Abstract
Laser induced damage threshold (LIDT) is an important optical indicator for nonlinear Potassium Dihydrogen Phosphate (KDP) crystal used in high power laser systems. In this study, KDP optical crystals are initially machined with single point diamond turning (SPDT), followed by water dissolution ultra-precision polishing (WDUP) and then tested with 355 nm nanosecond pulsed-lasers. Power spectral density (PSD) analysis shows that WDUP process eliminates the laser-detrimental spatial frequencies band of micro-waviness on SPDT machined surface and consequently decreases its modulation effect on the laser beams. The laser test results show that LIDT of WDUP machined crystal improves and its stability has a significant increase by 72.1% compared with that of SPDT. Moreover, a subsequent ultrasonic assisted solvent cleaning process is suggested to have a positive effect on the laser performance of machined KDP crystal. Damage crater investigation indicates that the damage morphologies exhibit highly thermal explosion features of melted cores and brittle fractures of periphery material, which can be described with the classic thermal explosion model. The comparison result demonstrates that damage mechanisms for SPDT and WDUP machined crystal are the same and WDUP process reveals the real bulk laser resistance of KDP optical crystal by removing the micro-waviness and subsurface damage on SPDT machined surface. This improvement of WDUP method makes the LIDT more accurate and will be beneficial to the laser performance of KDP crystal.
Highlights
Potassium Dihydrogen Phosphate (KDP) crystal is currently one of the important and irreplaceable optical components used in high power laser systems for inertial confinement fusion (ICF)research [1,2,3]
The magnetorheological finishing (MRF) [21,22] and water dissolution ultra-precision polishing (WDUP) [23,24,25] have been frequently studied recently, for they can remove the single point diamond turning (SPDT) machined marks left on the crystal surface
KDP crystals are processed to an ultra-precision surface with SPDT and WDUP methods
Summary
Potassium Dihydrogen Phosphate (KDP) crystal is currently one of the important and irreplaceable optical components used in high power laser systems for inertial confinement fusion (ICF)research [1,2,3]. For nanosecond scale pulsed-laser radiation widely used in fusion-scale experiment, laser induced damage in KDP is usually believed to be initiated at nanometer absorbing centers in the bulk of the material during crystal growth [6,7,8] and the small defects introduced during the technological fabrication processes [9,10,11]. They either absorb energy and conduct it to the surrounding host material leading to rapid heating and ionization (thermal explosion theory) [12], or create surface modulation effect on lasers and transmission. Polishing, aimed at removing the diamond tool turned waviness, is considered a feasible way to increase the LIDT of KDP optical components
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