Energy level decay and excited state absorption processes in dysprosium-doped fluoride glass

The primary excited state decay processes relating to the ⁶H_13/2→⁶H_15/2∼3 μm laser transition in singly Dy³⁺-doped fluoride μZBLAN glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the ⁶F_9/2, ⁶H_7/2 energy levels at 1125 nm and ⁶F_11/2, ⁶H _9/2 energy levels at 1358 nm established that the energy levels above the ⁶H_11/2 level, excluding the ⁴F _9/2 level, are entirely quenched by multiphonon emission in ZBLAN glass. The ⁶H_11/2 and ⁶H_13/2 energy levels emit luminescence with peaks at ∼1700 and ∼2880 nm, respectively, but at low quantum (luminescence) efficiencies. The quantum efficiency of the ⁶H_11/2 level and ⁶H_13/2 level is ∼9×10^-5 and ∼1.3×10^-2, respectively, for [Dy³⁺]=0.5 mol % based on calculations of the radiative lifetimes using the Judd-Ofelt theory. Excited state absorption (ESA) was detected by monitoring the rise time of the 1700 nm luminescence after tuning the probe wavelength across the spectral range from 1100 to 1400 nm. As a result of nonradiative decay of the higher excited states, ESA contributes to the heating of ∼3 μm fiber lasers based on Dy³⁺-doped fluoride glass. For [Dy³⁺] up to 4 mol %, we found no evidence of energy transfer processes between Dy³⁺ ions that influence the decay characteristics of the ⁶H_11/2 and ⁶H_13/2 energy levels.