There is disclosed a method of fabricating an optical waveguiding surface layer in an optically transparent crystalline substrate, such as lithium niobate, by selectively indiffusing metal ions, such as titanium, into the crystal to effect an increase in the index of refraction of a surface portion thereof....http://www.google.de/patents/US4284663?utm_source=gb-gplus-sharePatent US4284663 - Fabrication of optical waveguides by indiffusion of metals
