Rare-earth doped optical fibers are widely used as a heart of fiber lasers and amplifiers. In this paper we deal with study of the MCVD process of fabrication of preforms with low-phonon Sb2O3 and influence of achieved core composition on optical properties of thulium-doped optical fibers used for optical generation at 1900-2000 nm spectral region. The solution-doping method and nanoparticle-doping method extending the MCVD method were used for the fabrication of preforms. A maximum of 0.15 mol% Sb2O3 in the core of preforms was achieved. It corresponds to usual role of Sb2O3 as refining agent in glass fabrication The same spatial distribution of Sb2O3 together with Tm3+ ions and Al2O3 in the core was observed, so welcome localization of all dopants in nanometer-atomic scale can be expected. Tm3+ doped fibers exhibited good lasing characteristics at 1950 nm, fiber doped with Sb2O3 exhibited slightly higher SLE in comparison to fiber undoped with Sb2O3. The difference is not substantial but proportional to achieved level of doping.