The Research On The Gain Fiber Material And Pump Sources To The 4.3?m Mid-infrared Fiber Laser

The mid-infrared fiber laser has been widely used in environmental monitoring,medical diagnosis and military defense and etc because of its wavelength band in so-called?molecular fingerprint area‘.And as a member,4.3?m mid-infrared fiber laser due to its unique spectra and spectral characteristic of the position,has a very good application prospects on the volcano monitoring,research on carbon cycle and the greenhouse effect,the combustion diagnosis,airborne optoelectronic countermeasure and other fields.So far,however,reports of the 4.3?m mid-infrared fiber laser have been blank at home and abroad.The reason is that it is mainly limited by the lack of existing optical fiber materials and related pumping source devices.More specifically,there are two main bottlenecks in the development of infrared fiber lasers in 4.3?m as follows,the first one is the absence of high quality,high gain,low loss of rare earth doped glass and its optical fiber;the second one is the shortage of high efficiency 1.7?m pump waveband optical fiber laser.This thesis intends to study the above two key technical problems which hamper the development of 4.3?m mid-infrared fiber lasers by exploring the 4.3?m mid-infrared light-emitting characteristics of Dy³⁺-doped chalcogenide glass and its single mode of double cladding fiber preparation,1.7?m fiber laser with high power output and other aspects.The main contents in this dissertation are described as follow:1.The Ga-As-S system was selected as the rare earth doped glass substrate,and a series of glass with Dy³⁺concentration of 1000⁵000 ppm was prepared by the traditional melt quenching method.Based on the test to the physical and thermal stability of the Ga-As-S system glass,the visible and near infrared absorption spectrum and combining with J-O theoretical calculation and mid-infrared fluorescence spectroscopy and lifetime test,the 4.3?m infrared luminescence characteristic of the Dy³⁺-doped glass as well as the potential for 4.3?m mid-infrared fiber laser gain media glass material would be systematically studied and evaluated.The results of correlation experiment test analysis and J-O theoretical calculation show that:The solubility RE ions can be increased dramatically with the addition of gallium?Ga?to the host?As₂S₃?due to the presence of[GaS_4/2]tetrahedra and the optimal doping concentration is 3000 ppm,meanwhile the excellent thermal stability??T=182??is well maintained.The measured fluorescence lifetime?_mea and the stimulated emission cross-section(?_emi)of Dy³⁺in the glass at the wavelength of4.3?m are about 1.60 ms and 1.06×10^-20 cm²,respectively,and the laser quality factor(?_emi×?_mea)reaches 1.70×10^-23 cm²·s.2.The Ge₂₀Ga₅Sb₁₀S₆₅ glass with Dy³⁺concentration of 1000¹0000 ppm was prepared by traditional melt quenching method.And combining with the visible and near infrared absorption spectroscopy,J-O theoretical calculation,the infrared fluorescence spectra and fluorescence life decay curve test,the mid-infrared emitting characteristics of the glass and its potential to be used as 4.3?m mid-infrared fiber laser gain medium glass material could be systematically researched and evaluated.The results of correlation experiment test analysis and J-O theoretical calculation show that:The measured fluorescence lifetime?_mea and the stimulated emission cross-section(?_emi)of Dy³⁺in the glass at the wavelength of 4.3?m are about 1.41 ms and 1.86×10^-20 cm²,respectively,and the laser quality factor(?_emi×?_mea)reaches 2.62×10^-23 cm²·s;It has an excellent 4.3?m mid-infrared luminescence characteristic,which is the ideal for the gain dielectric glass material of the 4.3?m mid-infrared fiber laser.Besides,selection of doping concentration of 3000ppm Ge₂₀Ga₅Sb₁₀S₆₅glass,and for the first time,we adopt Cl₂ air to remove impurity of the glass with chemically process.it effectively eliminates the original glass impurities such as O-H?S-H,and the fluorescence lifetime of Dy³⁺at 4.3?m can be improved correspondingly from 1.41 ms to 1.82 ms.Then,on the basis of that,the single-mode double-clad Ge₂₀Ga₅Sb₁₀S₆₅ glass fiber with the core/cladding ratio?43.5:23:4 and 62.5:30:5.5?was prepared by extruding and rod-in-tube methods,respectively.The optical fiber is expected to be an ideal gain optical fiber material for mid-infrared fiber laser in 4.3?m.3.Adopt the ultraviolet dual beam interference method to get the optical fiber Bragg grating?FBG?with a special working wavelength of 1.7?m.On this basis,the structure of Thulium-doped fiber laser based on the FBG is built,and the output characteristics of 1.7?m Tm-doped fiber laser are studied systematically.By optimizing the dopant concentration,the length of the gain fiber and the reflectance of the output FBG,the suppression of reabsorption of the signal light and gain saturation effects was achieved.The short wavelength operation in the Tm-doped all-fiber laser is realized with a forward pumping.The laser yielded 1.28 W of continuous-wave output at 1707 nm with a narrow-linewidth of⁴4 pm and the maximum slope efficiency of 36.1%was obtained.On this basic,the bidirectional pump configuration for the ultra-short wavelength operation was designed and the laser yielded 3.15 W of continuous-wave output at 1706.75 nm with a narrow-linewidth of⁵0 pm and a maximum slope efficiency of 42.1%.4.Establishing a theoretical model of 4.3?m mid-infrared fiber cascade laser based on Dy³⁺-doped Ge₂₀Ga₅Sb₁₀S₆₅ glass fiber,and the effects of the pump wavelength?1319 nm?1707 nm?,pump structures?the forward,backward,bidirectional pump?,gain fiber parameters?fiber loss,fiber length and etc?on the laser performance have been systematically investigated to optimize the operation of fiber lasers at 4.3?m.The simulation results show that,the 1707 nm is the more effective pump wavelength,and the predicted maximum slope efficiency is determined to be¹5.1%for the pumping wavelength;among the three pumping structures,each of them has its unique advantage and some unavoidable shortcomings,in which the backward and bidirectional pumping show relatively better laser performances,however,the forward pumping own a more simple and feasible optical system.;The loss of gain fiber at 4.3?m should be keep below 6 d B/m,which is a prerequisite for the effective 4.3?m mid-infrared laser output.