Identification of the laser-induced damage mechanisms in KDP by coupling 355nm and 1064nm nanosecond pulses
2010
Nanosecond Laser-Induced Damage (LID) in potassium dihydrogen phosphate (KH 2 PO 4 or KDP) remains an
issue for light-frequency converters in large-aperture lasers such as NIF (National Ignition Facility, in USA) and
LMJ (Laser MegaJoule, in France). In the final optic assembly, converters are simultaneously illuminated by
multiple wavelengths during the frequency conversion. In this configuration, the damage resistance of the KDP
crystals becomes a crucial problem and has to be improved. In this study, we propose a refined investigation
about the LID mechanisms involved in the case of a multiple wavelengths combination. Experiments based on an
original pump-pump set-up have been carried out in the nanosecond regime on a KDP crystal. In particular, the
impact of a simultaneous mixing of 355 nm and 1064 nm pulses has been experimentally studied and compared
to a model based on heat transfer, the Mie theory and a Drude model. This study sheds light on the physical
processes implied in the KDP laser damage. In particular, a three-photon ionization mechanism is shown to be
responsible for laser damage in KDP.
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