Growth And Performance Study Of Nd³⁺;Tm³⁺;Er³⁺ Doped CaF₂/SrF₂ Single-crystal Fibers

In recent years,fiber lasers have rapidly developed toward high power,high energy,long wavelength,and ultrafast directions,and higher requirements have been placed on laser gain media.Glass fiber limits its further development due to its lower thermal conductivity,severe nonlinear gain,and lower laser damage threshold.As a new laser gain medium,single-crystal fiber combines the core idea of fiber laser and single-crystal gain,and has the advantages of high thermal conductivity,excellent mechanical properties,stable physical and chemical properties,and large specific surface area.Single-crystal fiber has wide applications in fiber laser,frequency multiplier and infrared detection.At present,researchers have developed a variety of laser single-crystal fibers but have not achieved practical applications.The growth method of single-crystal fiber mainly includes micro-pulling-down method and a laser-heating pedestal method,which are difficult to meet industrial production due to the low work efficiency and complicated process.Therefore,research on single-crystal fiber must adopt new materials,new processes,and new ideas.In view of the shortcomings in the field of single-crystal fiber,this paper innovatively designed porous graphite crucible,selected CaF₂/SrF₂ crystal as the matrix material,and realized the high-throughput preparation of rare-earth ion doped CaF2/SrF2 single-crystal fibers via temperature gradient method by optimizing the growth process,the main research contents are as follows:The single-crystal fibers grown by the temperature gradient method are transparent,complete in shape and free from cracking.They are characterized by high power microscope,EDS,ICP-MS and XRD.It is concluded that there are no large defects such as bubbles,holes and cracks inside the single-crystal fibers;the original crystal phase is maintained;the rare earth ions are evenly distributed,which proves the feasibility and reliability of the growth method.In addition,this method can not only meet industrial production,but also can be used for new material exploration.High-quality Nd³⁺,Gd³⁺:CaF₂ single-crystal fibers were grown by breaking the Nd³⁺ion clusters with Gd³⁺ions,and their spectral properties and laser properties were tested.Nd³⁺,Gd³⁺:The characteristic absorption peak and emission peak of Nd³⁺,Gd³⁺:CaF₂ single-crystal fibers are consistent with bulk single crystal;We tested the laser performance of Nd³⁺,Gd³⁺:CaF₂ single-crystal fiber with different transmittance under796 nm LD-pumping.When T=2%,the maximum output power of 1.49 W was obtained in 0.3 at.%Nd³⁺,5 at.%Gd³⁺:CaF₂ single-crystal fiber,the corresponding slope efficiency reached 40%.The quantum efficiency at 2?m of Tm ion can reach 200%due to the effect of cross relaxation.The high-quality Tm³⁺:CaF₂ single-crystal fiber were grown,and its spectral properties and laser performance test.The characteristic absorption peak and the emission peak of Tm³⁺:CaF₂ single-crystal fiber were consistent with the bulk single crystal;The 792 nm laser diode was used as the pump source to test the continuous laser performance of the Tm³⁺:CaF₂ single-crystal fiber at 2?m band under different transmittances,when T=2%,the maximum output power of 2.23 W was obtained in 3 at.%Tm³⁺:CaF₂ single-crystal fiber,and the slope efficiency was 64.4%,which was close to 2 times of Stokes efficiency.Due to the self-aggregation effect of Er ions at even a low concentration in the SrF₂ crystal,the Er³⁺:SrF₂ single-crystal fibers were grown by the same method.The spectral properties and laser properties of Er³⁺:SrF₂ single-crystal fibers were studied.The Er:SrF₂ single-crystal fibers had a wide emission band at 2.8?m;We studied the laser performance of the Er³⁺:SrF₂ single-crystal fibers at different transmittances under 972 nm and 976 nm LD-pumping,respectively.The highest output power of 860mW was achieved in 3 at.%Er³⁺:SrF₂ single-crystal fiber,and the slope efficiency was as high as 34.9%,which was close to Stokes efficiency.