Virtual Journal of Laser

Great increase in the output power of erbium-doped fluoride fiber lasers has recently been achieved, but the predicted high slope efficiency was not observed. We conduct an investigation into the possible underlying reasons, based on the newly available spectroscopic data, and our measurements on the running fiber laser are compared against computer simulation. The results indicate that interionic processes in the present fibers are much weaker than previously expected and only allow for an operation with 15–30% efficiency where the excited-state absorption is shown to have an important contribution.