Microscopic characterization of laser-induced damage performance of large-size KDP and DKDP nonlinear crystals

KDP and DKDP are unique materials for frequency conversion in large-aperture laser systems. Under high power irradiation, a threshold exists above which multiple damage sites are formed in the bulk of crystal plates thus obstructing beam propagation and creating undesirable beam modulations. Damage testing has focused on measuring the irradiation threshold fluences that lead to irreversible material modifications. However, small amounts of damage in optical components have been determined not to hinder system performance in large-aperture laser systems. In this work, we present a new approach to evaluating damage performance that provides statistics on damage pinpoint density, size and morphology as a function of fluence, wavelength and pulse duration and relates that to the resulting beam obscuration. We measure the size of damage sites for different wavelengths, pulse-lengths, and fluences. Different pulse-lengths are approximated by using multiple pulses appropriately delayed with respect to each other. We find that in KDP/DKDP crystals, the size of damage sites strongly depend on the pulse-length, with longer pulses creating larger damage sites. Also examined are ways of laser-annealing to increase the damage resistance.