Process for producing an optical waveguide

A process for producing an optical waveguide comprising contacting at least one optically smooth surface of a single crystal of K.sub.1-x Rb.sub.x TiOMO.sub.4 wherein x is from 0 to 1 and M is P or As with a molten salt or molten mixture of salts of at least one salt of Rb, Cs and Tl at a temperature of from about 200.degree. C. to about 600.degree. 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 starting crystal, and cooling the resulting crystal, with the provisos that

• (a) when x is from 0 to 0.8, said molten salt is at least one selected from the group consisting of RbNO.sub.3, CsNO.sub.3, TlNO.sub.3, and said molten mixture is at least one of RbNO.sub.3, CsNO.sub.3, TlNO.sub.3 with at least one nitrate selected from the group of Ca(NO.sub.3).sub.2, Sr(NO.sub.3).sub.2 and Ba(NO.sub.3).sub.2 or of RbCl with at least one chloride selected from the group of ZnCl.sub.2 and LiCl;
• and
• (b) when x is at least 0.8, said molten salt is at least one selected from the group consisting of CsNO.sub.3 and TlNO.sub.3 and said molten mixture is at least one of CsNO.sub.3 and TlNO.sub.3 with at least one nitrate selected from the group of Ca(NO.sub.3).sub.2, Sr(NO.sub.3).sub.2 and Ba(NO.sub.3).sub.2.