Refractive Index and Concentration Profiles of Proton-Exchanged LiNbO3 Waveguides

The extraordinary refractive index profile of proton-exchanged LiNbO3 waveguides as well as the proton and lithium distributions are investigated. The exchange is effected by pure benzoic acid and with lithium benzoate as an additive, on pure and magnesium doped LiNbO3 substrates. The refractive index profiles are reconstructed from measured effective indices by a very reliable improved inverse WKB procedure. Marked deviations from step-function-type profiles are found. The same holds for the lithium distribution which has been determined by SIMS. To check the reconstruction procedure, the waveguiding layer is gradually polished off, and the modes of the remaining guide are measured and fitted. Simultaneously, the infrared absorption by OH- stretching vibrations is recorded. Its cross section is determined to be (4 ± 2) × 10−19 cm2. The effect of adding lithium benzoate, of doping with magnesium as well as annealing, on the stability of the refractive index profile is discussed.