There is disclosed an optical waveguide comprising a K.sub.1-x Rb.sub.x TiOMO.sub.4 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0 to 1 and M is P or As, having at least one optically smooth surface wherein sufficient K.sup.+ and/or Rb.sup.+ have been replaced by ions selected from at least one of H.sup.+ and NH.sub.4.sup.+ and, optionally, at least one monovalent ion selected from Rb.sup.+, Cs.sup.+, Tl.sup.+, and/or at least one divalent ion selected from Ba.sup.+2, Sr.sup.+2, Ca.sup.+2 and Pb.sup.+2 change 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.sub.1-x Rb.sub.x TiOMO.sub.4 with an ion exchange medium capable of supplying said replacement ions for sufficient time at a temperature of from about 100.degree. C. to about 600.degree. C., 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 cam of crystal thickness across the z-surfaces of a z-cut K.sub.1-x Rb.sub.x TiOMO.sub.4 single crystal, in the presence of a proton source for sufficient time at a temperature of from about -40.degree. C. to 400.degree. C. A nonlinear optic device using the optical waveguide of the invention is also disclosed.