based on the standing-wave theory, a theoretical model with high energy absorption at surface and interfaces, is employed to describe lased-induced damage of optical multilayer films. The temperature distributions in two different reflective multilayer films are calculated, and the characteristic, modes and thresholds of damage are analyzed.