Shaping of picosecond laser pulses by second harmonic generation with time predelay

Abstract

Shaping and compression of short laser pulses in the course of a second harmonic (2H) generation is a well verified technique in the case of Nd:glass laser radiation converted in KTP crystal cut for type II phase-matching. The laser pulse, few ps in duration, is divided into two orthogonally polarized pulses and one is delayed in respect to the other. Both of them enter the nonlinear optical crystal, in which they propagate with different group velocities. As they are approaching each other, short 2H pulse is generated at their temporal overlap. Shaping of the 2H pulse is achieved by changing the predelay and input pulse intensities. For current high power picosecond laser sources of choice, Yb:YAG lasers, borate crystals have to be used for efficient harmonic generation because of their convenient thermal properties and long lifetime. Although the requirement on group velocities is not fulfilled for the LBO and BBO crystals, numerical calculations have revealed conditions where pulse shaping and even significant pulse compression occur. Fundamental input pulse of 1.8 ps was compressed down to 0.8 ps (FWHM) by 2H generation in LBO crystal. In this work, details about the calculations and measurements of shaping of 2H picosecond Yb:YAG laser pulses in LBO and BBO crystals in dependence on input intensities and predelay will be presented. The shaped 2H pulse measurements performed by an autocorrelator agree well with the calculations, achieving almost 3-fold compression. For the most interesting cases, 2H pulses are characterized by FROG in detail.