Thulium doped solid state lasers are well-known sources of radiation tunable around 2 μm. Obtainable wavelengths cover important spectral regions such as absorption lines of water, characteristic spectra of some gases or the atmospheric transmission bands. Tunable Tm3+ lasing was accomplished using various matrices, such as YAG, YAP or YLF . Recently an interest re-emerged in the mixed garnets as host materials. Their multi-site structure induces broadening of spectra of the active ions, improving overlap with pumping diode emission and extending emission spectra and tunability . In this submission, Tm3+ doped mixed gadolinium-gallium-aluminium-garnet Gd3(Ga5−xAlx)O12 (GGAG) is investigated. Focus is given to the Tm3+ concentration dependence of lasing and tunability parameters as a follow-up work to the first Tm:GGAG lasing report with one Tm3+ concentration sample . The Tm:GGAG crystal was grown using Czochralski method in a slightly oxidative atmosphere. Five planeparallel face-polished Tm:GGAG samples with concentrations varying from 1.4 at.% to 8.4 at.% (Tm/Gd) and thicknesses ranging from 2.7 mm to 3.6 mm were cut from the crystal. The samples were longitudinally pumped by fiber coupled (core diam. 400 μm, NA = 0.22) laser diode at wavelength 785 nm in QCW regime (pulse length 5 ms, repetition rate 10 Hz). The 145 mm resonator consisted of a flat pumping mirror (HR @ 1.85–2.11 μm, HT @ 0.78 μm) and concave output coupler (r = 150 mm, R = 97.5 % @1.8 – 2.10 μm). For the wavelength tuning, 1.5 mm thick birefringent SiO2 plate was inserted in the cavity at the Brewster’s angle.
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