Research On Key Technology Of Mode Multiplexing In Large Capacity Optical Fiber Transmission System

With the global data traffic increasing exponentially,the capacity of the single-mode fiber communication system is approaching its nonlinear Shannon limit.As a new and promising capacity expanding method,mode division multiplexing(MDM)technology can multiply the capacity of optical fiber communication system by exploring the modes as a new degree of freedom for transmission.However,in a practical MDM system based on few-mode fiber,mode coupling and differential mode group delay(DMGD)will deteriorate the transmission performance severely and thus limit the transmission distance.With the interplay between mode coupling and DMGD,the transmission process of the system is far more complex than the single-mode fiber system resulting that each output signal of the system is a super position of multiple linear convolutions between input signals and channel impulse response.Therefore,this paper focuses on the hot issues such as the multi-mode fiber-mode crosstalk and the effect of mode coupling and dispersion on the performance of the mode reuse system.The main work of this paper is as follows:(1)In this paper,the model crosstalk of the few mode fiber is simulated.Based on the theory of electromagnetic field,the mode of the few-mode fiber is analyzed and the mode field distribution of four modes in few-mode fiber is simulated.In addition,the orthogonality of the model,which is the theoretical basis of the mode multiplexing,is derived from the electromagnetic field theory.And then,we simulate the light trajectory of the few-mode fiber use the ray theory,and the mode crosstalk in the few-mode fiber is detailed analyzed on the basis of simulation.The following conclusions can be drawn from the simulation:when optical signal entering the multi-mode fiber,the more concentrated of the position is,the larger the incident angle is,the smaller the relative refractive index difference is,and the greater the mode crosstalk is.(2)Based on the simulation study on the mode characteristics of the few-mode fiber,the generation mechanism of the mode coupling and the delay mode group delay and its specific influence on the different modes are analyzed emphatically.Based on the above analysis,some important conclusions can be drawn.When the difference of the transmission constants of the two modes is small,the coupling mode field strength is large.For the two modes with large difference in transmission constants,the coupling between the modes is very small.The polarization mode dispersion of high-order mode is larger than the lower order mode.Compared to the mode dispersion,the polarization mode dispersion is negligible.In considering the damage factors above,we simulate the mode division multiplexing system with the use of the coupled mode theory method and transfer matrix method respectively.The following conclusion can be drawn by comparing the characteristics of the two methods.The matrix transfer method is more suitable for mode multiplexing system modeling.Then,the dispersion compensation and the MIMO constant modulus algorithm in the mode multiplexing system are analyzed.It is confirmed that the simulation system adopts the dispersion compensation fiber for dispersion compensation and MIMO equalization use the constant modulus algorithm.(3)A 2×2 coherent detection MDM simulation system is built on OptiSystem platform based on few-mode fiber link model.The following conclusions can be drawn by simulating the influence of damage such as dispersion and mode coupling on the performance of the system.Dispersion can make the signal distortion during transmission,so that the transmission signal cannot be reliably transmitted in the link.Dispersion compensation must be conducted in the system.As the mode coupling strength increasing,the mode crosstalk becomes more and more serious,the transmission signal is gradually distorted.MIMO equalization algorithm can eliminate crosstalk between mode signals and effectively improve the quality of the transmission signal.With the increase of coupling strength,equalization effect decreased.