Study On The Laser Heated Pedestal Growth And Properties Of Lutetium-based Oxide Single Crystal Fibers

As a kind of new-type functional crystal material,single crystal fiber(SCF)combines the advantages of bulk single crystal and fiber material.It has the advantages of excellent physical and chemical properties,high optical quality,high doping concentration,high temperature resistance,oxidation resistance,and large specific surface area.SCF has huge application prospects in many fields,such as fiber laser and temperature sensing.In recent years,the research of SCF has shown a hot trend,including the growth of SCF,cladding preparation technology and device development,etc.,which are constantly making breakthroughs.Compared with the international advanced research level,the research of SCF in our country started relatively late,and the related research is in the exploratory stage.Therefore,it is of great significance to carry out research on the growth and performance optimization of highquality SCF.In this thesis,the laser heated pedestal growth(LHPG)was used to explore the crystal growth of two types of lutetium oxide SCFs,Lu3Al5O12(LuAG)and rare-earth sesquioxide mixed crystal LuREO3(RE=Y or Sc),which were selected as the research object.And the basic properties of the grown SCF were characterized.Focusing on the application requirements of SCF in the field of fiber laser and temperature sensing,the laser performance of Yb:LuAG SCF was preliminarily verified.By using upconversion fluorescence intensity ratio(FIR)technology,the optical temperature sensing performance of Er,Yb:LuREO3,Ho,Yb:LuAG and Ho,Yb:LuREO3 SCFs were studied.The main research outlines and conclusions are as follows:(1)Optimized Growth of LuAG SCFThe growth optimization of LuAG SCFs by the LHPG method was studied.Corresponding solutions were proposed for typical problems such as stress concentration,component volatilization and diameter fluctuation encountered in the growth process of high melting point SCFs.The laser uniformity was optimized by installing water-cooling protection device of the focusing mirror and the laser beam was collimated,thereby solving the problems such as skewed molten-zone,crystal cracking and volatilization of raw materials.The diameter uniformity of the SCF was improved by exploring the suitable growth power,shaping source rods and improving the stability of the laser power.Bubble defects are easy to appear when using ceramic rods for crystal growth.Therefore,combined with the characteristics of the LHPG method,the effects of growth orientation and furnace pressure on bubbles and stress distribution were studied.Low pressure can limit the formation and transport of bubbles,and the bubble densities are decreased by nearly 50%with a chamber pressure of 0.3 atm.In addition,the<111>-oriented LuAG single-crystal fibers are easier to be doped with large-sized bubbles than the<100>-oriented fibers,and are more sensitive to changes of pressure.When growing SCFs at 0.3 atm,the bubble size and stress of the<111>-LuAG SCFs are decreased by almost 40%,and the crystal quality is better than that of the<100>-fibers.To sum up,low pressure is beneficial to reduce the density and size of bubbles and improve the optical properties of SCFs,especially for<111>-LuAG fibers,which provides a feasible way to reduce the bubble defects in single-crystal fibers grown by the LHPG method.(2)Characterization and Laser Performance of Yb:LuAG SCFFocusing on the application of SCF in the fiber laser,high-quality 10 at.%Yb:LuAG SCFs were grown by the LHPG method.The grown fibers were evaluated in terms of crystal crystallinity,doping uniformity,and diameter uniformity.The test results show that the has a good crystallinity,uniform Yb3+ ion doping,and a maximum dianeter fluctuation of 1.44%.The anisotropy of dislocation defect and optical transmission performance of Yb:LuAG SCFs were studied.The dislocation defect anisotropy of LuAG SCFs grown along the<111>,<100>and<110>orientations were studied by chemical etching method.The dislocation density of the<111>-LuAG SCF is 8.0×103 cm-2,which is one order of magnitude smaller than that of other two directions.The beam quality and optical loss tests under the 808 nm pump show that the<111>-LuAG SCF has better optical uniformity and smaller loss coefficient.On this basis,the continuous wave laser characteristics of the<111>-Yb:LuAG were studied,and the maximum output power of 268 mW with the slope efficiency of 15.82%is realized at 1050 nm.And the values of Mx2 and My2 at the highest output power are 1.13 and 1.25,respectively.The above results indicate that the Yb:LuAG SCF grown by the LHPG method has the ability to achieve laser output at above the 1μm wavelength.(3)Upconversion fluorescence thermometry performance of Er,Yb:LuREO3(RE=Y,Sc)SCFsHigh-quality 0.5 at.%Er3+,5 at.%Yb3+doped LuYO3 and LuScO3 SCFs were successfully grown.The temperature sensing experiment was designed,and the upconversion fluorescence temperature characteristics of LuYO3 and LuScO3 SCFs were studied.Under the excitation of 980 nm laser,both samples yield green and red emission.They consist of the following emission bands:H band(2H11/2→4I15/2),S-band(4S3/2→4I15/2)and F band(4F9/2→4I15/2).The FIR technique was used to study the temperature sensing performance of LuREO3 SCFs,with the intensity ratio of H-band and S-band as the indicator.And the effective temperature detection in the range of 298～923 K was realized.The maximum absolute sensitivities(Sa)of Er,Yb:LuYO3 and Er,Yb:LuScO3 at 298 K are 0.0096K-1 and 0.0066 K-1(298 K),respectively,and the maximum relative sensitivity(Sr)are 0.0167 K-1 and 0.0118 K-1,respectively.The above results show that the LuREO3 SCFs have a good prospect in the field of optical temperature sensing.(4)Upconversion fluorescence thermometry performance of Ho,Yb:LuAG and Ho,Yb:LuREO3 SCFsIn order to improve the temperature measurement performance,0.5 at.%Ho3+,5 at.%Yb3+doped LuAG,LuYO3 and LuScO3 SCFs were grown.Under the excitation of 980 nm laser,Ho,Yb:LuAG showed yellow emission,while Ho,Yb:LuYO3 and Ho,Yb:LuScO3 showed green emission.The non-thermal coupling energy levels of Ho3+(5F4,5S2→5I8 and 5F5→5I8)were used as indicator to study the upconversion fluorescence temperature characteristics by FIR technology.The maximum Sa values of Ho,Yb:LuAG,Ho,Yb:LuYO3 and Ho,Yb:LuScO3 single crystal fibers are 0.0140 K-1(300 K),0.1603 K-1(303 K)and 0.0886 K-1(300 K),respectively,and the maximum Sr values are 0.0066 K-1(341 K),0.0102 K-1(335 K)and 0.0108 K-1(331 K),respectively.LuREO3 SCFs show great potential in high-sensitivity temperature sensing applications.For the same matrix material,the temperature measurement performance of based on the non-therlally coupled energy levels of Ho3+ion has been greatly improved compared with the Er3+ion,which provides a promising route to improve the sensitivity of optical temperature sensors.