Influence of laser beam size and wavelength in the determination of LIDT and associated laser damage precursor densities in KH2PO4

The laser resistance of large optical components remains an important limitation for the performances and the maintenance costs in LMJ or NIF projects. For practical reasons LIDT studies are commonly performed with small samples and table top lasers whose characteristics change from one to another. In these conditions, it is necessary to know exactly the influence of the different experimental parameters (wavelength, spot size, ...) on the final data. These considerations are particularly true in heterogeneous materials as KDP crystals. Indeed the use of different laser beam sizes (from μm to hundreds μm) to plot laser damage probability curves had clearly shown that at 355nm in KDP, it is possible to exhibit a limit of irradiated area which permit to distinguish two different LIDT associated with two laser damage precursors densities. This prior result has put in evidence the influence of irradiated beam size in the discrimination of different kinds of defects in KDP. We present in this paper a systematic study of beam size effect in KDP for three different wavelengths: 355nm, 532nm and 1064nm. This study performed in 1:1 and R:1 mode will reveal precursors for each wavelength and their respective evolution under repetitive shots for small and large beams. This multi-parameters study will help us to highlight mechanisms involved in laser-induced damage in KDP crystal.