Role of energy transfer in concentrated Dy³⁺-doped fibers

Using high power quasi-cw pulse pumping, we show that energy transfer upconversion (ETU) processes in highly doped Dy³⁺ double clad ZBLAN fibers creates a pathway for significant excitation loss that clamps the gain. For a 4 mol.% Dy³⁺-doped fiber, we establish that the pump absorption is non-saturable up to a maximum launched (peak) pump power of 100 W. We propose that this arises from a co-operative three-ion ETU process. Additionally, the high power pulsed pumping of Tm³⁺, Dy³⁺-co-doped fiber produces laser relaxation spikes that appear after the pump pulse, suggesting that ETU dominates all other process during pumping.