Laser raster conditioning of KDP and DKDP crystals using XeCl and Nd:YAG lasers

Abstract

Laser conditioning by raster scanning KDP and DKDP crystals using Nd:YAG and XeCl excimer laser systems was demonstrated. The laser systems were evaluated to determine their respective feasibility of improving the damage thresholds of the harmonic materials for use on the National Ignition Facility (NIF). Crystals were first evaluated using an Nd:YAG laser (355 nm, 7.6 ns) by scanning 2 x 2 cm2 areas with sub-damage threshold fluences and then performing unconditioned (SA) damage tests at 355-nm in the respectively scanned regions. Subsequently, five KDP and DKDP samples of various damage quality were raster scanned in a similar fashion at MicroLas GmbH (Goettingen, Germany) using a commercial Lambda Physik Excimer system (XeCl, {lambda} = 308 nm, 20 ns). The samples treated in Germany were then tested at Livermore National Laboratory (LLNL) at 355 nm to demonstrate the excimer's potentia1 as an alternative conditioning source. The excimer scan results suggest that crystals can be treated at high fluence (50 Ycm2, 308-nm, 204s) levels without noticeable bulk damage. In addition, comparable conditioning is possible even with the fluence set at 30% of the 308-nm damage threshold. The laser damage tests with 355-nrn on the majority of the excimer laser-treated crystals demonstrates themore » effect of conditioning, by raising the SI1 threshold or by reducing the low fluence tail of the 355-nm S/1 damage probability curves. Furthermore, the high average power and flat top beam profile of an excimer laser makes it possible to laser condition a 42-crn NIF-size crystal in one day, compared to 41 days for a commercial table-top Nd:YAG system. The test samples were to be particularly susceptible to surface damage during excimer raster conditioning, possibly due to high levels of dust and/or contaminants in the laboratory environment.« less