高阶轨道角动量传输光纤设计及传输特性研究(内封底文章)

Design and transmission characteristics of high-order orbital angular momentum transmission fiber(inside back cover paper)

为解决一般轨道角动量传输光纤传输轨道角动量模式数量少、质量差的问题,提出了一种基于正六边形空气孔排列的新型光子晶体光纤结构。该光纤引入了空气填充率高的矩形空气孔以及采用高折射率材料填充环形传输区域,能够有效提高环形传输区域和包层间的折射率差,且正六边形排列空气孔有利于提高模间有效折射率差。经过结构优化得到最优光纤结构,有限元法的分析结果表明,最优结构下,该光纤在常用波段S+C+L+U波段上能够支持142种轨道角动量模式的传输,最高阶数达到36阶。且所提出光纤具有良好的传输特性,本征模式的最高限制性损耗为10−9 dB/m量级,与典型轨道角动量传输光子晶体光纤相比至少降低了一个数量级;最大有效模场面积能够达到206.18μm^(2),最小非线性系数低至0.397 W^(−1)∙km^(−1);色散平坦且最小色散变化低至1.4578 ps/(nm∙km);所有本征模式纯度均在93.4%~96.8%范围内。且该光纤具有较好的制备可行性,对制造精度要求不高。因此,该光纤在基于轨道角动量光纤的复用系统中具有广阔的应用前景,为提高通信容量提供了一种有效手段。

Objective Orbital angular momentum based multiplexing is a special form of space division multiplexing,different OAM modes are orthogonal to each other,based on which different modes can carry different information,and multiple OAM beams with different topological loads can be used as carriers for information transmission,which can greatly improve the channel capacity of the communication system without the need for additional bandwidth.Compared with the transmission of OAM in free space,the transmission of OAM modes in optical fibers can effectively avoid the interference of external factors,and ordinary optical fibers are unable to meet the requirements for the transmission of OAM modes.Photonic crystal optical fibers,as a kind of special optical fibers with high structural designability,offer the possibility of realizing the transmission of OAM modes.In order to achieve high quality transmission of more OAM modes,it is necessary to design photonic crystal optical fibers with suitable structures that can support the transmission of OAM modes.Methods In this paper,a novel photonic crystal fiber structure based on a positive hexagonal arrangement of air holes is proposed.The fiber introduces rectangular air holes with high air filling rate and high refractive index materials to fill the ring transmission region,which can effectively improve the refractive index difference between the ring transmission region and the cladding,and the hexagonal arrangement of the air holes is conducive to the improvement of the effective refractive index difference between the modes.Structure optimization and optimal structure verification take into account the number of OAMs that the fiber can support as well as the effective refractive index difference between modes and mode purity.The optimal fiber structure is obtained through structural optimization,and the performance of the fiber is analyzed using finite element analysis.Results and Discussions The results of the finite element method analyses show that the optimal optical fiber structure is optimized to support 142 OAM modes in the commonly used S+C+L+U band,with the topological charge ordering up to 36.Moreover,the proposed fiber has good transmission characteristics.The confinement loss is below 10-9 for all eigenmodes,which is at least one order of magnitude lower than the typical photonic crystal fibers;The maximum effective mode field area can reach 206.18μm^(2) and the minimum nonlinearity coefficient is as low as 0.397 W^(−1)∙km^(−1);Flat dispersion and minimum dispersion variation are as low as 1.4578 ps/(nm∙km);And purity are 93.4%-96.8%for all eigenmodes.Based on the effect of manufacturing errors on the performance of the optical fiber,it can be seen that the optical fiber does not require high manufacturing accuracy.Conclusions In order to achieve high-quality transmission of more number of OAM modes,this paper proposes a design method of photonic crystal fiber based on hexagonal structure by combining the two ways of rectangular air holes and the filling of annular transmission region with high refractive index materials.The introduction of rectangular air holes and high refractive index materials increases the refractive index difference between the ring transmission region and the cladding,which in turn facilitates the stable transmission of a larger number of OAM modes.The performance analysis of the optical fiber by Comsol Multiphysics finite element analysis software shows that the photonic crystal fiber can not only support a larger number of OAM modes,but also has the characteristics of low confinement loss,low nonlinear coefficient,flat dispersion change,and high mode purity,which is valuable for high-capacity optical fiber communication.