Theoretical And Experimental Research On The1.55μm Single-Frequency Narrow Linewidth Fiber Laser

Single-frequency （Single-longitudinal-mode） narrow linewidth fiber lasers havegreat potential applications in optical fiber communication system, fiber optical sensors,microwave photonics system and THz generation. In order to obtain the stablesingle-frequency fiber laser operation, some theoretical analyses and experiment studiesare performed on the design of fiber laser resonator, longitudinal-mode selection andmode-hopping restraining. The resonator structure of single-frequency narrow linewidthfiber lasers, which use narrow band mode selecting devices to reduce thelongitudinal-mode density greatly and use nonlinear two-wave mixing effect to suppressmode hopping and ensure the stable single-frequency operation, is presented. Then twodifferent single-frequency narrow linewidth fiber lasers, based on fiber saturableabsorber and self-feedback injection, respectively, have been theoretically andexperimentally investigated in detail. Finally, high-performance single-frequencynarrow linewidth fiber lasers are achieved.The main results are as follows:1. The spectrum characteristic functions of active fiber linear F-P resonantor andactive fiber ring resonantor are derived respectively. Based on these functions, theinfluence of the resonantor’s structural parameters including gain and loss intra-cavity,cavity length and cavity mirror reflectivity or coupling output ratio on the characteristicsof spectral linewidth is investigated in detail. Moreover, the linewidth limit formula ofsingle-mode laser is derived by taking the spontaneous emission into account, and thenthe influences of the resonantor’s structural parameters on the linewidth limit is furtherstudied. Finally, the theoretical basis to design the resonantor of single-frequency narrowlinewidth fiber laser is achieved.2. The fiber laser structure of single-frequency narrow linewidth fiber laser, whichuse narrow band mode selecting devices to reduce the longitudinal-mode density greatlyand utilize nonlinear two-wave mixing effect to suppress mode-hopping and ensure thestable single-frequency operation, is proposed. Furthermore based on rate equations andpower transmission equations, the model to analyze and design Er³⁺-doped fiber laser has been established. The factors affecting the power output characteristics of fiber laserhave been illustrated by means of numerical calculation.3. The single-frequency narrow linewidth fiber laser based on fiber saturableabsorber has been investigated. The influence of the fiber saturable absorber on outputpower and conversion efficiency is studied also. In this experimental device, a fiber ringfilter uses to reduce longitudinal-mode density and a segment Er³⁺-doped fiber withlength of10m serves as saturable absorber to constrain mode-hopping and ensuresingle-frequency. Satble single-frequency laser output at1549.98nm with linewidth of2.2kHz is observed. The fiber laser exhibits that output power is26.5mW, optical-opticalefficiency is20.9%, corresponding slop efficiency is23.6%.4. In order to improve conversion efficiency and compress linewidth for fiberlaser based on fiber saturable absorber, a new fiber laser structure combining FBG F-Petalon with polarization-maintaining （PM） fiber saturable absorber is presented. In thisfiber laser, a FBG F-P etalon is used as reflector and mode-selecting component toreduce the longitudinal-mode density greatly. Meanwhile, a segment un-pumped PMEr³⁺-doped fiber （EDF） with length of2m serves as saturable absorber to suppressmode-hopping and ensure the stable single-frequency operation. Using single-claddingEr³⁺-doped fiber as gain medium, the fiber laser output at1550.65nm exhibits that outputpower is43.7mW, optical-optical efficiency is29.5%, corresponding slop efficiency is33.4%and linewidth is about1.2kHz. Then using Er³⁺/Yb³⁺co-doped fiber as gainmedium, the fiber laser output at1549.83nm exhibits that output power is923.6mW,optical-optical efficiency is27.2%and spectral linewidth is about2.5kHz. To ourknowledge, it is the maximum power directly extracting from single-frequency narrowlinewidth fiber laser.5. A novel self-feedback injection single-frequency narrow linewidth fiber laserwith linear cavity is firstly proposed and demonstrated experimentally. The dynamicgain grating and refractive index grating, which induced by interfering between thefeedback light from the part of output light and the lasing light in intra-cavity,discriminates and selects laser longitudinal modes efficiently. With single-claddingEr³⁺-doped fiber as gain medium, satble single-frequency laser output at1549.79nm withlinewidth of2.2kHz is observed. The fiber laser exhibits that output power is30.6mW,optical-optical efficiency is27.3%. Then using Er³⁺/Yb³⁺co-doped fiber as gain medium, the fiber laser output at1550.65nm exhibits that output power is653.7mW,optical-optical efficiency is21.1%, corresponding slope efficiency is24.6%and spectrallinewidth is about5.0kHz. To our knowledge, it is the maximum power extracting fromsingle-frequency narrow linewidth fiber laser with linear cavity.