Growth,Spectroscopy And Laser Performance Of 2.79 μM Er:YSGG Laser Crystals

The radiative transition of trivalent erbium ions betweel 4I11/2 and 4I13/2 energy levels can generate 2.7～3 μm mid-infrared(MIR)laser,which plays an important role in biomedicine,air remote sensing,space military,and other fields due to the strong absorption by water vapor around this waveband and its low transmission loss at high altitudes.In recent years,Er3+-doped laser crystals have been extensively investigated and studied at home and abroad.The YSGG crystal has good mechanical properties and low phonon energy,which makes Er:YSGG crystal has great advantages and potential as the gain medium of 2.7～3 μm MIR laser.In this dissertation,the spectral characteristics of Er:YSGG crystals with different concentrations and the influence of Er3+concentration on Er3+energy level transition and laser wavelength have been studied and analyzed.The laser performances of Er:YSGG series crystals were investigated in laser diode(LD)end-and side-pumping mode,respectively,which can provide important reference for promoting the development of the high-power MIR solid laser.The research content and results of this dissertation mainly include the following five points:(1)Nine different concentrations of Er:YSGG crystals with high optical quality were grown by Czochralski(Cz)method.The Er3+energy level transition mechanism and effects of its concentration on the fluorescence spectra of Er:YSGG,as well as the fluorescence lifetime of upper and lower laser levels were analyzed.By Judd-OFelt(JO)theoretical calculation,the spectral parameters of Er3+ transition from the ground state to excited state were obtained.Judging from fluorescence spectra,the optimal erbium doping concentration in the Er:YSGG for MIR laser output might be about 40 at.%.(2)Based on the LD end-pumping structure,the laser performance of Er:YSGG crystal blocks with doping concentrations of 25 at.%,30 at.%,35 at.%,40 at.%,and 50 at.%was investigated.According to the absorption coefficients of Er:YSGG crystals with different concentrations at～970 nm LD pump wavelength,the crystal length was reasonably designed.The laser performance of crystal blocks with different concentrations under continuous-wave mode was studied and the influence of Er3+concentrations on laser output was analyzed.The results show that the output power can be improved by optimizing the doping concentration of Er3+and the length of crystal block.Finally,a high-power continuous-wave(CW)laser output of 1.37 W at 2.79 μm was achieved in 30 at.%Er:YSGG crystal.(3)The LD side-pumping structure was adopted to overcome thermal effects caused by the small pumping area and uneven pumping in the LD end-pumping system.The laser performance of Er:YSGG crystal rod was studied.By optimizing the length of the resonator and the transmittance of the output mirror,a high-power laser output of 25.74 W at 150 Hz was obtained.In addition,the methods of thermal bonding and grinding crystal end-faces into concave were adopted to compensate for the thermal effects under high repetition frequency and high pump power,respectively.And COMSOL Multiphysics simulation software was used to simulate the temperature distribution of traditional Er:YSGG crystal rod and YSGG/Er:YSGG/YSGG bonding crystal rod side-pumped by LD,and the effect of thermal bonding process on improving temperature distribution in laser crystal was analyzed.The laser experimental results show that the thermal bonding crystal rod and crystal rod with concave-end faces can both effectively compensate for the thermal lens effect of laser medium under high repetition frequency and high pump power.The saturation point of pump power can be extended and the output power can be increased compared with the conventional crystal rod.(4)The laser performance was further improved by combining thermal bonding and negative curvature of the end faces.The YSGG/Er:YSGG/YSGG thermal bonding crystals with negative curvature radii of 250,500,and 1000 mm were designed and fabricated,and the laser performance under LD side-pumping was compared.The results show that the thermal lensing effect can be further improved by the combination of thermal bonding and negative curvature of concave-end faces,and finally the quasicontinuous laser output power of 15.59 W at 500 Hz was achieved on bonding crystal rods with a negative curvature radius of 500 mm.Compared with the conventional Er:YSGG rod,the maximum output power was increased by 56.21%,and the beam quality was also improved.(5)The influence of rod diameter on laser output was analyzed,and the laser performance of Er:YSGG rod with diameters of 2,3,and 4 mm was compared under LD side-pumping condition.The results show that the laser output power and slope efficiency of the crystal rod with a smaller diameter were lower at low frequency and low pump power,but the laser performance was obviously superior at high frequency and high pump power.The innovative points of this dissertation are as follows:(1)The growth and spectral characteristics of Er:YSGG crystals with different concentrations were studied,and combined with energy level transition and fluorescence spectrum,the mechanism of laser output wavelength variation was explained reasonably(2)According to the absorption characteristics of the crystal,the 30 at.%Er:YSGG crystal element was optimized and fabricated,and the continuous-wave laser output of 2.79 μm with high beam quality was achieved at 1.37 W under LD end-pump condition.(3)A method of combining thermal bonding and grinding crystal end-faces into concave was proposed,and the thermal lensing effects of Er:YSGG crystal under high repetition frequency and high pump power were effectively improved and compensated by optimizing the radius of curvature;(4)The laser performance of Er:YSGG crystal rod with different diameters was studied under LD side-pump condition,and the variation of laser output with diameter was found,which has important potential application value.The research work of this dissertation has important reference value for the exploration of novel Er3+-doped laser crystals,the study of luminescence mechanism and concentration optimization,and also lays a good technical foundation for improving the laser performance of Er3+-doped laser crystals and developing high performance Er3+-doped solid laser.