Rapid lifetime testing of fused silica for DUV laser applications

Lifetime testing of fused silica's absorption degradation upon 193 nm is shortened by enhancing the twophoton absorption (TPA) induced generation of E′ and NBOH defect centers per laser pulse. Increasing the irradiation fluence from typical marathon test values Hb1 mJ/cm2 to H=10 mJ/cm2 gives a more efficient TPA process. In addition, the sample's temperature is lowered to −180 °C during irradiation yielding an increased breaking efficiency of weak Si–O bonds per pulse. A small sample length of 10 mm combined with the laser induced deflection (LID) technique for direct absorption measurements prevents microchannel (MC) formation, a common break-down criterion in marathon tests. For a UV grade fused silica sample (type III) the end of absorption degradation is found after about 1.2 *107 laser pulses. Absorption measurements between 3 and 25 mJ/cm2 before and after lifetime testing reveal that the laser induced absorption change decreases with decreasing fluence. The experimental results are in good agreement with a real marathon test at a fixed fluence.