Influence of the defect structure on the refractive indices of undoped and Mg-doped lithium niobate

We measured the refractive indices of undoped and Mg-doped lithium niobate by an interferometric technique in a wavelength range from 400 to 1200 nm. The composition of the undoped samples varied between 47 and 50 mol % Li₂O in the crystal whereas the Mg-doped samples were grown from congruent melt with up to 9 mol % Mg. Our results can be excellently described by a generalized Sellmeier equation which takes into account the defect structure of Li-deficient and Mg-doped lithium niobate. Calculations of the phase-matching conditions of several nonlinear effects in Mg-doped lithium niobate reveal good correspondence with the respective experimental results in a temperature range from -50 to 250 ^∘{}C.