Characterization of proton-exchange and annealed LiNbO_3 waveguides with pyrophosphoric acid

Low-loss optical waveguides in Z-cut LiNbO3 made by proton-exchange with pyrophosphoric acid are investigated. The index increase of 0.131 can be reduced by thermal annealing, which provides a low mismatch of optical fields between a Ti-diffused waveguide and a proton-exchange one. Fabrication conditions of single-mode waveguides are presented. Single-mode slab waveguides without and with annealing show low propagation losses at 0.25 and 0.15 dB/cm with an effective index of 2.2932 and 2.2068, respectively. These waveguides are designed to be just below the cutoff of the next higher-order mode. The diffusion coefficient resulting from both proton-exchange and annealing is also discussed. The fabrication conditions and the prescribed parameters give useful information for designing and fabricating waveguides by proton-exchange and annealing.