Compact, unstable cavity, cryogenically-cooled Yb:YAG q-switch laser for laser shock peening

Recently, we published a novel design approach for unstable laser cavities that overcomes the limitation of such a design to high gain media and presented first results of a laboratory prototype [1]. Based on this, we developed a larger prototype system suitable for applications in laser shock peening featuring a relatively simple layout and a compact footprint of about 0.8x0.6m². The presented laser is based on cryogenically cooled Yb:YAG ceramic equipped with a Cr:YAG absorber cladding to suppress amplified spontaneous emission. As the pump source we utilize a 7kW peak power laser diode module generating a high quality tophat beam with hexagonal lateral profile. With this the system achieves an output energy in excess of 1J in a 10ns to 20ns pulse in q-switch operation mode. The output beam profile adapts well to the hexagonal shape of the pump beam. The laser can be operated with up to 20Hz repetition rate limited only by the cooling capacity of the used cryostat. The application of the system in laser shock peening (LSP) was tested by treating aluminum plates within a test setup. The treated area was then examined regarding residual stress to verify the success of the LSP process. It was found that significant residual stress was generated with a penetration depth of up to 0.6mm and a pressure of 200MPa at the surface. [1] J. Korner, S. Zulic, D. Rostohar, A. Lucianetti, T. Mocek, “Novel unstable resonator configuration for highly efficient cryogenically cooled Yb: YAG Q-switched laser”, Optics Express, 27(15): 21622-21634, 2019