Absorption measurements on 5 mol. % MgO-doped and undoped congruent lithium niobate (LiNbO(3)) crystals revealed absorption bands in the wavelength range of 2500-2800 nm, caused by incorporated hydrogen ions. High-temperature annealing was applied to the congruent LiNbO(3) (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.