Research On Gain Equalization Of Few Mode Fiber Amplifier

As a potential solution to improve the transmission capacity of communication system,few-mode fiber communication has attracted much attention.Few-mode optical fiber communication system uses mutually orthogonal signal modes in the optical fiber as independent communication channels for information transmission,and realizes a substantial increase in the transmission capacity of the communication system.The few-mode erbium-doped fiber amplifier is a vital relay amplifier component in the few-mode fiber communication system.When the mode division multiplexing signal in the system is amplified by a uniform erbium-doped few-mode fiber amplifier,the signal will have a large differential mode gain,which will lead to a sharp decline in the transmission capacity of few-mode fiber communication system.Therefore,it is necessary to design the gain equalization of the few-mode erbium-doped fiber amplifier.The main work and innovation achievements of this paper are as follows:(1)Based on the fiber mode theory,the mathematical model of the few-mode fiber amplifier is analyzed.Four-order Runge-Kutta method and target shooting method are used to numerically solve the differential equations of the fiber amplifier,and the MATLAB simulation model of the few-mode erbium-doped fiber amplifier is established.(2)The gain performance of the few-mode fiber amplifier with different core pumping modes is studied by using the mathematical model of the few-mode fiber amplifier,the gain complementarity among the pumping modes is analyzed,and the best pumping mode combination of the few-mode erbium-doped fiber amplifier is determined.The erbium-doped ring structure of few-mode fiber is designed,and the principle and implementation process of genetic algorithm are described.According to the optimization conditions of fiber amplifier,the fitness function of genetic algorithm is defined,and the optimization parameters such as population size,crossover probability and mutation probability in genetic algorithm are determined.(3)The optimal three-layer erbium-doped structure of LP₀₂+LP₂₁ mode pumped four-mode group few-mode fiber amplifier is optimized by genetic algorithm.The average gain of C-band four-mode multiplexing signal is greater than 20d B,the differential mode gain is less than 0.5d B,and the differential mode gain of 1550nm signal is the minimum 0.103d B.The four-layer erbium-doped structure suitable for five-mode group few-mode fiber amplifiers is designed,and the optimal erbium-doped structure of LP₀₂+LP₂₁+LP₃₁ mode pumped five-mode group few-mode erbium-doped fiber amplifier is optimized by genetic algorithm.The average gain of C-band five-mode mode division multiplexing signal is greater than 20d B,the differential mode gain is less than 0.4d B,and the differential mode gain of 1550nm signal is the minimum0.016d B.(4)The signal mode gain,noise figure and C-band gain spectrum of two kinds of erbium-doped fiber amplifiers are analyzed respectively.The relationship between the gain performance of the few-mode fiber amplifier and the pump power,signal power and signal wavelength is studied.The influence of the fabrication error of the fiber erbium-doped structure on the gain performance of the fiber amplifier is calculated,and the most important structural parameter is determined.The research shows that the combination of pump control and erbium-doped fiber structure optimization can effectively reduce the differential mode gain of the mode division multiplexing signals in the few-mode fiber,and achieve the gain equalization of the few-mode erbium-doped fiber amplifier.