Preparation Of High Reflectance Reflectance Film And Its Laser Damage

With the inertial confinement fusion (ICF) experiments conducted in recent years, one of the most important subjects discussed is to improve the ability of damage-resist optical components to achieve the laser power that meets with the ignition conditions. As to improve both the peak power and reliability of such high power laser system, the study of the damage mechanism of optical film and how to improve the laser induced damage threshold seem to be particularly important. Gradient-index thin film is a kind of optical thin film whose refractive index varies continuously with its thickness. This special kind of films demonstrates the advantages of improved adhesion, reduced optical losses and internal stress in practice. Recent researches show that the gradient-index thin films also have very promising potentials in improvement of resistance to laser induced damage.The main contents of this paper is to use a two-zone ion beam sputtering technique to explore methods of depositing gradient-index film and a more detailed study of its laser damage characteristics. Concerning recent cutting-edge researches abroad, we designed a rugate mirror type of gradient index film and determined the two-zone ion beam sputtering refractive index calibration curve of Ta2O5and SiO2mixed material. Gradient-index high reflector samples were deposited under the optical broadband monitoring. Damage probability curves and damage morphologies of gradient-index and high-low stacks reflectors were compared. Gradient-index reflector reaches the same damage threshold level of the high-low stacks design. Initial damage morphology of rugate was defect ejection centered scald, while no delamination was found. When exposed to subsequent laser pulses, high-low stacks damage shows abrupt damage growth, while the damage site of rugate develops slowly and steadily. The damage characteristics of the mixed material monolayers showed:increasing content of SiO2in the mixed materials improves laser resistance, which contributes to the better damage performance of rugate. The graded design of rugate also smooth out the thermal stress built up in the layer structure under laser pulse, which avoid the catastrophic delamination.Our research showed that, with the two-zone ion beam sputtering technique, once the refractive index calibration curve of mixture is determined, complex gradient-index film can be deposited under the optical broadband monitoring. This kind of thin film demonstrated improved damage characteristics, which is very promising in high-power laser film. Study of this subject offers new ideas for the design of laser film to ensure the output power and stability of the high-power laser system.