Effect of waveguide uniformity on phase matching for frequency conversion in channel waveguides

We have studied the effect of nonuniformity in graded index, channel waveguide dimensions on the phase-matching condition for guided-wave nonlinear frequency conversion. Both theoretical and experimental results using normalized waveguide parameters and quasi-phase matching for second- harmonic generation in annealed proton-exchanged LiNbO3 waveguides show the existence of an optimum waveguide design which is insensitive to inhomogeneities in the waveguide dimensions. Application of such a waveguide design using normalized approach can significantly relax the fabrication tolerance, leading to nonlinear guided-wave devices with long interaction lengths and useful conversion efficiencies.