UV-induced aging leading to laser damage in the bulk of fused silica

Laser damage measurements with multiple pulses at constant fluence (S-on-1 measurements) are of high practical importance for design and validation of high power photonic instruments. Mimicking the usual operation conditions, they allow observing possible modifications of the laser damage behavior during operation. In fact, nanosecond S-on-1 tests often reveal the “fatigue effect”, i.e. a decrease of the laser damage threshold with increasing pulse number. When irradiating with ultraviolet wavelengths, the fatigue effect is caused by cumulative material modifications. Systematic improvement of the concerned optical materials can only be achieved if the material modifications operated by the laser irradiation are identified. In this presentation we will show our latest results on the material modifications observed by photoluminescence in the bulk of fused silica. Causing the modifications and pumping the photoluminescence at 266 nm, modifications in the color center concentrations can be observed before the occurrence of damage. These observations can thus help to predict imminent fatigue laser damage under certain irradiation conditions. The lifetime and the nature of the observed modifications differ for low OH and high-OH silica types. Although bulk fatigue damage is only limiting at 266 nm, we also made first investigations using 355 nm as modification wavelength. However, the lifetime of the modifications causing the reduced laser damage threshold is much longer than the lifetime of the modified color centers, indicating that the observed modifications only accompany the initial stage of the problematic and still unknown modifications that weaken the damage threshold.