Exploration And Physical-chemical Properties Of CNGS Laser Crystals In The 2-3?m Band

As more and more laser technology enters human life,research workers have become more and more extensive and in-depth research on laser technology.Among them,the laser gain medium at the core of the laser plays a vital role in the laser performance.The matrix material of the medium as the carrier of the activated ions determines whether the activated ions can smoothly and efficiently enter the laser material,and also determines the thermal properties,basic physical properties,and chemical properties of the laser material.The activated ions as the luminescent center have the rich energy levels and the movement characteristics of the extranuclear electrons can be used to radiate energy to the outside world by their energy level transitions,which in turn determines the spectral characteristics of the laser materialLGS(La3GasSiOi4)crystals and their homotype compounds are a class of piezoelectric materials with excellent properties.These electrical characteristics have also aroused strong interest in exploring whether such crystals can become versatile all-solid-state laser devices.Among the LGS series crystals,there is a kind of disordered structure-CNGS crystal in which two or more atoms occupy the same oxygen ligand.CNGS crystal belongs to trigonal crystal system,32-point group,P321 space group,non-centrosymmetric structure,so its structural characteristics can be judged that CNGS crystal has potential application value in nonlinear optics.Compared with LGS crystals,CNGS has the characteristics of low Ga content,excellent physical and chemical properties,good mechanical properties and easy growth.At present,2-3 ?m lasers are widely used in today's society.Among them,Tm3+corresponds to the energy level transition of 3F4?3H6 near 2 ?m,and corresponds to the spectrum transition of 3H6?3H4 near 800 nm,and its absorption band covers the wavelength range pumped by commercial LD diodes.The electrons of Tm3+ can absorb the LD pump to the 3H4 energy level,and will jump to the laser energy level on 3F4 in a non-radiative relaxation manner to form a particle number inversion,which greatly improves the quantum efficiency of ions.In addition,Tm3+ ions have a relatively long laser upper-level lifetime,which is a typical three-level system,and Tm3+ lasers are also important tunable laser light sources.In the 2.5-3 ?m wavelength range,Er3+ doped crystals can radiate 2.8 ?m mid-infrared laser output due to the energy level transition of 4I11/2?4I13/2,and Ho3+doped crystals can radiate due to the energy level transition of 5I6?5I7 2.8-3?m mid-infrared laser output.In view of the above-mentioned problems and research background,this paper combines CNGS crystal,a photoelectric material with potential versatility,with the luminescence characteristics of complex energy levels of rare earth ions to explore and study the material in the near infrared to mid infrared 2-3?m spectrum and laser performance.The main research contents are as follows:1.Re:CNGS(Re=Tm,Er,Ho/Pr,Ho/Pr/Yb)crystal growth process researchIn this paper,the high-quality single-doped 5 at.%Tm:CNGS,higher concentration 15 at.%Single-doped Er:CNGS,co-doped 1 at.%Ho/0.5 at.%Pr:CNGS and co-doped 10 at.%Yb/1 at.%Ho/0.5 at.%Pr:CNGS crystals were synthesized.The crystal growth process of different doping and doping conditions was explored.In the process of crystal growth,we adjusted the cation sites of Ca2+,Nb5+ and other cations in CNGS on the chemical composition,reformed the temperature field,and at the same time,adjusted the raw material ratio and solid-phase synthesis of the CNGS crystal growth in the early stage.The method and process parameters of the growth process were improved and optimized.By reasonably adjusting the growth atmosphere and crystal size,and analyzing the growth conditions and crystal defects of each crystal,a high-quality rare-earth ion-doped CNGS single crystal was finally grown2.Re:CNGS(Re=Tm,Er,Ho/Pr,Ho/Pr/Yb)crystal structure and basic performance characterizationIn the paper,we have measured and calculated the theoretical density and actual density of Tm:CNGS,Ho/Pr:CNGS,and Yb/Ho/Pr:CNGS crystals.In general,the doping concentration of CNGS has a positive correlation with its density.The phase of the crystal was analyzed by powder diffraction XPRD,and the basic structure consistency of the grown crystal and pure matrix crystal after doping with rare earth was determined.At the same time,the high resolution X-ray diffraction was used to confirm that the crystal has a high crystal quality The unit cell parameters of the Tm:CNGS,Ho/Pr:CNGS and Yb/Ho/Pr:CNGS crystals were fitted as a=b=8.080A,c=4.996A,V=282.47 A3;a=b=8.088A,c=4.98OA,V=282.181 A3;a=b=8.078A,c=4.9920A,V=282.1081 A3.The structure of Ho/Pr:CNGS crystal was analyzed by Rietveld method,and the data of atomic coordinates,interatomic distance,average bond length and other data in the unit cell structure were obtained to prepare for the subsequent theoretical calculation.Finally,by studying the chemical composition of the growing crystal and the growth quality of the crystal,the doping concentration and segregation coefficient of each ion after doping the crystal with rare earth ions were determined3.Characterization and research of crystal thermal propertiesAs a solid-state laser gain medium,good thermal properties are crucial properties of crystals.The medium has better thermal properties to prevent the crystal from adverse effects such as thermal lens effect,medium cracking,poor beam quality,and multi-mode output during laser oscillationThe theoretical molar specific heat Cv of Re:CNGS crystal is calculated to be 440.6J/(k·mol),while the measured constant pressure specific heat of Tm:CNGS is 0.662 J·g-1·K-1,slightly lower than 0.578 J·g-1·K-1 of pure CNGS crystals,while the specific heat of Ho/Pr:CNGS and Yb/Ho/Pr:CNGS crystals is reduced relative to pure CNGS small.At the same time,the paper studies the thermal expansion properties of Tm:CNGS crystals.The linear thermal expansion coefficients of Tm:CNGS along the X and Z axes of physics are ?X=5.88×10-6K-1 and ?Z=7.07×10-6K-1,compared with pure CNGS,the anisotropy of thermal expansion of Tm:CNGS is more obvious when the temperature rises.Compared with vanadate system and borate system,Tm:CNGS has a smaller thermal expansion coefficient and anisotropy of thermal expansion.In terms of thermal conductivity,the thermal diffusivity of Tm:CNGS crystals varies less with temperature,but the overall trend is that the higher the temperature,the smaller the thermal diffusivity of the crystal.At 300?,the thermal diffusion coefficients ?x and ?z of the Tm:CNGS crystal in the X and Z directions are 0.686 mm2 s-1 and 0.773 mm2 s-1 respectively,and the thermal diffusion coefficients are?x=2.963 W/m·K and ?z=3.338 W/m·K4.Characterization and research of the spectral performance of Tm:CNGS crystalBased on the fluorescence characteristics of Tm3+at 2 ?m,this paper measures the absorption spectrum,fluorescence spectrum and laser level lifetime of Tm:CNGS,and finally characterizes the continuous laser performance of Tm:CNGS.For the absorption characteristics of Tm:CNGS,from 400 nm to 2000 nm,the unpolarized absorption spectrum of Tm:CNGS has strong absorption peaks at 685 nm,793 nm,1211 nm,and 1669 nm,corresponding to 3H6?3H4 of Tm3+.The absorption coefficient,FWHM and absorption cross section of the energy level transition at the output wavelength of 793 nm of the commercial LD pump source are 2.96 cm-1,15 nm and 5.01×10-21 cm2,respectively.In terms of fluorescence characteristics,the emission peak near 1888 nm has the largest fluorescence intensity,its FWHM is about 164 nm,and the fluorescence attenuation around 2 ?m corresponding to the 3F4?3H6 transition of Tm3+is about 2.345 ms5.Tm:CNGS crystal continuous waves laser performance characterization and researchIn the Tm:CNGS continuous waves laser test,the optimal parameters can be obtained when the parameters of the pump source and the cavity are adjusted to a core diameter of 100 ?m,the focusing system is 1:2,the input mirror IM is R=-200 mm,and Toc=5%For output,when the pump power is 5 W,a 2 ?m near-infrared laser oscillation with a maximum power of 740 mW can be obtained,with a maximum light conversion efficiency of 14%and a slope efficiency of 17.07%.