Multipulse laser damage in potassium titanyl phosphate: statistical interpretation of measurements and the damage initiation mechanism

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Multipulse laser-induced damage is an important topic for many applications of nonlinear crystals. We studied multipulse damage in X-cut KTiOPO4. Using a 6-ns Nd:YAG laser with a weakly focused beam, a fatigue phenomenon was observed. We addressed whether this phenomenon necessarily implies material modifications. Two possible models were tested, both of them predicting increasing damage probability with increasing pulse number while all material properties are kept constant. The first model, pulse energy fluctuations and depointing, increases the probed volume during multiple pulse experiments. The probability to cause damage thus increases with increasing pulse number; however, this effect is too small to explain the observed fatigue. The second model assumes a constant single-shot damage probability p1, so a multipulse experiment can be described by statistically independent resampling of the material. Very good agreement was found between the 2000-on-1 volume damage data and this statistical multipulse model. Additionally, the spot size dependency of the damage probability is well described by a precursor presence model. Supposing that laser damage precursors are transient, the presented data explain the experimental results without supposing material modifications.