High Power Double-clad Fiber Lasers And Their Applications

Research of rare-earth-doped double-clad fiber(DCF) lasers is one of the subjects on the frontiers of optoelectronic technology.By using DCF and cladding-pumped technology,the conversion efficiency and output power of fiber lasers can be greatly improved,and a variety of nonlinear effects can be effectively inhibited as well.Therefore,it has become an important way to develop high power lasers.Since the Yb³⁺ ions are with simple structure of energy-levels,absence of excited-state absorption and concentration-quenching effects,and high quantum efficiency and optical conversion efficiency,Yb³⁺ -doped DCF(YDCF) lasers become the most popular DCF lasers.Since the technology of wavelength tuning using reflection gratings has been successfully applied to other kinds of lasers,it is expected that tunable YDCF lasers will be commercialized soon on the international market.The development of high power tunable YDCF lasers will have significant social and economic impact.As a member of the group working on the key scientific project of Fujian Province of "Tunable high-power Yb³⁺-doped double-clad fiber lasers",I have participated in the theoretical and experimental investigation of YDCF lasers,and the development of a prototype of high power tunable YDCF laser.In this thesis,the research and development of DCF lasers are reviewed firstly. The basic theory of YDCF lasers is presented,with the focus on the structure of DCF, energy-levels and spectral characteristics of Yb³⁺ ions,pumping and coupling methods of DCF lasers.Based on the rate equations under the steady-state condition, a model of strongly backward-pumped YDCF laser was constructed.The effects of cavity feedback,fiber length,scattering loss coefficient and lasing wavelengths on the characteristics of backward-pumped YDCF laser are analyzed numerically using Newton-Raphson method.The results provide theoretical guidance for design and fabrication of YDCF laser.An experimental set up was demonstrated with reference to the theoretical analysis results.Using a 975 nm laser diode(LD) as the pump source and a YDCF of around 14 m as the gain medium,a CW laser with a slope efficiency of 82%was demonstrated.The output laser power at the wavelength of 1088 nm is up to 85.1 W. the experimental results were in good agreement with those by numerical simulations.Based on an external cavity using lazed grating and Littrow configuration,a tunable YDCF laser was demonstrated with a tuning range of 80 nm. The wavelength of the single-mode output laser was tunable from 1041 nm to 1121 nm.The output laser beam quality factor was M²_x,y=1.23,1.20,which was near the diffraction limit.The performance specifications of the prototype of high power tunable YDCF laser developed in our group are at the national leading edge.The work reported in this thesis will be beneficial to the further research and development of YDCF lasers.