There is disclosed an optical waveguide comprising a K1-xRbxTiOMO4 single crystal substrate, wherein x is from 0 to 1 and M is P or As, having at least one optically smooth surface wherein sufficient K+ and/or Rb+ have been replaced by ions selected from at least one of H+ and NH¿4?+ and, optionally, at least one monovalent ion selected from Rb?+, Cs+, Tl+¿, and/or at least one divalent ion selected from Ba?+2, Sr+2, Ca+2 and Pb+2¿ to result in the surface index of refraction at least about 0.00025 with respect to the index of refraction of the single crystal substrate. One process disclosed for producing an optical waveguide comprises the steps of contacting at least one optically smooth surface of a single crystal of K¿1-x?RbxTiOMO4 with an ion exchange medium capable of supplying said replacement ions, at a temperature of from about 100 °C to about 600 °C for a time sufficient to increase the surface index of refraction at least about 0.00025 with respect to the index of refraction of the single crystal by replacing K?+¿ and/or Rb+ of said single crystal with said ions supplied by the exchange medium, and cooling the resulting crystal. Another process disclosed for producing an optical waveguide comprises the step of applying a DC voltage of from about 20 V per cm of crystal thickness to about 2000 V per cm of crystal thickness across the z-surfaces of a z-cut K¿1-x?RbxTiOMO4 single crystal, in the presence of a proton source at a temperature of from about -40 °C to 400 °C for a time sufficient to raise the index of refraction underneath the anode by at least 0.00025 with respect to the index of refraction of the single crystal by replacing K?+¿ and/or Rb+ of said single crystal with H+. A nonlinear optic device using the optical waveguide of the invention is also disclosed.