Research On Modal Crosstalk Characteristics Of Few-Mode Erbium-Doped Fiber Amplifiers

The channel capacity of single-mode fiber communication system is restricted by the nonlinear Shannon limit.Space division multiplexing(SDM)technology is considered to be an effective method to further increase the data transmission capacity.Few-mode erbium doped fiber amplifier(FM-EDFA),as one of the typical representatives of SDM technology,has become an indispensable key component in long-distance mode division multiplexing(MDM)systems.This thesis focuses on the mode crosstalk characteristics of the FM-EDFA.The main contents and innovations are as follows:1.The coupled-mode equations for the FM-EDFA are established by regarding the gain and refractive index disturbance of the few-mode erbium-doped fiber(FM-EDF)as perturbations,which is especially suitable for analyzing the mode crosstalk due to an imperfect splice between different fibers and the influence of differential mode gain(DMG)on mode crosstalk.The analytical expression for mode crosstalk is presented,and verified by numerical calculation.2.The crosstalk characteristics caused by the mode field mismatch during the fusion splicing of the few-mode transmission fiber and the FM-EDF are studied,which can be characterized by the effective mode mismatch factor(???),mode coupling coefficient(?12)and coupling length(Lc).It is shown that,when the ratio of ??? to?12 is greater than 10,or the product of ?12 and Lc is less than 0.2,the signal mode crosstalk is less than-20dB.The relationship between the maximum mode coupling efficiency and the connection loss between different optical fibers is also analyzed.3.The influence of the gain characteristics of the FM-EDFA on the mode crosstalk is investigated.Our calculation shows that a low DMG helps to restrain the mode crosstalk of the FM-EDFA.In other words,it can tolerate larger gain coupling or refractive index perturbation for the active fiber.For the case with DMG=2dB,when the ratio of ??? to g12 is greater than 11.3 or the product of g12 and Lc is less than 0.17,the mode crosstalk between two siganls is lower than-20dB,where g12 is the mode coupling coefficient related the gain.4.The mode crosstalk of the FM-EDFA composed of the mode selective photonic lantern is analyzed in detail.The power transfer or crosstalk characteristics of the photonic lantern and the active fiber section are obtained by experiment and simulation calculation,respectively.Our simulation shows that,for the case with DMG=2dB and Lc=3m,the crosstalk between degenerate modes resulting from the active fiber section is less than-36dB.According to the mode power transfer matrices of the active fiber section and the photonic lantern,the output signal-to-crosstalk ration of a three-mode erbium-doped amplifier with the fundamental mode gain of 20dB and the DMG of 2dB are calculated out with 7.76dB,6.79dB and 2.70dB for LP01,LP11a and LP11b modes,respectively.The analysis method for modal crosstalk is also suitable for other structures of FM-EDFA or mode multiplexing systems.