Planar LiTaO3 waveguides fabricated by proton exchange in concentrated and diluted pyrophosphoric acid with annealing

Annealed proton-exchanged z-cut LiTaO3 planar waveguides fabricated with pyrophosphoric acid have been characterized. For proton exchange, the extraordinary index increase Delta n(e) ranged from 0.0119 to 0.0141, depending on the exchange temperature. The effective diffusion coefficient D-e ranged from 0.1325 mu m(2)/h at 240 degrees C to 0.545 mu m(2)/h at 280 degrees C. Single-mode propagation losses were alpha=0.7 dB/cm. Compared with benzoic acid, pyrophosphoric acid produces waveguides with a higher Delta n(e) and a lower propagation loss. For proton exchange in lithium phosphate-diluted pyrophosphoric acid, a lower Delta n(e) was obtained, but D-e and propagation losses were not reduced. After proton exchange, the waveguides were annealed. The surface index initially increased. peaked, and then decreased. The waveguide depth d and the surface index n(s) were measured at regular intervals. Figures and empirical formulas relating the waveguide depth d and the surface index increase Delta n(s) to the anneal time, anneal temperatures, and the waveguide depth after proton exchange are given. An example is given in which, the desired waveguide parameters d and Delta n(s), the fabrication conditions could be calculated with the previously derived formulas. Propagation losses decreased to 0.4 dB/cm after prolonged annealing. (C) 1996 Optical Society of America