The exact "quantum limit" value of noise figure of EDFA in the high gain regime at quasi-two-level pumping is determined on the base of the McCumber theory. It is shown that in the unsaturated gain regime the ultimate spectral efficiency of information transmission depends on host material of EDFA and can serve as a figure of merit for comparison of host fibers. From the very beginning of development of the first erbium-doped fiber amplifiers (EDFA) an attention of many research groups is focused on the search and investigation of promising alternative materials for fiber amplifiers. Ever growing information capacity of long-haul communication lines and wide adoption of reconfigurable local optical networks require an application of fiber amplifiers with a larger bandwidth and smaller spectral nonuniformity of gain than that of common EDFA. Limited bandwidth and obvious spectral gain non-uniformity of EDFA based on widely used silicate fibers and low efficiency of thulium-and praseodymium-doped fiber amplifiers stimulate active research of the spectral properties of phosphate, fluorophosphate, bismuth, telluride and some other types of erbium-doped fibers [1][2][3][4][5][6].For practical application of new materials for fiber amplifiers in telecommunication industry they should meet a number of requirements: low contents of elements with noticeable absorption in the communication band; high solubility of the dopant in the fiber material; sufficient mechanical and chemical stability of both fiber core and fiber cladding materials. Moreover, the concentration of a dopant in the fiber material should allow to obtain acceptable values of gain and, at the same time, should be low enough to avoid significant nonradiative losses. The noise figure and the efficiency of pump power conversion of promising fiber amplifiers should not be worse, at least, than the characteristics of conventional amplifiers. It is also necessary to take into account technology problems and compatibility of new fibers and those already in use.At present the following figures of merit are used for the comparison of erbium-doped fibers: full width of emission spectrum at half maximum (FWHM), the lifetime of erbium ions in the excited state, the product of the peak value of the emission cross-section and FWHM, and some other. We propose to use a new criterion that characterizes fiber amplifiers from the viewpoint of their application in optical communication systems. It is the ultimate spectral