| In recent years,fiber lasers are widely used in laser processing,Biomedical,national defense and other fields due to high conversion efficiency,good beam quality,high flexibility,and good heat dissipation.In fiber laser system,fiber combiner is one of the core components to increase the power of the high-power fiber lasers and solve the bottleneck of a single fiber laser output power.As a typical fused-tapering optical fiber device,the main problems facing its development at present is the lack of mechanism understanding of the tapering process of large-diameter optical fiber device,and the control of process parameters often depends on experience.Therefore,this thesis focuses on the tapered shape formation mechanism and defect suppression measures for the fused-tapering technology,which is of great significance to the fabrication of high-power fiber combiners and the development of high-power fiber lasers.The main contents of this thesis include:(1)The importance of the tapered shape to the transmission characteristics of the fiber device is clarified by studying the propagation behavior of light in a tapered waveguide.The fiber fused-tapering numerical model is established based on the three-dimensional NavierStokes equation,and the rheological behavior of fiber device during the process of fused tapering is described comprehensively(2)Finite element model of the fused tapering process is established.Firstly,the heating process of the scanning heat source on the fiber device is analyzed,and the time required for the preheating of the fused tapering process and the key parameters of temperature field are studied.Then the coupled field of preheated temperature field and flow field are solved,the mechanism of fused-tapering rheology is investigated,and the influence of key parameters on the formation process is studied by applying different boundary conditions.Several defects of fused tapering including bending defect,collapse defect,ellipicity defect,size fluctuation defect and the depression defect are summarized and their formation reasons are studied.(3)Fused-tapering platform based on premixed propane flame is built,and the procedures of fused-tapering and the functions required by the platform are analyzed.Particularly,visual inspection system of the platform is analyzed,and the methods to improve the automation of the platform and complete the post-processing tasks with image processing algorithms such as canny operator and watershed algorithm are described.(4)The existence of fused-taper defects is verified through experiments,and the optimized tapering parameters of 1250μm-diameter glass capillary are explored from the perspective of defect suppression.Through the response surface optimization,a set of optimal process parameters that can suppress most of defects is obtained.By comparing the experimental results using the best process parameters with the simulation results,the results show that the numerical simulation model proposed in this thesis is more suitable for describing the process of fused-tapering rheology than the traditional model. |
