Influence of various imperfections on the conversion efficiency of second-harmonic generation in quasi-phase-matching lithium niobate waveguides

The generation of coherent blue light by frequency doubling of radiation from semiconductor lasers in quasi-phase-matching lithium niobate waveguides has recently been demonstrated, but with a conversion efficiency significantly lower than the theoretically predicted value. We have experimentally investigated the performance of some quasi-phase-matching waveguides by measurements of the second-harmonic power and the integral of the second-harmonic power with respect to the fundamental wavelength at a wavelength scan. These experimental results are combined with a theoretical analysis. It is concluded that the inhomogeneities in the effective index, the absence of domain inversion in some regions along the waveguide, and the suppression of the nonlinearity that is due to the proton exchange process are the most important imperfections to deal with in the future optimization of the domain inversion process and the waveguide formation.