Low-loss optical waveguides in lithium tantalate by vapor diffusion

Planar and channel optical waveguides have been produced in Y-cut LiTaO3 substrates by zinc diffusion from the gaseous phase at 800 °C. Prism coupler measurements on planar waveguides diffused for 6 h indicate effective index changes of 0.0063 and 0.0033 for TE and TM polarizations, respectively, with effective diffusion depths of 3.5 and 3.9 μm for the two polarizations. Losses in channel waveguides at 0.633 μm wavelength were 1.0±0.2 dB/cm for TE polarized fields, as compared with 4.7±1.0 dB/cm measured on companion samples produced by the conventional Ti indiffusion process at 1150 °C. Electro-optic modulation has been demonstrated in channel waveguide interferometers produced by this technique. Based on electron microprobe analysis, a surface Zn concentration of ∼1.3×1021 cm−3 has been determined.