The issues of laser-induced damage of transparent dielectric optics severely limit the development of large laser systems. In order to explore the mechanism of nanosecond laser damage on KDP surface, a multi-physics coupling dynamics model and a time resolved detection system were developed to obtain the transient dynamic behaviors of laser damage. The behaviors of laser energy transmission, thermal field distribution and damage morphology during nanosecond laser irradiation on KDP surface were simulated. It is found that the enhancement of light intensity caused by surface defect plays an important role in the initial energy deposition and damage initiation of the laser irradiation area. The evolution of the temperature field and fluid flow during subsequent laser irradiation contributes to the laser damage process. The simulated evolution of heat absorption source is verified by the transient images of local defect-induced laser damage captured by the ultra-fast experimental detection system. This work provides further insights in explaining the laserinduced damage by surface defects on KDP crystals.
KEYWORDS: Modulation, Crystals, Diffraction, Laser induced damage, Optical components, Laser crystals, Micro cutting, Micromachining, Near field diffraction, High power lasers
Micro-machining has been proved the most effective method to mitigate the laser-induced surface damage growth on potassium dihydrogen phosphate (KDP) crystal in high power laser systems. However, the phase contrast of outgoing laser beam, introduced by the mitigated KDP surface, would cause light propagating turbulence and downstream intensification with the potential to damage downstream optics. In this work, a Gaussian mitigation pit with width of 800μm and depth of 10μm is fabricated on KDP rear surface by micro-milling. The effect of the mitigation pit on downstream light intensification is analyzed through propagation calculations based on Fresnel diffraction integral theory. The light intensity modulations reach a peak value at the position of 10mm downstream from the rear surface, decrease sharply subsequently and get stable eventually. The results indicate that the modulations induced by Gaussian mitigation pits would change with various downstream locations. It is essential to notice the unacceptable downstream intensification and reduce the risk of laser damage on other optics by choosing an appropriate installation location.
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