We present a model for 455-nm thulium-doped fluorozirconate fiber lasers co-pumped at 645 nm and 1064 nm. Twelve radiative transitions are accounted for in our model, along with cross- relaxation and cooperative upconversion processes. Blue laser output power is computed using a rate equation analysis. Relevant spectroscopic data used in our model are given, including cross-section measurements that we have performed. The results of our simulation show a good agreement with previously published experimental data. The importance of cross-relaxation processes is discussed. The dependence of output laser power on fiber length, output mirror reflectivity, and pump powers is also addressed.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.