The thulium-doped ZBLAN fiber laser is emitting visible radiation through an upconversion process which allows for the generation of hundreds of milliwatts of output power at 480 nm. The main features of this laser are presented and analyzed on the basis of an equation rate analysis. Cross- relaxation processes are shown to play an important role in the dynamical evolution of the population levels. The problem of the onset of a photo-induced absorption in the fluoride-glass matrix is also addressed in connection with its detrimental effects on laser operation. Practical solutions to this problem are explored.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.