Abstract
In this thesis, by using type-I critical phase matching in Lithium Triborate (LBO), we demonstrate second harmonic generation of a high-power picosecond ytterbium-doped fiber laser system. Using the classical Boyd–Kleinman theory, we determined the optimal focal parameter to be 2.8 ~ 2.9 for second harmonic generation in focused beam case. However, the optimal focal parameter varies in different nonlinear crystals due to the walk-off effect. Type-I critical phase matching in Lithium Triborate has optimal focal parameter of 1.8 ~ 1.9. The corresponding walk-off angle is 0.4°. We get most efficient second harmonic generation by using lens of which the focal length was 125 mm. The maximum output power of our fiber laser system is 50 W, with a spectral bandwidth of 4.8 nm. Its polarization state is elliptical. With 18.8 W of linearly polarized incident light, we can generate 2.3 W of green light (λ=532 nm) with efficiency of 12.2 %. Due to self-phase modulation, the spectrum bandwidth broadens from 1 nm to 4.8 nm after fiber amplification. The conversion efficiency saturates in the high power region. The experimental data are in good agreement with simulation results using Boyd-Kleinman theory. By optimizing polarization state of the fundamental light at 50 W, we generate 5.5 W of the green output.