Yb-doped Double-clad Fiber Laser Theory And Experimental Research

Fiber lasers have attracted much attention from its emergence for its advantages of low threshold, high efficiency, beam quality, reliability, compactness, and thermal management and they have been widely used in optical communication, aviation and spaceflight, life sciences, military, and industry. Recently, the emergence of double-clad fibers has increased the output power of fiber lasers from milliwatts to kilowatts, and expanded the application of fiber lasers. Also, with the development of relevant components and technologies, performances and output power of double-clad fiber lasers are being improved. The emission spectrum of Yb~3+ ions includes 1μm wavelength range and can provide excellent conversion efficiency due to low quantum defect. These properties make Yb~3+-doped double-clad fiber lasers exceptional high-power sources in the 1μm wavelength range. In this thesis, theoretical and experimental study on CW Yb~3+-doped double-clad fiber lasers has been was presented. The main work includes:1 , Based on the model of Ido Kelson, theory of Yb~3+-doped double-clad fiber lasers was systematically studied. The influence of fiber length, doping concentration, pumping scheme and reflectivity of the output mirror on the output characteristics of laser were discussed in detail, theoretical instructions was provided to the design of double-clad fiber laser.2, The design of double-clad fiber laser includes: a plate for fiber polish was designed to polish the end of the double-clad fiber and excellent optical surface was obtained. According to the parameters of double-clad fiber and LD pump source, we developed a pump coupling system made up of lenses, and the coupling efficiency was up to 43 %. A 1.4W output Yb~3+-doped double clad-fiber laser with a F-P cavity operating at 1087nm was reported. The laser employed a homemade single mode Yb~3+-doped fiber with D shape inner cladding pumped by a fiber-coupled LD operating near 976nm as its gain medium and slope efficiency up to 25 % was obtained. 3, Narrow bandwidth output double clad-fiber laser, wavelength tuning by temperature changing and mode selection with a curved fiber were obtained experimentally, and theoretical analysis were also carried on. The main contents are as follows:(1) Narrow bandwidth output: Using a compound cavity structure, the laser produces a stable 0.14nm narrow bandwidth spectrum.(2) Wavelength tuning by temperature changing: Study of thermal effects on outputwavelength of Yb3+-doped double-clad fiber laser with low power output has been given in this paper. The output spectrum shifts to longer wavelength for 11.7nm with temperature change of 80 °C, while other characteristics of the laser does not change greatly. We give the detailed theoretical analysis in this paper, and a model of the relation between laser output wavelength and temperature has been demonstrated.(3) Mode selection with curved fiber: the influence of fiber curvature on double clad-fiber laser output was studied. Simulation shows laser using multimode double clad-fiber can have LPoi mode output by properly choosing radius of curvature. By this method, diffraction-limited laser output was obtained from our fiber laser with homemade multimode Yb3+-doped double clad fiber. 4, Theoretical and experimental study on the multi-position-pumped double-clad fiberlaser:(1) Numerical simulation results on multi-position pumped (end pumped and side pumped) double-clad fiber laser has been given in this paper. Simulation shows multi-position-pumping can smooth the distribution of gain and laser power in the double-clad fiber, and thus high power output could be realized by multi-position-pumping using low power LDs. In addition, this simulation is important for the design of double-clad fiber amplifiers as multi-position side pumping frees the ends of the fiber.(2) The research on the loss mechanism of the multi-position-pumped double-cladfiber laser includes:(a) Pump leak at the connection point of LD pigtail and inner clad;(b) Pump loss generated by the connection of LD pigtail and inner clad. Simulation shows connection angle determines the loss of the pump loss. Small connection angle (about 10 °) makes higher coupling efficiency. Increasing the inner clad NA and connection angle can improve the coupling efficiency.(3) We explored the side pump coupling by optical glue, and 5 % coupling efficiency was obtained. Factors that influence the coupling efficiency in experiment was analyzed.