Directly diode pumped cryogenically-cooled Ho:GGAG laser

The spectroscopy properties and lasing of directly diode pumped Ho-doped mixed gadolinium-gallium-aluminium garnet Gd3Ga3Al2O12 (Ho:GGAG) single crystal in dependence on temperature were investigated for the first time. The crystal was grown by Czochralski method in a slightly oxidative atmosphere using an iridium crucible. The tested Ho:GGAG sample was cut from the grown crystal boule perpendicularly to growth direction ( c-axis). For spectroscopy and laser experiments 9.8mm thick plane-parallel face-polished plate (without AR coatings) with Ho-doping 1.26 at.% Ho/Gd was used. It was mounted in a temperature controlled cupreous holder, placed inside the vacuum chamber of the liquid nitrogen cryostat. The spectroscopy data, including absorption and emission spectra and upper laser level lifetime were measured in temperature range from 80K up to room temperature. GaSb-based MIR laser diode (Dilas, BA-1908-1000-SE, power 1W at 1908 nm) was used for longitudinal Ho:GGAG pumping. The laser diode was operating in the pulsed regime (20 ms pulse length, 20 Hz repetition rate) or CW regime. The 145mm long semi-hemispherical laser resonator consisted of a flat pumping mirror (HR @ 2.02 - 2.15 mu m, HT @ 1.91 mu m) and a curved (r = 150 mm) output coupler with a reflectivity of similar to 90% @ 2.05 - 2.15 mu m. For the crystal temperature 80K the laser slope efficiency was 35% in respect to absorbed pumping power. The laser emission wavelength was 2089 nm.