New energy-transfer upconversion process in Er³⁺:ZBLAN mid-infrared fiber lasers

We report a new energy-transfer process in erbium doped ZBLAN glass, which is critical for optimizing the operation of lasers that use the 3.5 μm band ⁴F_9/2 to ⁴I_9/2 transition. The magnitude of this energy-transfer process is measured for two different doping levels in Er³⁺:ZBLAN and the requirement for low doping in these lasers established.