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Holmium-doped solid state lasers are important direct sources of coherent radiation at 2 μm with applications in medicine, spectroscopy, LIDAR technologies, or conversion to mid-infrared wavelengths. Co-doping with Tm3+ sensitizer enables resonant diode-pumping at 1.7 μm with aim to reduce lasing threshold and thermal loading and to increase efficiency and obtainable wavelengths range. The Gd3(Ga,Al)5O12 (GGAG) crystal investigated in this work belongs to a class of mixed or disordered garnets. Such crystals are actively researched host material for rare earth ions due to broadening of dopant spectral lines and preserving good thermal and mechanical properties of crystalline garnet hosts. Spectroscopic and laser properties and their doping concentration dependence of Tm, Ho:GGAG crystal were investigated under 1.7 μm diode pumping. The laser material was Tm3+ and Ho3+ co-doped Gd3(Ga,Al)5O12. It was grown by Czocharlski method from melt with initial composition Gd2.91Ho0.012Tm0.075Ga2.7Al2.3O12. The grown crystal boule was cut into eight face-polished crystal samples 5.4 mm thick and 8–14 mm wide in diameter. Tm3+ and Ho3+ content of samples was between 2.1–3.2 at.% Tm/Gd and 0.3–0.5 at.% Ho/Gd. Crystals were pumped at room temperature by fiber-coupled (NA = 0.22, core diameter = 400 µm) 30 W laser diode emitting at 1.7 µm in quasi-continuous regime. A hemispherical laser cavity was tested with OC curvature of -150 mm and reflectivity of 96.5 % at 2090 nm. All lasers emitted at 2084–2090 nm range in the untuned setup. Threshold absorbed power was in 0.1–0.3 W range and generated beam corresponded to TEM00 mode. Both the highest efficiency w.r.t. absorbed power of 37 % and the highest output power amplitude of 3.8 W were obtained for 3.0 at.% (Tm/Gd) 0.5 at.% (Ho/Gd) crystal. The total wavelength tunability range of 1936–2111 nm was obtained, with lower concentration samples tended to result in broader continuous tuning curves.
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