Second-harmonic generation based on quasi-phase matching: a novel configuration.

We consider the possibility of generating second-harmonic radiation by the interaction of two counterpropagating fundamental waves in a quasi-phase-matched second-order nonlinear medium in a waveguide. The conversion efficiency reaches a maximum at an optimum pump intensity. For a large range of pump intensities below a certain pump intensity the efficiency is larger than that for the copropagating configuration. The saturation intensities at which the efficiency reaches 72% for poled KTiOPO(4) and LiNbO(3) can be achieved with lasers now available.