Research On Technologies Of Two-wavelength Single-longitudinal-mode Fiber Laser With Narrow Line-width

Two-wavelength single-longitudinal-mode fiber laser is a new implementation form of dual-frequency fiber laser.In addition to the traditional advantages of fiber lasers,such as high beam quality,low oscillation threshold,high conversion efficiency,good temperature stability,it also has the unique advantages of large frequency difference and convenient frequency tuning Two-wavelength single-longitudinal-mode ytterbium-doped fiber laser can be widely utilized in laser interferometric system,optical f-iber sensing,high-speed coherent optical communication and lidar detection.In particular,as a source of the synthetic wave absolute-distance interferometric system,it has a sufficiently narrow spectral line-width for a dual-frequency laser to achieve a sufficiently long coherence length for the long-distance measurement.Therefore,it is of great significance to research the techniques of two-wavelength single-longitudinal-mode fiber laser.In this thesis,the scheme of two-wavelength single-longitudinal-mode ytterbium-doped fiber laser(YDFL)with narrow line-width has been proposed.A parallel structure consisting of a fiber polarization beam splitter and double fiber Bragg gratings(FBGs)have been used to realize the oscillation of two-wavelength laser.An unpumped ytterbium-doped fiber(UPYDF)has been employed as a saturated absorber to achieve the selection of single-longitudinal-mode.Therefore,the two-wavelength single-longitudinal-mode laser oscillation can realize.On this basis,by adopting the method of extra-cavity weak feedback,double different extra-cavity feedback structures have been designed and been appli’ed to narrow the laser line-width.The main contents of the thesis are included as follows:Firstly,the research status of two-wavelength single-longitudinal-mode narrow-linewidth fiber lasers have been summarized,and the characteristics of ytterbium-doped fiber and the principle of YDFL have been analyzed.Based on the analyses of the multi-wavelength and single-longitudinal-mode selecting methods and laser line-width narrowing methods,a technical scheme of two-wavelength single-longitudinal-mode narrow-linewidth YDFL has been proposed.The optimal length of the gain fiber,the minimum length of the unpumped ytterbium-doped fiber and the limited value of the arm-length difference of the embedded UPYDF fiber ring have been theoretically calculated.The simulation results show that the extra-cavity reflectivity is larger,and the laser line-width is narrower,when the extra-cavity length is within a certain rangeSecondly,the experimental system of two-wavelength single-longitudinal-mode YDFL has been established.The experimental results show that the proposed two-wavelength single-longitudinal-mode fiber laser can realize the stable oscillation output.The double wavelengths or single wavelength operation of the laser can be switched by controlling the polarization controller?whose switching characteristic is reliable.The threshold pump power of a two-wavelength single-longitudinal-mode laser is 58.52mW.Both wavelengths are linearly polarizedThirdly,the system of 1064nm single-frequency narrow-linewidth YDFL has been established.The double different extra-cavity structures have been designed adopting the methods of FBG feedback and coupler splitting feedback,respectively.For the FBG feedback laser system,when the center wavelength and bandwidth of the double FBGs as the internal and external mirrors of the laser are the same,the laser line-width can be narrowed.The experimental results indicate that when the extra-cavity length is 160.3cm,the laser linewidth is narrowed from 0.301pm to 0.273pm,and the line-width narrowing ratio is 9.11%.For the coupler split-light feedback laser system,the experimental results indicate that when the extra-cavity length is 320.2cm,the line-width is narrowed from 0.301pm to 0.258pm,and the line-width narrowing ratio is 14.37%In summary,the simulation and experimental results prove the feasibility of the designed line-width narrowing scheme of dual-wavelength single-longitudinal-mode YDFL based on FBG feedback and coupler splitting feedback,which has laid a solid foundation for further research and application of two-wavelength single-longitudinal-mode narrow-linewidth fiber laser technologies.