The development of telecommunications applications as well as rapid increasing volume of transmitted data make it necessary to increase the number of channels in wavelength division multiplexing (WDM) transmission system and improve utilization of wavelength range currently applied in communication systems. Erbium-doped fiber amplifiers (EDFA) that are widely used in modern fiber-optic transmission systems operate in C spectral range of WDM system (1530 nm–1565 nm). However, in today’s commercial communications systems EDFAs are generally made of silica-based glass fiber, where at the region of 1550 nm erbium (Er3+) ions show a limited stimulated emission bandwidth. Noise and gain characteristics of those amplifiers lead to the conclusion that conventional EDFA has almost reached its limits. This means that new solutions needed aimed at expansion of the optical amplifiers bandwidth and obtaining higher output power. One of the solutions is an investigation of a material with wider gain spectra. Doctoral thesis covers an evaluation of the effective area parameter of erbium and ytterbium doped optical fibers with transverse shift measurement method as well as the possibility of improving optical amplifier’s efficiency by optimizing the length of doped fibers. As a result of the research, a combined optical amplifier has been successfully realized, which makes it possible to extend erbium gain band from 47 nm to 82 nm. The thesis reflects the completed research results, which allows evaluating the physical parameters of rare-earth doped optical fibers and their application in the development of optical amplifiers.