Ultrahigh Yb-doped silica based optical fibers

In this paper we systematically study a limitation for maximum of Yb dopant concentration in silica based fibers. Two the most popular glass matrixes (F-Al₂O₃-SiO₂ and Al₂O₃-P₂O₅-SiO₂) were thoroughly investigated in this respect. A possibility to introduce ultra-high doping level of Yb₂O₃ (in excess of 2.5 mol%) with a relatively low optical losses in fibers was demonstrated. At the same time it was investigated that at ultra-high Yb concentration in the fiber core even with maintaining very low background losses (both initial and induced by photodarkening) such a fiber can nearly completely lose its active properties. Optimal glass matrixes and optimal concentration of Yb dopant in the glass core, which allow keep up lasing properties of the developed fibers high, were studied. An ultra-short length fiber amplifier (~3.5 cm) based on the developed Al2O3-P2O5-SiO2 glass core fiber doped with 1.2 mol% of Yb₂O₃ (Yb ions absorption was about 1000 dB/m at 920 nm) was created.