Suppression of mid-infrared light absorption in undoped congruent lithium niobate crystals

Absorption measurements on 5 mol. % MgO-doped and undoped congruent lithium niobate (LiNbO3) crys- tals revealed absorption bands in the wavelength range of 2500–2800 nm, caused by incorporated hydrogen ions. High-temperature annealing was applied to the congruent LiNbO₃ (CLN) crystals, which decreased the absorption significantly. Then the annealed CLN crystals were periodically poled. As an application of the low-loss annealed CLN crystal, the operation of a 1550 nm pumped singly resonant CW optical parametric oscillator, resonant around 2600 nm, using a periodically poled crystal was demonstrated.