Impact of internal defects on laser-induced damage in Nd³⁺, Gd³⁺ co-doped SrF₂ crystals

The comprehensive investigation into the distribution characteristics of internal defects across various growth areas of Nd, Gd: SrF₂ (NGSF) crystals is presented in this paper by photothermal weak absorption, stress-induced birefringence, and laser confocal microscopy. The stress-gathering and absorption bands resulting from the accumulation of internal defects predominantly consist of fractures, empty tubes, and inclusions with impurities, as observed through SEM. Furthermore, laser-induced damage characteristics were examined to explore the mechanisms of internal defect-induced damage by a 1064 nm pulsed laser, revealing the exponential-Chapman relationship between weak absorption and laser damage threshold fluence. Based on the distribution profile of weak absorption, stress birefringence, and damage points, the damage mechanism is analyzed and attributed to the absorption of laser energy by the internal defects, further emphasizing that these internal defects are precursors to laser-induced damage.