Refractive index profiles and exchange ratios of proton-exchanged waveguides in congruent and MgO-doped LiNbO3

By changing the melt composition the exchange ratio of proton-exchanged LiNbO3 was varied. The near surface exchange ratio was measured by elastic recoil detection, and it was related to the re fractive index increase. The exchange ratio influences the refractive index profile and the diffusion coefficient. Three crystal phase transitions are clearly detectable. The refractive index profiles are never simple step index profiles, but all of them have a marked index discontinuity connected with a phase transition. The critical values at which the phase transitions occur in both congruent and MgO-doped single crystals differ from those in powder. The temporal mode index instability is comparable for all waveguides whose crystal phase was changed during the exchange.