A tellurite glass composition doped with a first lanthanide, preferably Tm, and codoped with another lanthanide oxide, e.g. Ho, is provided. The glass includes 4-12 mole % of alkalihalide XY, X being selected from the group of Li, Na, K, Rb, Cs and Fr and Y being selected from the group of F, Cl, Br and I. A preferred glass contains about 10 mole % of the alkalihalide CsCl. The addition of alkalihalide XY results in an enhanced energy transfer from the first to the second lanthanide ion, whereby the lower energy level of the first lanthanide ion is depopulated. The ratio between upper and lower energy level lifetimes for the first lanthanide ion can be reduced to a value below one, enabling efficient amplifier fibers.