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.