Abstract

Ti indiffusion is commonly used to make optical waveguides in LiNbO₃. The typical Ti concentration is ~2%. The effect of such a high impurity level to the crystallinity of the substrate has not been studied. We have used an optical confocal microscope to study the surface morphology of Ti:LiNbO₃ waveguides. Confocal microscopy provides diffraction-limited resolution and a very narrow depth of view by using spatial filters. The surface morphology can be seen with excellent contrast. Each stripe waveguide is delineated by two dark bands indicating the formation of a bulge due to Ti indiffusion. In addition, there are V- shaped chevron fringes with apexes located at the center of the waveguides. These fringes bend approximately every 2-20 µm. They are primarily V-shaped. However, they are not perfectly regular. They only exist on the surface with Ti indiffused waveguides. We believe that these fringes are microcracks resulting from the incorporation of Ti into LiNbO₃. The large Ti concentration can only be accommodated by an increase in volume. A bulge is developed on the surface of the waveguides. As the bulge forms, microcracks along certain preferred crystal axes also form. Such microcracks lead to increased scattering, hence, reduced extinction ratio, and premature breakdown when a large modulation voltage is applied. They can only be eliminated by reducing the Ti concentration.

© 1989 Optical Society of America