Abstract: A generally circular or elliptical waveguide (2) is fabricated within a substrate (3) such that the waveguide is surrounded by two different indices of refraction, with the index inside the waveguide ring being greater than the index outside the ring. This results, in combination with a varying index of refraction profile across the width of the waveguide inself, in a tendency to confine mode energy away from the outer perimeter of the waveguide ring where leakage is most likely to occur. As a result the asymmetrical index profile compensates for the curvature of the waveguide, wave phase front velocities are made uniform, and the mode energy remains confined within the waveguide. A two step ion exchange technique is disclosed for selectively increasing the index of refraction of the substrate and for forming the variable index of refraction profile within the waveguide.