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The demonstration of a Yb:YAG chirped pulse amplification laser producing 1 J, 5 ps pulses at 500 Hz repetition rate [1] and recently 1 J pulses at 1 kHz repetition rate [2] relied on efficient thermal management and high performance multilayer dielectric coatings on the laser amplifier active mirrors. In the active mirror configuration, the Yb:YAG amplifier crystals use HfO2/SiO2 multilayer dielectric anti-reflection (AR) and high reflection (HR) coatings. The Joule-level amplifier is operated in vacuum and at liquid nitrogen boiling temperature (77 K) with 1030 nm, 220 ps duration laser pulses making four reflections from each HR coating and 8 passes through each AR coating. The LIDT performance of these coatings is crucial to the future scaling of these amplifiers. In this work we describe results of an investigation of the laser induced damage threshold (LIDT) of Yb:YAG active mirror laser amplifier disks at atmospheric, vacuum and cryogenic temperature conditions. The measurements were conducted for 220 ps pulses, the typical pulse duration of stretched pulses we are using to implement kW-class average power CPA laser amplifiers [1,2]. We measured the 1-on-1 (single-shot) and 3000-on-1 LIDT on Yb:YAG crystals with and without the coatings. The results show that the LIDT for single shot damage occurs near 20 J/cm2, and 100% damage probability occurs near 29 J/cm2 for either the uncoated or coated Yb:YAG crystal at atmospheric conditions. Similar results were obtained in the vacuum and cryogenic temperatures tests. This leads to the conclusion that the Yb:YAG material itself, and not the coatings, is the limiting factor in the LIDT. This work was performed under the auspices of the U.S. Department of Energy, Office of High Energy Physics, Accelerator Stewardship Program under Award DE-SC0016136. References [1] C. Baumgarten, M. Pedicone, H. Bravo, H. Wang, L. Yin, C. S. Menoni, J. J. Rocca, and B. A. Reagan, Optics Letters 41, 3339 (2016). [2] Brendan A Reagan, Cory Baumgarten, Elzbieta Jankowska, Han Chi, Herman Bravo, Kristian Dehne, Michael Pedicone, Liang Yin, Hanchen Wang, Carmen S Menoni, Jorge J Rocca, High Power Laser Science and Engineering 6, e11 (2018). |
