Study on the micro-mechanism of MG-doped lithium niobate crystals against photorefraction

The ultraviolet-visible and infrared absorption spectra of Fe2O3 and MgO co-doped lithium niobate (LiNbO3) crystals were studied. It was found that some Fe2+ ions were still in Li-sites though Fe3+ ions changed their site occupation from Li-sites to Nb-sites while the doping concentration of MgO was above the threshold (the first threshold). Defect chemical analysis shows that the amount of Fe2+ ions will decrease as the doping concentration of Mg increases, and when the doping concentration reaches. a suitable value (the second threshold) all Fe2+ ions in Li-sites will diminish. It is expected that LiNbO3 crystal doped with the second threshold will have an excellent ability to resist laser induced optical damage. This phenomenon is called as double thresholds effect.