Study Of Large Mode Area Fiber With Normal Group Velocity Dispersion

In recent years,ultrashort pulse fiber laser sources have been widely applied in many fileds such as material processing,gas detection,medical treatment and so on.The generation of ultrashort pulse with larger pulse energy and higher peak power has become a research hotspot.The normal-dispersion mode-locked fiber laser can effectively increase the pulse energy compared with the conventional soliton mode-locked fiber laser operating in the anomalous-dispersion regime.However,the further increase of pulse energy is still limited by the accumulation of nonlinear phase in the laser cavity.Therefore,the large-mode-area fiber components with normal group velocity dispersion(GVD)are important for realizing high energy ultrashort pulse laser sources.The main contents are as follows:(1)The basic theory of fiber design is prsented,including Maxwell’s equations,coupled-mode-theory-based analysis method for the supermodes in the multi-core fiber.Besides,the finite-element method for numerical calculation of fiber modes is discussed.Finally,the basic characteristics of the optical fiber and corresponding formulas are introduced,including GVD,loss,effective mode area and nonlinear parameters.(2)A novel coaxial-dual-core optical fiber with a central air hole is proposed.By introducing an air hole in the center core,the fiber can process a red-shifted zero dispersion wavelength as well as an increased effective mode area.The influence of fiber parameters on the GVD and effective mode area is further explored.As a result,the designed fibers with three different choices of parameter conditions can operate in the broadband normal-dispersion wavelength regions,which cover 1400-1800 nm,1700-2000 nm and 2000-2300 nm,respectively.The GVD profiles exhibit low and flat shapes with values of>-5 ps/nm·km,while the effective mode areas can reach up to296μm~2.(3)A multi-core fiber(MCF)consisting of identical cores is proposed.The contribution of the coupling between two adjacent cores to tailoring the GVD of the MCF can be analyzed by using the dispersive equation,which is evaluated by the coupled mode theory(CMT).Based on the analysis of the influence of the parameters on the GVD and effective mode area of MCF,19-core and 37-core multi-core fibers were designed respectively.Both of them exhibit extremely flat normal GVD profiles with values of close to 0 ps/nm·km in the wavelength range of 1500–2300 nm,while the effective mode area of the 37-core MCF can reach 440μm~2at 1900 nm.Finally,7-core photonic crystal fiber(PCF)and 19-core coaxial-dual-core MCF are proposed and their basic characteristics are analyzed.