Effects of plate thickness and stack distribution of quasi-phase-matched materials on nonlinear frequency generation

Analytical results indicate that the plate-thickness tolerance of the GaAs plates in a quasi-phase-matched (QPM) stack with respect to the coherence length is not a critical parameter. Rather, proper placement order of the plates of different thicknesses that make up the stack is essential for generating efficient nonlinear conversion; conversion efficiency approaching that from a stack of plates with plate thickness equal exactly to the coherence length can be obtained. In effect, such ordered stacking optimizes the relative phase of the nonlinear process as it propagates down the stack and enhances the conversion efficiency. Furthermore, the analysis shows that random-order stacking of plates of different thicknesses produces a large variation in conversion efficiency, varying from one random stack to another. This may not be desirable in practice; one would normally prefer to have a process that produces QPM stacks with good and predictable performance. A single-pass three-wave coupled nonlinear frequency interaction model with temporal and spatial pulse profiles is used to analyze the behavior of the nonlinear conversion process in a quasi-phase-matched stack. Second-harmonic generation of CO₂ laser radiation in GaAs plates is used as an example