The ultraviolet-visible and infrared absorption spectra of Fe_2O_3 and MgO co-doped lithium niobate (LiNbO_3) crystals have been investigated. The experimental results show that some Fe~(2+) ions are still in Li-sites while the doping concentration of MgO is above threshold. Our results show that the large reduction of defect (Mg_(Li)~+ -V_(Li)~-) in nearly stoichiometric LiNbO_3 is the key factor to reduce the content of (Fe_(Li)~+ -V_(Li)~-) for anti-photorefraction. Based on defect chemical analysis, we obtained that only 1.3 mol.% of MgO is needed to suppress the optical damage completely in a nearly stoichiometric LiNbO_3 crystal with [Li]/[Nb]=49.9/50.1 while 21.2 mol.% needed in congruent ones.
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