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

Abstract

The primary excited state decay processes relating to the 5I6 → 5I7 ∼ 2.9 μm laser transition in singly Ho3+-doped tellurite (TZBG) glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the 5I6 energy level at 1151 nm and 5I7 energy level at 1958 nm has established that the rate of energy transfer up-conversion between holmium ions excited to the 5I7 level is negligible for Ho3+ concentrations up to 4 mol. %. Excited state absorption was not observed from either the 5I7 or 5I6 levels and the luminescence from the 5I7 and 5I6 energy levels was measured to peak at ∼2050 nm and ∼2930 nm, respectively. The 5I6 level has a low luminescence efficiency of ∼8.9% due to strong nonradiative multiphonon relaxation. In contrast, decay from the 5I7 level is essentially fully radiative. A linear decrease in the decay time of the 5I6 level with Ho3+ concentration augmentation results from energy transfer to OH− ions in the glass (with NOH ∼ 8.2 × 1017 ions cm−3) and reduces the luminescence efficiency of the 5I6 level to 8% for [Ho3+] = 4 mol. %. Numerical simulation of a fiber laser incorporating 4 mol. % Ho3+ showed that a population inversion of ∼7.8% is reached for square pulses of 100 μs duration and a repetition frequency of 20 Hz at a moderate pump intensity of 418 kW cm−2 if energy transfer to OH− radicals is neglected.