Structure Design And Performance Analysis Of Large Mode Field And Bending Resistant Fiber For Single Mode Operation

High power fiber lasers are widely used in industrial processing,medical surgery and national defense security due to excellent beam quality,high conversion efficiency and compact structure.However,with the further increase in output power,the serious power density inside the fiber causes"core burning",non-linear effects and mode instability,which undoubtedly affects the transmission efficiency and reliability of fiber lasers.Increasing the effective mode field area of the fiber improves the optical damage caused by non-linear effects,Single-mode operation and great bending resistance can avoid mode competition and improve the quality of the beam.The design of large-mode-field and bend-resistant fibers under single-mode operation is therefore a viable way to ensure the continued development of high power fiber lasers,and this paper is based on the structural design and performance analysis of the relevant fibers.The specific innovations are as follows:1.A new high refractive index ring-assisted and step-refractive-index fiber has been designed.Proposed fiber consists of a two-layer core,a multi refractive index ring located at the inner core and a cladding layer.Double index rings at the inner core act as a confinement for the fundamental mode and fully couple with the higher-order mode to leak outwards into the cladding,which helps to increase the effective mode field area.The single refractive index ring at the cladding improves the bending performance of proposed fiber and enables stable single-mode operation.Simulation results show that effective mode field area can reach 1518.814μm²,the bending loss of fundamental mode is only 0.004073 d B/m,and the high-base loss ratio reaches 657 under 1550 nm operating wavelength and 10 cm bending radius.Proposed all-solid-state fiber has a symmetrical structure and does not need to consider the influence of the bending direction angle.2.A new air-hole assisted multilayer core fiber with triangular coupling rings has been designed.Proposed fiber consists of three layers of cores,a triangular coupling ring located in the third layer of cores and six periodically arranged air holes.Air holes ensure stable single-mode operation and the triangular coupling ring further increases the effective mode field area.Simulation results show that effective mode field area can reach 1002.732μm²,the bending loss of fundamental mode is only 0.000827 d B/m,and the high-base loss ratio is 7240 under an operating wavelength of 1550 nm and a bending radius of 10 cm,which demonstrates excellent bending resistance.The fiber bending performance at different bending angles is then discussed,demonstrating the bending reliability of proposed fibers.3.A BP neural network-based prediction model for optical fiber performance metrics has been established.The model uses seven fiber structure parameters as input variables and two fiber performance indexes as output variables.By comparing the error between the predicted and actual values of the neural network model,relative errors of within 15%were found for fundamental mode loss and within 0.6%for mode field area.Both of which were within acceptable limits,which verified the accuracy of the prediction model.The prediction model based on BP neural network is able to obtain relatively reliable numerical results in a very short period of time,which provides a new idea for the subsequent fiber structure parameter adjustment.Under the condition of 1550nm working wavelength and 10cm bending radius,effective mode field area exceeds 10³μm²,and the bending loss of fundamental mode is less than 10^-2 d B/m.Two fiber structures designed in this thesis have reached the international advanced level and show potential application scenarios.