The Research Of Large Mode Area Single Mode Chrysanthemum Core Distributed Fiber

Since the effective mode area of fiber laser is proportional to the threshold power of the nonlinear effect, the large mode area fiber is one of the easiest and most effective methods to solve the problems of nonlinear effects when the power density of the fiber laser is enhanced and the optical damage and so on. However, large-mode-area single-mode fiber has been researched for decades, significant progress has been obtained, but most researches are still theoretical analysis and experimental research, the realization of the commercialization need a certain amount of time, because the production yield of large-mode-area single-mode fiber is too low.Large mode area of single mode Chrysanthemum core distributed fiber not only can overcome the defects which are low production yield of the large-mode-area single-mode fiber and the limited diameter of segmented cladding fiber, but also increases the effective mode area and suppresses of the nonlinear effect. Therefore, simulating by using numerical simulation software COMSOL Multiphysics in this thesis, large mode area of single mode Chrysanthemum core distributed fiber is researched and analyzed. The main contents of this thesis are as follows:(1) Based on the finite element method, the use of finite element method and simulation processes of COMSOL Multiphysics is given. Simulating by numerical simulation software COMSOL Multiphysics, the mode field characteristics, mode field area and power ratio between the core and the optical fiber of step index single-mode fiber and 4 chrysanthemum core large mode area single-mode chrysanthemum core distributed fiber are obtained. The comparison between the two indicate that changing the refractive index of the chrysanthemum core of large mode area single-mode chrysanthemum core distributed fiber will reduce the beam quality, but will significantly increase the mode area of synthetic base mode.(2) Several common beam quality evaluation methods are given. Comparing them, modified beam propagation factor BPF is selected as evaluation criteria of beam quality of large mode area single-mode chrysanthemum core distributed optical fiber in this thesis finally.(3) Simulating by COMSOL Multiphysics, when beam quality is same, the mode field area will increase with increasing the radius of the chrysanthemum core and the distance between the chrysanthemum core and the round core. The mode field area will not increase with increasing the number of fiber cores, but the sensitivity of mode field area and beam quality to the refractive index of chrysanthemum core can be effectively reduced. In the same conditions, the greater the wavelength is, the larger the mode field area is and the worse the beam quality is. Bending loss first decreases and then increases with increasing the number of chrysanthemum cores. When the distance between round core and chrysanthemum core increases, bending loss will decrease. Bending loss first increases slowly and then increases rapidly with increasing the radius of chrysanthemum core.