We present a temperature influence (in range from 78 up to 300 K) on the spectroscopic and laser properties of Tm:SrF2 crystal doped with 2 at. % of Tm3+. The sample was grown using the temperature gradient technique in shape of a single-crystal fiber (d= 2 mm, l = 5 mm) with plane-parallel face-polished without any AR coating. The Tm:SrF2 crystal was mounted in a temperature-controlled copper holder of the liquid nitrogen cryostat. The measured absorption and emission spectra remained broad even at low temperature. The fluorescence lifetime was fitted with a double exponential function, and the measured lifetime changed significantly with temperature decrease. The 147 mm long semi-hemispherical laser resonator consisted of a flat pumping mirror (T < 95 % @ 763 nm, HR @ 1750-2100 nm) placed inside the cryostat and a curved output coupler (r=150 mm, R=97.5 % @ 1750-2100 nm) placed outside the cryostat. For longitudinal pumping, a fiber coupled laser diode was used. The diode was operating in the pulse regime (5 ms pulse length, 10 Hz repetition rate) at wavelength 763 nm. At room temperature, the laser emission was achieved at 1949 nm with a high 38 % slope efficiency. With a temperature decrease, the slope efficiency increased, and the laser threshold decreased, and the laser output wavelength shifted.
We present a temperature influence (in range from 78 up to 300 K) on laser properties of Tm:LuVO4 vanadate crystal doped with 2 at. % of Tm3+. The sample was 2 mm thick plane-parallel face-polished single-crystal without any AR coating. The Tm:LuVO4 crystal was mounted in temperature-controlled copper holder of the liquid nitrogen cryostat. The 107 mm long semi-hemispherical laser resonator consisted of a flat pumping mirror (T < 95 % @ 788 nm, HR @ 1750-2100 nm) placed inside the cryostat and a curved output coupler (r=150 mm, R=97.5 % @ 1750-2100 nm) placed outside the cryostat. For longitudinal pumping, a fiber coupled laser diode was used. The diode was operating in the pulse regime (5 ms pulse length, 10 Hz repetition rate) at wavelength 788 nm. At 300 K, the laser emission was detected at 1931 nm with negligible power. With a temperature decrease the laser wavelength shifted to 1868 nm and the slope efficiency doubled from 7 % at 200 K up to 15 % at 78 K, while the laser threshold decreased more than five times. The maximum power amplitude of 0.8 W was achieved at 78 K. The quality of laser output beam was very good close to TEM00 mode. Using the SiO2 birefringent filter, we were able to switch the output wavelength between 1868 and 1931 nm.
The spectroscopic and laser characteristics of optically polished Cr:ZnSe single crystals in four different lengths of 2, 3, 5, and 10 mm were investigated when pumping with a laser diode generating radiation at the wavelength of ~1.69 μm. The optimization from the point of laser efficiency was done in dependence on the crystal samples active length, effect of antireflection (AR) coatings, pumping pulse duration, and repetition rate. As a result, a 3 mm thick anti-reflection coated Cr:ZnSe crystal pumped by a laser diode with a pulse length of 20 ms and repetition rate of 10 Hz was chosen as the optimal. Using a 0.8 mm thick MgF2 birefringent filter the laser oscillations were continuously tunable from 2.1 μm up to 2.7 μm with the narrow spectral linewidth of ~5 nm and Gaussian beam profile. Maximum mean output power of ~0.35 W and maximum optical-to-optical slope efficiency of ~39% was obtained for 3 mm thick AR-coated sample which was ~28% and ~10% higher than that for uncoated sample of the same thickness, respectively. Continuously tunable laser oscillation wavelength was measured in the range of ∼2.1-2.7 µm with the mean output power of P ≥ 150 mW within a ∼5 nm linewidth. The absorption and fluorescence spectra as well as fluorescence lifetime were also measured. The Cr2+ fluorescence lifetime of ∼7.2 µs measured for the 1 ns pulse excitation at room temperature (RT) was constant within the whole absorption band of 1.35–2.25 µm.
We present the influence of optically inactive Gd3+ ions codoping on the spectroscopic and laser properties of a novel fluoride Tm,Gd:SrF2 crystal. Eight crystals in shape of single-fiber (r = 2 mm, l = 5 mm) with different low Gd3+ (0, 0.5, 1, 2 at.%) and Tm3+ (2 and 3 at. %) concentrations were tested. The addition of an inactive ion Gd3+ had an influence on the absorption spectra, especially on the value of the absorption coefficient in the 0.76 μm pumping band. The fluorescence spectra excited at 763 nm have been influenced only slightly promoting lower wavelength peaks of 3F4→3H6 transition. The 2 μm fluorescence decay time was measured. The decay curves exhibited double exponential behavior, and two components of lifetime were estimated. While the faster component of about 5.5 ms increased with the Gd3+ addition, the slower component of about 19.7 ms was not affected. The Tm,Gd:SrF2 quasi-cw laser was tested. The pumping diode emitting at a wavelength of 763 nm was operating in the pulse regime (5 ms pulse duration, 10 Hz repetition rate). A 148 mm long hemispherical resonator formed by a flat pump mirror and a curved (r = 150 mm) output coupler was used. The Gd3+ addition had not a strong effect on the laser performance and the high slope efficiency up to 57 % with maximum output power amplitude up to 5.5 W at 1949 nm was achieved with all samples. For wavelength tuning, a MgF2 birefringent filter was used and the wide tunability range of up to 199 nm (1807 – 2006 nm) was reached.
In this research we focused on experimental measurement and modeling of the thermal lens in the active medium of diode pumped solid-state lasers at low temperature. The numerical model included the temperature and stress distribution leading to change of the index of refraction distribution and bulging of the faces of active medium. Evaluation of the influence of the main thermomechanical and thermo-optical properties of the active medium is presented. The important dependence of these parameters on the temperature and the doping concentration which leads to nonlinear equations is emphasized. The numerical model is compared with commonly used simple analytical solution. The results of the simulations are confronted with the experimental results of the measurement of refractive power of thermal lens formed in diode pumped Yb:LuAG and Yb:YAG lasers. In the experiments the crystals with different doping concentration were longitudinally pumped by fiber coupled CW laser diode at 0.930 μm with the focal point 0.2 mm in diameter. The 38 mm long semi-hemispherical laser resonator was used. The output laser oscillation wavelength was 1.03 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. The experiments showed strong dependence of refractive power on doping concentration at higher temperatures and importance of temperature and doping concentration dependence of material parameters for simulation which is often neglected.
We present temperature influence (from 78 to 300,K) on tuning and laser properties of thulium doped solid solution CaF2-SrF2 crystal (Tm:CaF2SrF2). The sample was 8.5 mm thick Tm:CaF2SrF2 block cut and face polished parallel to growing axis without any AR coating. The composition of sample was 60 mol% CaF2, 38 mol% SrF2 doped with 2 mol% TmF3. The sample was mounted in temperature controlled copper holder of the liquid nitrogen cryostat. The 148mm long semi-hemispherical laser resonator consisted of flat pumping mirror (HR @ 1.80 - 2.10 μ m, HT @ 0.78 μm) placed inside cryostat, and curved output coupler (r=150mm, R = 98:0% @ 2 μm) placed outside. For longitudinal pumping a fiber coupled laser diode was used. The diode was operating in the pulse mode (10 ms pulse length, 10 Hz repetition rate) at wavelength 763 nm. The 2mm thick MgF2 birefringent filter was placed at Brewster angle inside the resonator for laser wavelength tuning. With decreasing temperature the output oscillation wavelength shifted to shorter wavelength and range of tunability decreased from 117nm at 300K to 89nm at 100K following the fluorescence spectrum narrowing. The overall tunability was from 1795nm at 79K to 1944nm at 300 K. The highest output pulse energy 6.8mJ with slope efficiency 10% was obtained at 1814nm for T = 78 K. The temperature of Tm:CaF2SrF2 was found to have significant influence on laser output wavelength and tunability.
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.
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