Study On The Criterion And Test Method Of Laser Induced Thin Film Damage

In the study on laser constraint fusion, laser is required to provide high power density as 1015~10l8W/cm2. But when laser applies to optical element, laser induced thin film damage will take place. Laser induced damage of optical element, especially optical thin films, restricts laser power density. In order to improve and measure the laser radio resistance of optical elements, the laser induced damage mechanism and some important factor in the accurately measurement of laser induced damage threshold (LIDT) is studied in the thesis.The theoretical treatment of laser induced damage to optical thin films has followed the three distinct paths of avalanche ionization, multiphoton ionization and impurity induced damage. Avalanche ionization model is described as three stages. First, the first few electrons are raised to the conduction band by, in general, nonlinear optical processes which depend on material characteristics and laser photon energy. In the second stage, initiating electrons absorb laser energy and it can produce a secondary electron by impact ionization of a lattice atom. Thus avalanche ionization takes place. Finally, heat is deposited to the lattice by collision. The model of multiphoton absorption considers wide-gap dielectrics at high power light. Under this condition, an electron may absorb two or more photons and jumpto conduction band. Impurity model considers impurities absorb the incident radiation and their temperature rise, which ultimately produces melting, vaporization, or stress fracture of the film material around the impurity.The criterion of laser induced thin film damage is the most important part of laser induced damage test. Phase contrast microscope observation, transmission and reflection method, scattered light detection method, and plasma spark method were studied with experiment. Plasma spark method is based on laser induced damage theory, but plasma spark intensity varies with different film material and plate fashion, and it is difficult to detect on line. Phase contrast microscope observation method is the international standard detection method, but it can't detect some damages and is difficult to detect on line. Scattered light detection method uses He-Ne laser to illuminate the irradiation sport and collects the scattered light to judge damage condition. It can be used in on-line detection. Transmission and reflection method reaches the sensitivity of phase contrast microscope observation method and it can be used in on-line detection. From the experiment, we concluded that scattered light detection method and transmission and reflection method are valuable to develop.The LIDT test methods, such as R-on-1 method and 1-on-l method, are studied. R-on-1 method can get more valuable experimental data and can be used in the analysis of threshold steadiness. But the threshold is not accurate enough for laser conditioning effect. 1-on-l test can get more accurate threshold, but it require more test area and can't get threshold distribution.Diameter effect and laser conditioning effect which affect threshold accuracy are studied too. It can be concluded that at a diameter of 1 mm the test result is at a high consistency and comparability. After laser conditioning, optical surface changes more smooth and LIDT changes greater.