355 nm and 1064 nm-pulse mixing to identify the laser-induced damage mechanisms in KDP
2011
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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