Laser-induced modifications in fused silica up to damage initiation caused by multiple UV nanosecond pulses

Fatigue effects in fused silica have been largely studied in the past years, as this phenomenon is directly linked to the lifetime of high power photonic materials. Indeed, in the UV regime, we observe a decrease of the LIDT when the number of laser shots increases and this has been attributed to laser-induced material modifications. Under 266 nm laser irradiation, with nanosecond pulses of constant fluence, we observed that the photoluminescence is modified until damage occurs. High-OH fused silicas like Suprasil, "UV fused silica" or Herasil show NBOHC (Non-Bridging Oxygen Hole Center) luminescence at 664 nm (1.87 eV) whereas low-OH fused silica like Infrasil shows ODC (Oxygen Deficient Center) luminescence at 404 nm (3.07 eV). We found that the laser-induced density of NBOHCs increased until bulk damage occurred while the ODC's density decreased. We propose a new representation of the experimental S-on-1 breakdown data which allows predicting the occurrence of material breakdown consuming fewer sample surface and saving time compared to the classic representation N-d (Number of shots before damage) versus F (Fluence). The link between LIF and the modifications leading to breakdown is however modified if a break is used during the irradiation.