Main Cause of Surface Waveguides Formed under LiNbO3 Crystal Surface during Thermal Treatment

The formation of a surface waveguide on LiNbO3 crystals at about 1000°C was examined by systematic experimentation with respect to the content of H2O vapor in the heating atmosphere and OH- ions contained in the crystals before heating. The thermal diffusion of OH- ions led to the formation of inhomogeneous layers which had a refractive-index fluctuation around n S. We concluded that the main cause of the surface waveguide formation was OH- ions. The formation of a surface waveguide and its prevention by H2O vapor are concluded to be regarded as the same mechanism.