Characterization of annealed proton exchanged LiNbO3 waveguides for nonlinear frequency conversion

A thorough and detailed characterization of annealed proton-exchanged (APE) waveguides in Z-cut LiNbO₃ is described. The mode index measurements in planar waveguides as a function of wavelength and annealing time are reported, including useful analytical relations for the refractive index change, its dispersion, and the depth profile as a function of annealing parameters. Analytical expressions for the mode propagation characteristics are presented and experimentally verified with reasonable accuracy. It is shown that the planar waveguide characterization results can be used to model the channel waveguide characteristics accurately. The model provides closed-form expressions for the mode index and the mode field profile, and the theoretical results are in excellent agreement with the measured data. The technique is used to accurately predict the phase mismatch between the fundamental and second harmonic modes in frequency-doubling experiments using APE channel waveguides. An optimum waveguide geometry for which the phase mismatch is relatively insensitive to the waveguide nonuniformity was predicted and verified experimentally