| Fiber laser devices possess merits such as high beam quality and maintenance free owing to the laser is both generated and delivered by an inherently flexible medium,which allows easier laser delivery and thermal management.These merits are crucial in the fields of remote sensing,atmospheric monitoring,light detection and ranging（LIDAR）,and biomedicine.The core working medium in a fiber laser is rare-earth（RE）doped laser glass fiber.RE-doped silica glasses are mechanically strong and resistant to thermal shocks due to their three-dimensional highly polymerized rigid structures;hence they are the most prominent host materials in fiber laser devices.Furthermore,as the production process is well-established,silicate fibers are readily commercialized.Nonetheless,the rigid structure of silica glass limits the doping concentration of RE ions(～1019 ions/cm3).Moreover,due to the strong Si-O bond,its phonon energies(1200 cm-1)are among the highest in various glasses.Tellurite glass,which mainly consists of Te O2,has relatively low phonon energies(～750 cm-1),high RE solubility(～1021ions/cm3),large infrared transmission range（～6μm）and large absorption/emission cross-sections(～10-20 cm2).These properties make tellurite glasses an ideal host material for near-and mid-infrared range lasers.Therefore,it is imperative to explore novel RE-doped tellurite laser glasses and in addition,their respective lasing properties in the near-to-mid infrared region.Based on these research backgrounds,a systematic study of novel RE doped tellurite laser glasses and their corresponding optical fiber was carried out.Experiments concerning efficient emission and lasing properties of tellurite glass were conducted.This dissertation is divided into five chapters.The research progress of RE-doped laser glass and fiber laser are reviewed in chapter 1.Characteristics of multicomponent glass substrates,especially tellurite glass and emission properties of various RE ions are summarized.Additionally,research status,existing issues and possible solutions are introduced.In chapter 2,the production procedures and characterization methods of tellurite glass and fiber samples are presented.In chapters 3 to 5,the theoretical prediction and experimental verification of the glass forming regions（GFRs）of tantalum tellurite glass（TTZ）and gallium tellurite（TGZ）glass are studied in detail,as well as the influence of melting conditions on the GFR and physical and luminescent properties of tellurite glass,the near-and mid-infrared spectral characteristics of RE doped tellurite glass,and the preparation and process optimization of gain fiber and the laser output properties.The main results of this thesis are as follows:（1）Trial-and-error method has been the predominant approach to determine the GFR,which was inefficient and blind.To solve this problem,the GFRs of two new tellurite glasses（tantalum tellurite glass and gallium tellurite glass）are predicted and experimentally verified by a thermodynamic calculation method based on the glass forming rule.The effects of platinum crucible and alumina crucible on the GFR and the properties of tellurite glass are also studied.The results show that the theoretical GFRs are all located in the actual GFRs with high coincidence and proves that the thermodynamic calculation can accurately predict the GFRs of tantalum tellurite glass and gallium tellurite glass,which guides for the exploration of new tellurite glass and the compositional design of active fiber.（2）Utilizing physical property analysis and structure characterization of Raman spectra,the reasons for the differences in the GFRs of tellurite glass under different melting conditions were investigated.The influence of network modifiers on the structure of tellurite glass and the evolution process was explained by combining theoretical calculation and characterization results.It is found that the GFR of TTZ glass is enlarged and deviated from the predicted GFR by using an alumina crucible.Elemental analysis and structural information show that when the alumina crucible is adopted,a large amount of Al2O3 is introduced into TTZ glass,even if the preparation temperature is much lower than the melting temperature of alumina,leading to the breakdown of the chain structure formed by[Te O4]and the production of more terminal[Te O3]units containing Te=O double bonds.Eventually,the density and refractive index of TTZ（Al2O3）glass are lower than that of TTZ（Pt）glass.Compared with TTZ（Al2O3）glass,TTZ（Pt）glass has higher local symmetry of RE ions,larger absorption and emission cross-sections,and longer lifetime.（3）Based on the above results,ternary tellurite glasses with large GFR,high resistance to crystallization,large RE ion doping concentration and low phonon energy were further selected to study the luminescence properties of Tm3+-and Ho3+-doped glass at 2μm and Er3+-doped and Yb3+/Er3+co-doped glass at 1.5μm and 2.7μm,respectively.By doping incremental concentrations of RE ions and characterizing their emission spectra and fluorescence attenuation curves,the luminescence regularity and optimal doping concentration of RE ions in the new system were obtained.Based on the Judd-Ofelt theory,the optical spectral characteristic parameters of the above RE doped tellurite glass were evaluated.The results show that the fluorescence transition probability of Tm3+:3F4→3H6 in the singly Tm3+-doped TTZ glass is 400.43 s-1,which is higher than that of germanate glass(311.50 s-1),mainly due to the high refractive index of tellurite glass.In TTZ glass,high-efficiency luminescence at 2μm is achieved by pumping singly Ho3+-doped TTZ glass with a 1940 nm semiconductor laser.The absorption cross-section(σaMC),emission cross-section(σeMC),spectral quality factor（FOM）and gain coefficient are 3.63×10-21 cm2,4.56×10-21 cm2,6.83×10-24 cm2·s and 0.9 cm-1,respectively.In Er3+-doped TTZ glass,the luminescence at 2.7μm is enhanced when the concentration of Er2O3 is increased to 3 mol%.The absorption cross-section（σa）,emission cross-section(σeFL),FOM and gain coefficients are 0.91×10-20 cm2,1.01×10-20 cm2,3.18×10-24 cm2·s and 2.25 cm-1,respectively.The above results indicate that TTZ glass can be used as the substrate material for near-and mid-infrared fiber lasers.（4）The melting process of bulk tellurite glass was explored and the key parameters such as glass composition,melting temperature and melting time were obtained.Large transparent tellurite cladding and core glasses without crystallization were prepared,respectively.The optical properties of the prepared optical fiber were measured by transmission loss and amplified spontaneous emission（ASE）,and various optical parameters of the optical fiber were obtained.Under the guidance of theoretical calculation,Tm3+-doped TTZ glass fiber with a numerical aperture（NA）of 0.21 was designed.The fiber preform rod and Tm3+-doped single-mode fiber were prepared by the rod-in-tube method,with a core diameter of about 8μm and a cladding diameter of about 125μm.The Ho3+-doped TTZ glass fiber was prepared,and the average transmission loss of the fiber at 1310 nm was 0.12 d B/cm by a cut-back method.A Yb3+/Er3+co-doped single cladding optical fiber with a NA of 0.16 was designed.The preform rods of TTZ glass and the multi-mode and single-mode Yb3+/Er3+co-doped TTZ glass fibers were fabricated by the suction method and rod-in-tube method,respectively.Finally,1.5μm band laser output was realized in the Yb3+/Er3+co-doped TTZ fiber.The above results prove that TTZ glass has the potential to be the working medium of high-gain fiber laser in the near-and mid-infrared band. |
