Theoretical And Experimental Studies On Damage Of K9 Glass And Fused Slica Irradiated By Excimer Laser

It has very important application value to investigate the damage mechanism of typical optical materials irradiated by ultraviolet excimer lasers. As significant optical materials, K9 glass and fused silica both have been widely used in the military field. With the rapid development of new style laser arms, all kinds of military detectors will almost face the serious threatening of laser arms. Because ultraviolet excimer lasers present favorable characteristics such as wide wavelength range, high photon energy, high peak power and high coupling efficiency, the high-power excimer lasers are expected to be widely applied in precision laser machining and military fields. Therefore, the research on the damage of K9 glass and fused silica irradiated by excimer lasers has important military value. This paper will provide great significance in deeply understanding the interaction between ultraviolet laser and material, offer important reference in laser precision machining and laser attack-defense field, and also provide theoretical basis for the anti-laser damage ability of optical components.In this paper, the theoretical analysis and damage experiment of K9 glass and fused silica were studied by using the three most representative excimer laser, which consist of the deep ultraviolet 193nm ArF laser, the solar-blind 248nm KrF laser and the 308nm XeCl laser in the wavelength of atmospheric window. The theoretical model of K9 glass irradiated by excimer laser and the theoretical model concerning the impurity particles induced damage of fused silica were established, respectively. A numerical simulation was performed to calculate temperature and thermal stress fields in K9 glass sample using finite element method. The thermal and mechanical damage in K9 glass irradiated by excimer laser was investigated, and the relationship between laser parameters and the damage threshold were also studied. Furthermore, the experiment of K9 glass and fused silica damaged by excimer laser with different wavelength of 193nm,248nm and 308nm were carried out to obtain the damage morphology and damage threshold. The damage mechanism and the damage growth of K9 glass and fused silica irradiated by different wavelength of excimer laser were studied. The research results in this paper are as follows:1) The damage mechanism of K9 glass irradiated by KrF excimer laser were melting damage and stress damage, and the stress damage first appeared inside the K9 glass sample, and which was dominated by circular tensile stress. As the laser fluence increased gradually, the compressive stress tended to induce the stress damage and melting damage on the spot area in the surface of material. Furthermore, on the area surrounding the edge of spot, the tensile stress caused stress damage. Thermal stress usually propagated inside the material in the form of thermal shock wave. Its maximum value appeared in the time when the laser pulse completed and afterwards, reduced rapidly and changed with time. The repeated impacts of thermal stress were prone to result from the increasing damage to optical materials, and increased the time for the damage of materials. Thus, the materials were more inclined to be fractured.2) Using the theoretical model, the tensile stress damage threshold, the compressive stress damage threshold and the melting damage threshold of K9 glass irradiated by KrF excimer laser were calculated to be 0.64J/cm2,0.76J/cm2, and 1.05J/cm, respectively. Besides, the laser parameters exhibited a great effect on damage effect and damage threshold. The high-repetition rate laser was more prone to damage the optical material.3) In the experiment, the damage thresholds of K9 glass and fused silica irradiated by excimer laser with three different wavelengths were measured. The relationship between the damage thresholds and wavelength was investigated. Moreover, the damage thresholds of K.9 glass irradiated by ArF, KrF, and XeCl excimer lasers were 0.44J/cm2,1.8J/cm2 and 3.68J/cm2 respectively by using a plasma flash evaporation method; the damage thresholds of zero probability for the excimer laser to K9 glass with three different wavelengths were 0.21J/cm2, 1.24J/cm2 and 2.45J/cm2 respectively. The damage mainly was found on input surface of K9 glass. The wavelength effect was shown to be obvious:the shorter the wavelength of incident laser, the smaller the damage threshold. As fused silica was irradiated by ArF, and KrF excimer lasers, their input surface were firstly damaged. The damage thresholds measured by using a plasma flash evaporation method were 0.83J/cm2 and 3.8J/cm2 separately, while corresponding damage thresholds of zero probability were 0.4J/cm2 and 2.7J/cm2 respectively. However, when fused silica was irradiated by XeCl excimer laser, its damage first occurred to its output surface. Besides, the damage threshold of output surface was smaller than that of input surface:the thresholds measured by a plasma flash evaporation method, of both were 2.2J/cm2 and 1.7J/cm2, respectively. It could be found both presented a great difference (about 1.3 times).4) The damage mechanism of K9 glass irradiated by ArF and KrF excimer lasers was proven to be a thermal-mechanical coupling mechanism; while the damage mechansim of K9 glass irradiated by XeCl excimer laser mainly followed the thermal damage arising from surface impurities. Under multiple-pulse irradiation, the damage growth of K.9 glass irradiated by the excimer laser with different wavelengths agreed with the exponential growth, and the apparent cumulative damage effect was also observed.5) The damage mechanism of fused silica irradiated by ArF excimer laser consisted of the multiphoton ionization induced damage mechanism, the thermal-mechanical coupling mechanism, and the surface impurities mechanism; while the damage mechanism of fused silica irradiated by KrF excimer laser was demonstrated by the thermal damage resulted from the strongly adsorbed impurities on surface; the damage of XeCl excimer laser to fused silica was realized by the thermal damage caused by self focusing and the adsorbed surface impurities. In the case of multiple-pulse irradiation, the excimer laser with varying wavelengths exerted an obvious cumulative damage effect on fused silica. The damage growth mechanism was primarily attributed to the shock impact of high temperature plasma and the redeposition of laser energy. Although the damage mechanism of fused silica was induced by joint effects of multiple mechanisms, the thermal damage induced by surface impurity was always present no matter which mechanism played a dominant role. The treatment of the surface impurities on the surface of fused silica was the key part affecting the anti-laser damage ability of optical components.6) Subject to the variation of wavelength, the damage growth of fused silica irradiated by excimer laser was able to satisfy different relationships. Furthermore, the input and output surfaces of fused silica also presented different laws regarding damage growth. The damage of fused silica irradiated by ArF excimer laser exponentially grew and no damage was found on the output surface; the KrF excimer laser induced damages on the input and output surfaces of fused silica were generally consistent in initial stage; however, the subsequent damage growth showed different laws:the damage on the input surface grey exponentially, while the damage on the output surface increased slowly; in terms of damage morphology, both surface presented similar damage morphology in initial stage, however, with the experiment proceeded, the damage morphology of the input surface was found to be greatly obvious than that of the output surface; the damage of XeCl excimer laser to the input surface of fused silica was subject to the law of linear growth, while the damage of output surface was shown to grow exponential and its degree was significantly higher than that of input surface. The main reason leading to the damage difference between the input and output surfaces of fused silica under different wavelengths was due to the shielding effect of laser wavelength on plasma. The shorter the wavelength, the less obvious the shielding effect on plasma.7) Unlike far-infrared TEA CO2 laser, the ultraviolet excimer laser with short wavelength exhibited better damage effects on the same kind of optical materials. As seen from the results, the wavelength could significantly influences the damage effect, especially in ultraviolet band. With the wavelength reduced, photon energy grew, and the effects of multiphoton ionization including single-photon and two-photon tended to be more apparent:it can be directly reflected by the decrease of damage threshold.8) The theoretical model for the damage of excimer laser to optical materials was constructed. On this basis, the theoretical model concerning the impurity particles induced damage of fused silica was established. Based on the theoretical models, the mechanisms of the melting and surface cracks in the experiment of K.9 glass damaged by KrF excimer laser were analyzed. Besides, the thermal damages of XeCl excimer laser to the strongly adsorbed impurities on the surface of fused silica were also explored. The theoretical analysis was in agreement with the experimental results, which validated the scientificity of the theoretical model.