Optimization of second harmonic generation in a Yb:KGW femtosecond laser

Efficient second harmonic generation (SHG) of a Yb:KGW laser with the femtosecond (fs) pulsed output using lithium triborate (LiB3O4, LBO) and barium borate (BaB2O4, BBO) as nonlinear optical (NLO) crystals is reported. Numerical calculation for frequency conversion efficiency as a function of the waist beam size and NLO crystal length shows that efficient SHG of the fs laser beam could be achieved using a relatively short crystal length with a tightly focused beam. We have applied this numerical calculation to SHG of the mode-locked Yb:KGW laser with an 175 fs pulse width and an 11.5 nm linewidth at 1036 nm. Predicted values for the conversion efficiency and its output spectrum are in good agreement with experimental results. Prospects for improvements of the SHG conversion efficiency and, furthermore, sum frequency generation using fundamental and SHG beams will be discussed. (C) 2018 The Japan Society of Applied PhysicsThis work was supported by the Korea Electrotechnology Research Institute Primary research program (18-12-N0101-41) through the National Research Council of Science and Technology (NST) funded by the Ministry of Science, ICT and Future Planning (MSIP) and the Basic Science Research Program (NRF-2017R1A2B4005806) funded by National Research Foundation of Korea (NRF)