Suppression of degenerate parametric processes limiting frequency-doubling efficiency of crystals

A theoretical and experimental investigation was made of the competition between second harmonic generation and degenerate parametric amplification of the fundamental-frequency wave in three-wave interactions in two crystals arranged in series. It was found that the angular spectrum of the second harmonic and conversion efficiency were governed by the signs of the coupling coefficients and phase mismatch (which depended on the relative orientation of the crystals), as well as by the dispersion in the air gap between the crystals. All types of relative orientation of crystals of the $\bar42m$ class were investigated. The results were used to find the orientation ensuring maximal suppression of the parametric amplification of the fundamental-frequency wave, which limited the frequency-doubling efficiency.