In situ Investigation of $OH^{-}$ Absorption in $LiNbO_{3}$ and $LiNbO_{3}:MgO$ Crystals during Reducing/Oxidizing Annealing

The work presents experimental results of an in situ investigation of the $OH^{-}$ absorption in pure and MgO-doped $LiNbO_{3}$ crystals during reducing (95% Ar + 5% $H_{2}$) and oxidizing $(O_{2})$ high-temperature treatments in the temperature range from room temperature to 820 K. The absorption spectra measured in situ at high temperatures in reducing/oxidizing atmospheres have been analyzed. The origin of the changes in optical absorption caused by heating of the crystal is discussed in terms of the OH-bonds orientation change.