Dramatic change of guiding properties in heavily Yb-doped, soft-glass active fibers caused by optical pumping

The authors report experimentally observed qualitative changes of the waveguiding properties in ytterbium-doped fibers with high concentration of doping when the fibers are optically pumped with light at the 975nm wavelength. They found that when concentration of Yb2O3 in a single-mode fiber exceeds ∼10wt.% and the pump power is gradually increased, the fiber first looses index-induced guidance, and then regains guidance as the population inversion in Yb+3 ions becomes sufficient for gain guiding. The associated variation of the mode-field diameter is a practically important effect that will complicate design and operation of fiber lasers and amplifiers based on highly Yb-doped glass.