1.7 μm diode-pumped Tm:GGAG and Tm, Ho:GGAG 2.0-2.1 μm laser

The lasing of Tm or Tm,Ho-doped mixed gadolinium-gallium-aluminium-garnet (Tm:GGAG and Tm,Ho:GGAG) under 1.7 mu m 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 two Tm, Ho:GGAG sample with concentrations 0.4 at.% (Ho/Gd), 4 at.% (Tm/Gd) and 0.7 at.% (Ho/Gd), 7.7 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 mu m, NA= 0.22) coupled laser diode (Bright Lase Ultra - 100) was used to longitudinally pump each crystal placed in a water cooled (16 degrees C) heat-sink. The laser diode was operated in a pulsed regime (up to 15 ms pulse duration, 10 Hz repetition rate, 1684 nm wavelength, output power amplitude up to 25 W) and CW regime (1690 nm wavelength, power up to 30 W). A hemispherical resonator (30 mm length for CW regime, 120 mm for pulsed regime) was assembled using a flat pumping mirror (HR @ 1.9-2.3 mu m, HT @ 1.7 mu m) and concave output couplers: r = 150 mm, reflectivity approximate to 97.5 % A1.85-2.05 mu m for Tm:GGAG; r = 100 mm, reflectivity approximate to 99 % @ 1.8 - 2.11 mu m for Tm, Ho:GGAG. The 1.4 at.%, 2.5 at.%, 5.1 at.% and 6.9 (Tm/Gd) Tm:GGAG laser efficiencies with respect to the absorbed power were 50 %, 35 %, 26 % and 27 %, respectively, with maximum achieved output power amplitude of 750 mW, 1.9 W, 3.0 W and 4.4 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 56 % and 47 % respectively. High slope efficiency of 36 % and broad tuning range 1932-2115 nm was measured for the lower (0.4 at.% (Ho/Gd), 4 at.% (Tm/Gd)) Tm, Ho:GGAG sample. Considering these results, the 1.7 mu m 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.