High-power femtosecond (fs) lasers in the visible wavelength regime have numerous applications in areas including micro-machining, medical eye surgery, communication, spectroscopy, etc. To generate this laser beam, frequency conversion, especially second-harmonic generation (SHG), of near-IR lasers using nonlinear optical crystals is known to be the most standard technique. However, the use of a long-length crystal, which is preferred to achieve high SHG conversion efficiency for long-pulse or cw lasers, cannot be applied to the fs laser with broad linewidth due to the tight phase matching condition and the exacerbated walk-off effect. Thus the conditions of the nonlinear optical crystal should be optimized to achieve efficient SHG generation and hence, the high power visible fs laser pulses. There are many reports for the efficient SHG of the fs lasers but not many reports about influence of the crystal length on the SHG process, such as the pulse width and the linewidth and the conversion efficiency.
Here, we report efficient SHG of femtosecond Yb lasers at 1 um by optimizing the conditions of nonlinear optical crystals. The SHG pulse and the conversion efficiency were numerically calculated to find the optimized conditions of the nonlinear optical crystals for the high power fs laser pulses with different pulse widths. Preliminary experiments were conducted using a Type I LBO crystal and the femtosecond Yb laser at 1 um, which was in good agreement with the theoretical results. The theoretical and the experimental results for LBO and BBO crystals will be reported in detail.