Structural And Spectroscopic Characteristics Of Yb³⁺/Ho³⁺ Co-doped Bismuth-tellurite Glasses By WO₃ Adjustment

Mid-infrared fiber lasers have attracted wide attention in nonlinear optics and high-power fiber lasers because of their high beam quality and high conversion efficiency.In particular,fiber gain media that can provide shorter pulse duration,wider spectrum span,and greater tunable range have been the goal of researchers.However,there are still many pain points in the gain medium of mid-infrared fiber lasers,such as low luminous efficiency and limited tunable range of the mid-infrared light source.Therefore,the above problems faced by the rare-earth-doped fiber with a 2μm band are studied in this paper.The first step is to design and optimize the Te O₂-Bi₂O₃-Zn O glass system,study the physicochemical and structural properties of the glass,and analyze the radiation and gain characteristics of Ho³⁺/Yb³⁺co-doped tellurium bismuth glass and the energy transfer mechanism between rare earth ions.In the second step,the network modifier WO₃ was added to the glass to regulate the topological cage structure,and the changes in the glass mesh structure and luminescent center activity were qualitatively analyzed,so as to explore the influence of topological cage changes on the absorption characteristics,radiation characteristics,gain characteristics and energy transfer process between rare earth ions in the rare-earth ion co-doped glass system.On this basis,sensitized ion Ce³⁺was added into Ho³⁺/Yb³⁺co-doped system to further enhance the 2μm emission property of fiber core glass by forming a special cross-relaxation process.Finally,a mathematical model of Ho³⁺/Yb³⁺co-doped 2-3μm band fiber laser was constructed based on the rate equation and the optical transmission equation.The theoretical relationship of pump optical absorption efficiency,signal optical power,and slope efficiency with fiber length was obtained based on the fourth-order Runge-Kutta method.The main contents and conclusions are as follows:1.Design,optimization,preparation,and mid-infrared luminescence characteristics of Ho³⁺/Yb³⁺co-doped tellurium bismuth glass were carried out.（1）In order to enhance the mid-infrared emission performance of the fiber-core glass,the composition of bismuth-tellurite glass was designed and optimized,and85Te O₂-5Bi₂O₃-10Zn O（TBZ）matrix glass and Ho³⁺/Yb³⁺co-doped TBZ glass were prepared high-temperature melting method.The physical properties of TBZ glass were investigated by density,refractive index,and DSC.The refractive index curve of TBZ glass at the infrared band is obtained by refractive index test and second-order Sellmeier fitting.The refractive index and nonlinear refractive index at the 2μm band are both higher than that of conventional tellurate glass.The results of the differential thermal curve test show that TBZ glass has good thermal stability（ΔT=106℃）.The structural units included in TBZ glass were determined by FT-IR spectra,including[Te O₄],[Te O₃₊₁],[Bi O₃],and[Bi O₆].Further Raman spectroscopy and photoelectron spectroscopy XPS analysis have obtained the content of BO in the TBZ matrix glass（about 8.51%）,and the[Te O₃]structural unit is determined as the main structure,and the maximum phonon energy of the matrix glass is determined to be about 758 cm^-1.（2）The emission characteristics,gain characteristics,and energy transfer mechanism of Ho³⁺/Yb³⁺(Yb³⁺and Ho³⁺with fixed content of 2 mol%and 0.1,0.25,0.5,0.75,and 1 mol%respectively,and named TBZ1-5)co-doped TBZ glass at different concentration ratios were thoroughly studied.The fluorescence spectra of TBZ1-5 glasses show that with the increase of Ho³⁺concentration,the emission intensity at the 2μm band enhances first and then decreases,and the optimal doping ratio of Yb³⁺/Ho³⁺is 2 mol%:0.5 mol%(Ho³⁺concentration is 1.97×10²⁰ ions/cm³).Based on the measured absorption spectra and Judd-Ofelt theory,the?₂ value of85Te O₂-5Bi₂O₃-10Zn O-2Yb₂O₃-0.5Ho₂O₃（TBZ3）glass is obtained to be 4.65×10^-20cm².On this basis,it is calculated that the maximum emission cross section of Ho³⁺:⁵I₇to ⁵I₈ energy transition in TBZ3 glass is about 9.92×10^-21cm²,and the gain coefficient is 1.95cm^-1.Finally,the energy transfer mechanism between Ho³⁺/Yb³⁺was quantitatively analyzed based on the spectral overlap method,and the forward energy transfer coefficient C_D-A between Ho³⁺/Yb³⁺was calculated to be 0.36×10^-40 cm⁶/s,indicating that an effective energy transfer process could be obtained in TBZ3.2.Under the determination of the Ho³⁺/Yb³⁺doping concentration ratio,the influence of WO₃ content on the luminescence characteristics and structural properties of Ho³⁺/Yb³⁺co-doped tellurium bismuth glass was investigated,and the energy transfer process（ET）was discussed.（1）WO₃-regulated matrix glass was prepared by the high-temperature melting method,and its basic physical properties were studied.The data of density and refractive index were analyzed.With the increase of WO₃ concentration from 0,5 mol%,and 10mol%by 15mol%,the refractive index was significantly enhanced,and the nonlinear refractive index and density of the glass were both enhanced.When the content of WO₃ was 10 mol%,the molar volume V_m reached the maximum（30.21cm³/mol）,indicating that the gap of the glass network was larger,and it was easier to achieve high concentration doping.With the increase of WO₃ content,the refractive index of glass is slightly enhanced,and due to the increase of O^2-content in the glass,the oxygen packing density OPD value increases from 62.52 cm^-3 to 66.29 cm^-3,and the glass mesh structure tends to be dense.DSC test shows that WO₃ can effectively enhance the thermal stability of the bismuth-tellurite glass.（2）The spectral properties and energy transfer mechanism of Ho³⁺/Yb³⁺co-doped Te O₂-Bi₂O₃-Zn O system glasses with different WO₃ concentrations were systematically studied.In the（85-y）Te O₂-5 Bi₂O₃-y WO₃-10Zn O（y=0,5,10,and 15,respectively named TBZ-TB15WZ）system,the glass transmittance is obviously enhanced with the increase of WO₃ content,and the absorption edge of the glass has obvious blue shift.Direct optical band gap Edopt and indirect optical band gap Eindopt increased from 2.91 and 2.83 e V to 3.12 and 3.02 e V respectively,and the glass network structure became dense.According to the absorption spectrum and Judd-Ofelt theory,it is found that with the increase of WO₃ content,?₂ increases from 4.65×10^-20 cm² to5.77×10^-20 cm².The larger?₂ value confirms the good structural stability of TBWZ glass,and the larger?₆ value is obtained when WO₃ content is 10 mol%.In this case,the effects of Stark energy level splitting and crystal field on Ho³⁺are weak,and it is easy to obtain a wide fluorescence emission bandwidth.The optimal concentration of WO₃ was 10 mol%by comparison of fluorescence spectra.In addition,the energy level lifetime,absorption and emission cross-section,and gain coefficient at 2μm and 3μm bands are enhanced after the introduction of WO₃.After the introduction of WO₃,the energy transfer coefficient C_D-Abetween Yb³⁺/Ho³⁺increases from 0.36×10^-40 cm⁶/s to 1.11×10^-40 cm⁶/s,indicating that the introduction of WO₃ can effectively enhance the energy transfer process of Ho³⁺/Yb³⁺co-doped bismuth tellurite glass.（3）The regulation effect of WO₃ structural units on the topological cage structure of Te O₂-Bi₂O₃-Zn O glass mesh was studied.According to the test results of FT-IR,Raman spectroscopy,and XPS,the introduction of WO₃ leads to the increase of bridging oxygen content,resulting in the transition from[Te O₃]to[Te O₄]structural unit,and the increase of phonon energy of matrix glass,which partially makes up for the energy mismatch between Ho³⁺/Yb³⁺levels.Therefore,it shows the enhancement of mid-infrared emission characteristics.On the other hand,the change of topological cage effectively inhibits the interaction between rare earth ions and enhances the mid-infrared luminescence performance.3.The 2.0μm luminescence enhancement of Ce O₂ on Yb³⁺/Ho³⁺co-doped75Te O₂-5Bi₂O₃-10WO₃-10Zn O（TB10WZ）glasses were studied,and the energy transfer process was discussed:（1）Mesh modification of TB10WZ glass was carried out by Ce³⁺,and the introduced contents of Ce O₂ were 0.1,0.3 and 0.5 mol%,respectively.It has been found that a small amount of Ce³⁺can form an effective cross-relaxation with Ho³⁺,inhibiting Ho³⁺up-conversion,and promoting the energy level population of Ho³⁺:⁵I₆ enhances the emission performance of 2μm band,and when the doping amount of Ce O₂ reaches0.3 mol%,the luminescence gain intensity reaches,and the maximum emission cross-section of 1.21×10^-20cm² and gain coefficient of 2.28 cm^-1 can be obtained at 2μm band.Compared with the unintroduced TB10WZ sample(the maximum emission cross-section is 1.15×10^-20cm²),the increase is about 1.05 times.（2）The spectral overlap method was used to quantitatively analyze the energy transfer process between Ho³⁺/Yb³⁺/Ce³⁺,and it was found that the proportion of 0-phonon assist between Ho³⁺/Ce³⁺was 1.51%,and the proportion of 1-phonon assist was as high as 98.49%.The microscopic energy transfer parameter C_D-A between Ho³⁺/Ce³⁺is 7.32×10^-40cm⁶/s.The results show that the Ce O₂-enhanced Ho³⁺/Yb³⁺co-doped bismuth-tellurite core glass is an ideal gain medium for achieving 2.0μm laser output.4.When the Ho³⁺/Yb³⁺doping concentration ratio is determined as 0.5 mol%:2mol%,the rate equation and optical power equation model of 2μm and 3μm bands are constructed,and the changes of pump optical power,signal optical power,and slope efficiency at 10 mol%WO₃ content are systematically analyzed.（1）The change in pump light absorption efficiency before and after the introduction of WO₃ is studied.In the two models,it is found that when the pump power is set to 10W,with the gradual increase of fiber length,the minimum value of forward pump power（P_f）gradually decreases and reaches the minimum in TBZ3 and TB10WZ systems,and the pump light absorption efficiency is close to saturation.In this case,the fiber lengths corresponding to TB10WZ are 25.12 cm and 9.52 cm in the 2μm and 3μm models,respectively,while those of the TBZ3 system are 29.96cm and 11.98cm in the 2μm and 3μm models,respectively.It can be seen that in the system containing WO3,the fiber lengths required to reach the saturation state are relatively shorter.（2）The variation trend of signal optical power in different models is studied.It is found that a theoretical optimum fiber length L_op exists in the fiber to maximize the optical power of the forward signal.Compared with the TBZ3 system without WO3,the length of the gain fiber is shorter（26.3 cm and 11.5 cm in the 2μm and 3μm models,respectively）when the optical power S_f of the forward signal reaches the maximum.Moreover,the maximum optical power of the signal increases obviously（about 0.21 W and 0.07 W in the 2μm and 3μm models,respectively）.（3）The slope efficiency changes in the two models are compared systematically.It is found that the introduction of WO₃ can effectively improve the slope efficiency of the fiber（about 5.6%and 7.0%in the 2μm and 3μm models,respectively）.The calculation results show that the introduction of WO₃ can realize the laser output in the mid-infrared band more effectively.