Research On Damage Characteristics And Mechanisms Of Optical Film Components Irradiated By Pulsed Lasers With Different Durations

The damage characteristics and mechanisms of optical film components induced by pulsed lasers at different regimes (fs, ns, ms) are theoretically investigated, as well as numerical simulations and experiments.An improved model consist of surface defects and subsurface defects is proposed to describe the millisecond laser damage based on the damage experiments of two different kinds of optical coatings (AR and HR coatings). The thermal and stress analysis shows that AR is damaged by surface defects and HR by subsurface defects. To go further, the integrated absorption rate between films and substrate (η) is used to decide which kind of the defects actually dominates the laser damage for thin films of different structures. The results show that when η<1, surface defects dominate, otherwise subsurface defects dominate.A one-dimensional semi-infinite model for nanosecond laser interaction with optical film components is built. The temperature profile in the component is solved by a matrix system from Laplace transform of heat transfer equations, while considering about the electric field distribution in the film. The solving results are compared to that of numerical method which is widely acceptable as well as the experimental ones. The results show that our solution method can be applicable to the analysis of laser damage as well as works fast and be useful to big a big size of model.An approach for the theoretical evaluation of the femtosecond damage threshold in optical interference coatings that combine metal and dielectric films is presented. The model that is used combines a matrix formalism to describe the film system to the two temperature model that describes the energy transfer and the temperatures of electrons and lattice in a solid submitted to a laser irradiation at the femtosecond time scale. With this approach the thermal consequences due to the ultrafast absorption of the metal film can be evaluated in the multilayer stack. Some applications are presented for the case of broadband mirrors for ultrashort pulses. Particularly the impact of the metal film (element, thickness) and the design on the laser induced damage threshold is studied.Much significant information is derived from comparison of damage morphologies and damage thresholds among three kinds of lasers (femtosecond laser, nanosecond laser and millisecond laser). The results show that one temperature model should be used in femtosecond laser damage of metal films while two temperature model should be used in nanosecond and millisecond lasers. Femtosecond laser cause breakdown of dielectrics while nanosecond and millisecond lasers also cause thermal damage. There are some scale range of laser pulse length in which the damage mechanism for metals change from1-T model to2-T model, and the damage for dielectrics change from breakdown to thermal effects. Moreover, it is found that millisecond laser damage film substrate seriously and is less sensitive to defects than nanosecond laser.The research results in this paper can be a basis of going further study of damage properties of millisecond laser with optical film components, provide a method to solve heat transfer equations in nanosecond regime more quickly, as well as helping design metal/metal-dielectric mirrors with high damage resistance performance. Moreover, the results of comparison between three lasers can provide a reference for selecting appropriate lasers in an appropriate industrial area or military area.