  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.
