Yb-doped Double-clad Fiber Laser

Fiber laser has become one of the hotspot of laser field for its advantages, such as high efficiency, beam quality, reliability, compactness and fast heat dissipation and they have been widely used in telecommunication, industry processing, military, and medical operation ect. Recently, the appearance of double-clad fiber has increased the output power of fiber laser from milliwatts to kilowatts, and significantly extended the application of fiber laser. The emission spectrum of Yb³⁺ covers 1μm wavelength range and can provide excellent conversion efficiency due to low quantum defect. So Yb³⁺-doped double-clad fiber laser are the first choice as high-power sources in the 1μm wavelength range. This thesis presented both theoretical and experimental studies on CW Yb³⁺-doped double-clad fiber laser.The relationships between fiber laser output power and the doped parameters, core or inner cladding parameters are discussed in detail. The performance of cladding pumping fiber laser with different inner cladding structure is calculated through the geometry and fiber optics methods as the theoretical basis of optimizing the structureof a double clad fiber to improve the pumping efficiency.Based on the rate equations and transmission equations, an analytical model of Yb³⁺- doped double clad fiber laser is developed. The relationships of output power, threshold pump power and slope efficiency of P-F cavity fiber laser are obtained by solving the rate equations.The effects of fiber length, mirror reflectivity, pumping scheme, pump wavelength and Laser wavelength on performances of fiber laser are discussed in detail. The analysis shows that the most optimal Laser wavelength is 1090nm. These results are very important for optimizing the structure of a double-clad fiber laser.Based on the rate equations, the effect of self-pulsing and pumping perturbation on the stability of Yb³⁺-doped fiber laser is studied theoretically .Output end feedback or doping concentration increasing restrain the self pulsing is predicated in the study. The undulation of the laser photon density contains relaxation oscillation frequency itself and that of pumping. The oscillation will be resonated if those two frequencies are closed to each other. SBS and SRS in Yb³⁺- doped double clad fiber laser, the effects of fiber core diameter, fiber length, and wavelength on performances of threshold and pumping scheme on the characteristics of fiber laser are discussed systematically. It is shown that either increasing the fiber core diameter, decreasing fiber length or wavelength width may weaken the SBS and SRS.The experimental system of end pumped Yb³⁺-doped double clad fiber laser is set up. Some problems, such as end coupling, polishing of fiber end and adjusting light path, are solved. The single transverse mode double clad fiber laser with output power of 5.1W and slope efficiency of 76% is achieved at 6.9 W pump power. The output properties of fiber laser with different fiber lengths are investigated. Several output properties of the laser, including longitudinal mode, power and slope efficiency are analyzed for further improvement suggestions.By decreasing the reflectivity of input mirror and separating the distance between the mirror and fiber end, a Fabry-Perot filter is formed. A novel narrow linewidth fiber laser with 1.34 W output power and 25% slope efficiency is obtained by adjusting the filter.