The basic spectroscopic parameters of Ho3+-doped fluoroindate glass for emission at 3.9 μm

This report details the first study of the fundamental spectroscopic properties of a new optical material for prospective application as a gain medium for fiber laser emission at 3.9 μm. We have investigated the decay processes that are relevant to the excited states of the 5I5 → 5I6 transition in singly Ho3+-doped InF3 (fluoroindate) glass using time-resolved fluorescence spectroscopy. The HoF3 concentration in the glass was 10 mol.%. We excited the 5I7 and 5I6 energy levels using selective excitation at 1982 nm and 1150 nm, respectively. We have established that a strong energy-transfer upconversion process by way of the well-known dipole-dipole interaction between two holmium ions excited to the 5I6 level populate the 5I5 (upper laser) level of the 3.9 μm transition. The 5I6 and 5I5 energy levels emit luminescence with measured peaks located at 2.85 μm and 3.92 μm, respectively and the luminescence efficiencies of these emissions are 78% and 0.2%, respectively. Results from numerical simulations show that for this high Ho3+ concentration, a population inversion for the 3.9 μm transition is reached only for a short time (t < 100 μs) after direct upper laser level pumping at a wavelength of 889 nm.