Steady photo-darkening of thulium alumino-silicate fibers pumped at 1.07~μ m: Quantitative effect of lanthanum, cerium and thulium

By pumping thulium-doped silica-based fibers at 1.07~\mu m, rapid generation of absorbing centers leads to photo-induced attenuation (PIA). This detrimental effect prevents exploiting laser emissions in the visible and near infrared. We report on the characterization of the PIA versus the fiber core composition, particularly the concentration of thulium (Tm), lanthanum (La) and cerium (Ce) ions. We show that UV emision induced by Tm-Tm energy transfers is the source of photo-darkening, and that lanthanum and cerium are efficient hardeners against PIA.