It has been shown that doping lithium niobate with some metal ions, e.g. Fe, Mn, Cu, an increase of its photorefractive response can be achieved. Under specific circumstances, the photorefractive response ruins the optimum conditions for the Lorentz- oscillator-type nonlinear optical effects, and one cannot therefore obtain the highest nonlinear optical parameters. However, doping with Mg, Zn, or In, the photorefractive effect in lithium niobate almost vanishes giving the opportunity of taking advantage of its oscillator-type nonlinear optical properties. In the case of LiNbO3:Mg a threshold in magnesium content exists at which the photorefractive effect is almost destroyed. This threshold value is approximately 4.6 mol % MgO in congruent melt.
It is generally accepted that an increase in the photoconductivity of magnesium- doped lithium niobate leads to an inhibition of its photorefractive response when above threshold. However, a direct relationship between the increase of photoconductivity and the photorefractive inhibition was not observed in this work.