Influence of the laser beam size on laser-induced damage in KH2PO4

For large aperture solid state lasers, the laser resistance of the optical component remains an important limitation for the performances and the maintenance costs. Since decades, laser induced damage has been intensively studied in order to understand and control the origin of the phenomenon. LID measurements are commonly performed with table top lasers whose characteristics change from one to another and, sometimes, the scaling laws do not permit to explain the experimental differences. For example, we have previously demonstrated that, in KH2PO4 (KDP) crystals, the laser beam size can influence strongly the determination of the damage probability. Here, we present a systematic study realized on KDP crystal to quantify the influence of the beam size on the LIDT (Laser Induced Damage Threshold) measurement at 355 nm. The use of an unique Gaussian beam ranged from micronic to sub-millimetric sizes permits to highlight different types of laser-damage precursor. LIDT measurements realized with beams of small (lower than 100 pm at 1/e(2))or large (upper than 400 mu m at 1/e2)dimensions give information about the behavior of material regarding precursor defects.