We present temperature influence (in range from 78 up to 400,K) on spectroscopic properties and laser performance of new Yb-doped mixed garnet Gd3GaxAl5-xO12 (Yb:GGAG). The sample was 2.68 mm thick plane-parallel face-polished Yb:GGAG single-crystal plate which was AR coated for pump (930 nm) and generated (1030 nm) laser radiation wavelength. The composition of sample was Gd3.098Yb0:0897Ga2:41Al2.41O12 (3 at % Yb/Gd). The Yb:GGAG crystal was mounted in temperature controlled copper holder of the liquid nitrogen cryostat. The 138 mm long semi-hemispherical laser resonator consisted of a flat pumping mirror (T > 90 % @ 930 nm, HR @ 1030 nm) placed inside cryostat, and a curved output coupler (r = 150 mm, R = 94.5 % @ 1030 nm) placed outside cryostat. For longitudinal pumping a fiber coupled laser diode was used. The diode was operating in the pulse regime (5 ms pulse length, 20 Hz repetition rate) at wavelength 928.5 nm. The absorption spectrum was measured for the temperatures from 78 to 400 K, and absorption lines narrowing was observed with temperature decrease. Zero-phonon line at 970 nm has width 1 nm (FWHM) at 100 K. The fluorescence intensity decay time was measured and it increased linearly with temperature from 864 μs @ 78 K to 881 μs @ 300 K. The temperature of active medium has strong influence mainly on laser threshold which was 5 times lower at 100 K than at 300 K, and on slope efficiency which was 3 times higher at 100 K than at 300 K.
The goal of this work was an investigation of the temperature influence (in range 77 - 300 K) on laser properties of Tm:SrF2-CaF2 solid-solution, which is suitable as a gain medium for generation of radiation at 1.8-2 μm. The tested Tm:SrF2-CaF2 sample (60 mol% CaF2, 38 mol% SrF2) was doped with 2 mol% of TmF3. The diameter of the grown boule was 10 mm. The sample was cut and optically polished parallel to growing axis. The polished sample thickness was 8.5 mm. It was fixed in temperature controlled cupreous holder, placed inside vacuum chamber of the liquid nitrogen cryostat. A fiber coupled laser diode, operating in pulsed regime (10 ms pulse length, 10 Hz repetition rate) at wavelength 764 nm, was used for longitudinal sample pumping. The 142mm long semi-hemispherical laser cavity consisted of at pumping mirror (HR @ 1:8 - 2:0 μm, HT @ 0.77 μm) and curved (r = 150mm) output coupler with a reflectivity of ~ 92% @ 1:8 - 2:0 μm. From the results it follows that the temperature of the active medium has a strong infkuence on laser slope efficiency. The highest slope efficiency (42% in respect to absorbed power), obtained for temperature 77 K, was more than five times higher than slope efficiency for 300 K. The threshold decreased twice with the temperature lowering from 300 to 77 K. Laser output power amplitude 5.5W at wavelength 1856nm was reached for absorbed power 15.8W at 77 K.
The aim of this study was to investigate whether refractive power of thermal lens for Yb:LuAG crystal at cryogenic temperatures depends on Yb doping concentration which has not been examined yet. The three measured Yb:LuAG laser rods samples (length of 3 mm, diameter 3 mm, AR @ 0.94 μm and 1.03 μm, doping concentration 5.4, 8.4 and 16.6 at. % Yb/Lu) were mounted in the temperature controlled copper holder of the liquid nitrogen cryostat. Samples were longitudinally pumped with fiber coupled CW laser diode at 0.930 μm with the focal point 0.4 mm in diameter. The 38 mm long semi-hemispherical laser resonator consisted of a flat pump mirror (HR @ 1.03 μm and HT 0.94 μm) and curved output coupler (r=500 mm) of reflectivity 94 % @ 1.06 μm. The refractive power of thermal lens was estimated indirectly by measuring of change in the position of focused laser beam focal point. The measurement was performed for constant absorbed power of 10 W in temperature range from 80 up to 240 K. It was observed that cryogenic cooling caused reduction of thermal lens power, which increased linearly with increasing temperature. For temperatures from 80 to 160 K refractive power was identical for all concentration. For higher temperature the refractive power of thermal lens increased with increasing Yb3+ concentration. Presented study shows that application of cryogenic temperature leads to reduction of thermal effect even for high dopant concentration in Yb:LuAG crystal. This is essential for reaching of high output power while maintaining high beam quality.