Effect of the annealing process on active properties of proton-exchanged optical waveguides in erbium-doped lithium niobate

We report about our study of the annealing effect on the resultant chemical composition of the APE (Annealed Proton Exchange) layers and their optical properties, with emphasis on their potential active function. The samples were annealed at various conditions and characterised by a number of nuclear analytic methods (NDP; ERDA; HIERDA) to investigate concentration profiles of the exchanged ions. The content of OH groups, which are undesirable in the active waveguiding layers owing to their effect of erbium excited state lifetimes shortening, was studied by IR absorption spectrometry. The waveguiding properties (number of guided modes, refractive index vs. depth profile) were measured by mode spectroscopy at 632.8 nm. We found out that hydrogen introduced to the surface layers of LiNbO3 by PE (Proton Exchange) moves deeper into the substrate during A (Annealing), lowering thus total refractive index increment. Consequently, the crystallographic phase of the exchanged layers changes towards the a-phase. The lowest amounts of OH groups were found when highest annealing temperatures were used; however, a limitation exists there as temperatures above 400°C cause degradation of the waveguiding properties.