Design And Transmission Characteristics Analysis Of Trench-assisted 19-core Heterogeneous Fiber

High-speed and high-capacity transmission is an inevitable trend in the future develop-ment of optical communications.The system capacity of conventional single-mode optical fibers is already approaching its physical limit,so there is an urgent need to develop new optical fiber technologies to realize a substantial expansion and upgrade of communication capacity.Multi-core fiber（MCF）and Multi-core Few-mode fiber（MC-FMF）based on the space division multiplexing principle have been proposed as one of the effective solutions to the communication capacity crisis.while MCF and MC-FMF can effectively increase the transmission capacity,it comes with the problem of inter-core crosstalk（ICXT）.Severe ICX-T can cause interference between channels,which prevents the energy in the fiber from being transmitted properly.Therefore,how to suppress ICXT is an urgent scientific and techni-cal problem for practical applications in MCF and MC-FMF.In this paper,we propose two low-crosstalk structures,a 19-core single-mode fiber and a 19-core four-mode fiber,from the point of view of suppressing crosstalk between multiple-core fibers.The details are as follows:Firstly,the mode theory of optical fiber and the regulation mechanism of the number of modes are introduced,and the method of controlling the number of modes and the criterion of judging whether the modes are cut off are expounded.In addition,we analyze the com-putational methods for inter-core and inter-mode crosstalk in multi-core optical fibers,and describe the important parameters of space division multiplexing optical fiber for communi-cation,such as dispersion,nonlinear coefficients,and effective mode field area.Simultane-ously,the solution idea and procedure of the full vector finite element method are described.It provides a theoretical basis for multi-core fiber design and performance analysis.Secondly,a kind of trench-assisted 19-core single-mode fiber is designed.The design idea is to combine the low-index trench with the heterogeneous fiber core to achieve stronger crosstalk suppression effect.By optimizing the core parameters,when the working wave-length is 1.55μm,the crosstalk between the cores of the 19-core single-mode fiber is the lowest of-39.52 d B/100km,and the crosstalk between the three kinds of cores is lower than-30 d B/100km in the E+S+C+L band,all 19 core fibers can be independently transmitted in the operating band.By adjusting the core and assisted structure,the mode of LP₁₁is cut off before the E band,and only one mode of the fundamental mode is transmitted in the fiber.At the same time,when the bending radius of the 19-core fiber reaches 80 mm,the crosstalk between the cores will not change,and it has good bending resistance.Finally,in order to further expand the communication capacity of optical fiber,the core parameters are adjusted on the basis of the 19-core single-mode fiber.The 19-core fiber is capable of stable transmission of four LP modes without changing the core distance.The crosstalk between the three fiber cores is below-30 d B/100 km and the effective refractive index difference of each mode in each fiber core is greater than 10^-3.Inter-core and inter-mode crosstalk both meet the requirement of independent transmission.When the operating wavelength is 1.55μm,the lowest inter-core crosstalk reaches-139.80 d B/100km.The ef-fective mode field area of the four modes is greater than 80μm².The large effective mode field area effectively suppresses the nonlinear effects of the fiber,and the coefficient of non-linearity is lower than 1.28 W^-1·km^-1for the 19-core four-mode fiber.Moreover,when the bending radius is larger than 160 mm,the bending state has less influence on the inter-core crosstalk and the fiber has better bending insensitivity.