Effect of local phase-mismatch on frequency doubling of high-power femtosecond laser pulses under quasi-phase-matched conditions

Abstract

We investigate the second harmonic generation of high-power laser pulses under quasi-phase-matched conditions in periodically poled materials. The local phase-mismatch existed in every single domain of a periodical structure is found to affect the performance of the doubling process. For frequency doubling of high-power femtosecond pulses, this effect can be so severe that back conversion occurs and the doubling efficiency drops dramatically. We also propose a method to suppress the undesired effects by introducing an overall phase-mismatch. The doubling efficiency and the quality of the second harmonic pulses are significantly improved by compensation between the local and overall phase mismatches. Furthermore, we show by numerical simulations that the pulse duration of the back-converted fundamental pulse can be reduced by a factor of two, resulting from the interplay of the phase mismatches and the existence of group-velocity mismatch between the fundamental and the harmonic.