The up-converted luminescence at 525, 550 nm and 670 nm wavelengths, as well as the ordinary luminescence near
1600 nm wavelength of the LiNbO<sub>3</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup> crystal are investigated under optical excitations at 808 nm and 980 nm.
Dependencies of luminescence on intensities of optical pumping at both wavelengths are presented. The possible
mechanisms of up-converted emissions obtaining as well as population mechanisms of the Er<sup>3+</sup> ion <sup>4</sup>I<sub>13/2</sub> energy level in
the LiNbO<sub>3</sub> matrix are discussed.
One of the promising ways to improve the efficiency of lasers operating at wavelengths near 1600 nm and 3000 nm on
base of garnets doped with Er<sup>3+</sup> ions is their sensitization with Ce3+ ions. In this work the investigation results of spectral
characteristics of YAG:Er<sup>3+</sup>, YAG:Ce<sup>3+</sup> and YAG:Ce<sup>3+</sup>, Er<sup>3+</sup> crystals at room temperatures are presented. Dependencies
of these characteristics on the optical excitations at 457 nm, 808 nm and 980 nm wavelengths are studied. The influence
of Ce<sup>3+</sup> sensitizer ions on the efficiency and mechanisms of the various energy levels' population processes of Er<sup>3+</sup>activator ions in the YAG matrix is discussed.