Nonlinear optical investigation of the optical homogeneity of KNbO_3 bulk crystals and ion-implanted waveguides

We report second-harmonic generation (SHG) measurements in KNbO3 bulk single crystals and ion-implanted KNbO3 planar waveguides. The SHG properties of the bulk crystal and the waveguide are compared, in particular, with respect to the birefringence homogeneity. Two methods are employed: analysis of wavelength tuning curves obtained with collinear SHG and investigation of temperature tuning properties observed with noncollinear SHG. The wavelength dependence of the SHG conversion efficiency is explained by simple models based on the nonuniformity of the birefringence. The calculations are in good agreement with the measured changes of the birefringence. Except for one out of six planar waveguides, the ion-implantation process does not induce any considerable nonuniformity in addition to that already present in the bulk material.