The recent increased interest in high-concentration erbium-doped fibers (EDFs) has resulted from their potential use as single-frequency fiber lasers and compact optical amplifiers. Earlier study of multicomponent glasses and fibers on base of sodium-silicate glasses with high concentration of Na_2O (20-30 mol percent) showed the possibility of realization of high-concentration EDFs possessing better amplification properties than the standard silica glass EDFs.~1 However, low melting temperature, excess loss, as well as other limitations do not allow the wider use of these fibers in usable erbium-doped fiber amplifiers (EDFAs). It explains the vivid interest in the possibility of implementation of alkali doping into conventional methods of EDFs' fabrication, particularly into MCVD and solution techniques.
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