A method of forming graded index, nitrogen-doped optical waveguides. A cylindrically-shaped porous preform comprising at least two oxides is initially formed. One of the oxides, which more readily reacts with nitrogen, has a greater concentration near the center of the preform than at the outer portion...http://www.google.com/patents/US4203744?utm_source=gb-gplus-sharePatent US4203744 - Method of making nitrogen-doped graded index optical waveguides
