Er- and Yb-doped congruent LiNbO3 (CLN) single crystals and Cr3+- and Mg2+-doped stoichiometric LiNbO3 (SLN) single crystals were grown by the Czochralski method using additional liquid charging. Changes in the absorption spectra of the transitions F-2(7/2) -> F-2(5/2) in Yb3+ ions and I-4(15/2) -> H-2(11/2) in Er3+ ions along the length of a single-crystal plate correlate with a specific gradient of concentration of Yb3+ and Er3+ ions in the crystal. The correlation of the absorption spectra of the optical centers of Cr3+ ions in stoichiometric LiNbO3 single crystals with the content of magnesium along the length of the crystal is reported. (C) 2008 Elsevier B.V. All rights reserved,