Numerical study of sum frequency ultrashort pulse compression in borate crystals

Abstract

Second harmonic generation (SHG) of picosecond pulses in type II phase-matched nonlinear optical crystals is not widely employed because of the lower efficiencies compared to the type I phase-matching scheme. The limited efficiencies come from the difference between group velocities (GV difference) of the ordinary and extraordinary polarized input pulses. However, if the input pulses are delayed before the nonlinear crystal, a short second harmonic pulse can be generated with a slowly widening temporal overlap. Furthermore, this GV difference can be controlled by the tilting of the pulse fronts, and optimal GV difference can be obtained to achieve powerful output pulses with durations an order of magnitude lower than those of the input pulses. In this work, we present numerical results of SHG pulse compression in a beta barium borate (BBO) nonlinear crystal, which is ideal for SHG of high-power 1030 nm thin-disk lasers. The pulse compression is controlled by predelay and tilting of the pulse fronts. We find optimal parameters to achieve five-fold increase in output power and 20-fold pulse compression of 1.7 ps input pulses. Finally, we consider the experimental aspects of the group velocity control.