Statistical interpretation of S-on-1 data and the damage initiation mechanism

Abstract

Multipulse laser induced damage optical materials is an important topic for many applications of nonlinear crystals. We studied multi pulse damage in X-cut KTiOPO₄. A 6ns Nd:YAG laser has been used with a weakly focused beam. A fatigue phenomenon has been observed and we try to clarify the question whether or not this phenomenon necessarily implies material modifications. Two possible models have been checked, both of them predicting increasing damage probability with increasing pulse number while all material properties are kept constant: (i) Pulse energy fluctuations and depointing increase the probed volume during multiple pulse experiments. The probability to cause damage thus increases with increasing pulse number. However, this effect turned out to be too small to explain the observed fatigue. (ii) Assuming a constant single shot damage probability p1 a multipulse experiment can be described by statistically independent resampling of the material. Very good agreement has been found between the 2000-on-1 volume damage data and the statistical multipulse model. Additionaly the spot size dependency of the damage probability is well described by a precursor presence model. Supposing that laser damage precursors are either transient or, if they are permanent, irradiation of the precursor above its threshold only causes damage with a small probability, the presented data can be interpreted without supposing material modifications.