Energy level decay processes in Ho³-doped tellurite glass relevant to the 3 μm transition

The primary excited state decay processes relating to the⁵I₆→⁵I₇∼ 2.9 m laser transition in singly Ho³-doped tellurite (TZBG) glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the⁵I₆energy level at 1151 nm and⁵I₇energy level at 1958 nm has established that the rate of energy transfer up-conversion between holmium ions excited to the⁵I₇level is negligible for Ho³concentrations up to 4 mol. Excited state absorption was not observed from either the⁵I₇or⁵I₆levels and the luminescence from the⁵I₇and⁵I₆energy levels was measured to peak at ∼2050 nm and ∼2930 nm, respectively. The⁵I₆level has a low luminescence efficiency of ∼8.9 due to strong nonradiative multiphonon relaxation. In contrast, decay from the⁵I₇level is essentially fully radiative. A linear decrease in the decay time of the⁵I₆level with Ho³concentration augmentation results from energy transfer to OH^-ions in the glass (with N_OH∼ 8.2 10¹⁷ions cm^-3) and reduces the luminescence efficiency of the⁵I₆level to 8 for [Ho³] 4 mol. Numerical simulation of a fiber laser incorporating 4 mol. Ho³showed that a population inversion of ∼7.8 is reached for square pulses of 100 s duration and a repetition frequency of 20 Hz at a moderate pump intensity of 418 kW cm^-2if energy transfer to OH^-radicals is neglected.