Enhanced Cascaded SHG+DFG Process of Femtosecond Pulses Using Chirp Quasi-Phase Matching Waveguide

This study presents a chirp quasi-phase matching (QPM) waveguide to increase the conversion efficiency of the cascaded SHG+DFG effect for femtosecond pulses. The coupled mode theory is used to analyze the cascaded SHG+DFG process. In this paper, a constructive interference zone (or cascaded SHG+DFG region) is defined as when the conversion wave and its coupling component constructively interfere with each other and have a phase difference ranging from 0 to 0.5pi or from 1.5 to 2pi. On the other hand, the region is called a destructive interference zone (or back conversion region) when the phase difference ranges from 0.5 to 1.5pi . The proposed chirp QPM waveguide is designed to extend constructive interference length and shorten destructive interference length over the waveguide. The simulation results demonstrate that for a pulsewidth of 0.6 ps, the maximum conversion efficiency of a 50-mm chirp QPM waveguide can be increased to 25 times that of a uniform QPM waveguide.