Influence of the method of strong doping on composition uniformity and optical properties of LiNbO$_3$ : Mg single crystals

Raman spectroscopy, laser conoscopy and photoinduced light scattering methods have been applied to comparatively study composition uniformity of strongly doped LiNbO$_3$ crystals with a magnesium concentration close to a threshold value $\sim$ 5 mol% MgO, grown from a charge synthesized using precursor Nb$_2$O$_5$ : Mg (homogeneous doping method) and at direct addition of magnesium to the melt (direct doping method). It has been shown that application of homogeneous doping method allows one to obtain compositionally more homogeneous heavily doped LiNbO$_3$ : Mg crystal than direct melt doping method.