The absorption spectrum of Yb 3+ :YScO3 crystal fiber was measured on the 2 F7/2 → 2 F5/2 transition of Yb 3+ ion at a temperature of 77K. In Figure , curve 1 shows one narrow peak with a maximum at νmax = 10246 cm -1 . Additionally, there are three broad bands with weakly resolved structure and maxima at 11111, 10753, and 10582 cm -1 . This spectrum is compared to the energy levels of Yb 3+ ions with local symmetries of C2 and C3i in Y2O3 and Sc2O3 crystals as a reference . The optical centers of Yb 3+ ions in the Sc2O3 crystal exhibit the highest values of Stark splitting of the 2 F5/2 level being measured as ΔE = 856 cm -1 and ΔE = 1126 cm -1 for Yb 3+ centers with local symmetries of C2 (Figure ) and C3i (Figure ), respectively. In Yb 3+ :Y2O3, the Stark splitting values (ΔE) are equal to 726 см -1 and 1062 см -1 for Yb 3+ optical centers with local symmetries of С2 (Figure ) and С3i (Figure ), respectively. Furthermore, in YScO3 crystal fiber spectra, the Stark splitting of 2 F5/2 transition is determined to have a value of ΔE = 864 см -1 (Figure , a curve 1). The analysis of the Stark splitting values for the 2 F5/2(1', 2', 3') level in the sesquioxide materials reveals that the YScO3 crystal fiber doped with Yb 3+ ions exhibits splitting intermediate between the Yb 3+ :Y2O3 and Yb 3+ :Sc2O3 crystals. This observation is consistent with an intermediate lattice parameter value of the YScO3 crystal fiber, as it is a solid solution. However, determining of the precise position of the Stark level components is complicated due to the presence of multiple types of Yb 3+ optical centers in the YScO3 crystal fiber, similar to those formed in Y2O3 and Sc2O3 crystals . It is noteworthy that the absorption bands corresponding to the Stark components of the Yb 3+ optical centers in YScO3 exhibit broadening, which is presumably related to disorder in the crystal structure and the statistical distribution of Y 3+ and Sc 3+ ions in two structural positions, where they are partially substituted by Yb 3+ . The spin-orbit interaction within the 2 FJ multiplet remains unchanged. In Figure , the Stark sublevels associated with the 2 F5/2 excited state denoted by bars, and the sublevels of the 2 F7/2 ground state are represented by unbarred numbers. The numbering of levels within the multiplet follows a bottom-to-top sequence.