Efficient Mid-infrared Luminescence At 2.0?3.0 ?M From Rare-earth-doped Multi-component Germanate And Tellurite Glass Fibers

Lasers operating at 2.0-3.0?m mid-infrared?MIR?region can be widely used in material processing,atmospheric pollution monitoring,remote sensing,medical surgery,and military defense due to their excellent atmospheric transmission characteristics,strong ability of penetrating the smoke,“eye-safe”for human,and strong absorption band for water molecules,etc.Traditionally,silica and fluoride glasses are frequently used as glass hosts for achieving 2.0?m and 3.0?m laser output,respectively.However,silica glass has a lower rare earth?RE?doping concentration,narrower infrared transmission range,larger phonon energy,lower luminescent efficiency,while fluoride glass faces other issues such as poor physical and chemical performance,lower laser damage threshold,and difficulty of welding with silica optical fiber.Therefore,it becomes one of the current hottest issues of science field that how to analyze and realize a new type of high gain glass fiber and fiber lasers and greatly improve the performance of fiber materials and laser devices operating at 2.0-3.0?m.Based on the above background,this thesis aims to explore novel multi-component glass with high glass transition temperature,excellent thermal stability and fiber drawing property,and especially with high efficient luminescent at 2.0-3.0?m.Furthermore,multi-component glass fibers with low loss and high gain property are prepared through determining the related experimental and technical parameters based on a suction technology,and the features of the fiber lasers are also preliminary evaluated.This dissertation is divided into five chapters.In chapter 1,the characteristics,the applications as well as the latest research progress of the fiber lasers operating at 2.0-3.0?m have been reviewed.Afterwards,the glass hosts,the RE ions,and the mechanism for achieving 2.0-3.0?m luminescence and laser are summarized,and finally,the research purpose and research content are represented.In chapters 2-5,high efficient mid-infrared luminescence at 2.0-3.0?m based on fluorogermanate and barium tellurite glass fibers have been studied.Achievements of this thesis include:?1?In order to solve the problem of Yb³⁺/Tm³⁺co-doped fiber lasers with a lower laser efficiency,the competition between 1.8?m downconversion?DC?and visible upconversion?UC?emission is studied both qualitatively and quantitatively in Tm³⁺and/or Yb³⁺/Tm³⁺co-doped fluorogermanate glasses.First,the glass forming region of the germanate glass is predicted through the glass phase diagram model in order to choose an optimal glass composition.Afterwards,based on the absorption spectra and Judd-Ofelt theory,various optical parameters involving the oscillator intensity parameters,stimulated absorption cross-section,emission cross-section,the figure of merit?FOM?,and saturated gain coefficient are calculated and analyzed.The fluorescence characteristic and energy transfer?ET?mechanism of the glass under 808 nm and 980 nm laser diodes?LDs?are also comparatively investigated.Finally,further enhanced 1.8?m emission is obtained by introducing fluoride and the treatment of purified oxygen bubbling and protection during the melting process.A quantitative study of ET process between Yb³⁺and Tm³⁺in fluorogermanate glass suggests that the forward transfer parameter is larger than two orders of magnitude than the backward one and the ET efficiency is more than 80%.Moreover,it is found that the UC emission,especially the emission at 800 nm is the main path of energy loss in the system relative to the 1.8?m emission,while this adverse effect can be lowered in the case of Tm³⁺heavily doped.This privides a new method to improve the laser efficiency of Yb³⁺/Tm³⁺co-doped fiber lasers.?2?In the common fiber lasers,the pumping wavelength is very single and the pumping ways are not flexible and diversified,here we propose a new idea to sensitize the rare-earth by the transition metal with broad absorption in the range of visible to near-infrared for obtaining efficient mid-infrared laser.As an example,the Cr³⁺ion is utilized to sensitize Tm³⁺for obtaining an efficient 1.8?m emission.The spectroscopic investigations including absorption spectra,excitation spectra,emission spectra,and luminescent lifetime measurements indicate that the intensive and broad absorption band of Cr³⁺can provide a selective multi-wavelength pump scheme and a possible ET process from Cr³⁺to Tm³⁺,namely use a variety of pump sources such as visible lasers,Optical Parametric Oscillator?OPO?,commercial 808 nm and980 nm LDs.This ET process can be confirmed by the decreased luminescent lifetime of Cr³⁺and the enhancement of Tm³⁺emission at 1.8?m.Attractively,the highest ET efficiency from Cr³⁺to Tm³⁺is calculated to be 76.97%,manifesting that this Cr³⁺/Tm³⁺co-doped fluorogermanate glass is an excellent matrix for 1.8?m emission and may find applications in MIR fiber lasers and amplifiers.Similarly,an enhanced 1.8?m emission of Tm³⁺can also be achieved through Bi sensitization and the ET efficiency is as high as 95.42%.The underlying Bi?Tm³⁺ET mechanism is rationally proposed and discussed according to the static-to-dynamic luminescence spectra.These two different sensitization ways provide a novel approach for obtaining efficient 1.8?m emission of Tm³⁺.?3?Within the problem of fiber lasers with a lower gain and narrower tunable bandwith,we adopt the approach of rare-earth co-doping and optimal concentration and ratio and finally achieve an intense ultra-broad?FWHM=382 nm?tunable emission at 2.0?m in the Tm³⁺/Ho³⁺co-doped barium tellurite glass fiber upon the excitation of 808 nm.Raman,Differential Scanning Calorimetry?DSC?,and X-Ray Diffraction?XRD?measurements consistently present the evidence of multiple structural sites in barium tellurite glass system,which allows a heavily RE doping concentration without sacrificing the glass formation ability and thermal stability.Moreover,the investigation on radiative properties of both Tm³⁺and Ho³⁺show that the emission cross section of Ho³⁺(1.20×10^–20 cm²)is nearly double that of Tm³⁺(0.63×10^–20 cm²).The gain coefficients of Tm³⁺and Ho³⁺are also as high as 6.66cm^–1 and 1.05 cm^–1,respectively,which is much larger than the common silicate(1.50 cm^-1and 0.84 cm^-1)and phosphate(0.9 cm^-1 and 0.8 cm^-1)glasses.These results manifest that this barium tellurite glass fiber is a very promising candidate for potential application in MIR ultra-broad tunable fiber lasers.?4?In the aspect of multicomponent glass fiber preparation,the traditional rod-in-tube and stack-and-draw methods are often complex and time-consuming,especially a higher fiber loss impedes the implementations of laser output with new wavelength from the fiber.The suction technique for fabricating Er³⁺-doped barium tellurite glass fiber with low loss has been explored in order to achieve the 2.7?m laser output from multi-component oxide glass fiber.The transmission loss of the fiber at 1310 nm is measured to be 3.46 dB/m by the cutback method.The higher spontaneous transition probability(50.84 s^–1),emission cross section(0.79×10^–20 cm^–2),and figure of merit(3.18×10^–24 cm^–2·s)provide a solid foundation for obtaining intense 2.7?m emission.Upon the excitation of 980 nm LD,an intense 2.7?m amplified spontaneous emission?ASE?from Er³⁺-doped tellurite fiber is observed.Furthermore,the feasibility of Er³⁺-doped tellurite fiber lasers operating at 2.7?m has been theoretically predicted based on the rate equation and propagation equation,the effect of pumping configuration and fiber length on the laser output power,slope efficiency,laser threshold,and intracavity pump and laser power distributions have been analyzed in detail.The potential laser performance of the present tellurite fiber is also compared and analyzed with conventional Er³⁺-doped fluorozirconate?ZBLAN?fiber lasers.Furthermore,the two-and three-dimensional laser field distributions are further carried out to reveal the characteristic of this multi-mode step-index tellurite fiber.This work will contribute to design high-efficient 2.7?m multi-component oxide fiber lasers.