Rapid lifetime testing of fused silica for DUV laser applications

Abstract

Lifetime testing of fused silica's absorption degradation upon 193 nm is shortened by enhancing the two-photon absorption (TPA) induced generation of E′ and NBOH defect centers per laser pulse. Increasing the irradiation fluence from typical marathon test values H < 1 mJ/cm 2 to H = 10 mJ/cm 2 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 * 10 7 laser pulses. Absorption measurements between 3 and 25 mJ/cm 2 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.