Influence of dry-oxygen-annealing on the residual absorption of lithium niobate crystals in the spectral range from 500 to 2900 nanometers

Recent highly sensitive absorption measurements of lithium niobate (LiNbO3) show that within the spectral range between 500 and 2900 nm the transparency is limited by impurities such as iron or hydrogen. In order to reduce the residual absorption, 5-mol.-%-MgO-doped and undoped congruent as well as undoped stoichiometric LiNbO3 crystals are annealed in dry oxygen atmosphere. The extinction coefficient of the treated crystals is measured using whispering-gallery-resonator-based absorption spectroscopy. The conducted measurements show that the treatment of stoichiometric crystals leads to scattering centers. For the congruent material residual metallic ions like iron and copper dominate the absorption in the spectral region from 400-2000 nm, and oxidization only shifts the center of the absorption in our case from that of iron to that of copper, thus inhibiting reaching the theoretical loss limit. Nevertheless, starting from 2000 nm, where absorption caused by hydrogen dominates, annealing leads to a significant drop in absorption, narrowing here the gap to the theoretical loss limit.