Behavior of fused silica irradiated by low level 193 nm excimer laser for tens of billions of pulses

Fused silica samples from seven suppliers have been irradiated with low-level (0.1 mJ/cm2) 193 nm irradiation using an ArF excimer laser. At high pulse counts of tens of billions of pulses, a decrease of the optical path length is observed in the irradiated area. Two of the samples were irradiated with a higher fluence source of 3 mJ/cm2 for 100 million pulses in a separate area. That higher level exposure produced an increase in the optical path length as expected due to compaction. Birefringence measurements were also made which show that the character of birefringence is different for the high fluence and low fluence irradiated areas. A description of the test and measurements is presented along with data covering pulse counts to 40 billion pulses.