Intermediate excited-state absorption in erbium-doped fiber strongly pumped at 980 nm

We report the observation of fluorescence at 530 to 550 nm from erbium-doped germanosilicate fiber under strong pumping in the 980-nm band. The observed nonlinear dependence on pump power is shown to arise from a sequential two-photon absorption from the ground state to the (4)F(7/2) state through the short-lived (4)I(11/2) intermediate state. Rate-equation modeling shows that the pump power dependence of this intermediate excited-state absorption is influenced by population cycling induced by amplified spontaneous emission in the fiber from the metastable (4)I(13/2) state.