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The lasing of Tm or Tm,Ho-doped mixed gadolinium-gallium-aluminum-garnet (Tm:GGAG and Tm,Ho:GGAG) under 1.7 μm pumping was investigated for the first time. Three Tm:GGAG samples with Tm3+ concentrations 1.4 at.%, 2.5 at.% and 5.1 at.% (Tm/Gd) and one Tm, Ho:GGAG sample with concentration 0.4 at.% (Ho/Gd) and 4 at.% (Tm/Gd) in a form of face-polished plane-parallel plates were available. The original crystal boules were grown by Czochralski method in a slightly oxidative atmosphere. A fiber (core diameter 400 μm, NA= 0.22) coupled laser diode (Bright Lase Ultra - 100) was used to longitudinally pump each crystal placed in a water cooled (16 °C) heatsink. The laser diode was operated in a pulsed regime (up to 20 ms pulse duration, 10 Hz repetition rate, 1684 nm wavelength, output power amplitude up to 24 W) and CW regime (1690 nm wavelength, power up to 30 W). A hemispherical resonator (30 mm length for CW regime, 100 mm for pulsed regime) was assembled using a at pumping mirror (HR @ 1.9-2.25 μm, HT @ 1.7 μm) and concave output couplers: r = 150 mm, reflectivity _ 97.5 % @1.85-2.05 μm for Tm:GGAG; r = 100 mm, reflectivity _ 99 % @ 1.8 - 2.11 μm for Tm, Ho:GGAG. The 1.4 at.%, 2.5 at.% and 5.1 at.% (Tm/Gd) Tm:GGAG laser efficiencies with respect to the absorbed power were 57 %, 37 % and 33 % respectively with maximum achieved output power amplitude of 750 mW, 1.9 W and 3.0 W. The 1.4 at.% and 2.5 at.% sample lasers operated also in CW regime with output powers of 0.6 W and 1.8 W and efficiencies 54 % and 38 % respectively. Tm, Ho:GGAG laser had a slope efficiency of 35 % and output power amplitude of 2.1 W in pulsed regime and efficiency of 19 % and output power of 180 mW in CW regime. Considering these results, the 1.7 μm diode pumping appears to be a viable alternative for Tm and Tm, Ho-doped solid-state lasers, especially for lower concentration of the active ions.
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