Laser-Induced Damage in DKDP Crystals under Simultaneous Exposure to Laser Harmonics

While KDP and DKDP crystals remain the only viable solution for frequency conversion in large aperture laser systems in the foreseeable future, our understanding of damage behavior in the presence of multiple colors is very limited. Such conditions exist during normal operation where, for third harmonic generation, 1ω, 2ω and 3ω components are present with different energy ratios as they propagate inside the crystal. The objective of this work is to shed light into the damage behavior of frequency conversion crystals during operational conditions as well as probe the fundamental mechanisms of damage initiation. We have performed a series of experiments to quantify the damage performance of pristine (unconditioned) DKDP material under simultaneous exposure to 2ω and 3ω laser pulses from a 3-ns Nd:YAG laser system as a function of the laser fluences at each frequency. Results show that simultaneous dual wavelength exposure leads to a much larger damage density as compared to the total damage resulting from separate exposure at each wavelength. Furthermore, under such excitation conditions, the damage performance is directly related to and can be predicted from the damage behavior of the crystal at each wavelength separately while the mechanism and type of defects responsible for damage initiation are shown to be the same at both 2ω and 3ω excitation.