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

We investigate the second harmonic generation of high-power laser pulses un der quasi-phase-matched conditions in periodically poled materials. The loc al phase-mismatch existed in every single domain of a periodical structure is found to affect the performance of the doubling process. For frequency d oubling 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 o verall phase-mismatch. The doubling efficiency and the quality of the secon d harmonic pulses are significantly improved by compensation between the lo cal and overall phase mismatches. Furthermore, we show by numerical simulat ions 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 mism atches and the existence of group-velocity mismatch between the fundamental and the harmonic. (C) 1999 Elsevier Science B.V. All rights reserved.