Research On Mode And Crosstalk Characteristics Of Heterogeneous Few-mode Multicore Fiber

At present,the rapid development of information technology has caused a sharp increase in the amount of data in optical communication network,and the transmission capacity of standard single-mode optical fiber has approached the nonlinear Shannon limit.In order to solve the communication capacity crisis,multicore fiber,few-mode fiber and few-mode multicore fiber based on space division multiplexing make full use of space dimensions and are expected to solve the problem of single-mode fiber transmission tolerance.In particular,few-mode multicore fiber can expand the number of spatial channels from both the core and mode,which has great potential in terms of communication capacity expansion.The inter-core crosstalk and the intra-core crosstalk are important indicators to measure the transmission performance of few-mode multicore fiber,and the bending resistance of the fiber and the effective mode area are the key parameters to ensure the practicability of the fiber.In view of the above characteristics,the fiber is modeled and numerically calculated by the finite element method and coupled power theory.By optimizing the fiber structure,it achieves a low crosstalk,large effective mode area and bend insensitive few-mode multicore fiber design.The main research work is summarized as follows:(1)The coupled power theory is used to study the mode and inter-core crosstalk characteristics of few-mode multicore fiber.Research shows that when adjacent cores have different geometrical dimensions and refractive index distributions,which are heterogeneous structure,the propagation constants of the same mode group in different cores are different,which can effectively suppress inter-core crosstalk,and in the phase mismatch region,the inter-core crosstalk is independent of bending radius.(2)A heterogeneous ring-core 3-LP mode 12-core fiber is proposed.The fiber cores are arranged with square lattice structure and the adjacent cores are in a heterogeneous ring structure with different geometric parameters and refractive index distributions.Each core can support three modes of transmission.Due to the large difference in the mode effective refractive index of the heterogeneous cores,the fiber has the performance advantages of low crosstalk and bend insensitive.In addition,the center refractive index depression of ring-core can adjust the mode field,which can increase the effective mode area and the effective refractive index difference of the modes in the core,and suppress intra-core crosstalk.The results indicate that the designed fiber cladding diameter is 213.8?m,and the relative core multiplexing factor is46.2.When the core pitch is 36?m,the inter-core crosstalk of adjacent cores of LP₀₁mode,LP₁₁ mode,and LP₂₁ mode are lower than-86,-74,and-48d B/100km,respectively.The effective refractive index difference between modes in the core is greater than 1×10^-3,and the mode spacing is large.(3)To further improve the crosstalk suppression capability and the number of channels of few-mode multicore fiber,a trench-assisted heterogeneous ring-core 3-LP mode 24-core fiber is proposed.Combining heterogeneous structure and trench-assisted method to further reduce inter-core crosstalk.The results indicate that the designed fiber cladding diameter is 218.97?m,and the relative multiplexing factor is 86.6.When the core pitch is 30?m,the inter-core crosstalk of adjacent cores of LP₀₁mode,LP₁₁ mode,and LP₂₁ mode are lower than-80,-73,and-60d B/100km,respectively.Compared with the core structure without trench assistance,it achieves a smaller core pitch and a higher core reusability while ensuring a great performance of inter-core crosstalk.