Design And Analysis Of Bend-resistant Large Mode Area Single-mode Fiber

In recent years,the output power of high-power fiber lasers has exceeded the kilowatt and myriawatt levels,and has been widely used in military,medical and industrial fields.However,with the increasing output power of fiber laser,nonlinear effects and mode competition in gain fibers have become major limiting factors for the further development of fiber lasers.So the implementation of large mode area fiber with stable single-mode transmission capability is an effective way to solve this problem.In this paper,starting from overcoming mode competition and increasing the effective mode area,an ultra-low-loss single-mode large mode area fiber with introducing the core with the gradient refractive index distribution and with the simple trench-assisted structure,and two types of stressed single-mode large mode area fibers with high refractive index auxiliary rods at the core axis were designed and analyzed.The optimal features of these three optical fibers were determined by structural parameter optimization.Meanwhile,the Mode-field migration,mode evolution in straight-bending transition and practical fabrication methods of these three fibers were investigated separately.The main research contents and conclusions are as follows:（1）First,in this paper,a novel single-trench bending-resistant ultra-low-loss gradient refractive index large mode area fiber was proposed,and the structural parameters of the fiber were optimized by using the finite element method.The results show that when the input wavelength is 1064 nm,the fundamental mode loss of the fiber can be maintained at 1.21×10^-5d B/m at a bending radius of 7 cm,and the ratio of the least loss higher-order mode loss to fundamental mode loss is as high as 1.93967×10⁵,and the effective mode area can reach 479.1μm².In addition,the mode evolution in the straight-bending transition of this fiber was also analyzed,and the results show that the self-coupling efficiency of the fundamental mode in the straight-bending transition of this fiber can reach 0.862 at the bending radius of 7 cm.Finally,the core radius of the fiber can be extended to 82.5μm with the premise of single-mode transmission,and the effective mode area can reach 1500.7μm².Therefore,this fiber can be used for high power output of single-mode fiber lasers.（2）A novel bending stress resistant segmented cladding single mode large mode area fiber was proposed.The novel cladding structure consisting of 12 low-refractive-index auxiliary rods combined with the introduction of stress rods in the fiber core not only effectively reduces the difficulty of fiber fabrication,but also greatly improves the single-mode transmission capability of the fiber.The simulation results show that the fundamental mode loss of the fiber remains at5.5×10^-3 d B/m,and the loss ratio is higher than 210 in any bending direction,and the effective mode area can reach 1050.1μm²,when the input wavelength is 1064 nm at the bending radius of 24 cm.In addition,through the mode evolution analysis of the fiber in the straight-bending transition,it can be concluded that the self-coupling efficiency of the fundamental mode in the straight-bending transition can reach 0.85 at the bending radius of 24 cm.Therefore,this fiber has potential applications in compact fiber lasers and amplifiers.（3）A bending-stress-resistant step-core single-mode large mode area fiber with the step refractive index distribution of the core was proposed.The fiber achieves not only excellent low-loss single-mode transmission capability by introducing the core with the step refractive index distribution,but also achieves the large effective mode area.The simulation results show that when the input wavelength is 1064 nm and the bending radius is 20 cm,the effective mode area can reach 1406.8μm²,the fundamental mode loss can be maintained at 5.9×10^-3 d B/m,the loss ratio can reach 403.2,and the self-coupling efficiency of the fundamental mode in the straight-bend transition is 0.42.Therefore,this fiber can be used for single-mode high power fiber lasers and amplifiers.