面向高速模分复用传输的多模空芯光纤研制

The capacity of the optical transmission network is accelerating toward the order of Pbit/s, being very close to the nonlinear Shannon limit. Giant data centers offering state-of-art information services (e.g., cloud computing) and high-speed financial data transmission require reduced-latency, large-capacity communication. Regarding the optical network based on conventional multiplexing approach (e.g., the wavelength-division-multiplexing), the single-mode optical fiber suffers from the rapid growth of spatial spectral efficiency. Therefore, excellent optical fibers are in huge demand to achieve novel multiplexing techniques based large-capacity, high-speed communications. Hollow-core optical fibers, due to its unique air-core structure and mode guidance condition, hold merits of low latency and low nonlinearity, which are different from solid-core ones. Moreover, the sizes of optical fibers can be reduced by exploiting recently emerged materials and structures (e.g., metamaterials) in fiber cladding. Such size-reduced waveguide devices will further improve the density of high-speed communication system. In this research, we aim to design, optimize and fabricate multimode hollow-core optical fibers, particularly the novel structures and materials of cladding, toward high-speed mode-division-multiplexing communications. Using both analytical and numerical methods, we will investigate the propagation characteristics of high-order modes, as well as corresponding orbital-angular-momentum modes and high-order polarization-maintaining modes. Based on partial swarm optimization results, we will experimentally realize and characterize the proposed hollow-core optical fibers.

光传输网络正在向Pbit/s量级快速发展，并逐步逼近非线性香侬极限。承载云计算等新兴信息服务的大数据中心以及金融行业数据的高速传送均要求大容量、低时延通信技术。基于传统复用方式（例如波分复用）的光网络中单模光纤所承载的空间谱效率急剧增加，因此面向新型复用技术的高性能光纤就成为大容量、超高速光网络急需解决的关键科学问题。空芯光纤由于其特殊的空气芯子结构和模式传导条件，具有低时延、低非线性等实芯光纤所不具备的特性。另外，将超材料等新型结构和材料用于空芯光纤的包层设计可有效缩减光纤尺寸，小型化波导器件将进一步提升高速通信系统的集成度。本研究将通过合理设计多模空芯光纤，特别是深入挖掘包层的新型结构和材料，采用解析、数值相结合的方法，分析光纤中的高阶模式、及其对应的轨道角动量模式和高阶偏振保持模式的复用传输性能，利用粒子群算法优化光纤结构设计，研制出适用于高速模分复用通信技术的空芯光纤。

本项目面向模式复用技术开展了反谐振反射偏振保持空芯光纤、偏振保持椭圆环形空芯光纤、金属丝辅助单模单偏振光纤以及输出轨道角动量模式的多波长激光器的相关研究。1）通过在空芯反谐振反射光波导的包层毛细管中引入多层结构并调整正交方向毛细管壁厚度，设计的反谐振反射偏振保持空芯光纤能够在1425-1725 nm波长范围内实现偏振保持传输，模式间有效折射率高于1x10-4，传输损耗可低至0.002 dB/m，色散小于45.51 ps/nm/km；2）基于包层介质管的四阶旋转对称分布，提出了由半椭圆介质管构成的反谐振反射偏振保持太赫兹空芯光纤结构，该光纤在0.8-1.05 THz频率范围内时，两个模式的衰减系数约为1 dB/m，其有效折射率差为1.3x10-4-3.2x10-4，群速度色散为0.2-0.6 ps/THz/cm；3）提出了偏振保持椭圆环形空芯光纤，通过优化设计，光纤能够在1510-1630 nm波长范围内支持14个偏振保持模式，相邻模式有效折射率差高于1.33x10-4，色散值幅度不高于60 ps/nm/km；4）为降低高折射率芯子的制作成本，在椭圆环形空芯光纤的芯子外部引入4个空气孔，该光纤在1520-1600 nm波长范围内支持10个偏振保持模式，相邻模式有效折射率差高于1x10-4，色散值幅度不高于51 ps/nm/km；5）研究了1-2 µm波段内金属材料和负介电常数介质材料的性质，设计的金属丝辅助光纤在1550 nm波长处支持单模单偏振模式传输，正交方向两个模式的损耗比值为25.28；6）基于Mach–Zehnder滤波器、Sagnac环滤波器和模式选择耦合器，研制了能够输出轨道角动量模式的多波长可调谐光纤激光器，轨道角动量模式纯度高达95%、最大波长调谐范围22.64 nm。在项目研究过程中，发表学术论文15篇，授权国家实用新型专利1项、申请国家发明专利1项，培养在读硕士生4名。