| Orbital angular momentum（OAM）can effectively improve the capacity of optical fiber communication and the utilization ratio of frequency spectrum by taking advantages of its infinite topological charges and orthogonal electromagnetic waves of different topological charges.Photonic crystal fiber is a special optical fiber which can be designed flexibly and can effectively avoid the coupling between vector modes.It greatly improving the capacity of optical fiber communication system.At present,PCF has become an ideal waveguide for OAM modes transmission.In order to solve the problems of loss and relay in the transmission process of OAM modes,a novel PCF is proposed.Based on the proposed PCF,a three-dimensional Erbium-Ytterbium co-doped amplifier is set up.The main works are as follows:（1）A novel special optical fiber is proposed,which is a photonic crystal fiber with graded rectangular air holes in cladding.The structure of the PCF is optimized in order to obtain better propogation and optical properties.The optimal PCF structure is obtained,which can prolong the modes transmittion distance and be used as the amplifier element of the optical fiber amplifier.（2）We analyzed the properties of the PCF by using the finite element method（FEM）.The results reveal that the photonic crystal fiber supports 30 orbital angular momentum modes in C-band（1530 nm-1565 nm）,and the effective refractive index difference of vector modes is greater than 10-4.The nonlinear coefficient,confinement loss and dispersion are low,and the mode stable transmission distance can reach 1000 km.（3）A three-dimensional Erbium-Ytterbium co-doped fiber amplifier（EYDFA）is set up based on the proposed special optical fiber.The fiber length,pump power and signal power of the amplifier are optimized according to the changes of amplifier properties.The results reveal that the proposed amplifier has a flat gain of about 47 d B,a differential mode gain of not greater than 0.3 d B and a noise of not greater than 3.4 d B under the optimal parameters.In addition,this work considers the uncertainty of photonic crystal fiber manufacturing.The results reveal that the uncertainty generated in PCF manufacturing has a little influence on the amplifier properties and can be ignored.Therefore,the requirements for accuracy of photonic crystal fiber manufacturing are greatly reduced and the manufacturability is improved.The photonic crystal fiber and amplifier proposed in this work can effectively improve the capacity and propagation distance of the communication system.This work provides can provide a reference for OAM-based long-distance communication system. |
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