Laser damage thresholds of underwater anti-reflection coatings for face-cooling of high power Yb:YAG laser disks

Achieving high laser damage thresholds of face-cooled amplifier slabs is an important issue to develop high average power lasers. We report an investigation of the Laser Induced Damage Threshold (LIDT) of Ion Beam Sputtering (IBS) hafnium oxide anti-reflection layers deposited on Yb:YAG crystals, designed to be embedded in a flowing water coolant while submitted to 1 μ m laser irradiation, and compare the damage thresholds in air and underwater. The measured damage thresholds turn out to be consistent with the best values obtained on similar coatings, and are at least equal, or even higher, underwater than in air. An electromagnetic model is presented based on water permeation into the coating, that displays stronger field enhancements in air than underwater next to the sub-surface damage precursors, offering a possible explanation. Our results validate the possibility to use anti-reflection coatings for Yb:YAG crystals face-cooled in water, at fluences equal to or higher than the saturation fluence of Yb:YAG at room temperature. • LIDT of Yb:YAG crystals with antireflection coatings is as high underwater as in air. • One sample exhibited higher LIDT underwater. • The LIDT reaches the Yb:YAG saturation intensity at room temperature. • Water permeation into the coating may lead to reduced field intensities on defects.