Adiabatically tapered periodic segmentation of channel waveguides for mode-size transformation and fundamental mode excitation

We report modeling, fabrication, and characterization of tapered waveguides, using periodically segmented annealed proton exchange in LiNbO(3). For a taper transforming the 1/e full width (intensity) mode size from 6.0 microm x 4.4 microm to 2.0 microm x 1.3 microm, 0.4-dB excess loss was observed. The large, slightly elliptical mode can ease coupling to single-mode fibers and circular free-space beams. The same taper structure was used to excite a highly multimoded (13 modes) waveguide with more than 95% of the power stably launched into the lowest-order mode. This function is useful for nonlinear waveguide devices, such as difference-frequency generators, in which modes at highly disparate wavelengths interact.