| Laser is a very important invention in human history.Since T.Maiman invented the first ruby laser in 1960,laser has played an irreplaceable role in display,advanced industrial manufacturing,remote sensing communication,medical detection,military security and other fields after 60 years of development.At present,several forefronts of research in laser field are:visible laser applied to the field of display and optical lithography;high-power and high-energy laser with 1 μm band applied to the field of industrial manufacturing;mid-infrared laser applied to the fields of medical treatment and military security;LD-pumped ultrafast laser.Among them,the 2.0 μm and 3.0 μm band,which belong to the mid-infrared band,are the hotspots of research in the world because they are related to the fields of medical treatment and military security.The development of laser technique is inseparable from the continuous upgrade of laser device.Now the solid-state laser has been successfully industrialized,and the laser crystal is a core component of the solid-state laser.Therefore,it is of great significance to explore laser crystals with better thermal and optical properties for the development of laser.In recent years,the disordered crystal is getting more and more attention form researchers.In the structure of the disordered crystal,two or more two kinds of cations occupy the same part of the crystal inside randomly and there is a difference in the crystal structure.On the one hand this change makes the coordination environment of the activated ions inside the crystal varied and finally leads to the phenomenon of spectral broadening.On the other hand,at the "macro" level,the concentration ratio of doped ions meets the law of statistics.So the crystal structure is still orderly.This kind of disordered crystal can combine the advantages of wide spectrum,long fluorescence life of glass and excellent thermal properties of crystal,which has unique application value in the field of laser.In this paper,the research direction of laser is the 2.0 μm and 3.0 μm bands,which belong to the mid-infrared band,the rare earth ions doped laser crystals with disordered structure are taken as the research object,and two kinds of disordered crystals with different structures are grown by the Czochralski method,including Tm,Ho:GAGG crystal with garnet structure and Er,Yb:CGA crystal with K2NiF4 structure.The thermal and spectral properties of Tm,Ho:GAGG crystal are characterized,and the continuous laser output at 2.0 μm band is measured.For Er,Yb:CGA crystal,the spectral properties are measured at 3.0μm band.The above experiments provide data support and reference for the application of two kinds of rare earth ion doped disordered crystals in the field of mid-infrared band laser.The paper include the following five chapters:In the first chapter,the development trend of laser crystal,the research progress of 2.0 μm,3.0μm band crystals and the disordered crystal are reviewed.A crystal growth method,the Czochralski method,is introduced.Finally,based on the above summary,the significance,purpose and the main research content of this paper are proposed.In the second chapter,the experimental processes of growing Tm,Ho:GAGG crystal by Czochralski method,including the synthesis of polycrystalline raw material and crystal growth processes,are introduced,and several influence factors of crystal growth are discussed.For the growth atmosphere,the volatilization of Ga2O3 in raw materials is effectively inhibited with the mixture gas of 50%Ar and 50%CO2,and this kind of atmosphere also protects the iridium crucible from oxidation.Finally,the Tm,Ho:GAGG crystal with the size of Φ 26×40 mm3 is successfully grown by using the YAG seed crystal with<111>crystal orientation.The XRD pattern of the Tm,Ho:GAGG crystal is consistent with the standard XRD pattern,which indicates that the Tm,Ho:GAGG crystal grown by Czochralski method is single crystal.The XRF test shows that the doping concentration of Tm3+is 5.2 at.%and that of Ho3+ is 1.8 at.%,and the effective segregation coefficients of Tm3+and Ho3+,hich are calculated by the XRF result are 0.85 and 0.88.These calculation results also indicate that the Tm3+ and Ho3+in GAGG crystal are uniform distribution.The high resolution X-ray diffraction test shows that the rocking curve of Tm,Ho:GAGG crystal is smoothing,symmetrical and unsplit.The FWHM of the rocking curve is 31",which shows that the Tm,Ho:GAGG crystal has perfect crystalline quality.In the third chapter,the thermal and spectral properties of Tm,Ho:GAGG crystal are characterized,and the continuous laser output performance of the crystal is tested.The average density of Tm,Ho:GAGG crystal is 6.66 g/cm3.The specific heat value of the crystal at the room temperature is 0.759 J·g-1·K-1,and with the increase of temperature,its value shows an increasing trend.When the temperature reaches 300℃,the specific heat of the crystal is 0.956 J·g-1·K-1.The thermal diffusivity of Tm,Ho:GAGG crystal is 3.123 mm2/s at room temperature,and then it will decreases with the increase of temperature.The average thermal expansion coefficient of the crystal is 6.089×10-6 K-1.The thermal conductivity of the crystal at room temperature is calculated as 8.532 W·m-1·K-1 and it decreases gradually as the temperature rises.Then we test the spectral properties of Tm,Ho:GAGG crystals.The two major absorption peaks of the crystal appear at 682 nm and 784 nm with FWHM of 5 nm and 26 nm.The peak of fluorescence spectrum of the crystal appears at 2080 nm with FWHM of 68 nm,and its fluorescence lifetime is 10.67 ms.The absorption and emission half peak width and fluorescence lifetime of Tm,Ho:GAGG are better than those of Tm,Ho:GGG.This means that compared with the GGG crystal,the GAGG crystal is more suitable for Tm3+ and Ho3+ doping due to its disordered structure,and it is more suitable for matching LD pump after doping Tm3+and Ho3+.The thermal properties of Tm,Ho:GAGG crystal are better than those of YAG,YLF and YVO4 crystals which are doped with the same kind of rare earth ions.In terms of spectral properties,Tm,Ho:GAGG crystal also shows excellent comprehensive advantages compared with the frequently reported crystals such as Tm,Ho:LuAG,Tm,Ho:LuLF and Tm,Ho:YLF.The thermal and spectral properties of Tm,Ho:GAGG crystal show that it has potential value in the field of 2.0 μm band laser.The continuous laser output test of Tm,Ho:GAGG crystal shows that the output power increases linearly with the increase of 787 nm pumping light power absorption.When the pump power absorbed by the crystal sample reaches 2.91 W,the output power is 218 mW.The slope efficiency is 9.2%through linear fitting the data of absorption power and output power.It can be seen that the output power and slope efficiency of the crystal still need to be improved.The reasons may be that the crystal is not coated,the sample length is short,the doping conceLtration qf Tm3+ is still lowly,or the laser resonator structure needs to be further improved,and these are what we need to do next.In the fourth chapter,the growth processes of Er,Yb;CGA crystal grown by Czochralski method are introduced.The Er,Yb:CGA crystal with the size of Φ26×40 mm3 is successfully grown by optimizing growth parameters and atmosphere.The doping concentration of Yb3+is 0.95 at.%,and that of Er3+is 18.74 at.%.The effective segregation coefficient of Er3+ is 0.62.This indicates that when the crystal is grown by the Czochralski method,some parts of Er3+ accumulate at the solid phase interface,and the concentration of Er3+ in the latter half of the crystal is higher than that in the former half.The absorption spectrum of Er,Yb:CGA crystal with c crystal orientation shows that the crystal has a wide absorption band near 967 nm,which is caused by the superposition of the absorption peak of Er3+ at 967 nm and the absorption peak of Yb3+at 981 nm.The result of the absorption spectrum also indicates that the Er,Yb:CGA crystal can match the commercial InGaAs diode pump perfectly,which is beneficial to the practical application.The strongest fluorescence peak of Er,Yb:CGA crystal with c crystal orientation appears at 2730 nm,and the FWHM is 141 nm,which is wider than that of Er:GGG crystal and Er,Pr:GGG crystal at 2700 nm.It can be seen that the multiple disordered structure of Er,Yb:CGA crystal results in spectral broadening.This also shows that the Er,Yb:CGA crystal has great potential in the field of tunable laser.The fluorescence intensity of Er,Yb:CGA crystal with c crystal orientation in 4I11/2 and 4I13/2 levels decays exponentially over time.The fluorescence lifetime of 4I11/2 is 0.34 ms,and that of 4I13/2 is 3.18 ms.Compared with some kinds of Er3+ doped laser crystals with excellent spectral performance,the fluorescence lifetime of the upper level 4I11/2 in Er,Yb:CGA crystal is close to that of Er:CGA crystal(0.45 ms)and Er:CaYAlO4 crystal(0.35 ms).Er,Yb:CGA crystal has potential application value in the field of 3.0 μm mid-infrared tunable laser.In the fifth chapter,the main conclusions of this paper are summarized,and the main innovation points and the problems for further research are put forward. |
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