Design And Fabrication Of Ho³⁺/Tm³⁺ Co-doped Bismuthate Glass Microstructure Fiber And Its Performance

2?m band laser has become a research hotspot at home and abroad because of its wide applications in medicine,environmental monitoring,space communication,military and so on.In this paper,pointing at two scientific problems of the 2?m band rare-earth doped optical fibers in practical applications of fiber lasers,that is,the rare-earth doping concentration of high gain fiber,the optimization of fiber structure and single-mode transmission,the researches were carried out.Firstly,optimal Bi₂O₃-B₂O₃-Al₂O₃-Ba F₂ glass system was designed,the physicochemical and structural properties of these glasses were studied,the radiation characteristics,gain characteristics and energy transfer processes of rare-earth ions of Tm³⁺single doped and Ho³⁺/Tm³⁺co-doped Bi₂O₃-B₂O₃-Al₂O₃-Ba F₂ glasses were analyzed,the core and cladding materials of microstructure optical fiber with good performances were optimized and determined.Secondly,for two kinds of microstructure fibers,the effects of waveguide structure parameters on the mode,transmission and band characteristics were analyzed.And the waveguide parameters suitable for practical fabrication of bismuthate glass microstructure fibers were studied and determined.In the third step,the Ho³⁺/Tm³⁺co-doped 50Bi₂O₃-35B₂O₃-10Al₂O₃-5Ba F₂ glass microstructure fiber was prepared,and its 2?m band laser characteristics were studied.The main research contents and results of this dissertation are as follows:(1)On the basis of differential scanning calorimetry(DSC),Raman,XRD,FT-IR,and refractive index measurements and analyses,the physicochemical as well as structural properties of Bi₂O₃-B₂O₃-Al₂O₃-Ba F₂ glasses were studied systematically.The results showed that the boron aluminum anomalous point of Bi₂O₃-B₂O₃-Al₂O₃-Ba F₂ glass system is 35 mol%B₂O₃/10 mol%Al₂O₃.And this glass system has relatively large linear and nonlinear refractive index,as well as good crystallization resistance(?T?184?).Moreover,the maximum phonon energy of this glass system is 1150 cm^-1,and there are many structural units including[Bi O₃],[Bi O₆],[BO₃],[BO₄],[Al O₄],and[Al O₆]in the glass network,which provide structural basis for the realization of high concentration and homogenous doping of rare-earth ions.From the study of physicochemical and structural properties of the host glasses,it can be seen that the solubility of rare-earth ions in Bi₂O₃-B₂O₃-Al₂O₃-Ba F₂ system glass was much increased by effective control of Al₂O₃ content.(2)The radiative and gain characteristics of Tm³⁺doped 50Bi₂O₃-35B₂O₃-10Al₂O₃-5Ba F₂ glasses were systematically studied based on the absorption and emission spectrum.Note that the strongest 1.8?m emission can be obtained when the Tm₂O₃ doping concentration is 8 mol%,meanwhile the concentration of Tm³⁺in the prepared glass is 2.05×10²¹ ions/cm³,which is larger than that of germanate(7.6×10²⁰ ions/cm³)and silicate(8.35×10²⁰ ions/cm³)glasses reported so far.For the presented 50Bi₂O₃-35B₂O₃-10Al₂O₃-5Ba F₂-8Tm₂O₃ glass,the spontaneous radiative probability A_rad is 266.58 S^-1,the radiative lifetime?_rad is 3.15 ms,the maximum emission cross section is 5.29×10^-21 cm²(1864 nm),and the gain coefficient reaches 10.87 cm^-1(1864 nm).The macro energy transfer rate between Tm³⁺is 214.80×10^-20 cm³/s,which is much larger than that of Tm³⁺doped silicate glass,indicating that favorable cross relaxation process between Tm³⁺can be achieved in this glass sample.(3)The radiative and gain characteristics of Ho³⁺/Tm³⁺co-doped 50Bi₂O₃-35B₂O₃-10Al₂O₃-5Ba F₂ glass were systematically analyzed.Note that,in 50Bi₂O₃-35B₂O₃-10Al₂O₃-5Ba F₂-3Tm₂O₃-4Ho₂O₃ glass,the radiative lifetime?_rad of Ho³⁺is 5.42 ms,the emission cross section is 7.35×10^-21 cm²(2050 nm),the maximum gain coefficient is 7.81 cm^-1(2050 nm),which is much larger than that of tellurite glass(1.05 cm^-1).And large values of?_emi×??_eff(39.84×10^-26 cm³)and?_emi×?_rad(16.10×10^-21 cm²ms)indicate that Ho³⁺/Tm³⁺co-doped glass has good gain characteristic and gain bandwidth.Finally,the energy transfer process of Tm³⁺:³F₄(?)Ho³⁺:⁵I₇ was analyzed,the ratio of the forward energy transfer coefficient to the reverse energy transfer coefficient was calculated to be 90.44,which proves that Tm³⁺has effective sensitization on Ho³⁺in the glasses and ensures a strong 2?m fluorescence emission.The results showed that Ho³⁺/Tm³⁺co-doped 50Bi₂O₃-35B₂O₃-10Al₂O₃-5Ba F₂glass is a kind of laser gain material with good performances.(4)The mode,transmission and band characteristics of two kinds of 50Bi₂O₃-35B₂O₃-10Al₂O₃-5Ba F₂ glass microstructure fibers,MF1 and MF7,were studied.Mode analyses showed that the normalized effective core radius(R_eff/?)of Bi₂O₃-B₂O₃-Al₂O₃-Ba F₂ glass microstructure fiber with one single missing airhole in the center(MF1)is about 2.3 times of that of the microstructure fiber with seven missing airholes in the center(MF7),when the airhole pitch?,dimensionless parameter d/?and normalized wavelength?/?are exactly the same.Based on the study of R_eff/?,the endless single mode condition of MF1 is d/?<0.4,however,the critical d/?for achieving endless single mode in MF7 is much less than 0.1.Therefore,it is very difficult to fabricate the endless cutoff single-mode MF7.The study of transmission characteristics indicated that the confinement loss of MF1 is less than 10^-5 d B/m when the duty cycle d/?is 0.35,?is 2000 nm,and?is larger than 12?m,which can be used in practice.In addition,the chromatic dispersion characteristics of MF1 are also studied,while d/?of MF1 is 0.35,the peak negative dispersion coefficient of MF1 gradually decreases,and the peak position gradually shifts red with the increase of?.Note that MF1has flexible dispersion characteristics and is expected to be used in dispersion shifted fiber.Further study showed that the energy band of the presented microstructure fiber is only related to its dimensionless parameter d/?,and there is no complete photonic band gap in the studied microstructure fiber,indicating the designed microstructure fiber is a total internal reflection type fiber.Based on the above study and analysis,MF1 was selected as the basic structure model to build Ho³⁺/Tm³⁺co-doped 50Bi₂O₃-35B₂O₃-10Al₂O₃-5Ba F₂ glass microstructure fiber preform and prepare the optical fiber.(5)The fabrication process and laser performances of Ho³⁺/Tm³⁺co-doped microstructure fiber were studied.Based on the study of glass materials and the theoretical analysis of the microstructure fiber,Ho³⁺/Tm³⁺co-doped 50Bi₂O₃-35B₂O₃-10Al₂O₃-5Ba F₂glass microstructure fiber with hole pitch of 12?m,dimensionless parameter(d/?)of 0.35,core diameter of 15.2±0.2?m and cross section diameter of 125±0.5?m was prepared.Finally,under the pumped of 790 nm laser diode,a laser output at 2112 nm was achieved in the prepared microstructure fiber,yielding a maximum output power of 236 m W and a slope efficiency of 12.2%,as well as the beam quality factor was 1.35.