Accelerated life time testing of fused silica upon ArF laser irradiation

摘要

We report on two approaches to strongly shorten life time testing of fused silica's absoption degradation upon 193 nm laser irradiation. Both approaches are based on enhancing the two photon absorption (TPA) induced generation of E' and NBOH defects centers in fused silica compared to common marathon test irradiation parameters. For the first approach the irradiation fluence is increased from typical values H<1 mJ/cm~2 to H=10 mJ/cm~2, therefore increasing the peak laser power for a more efficient TPA process. To avoid microchannel formation in the samples, being a common break-down criterion in marathon tests based on transmission measurements, a small sample of 10 mm length is irradiated and the absorption is measured directly by the laser induced deflection (LID) technique. For comparing the experimental results with a real marathon test at H=1.3 mJ/cm~2, an experimental grade sample with very low hydrogen content, i.e. fast absorption changes due to reduced defect annealing, is choosen. During the fluence dependent absorption measurements after the prolonged irradiation at H=10 mJ/cm~2 it is found, that both experiments reveal very comparable absorption data for H=1.3 mJ/cm~2. For investigating standard material with high hydrogen content, i.e. slow absorption increase due to effective defect annealing, a sample is cooled down to -180 ℃ in a special designed experimental setup and irradiated at a laser fluence H=10 mJ/cm~2. To control the increase of the defect density and to determine the end of the TPA induced defect generation, the fluorescence at 650 nm of the generated NBOH centers is monitored. Before and after the low temperature experiment, the absorption coefficient is measured directly by LID technique. By applying both, elevated laser fluence and low temperature, the ArF laser induced generation of E' and NBOH centers in the investigated sample is terminated after about 1.2~*10~7 laser pulses. Therefore, a strong reduction of irradiation time is achieved in comparison to about 10~(10) pulses required in common marathon test applications.