用于产生和传输涡旋光束的微结构光纤设计及制造技术研究

The data-carring capacity of single-mode optical fibers has increased by four orders of magnitude in the past three decades, primarily because of multilplexing techniques that use wavelength, phase, and polarization of light to encode information. As the capacity of the current optical fiber systems reaches limits imposed by nonlinear effects，the possibility of spatial-division- multilplexing methods by use of multicore and multimodes fibers emerged to address the forthcoming capacity crunch. Although multicore fibers and potentially require more complex manufacturing than do circularly symmetric multimode fibers, conventional multimode fibers suffer from mode coupling caused by random perturbation in fiber. Methods that have been developed to address the problem of mode coupling so far have been dependent on computationally intensive digital signal processing algorithms, and have been based either on adaptive optics feedback or complex multiple-input multiple-output methodologies. Recently,prof.Siddharth Ramachandran in Boston University, Prof.Alan E. Willner1in University of Southern California, and Moshe Tur in Tel Aviv University proposed and demonstrated muxing, transmission and demuxing of 2 orbital angular Momentum beams over 10 wavelength channels through 1.1-km of vortex fiber. An aggregated total capacity of 1.6-Tbit/s is achieved by using 20-Gbaud/s 16-QAM signal on each channel. One of the key technologies for OAM-based new fiber communication is structure design and fabirication technique of a new type fiber,named vortex fiber. We will design and fabricate a kind of microstructured fiber structures,with a square core and ring refractive index profile that converts an input circular polarized Gaussian mode into optical orbital angular momentum (OAM) modes. By breaking the circular symmetry of the waveguide, the input circularly polarized fundamental mode in the square core can be coupled into the ring region to generate higher-order OAM modes, corresponding to the transference of spin angular momentum and orbital angular momentum. We will also design and fabricate another kind of microstructured optical fiber with a step-index central core and a annular high-index profile.The annular high-index profile that we expect to help with mode stability. the step-index central core,does not detract from the design philosophy, instead it allows for the fundamental mode to be Gaussian shaped, enabling low-loss coupling, either from free-space lasers or conventional single-mode fibers. Here,proposed microstructured fibers will open the door to exploiting microstructured fiber to generate and transmite vortex beams. The fabrication method for the microstructured vortex fiber has many merits such as only using one materials,big size preform,suitable to mass-fabrication and so on.

具有特殊螺旋波前结构和光子轨道角动量的涡旋光束，在新一代光通信技术及量子信息传输、成像新技术等领域的应用前景十分广阔。本申请针对我国开展涡旋光纤新技术研究急需的关键纤维光学材料开展研究工作。拟设计并制造两种基于微结构的涡旋光纤。一种是能够将圆偏振高斯光束转换成涡旋光束，具有方形内芯和环形外芯结构的微结构光纤。方形芯设计可以打破波导的圆对称结构，使输入方芯中的圆偏振基模可以被耦合到环形芯区，借助于自旋角动量和轨道角动量间的转移，产生涡旋束。另一种是能够高效传输自旋相反的圆偏振高斯模和不同拓扑荷的涡旋光束，具有圆形内芯和环形外芯结构的微结构光纤。圆形芯可以传输两种自旋相反的圆偏振光，而环形芯区传输拓扑荷为±1的涡旋束。采用微结构设计，可以消除因多种材料热学、力学性质的差异，导致的大尺寸预制棒难以制备的弊端，使涡旋光纤制造更容易实现规模化制造。本申请提出的两种微结构涡旋光纤，未见国内外报道。

基于轨道角动量（OAM）的空分复用（SDM）技术可以极大程度上提高通信的容量，有效地解决带宽问题。而涡旋光束的产生和传输器件是OAM光纤通信系统中的两个关键问题。本项目开展了用于涡旋光产生的纳米金属超表面结构的设计与制造技术的研究、用于涡旋光传输的基于聚合物材料的中空环芯光纤结构的设计与研究。在涡旋光产生方面，设计了以金、银为材质的超表面结构来产生多级涡旋光。初级的以金、银为材质的超表面结构可以产生拓扑荷为-4到4的涡旋光，且OAM模式纯度均大于95%；更进一步地，我们在初级以金为材质的超表面结构后加入四分之一波片和左旋圆偏振波片，该系统可以同时产生拓扑荷为+2、-2的涡旋光；在初级超表面结构后加入右旋圆偏振波片，该系统可以同时产生拓扑荷为+1、-1的涡旋光，且纯度高达92%。上述超表面结构的通讯波段均为850、1310、1550nm，具有波段宽、高容差、体积小、易集成、易设计等优点，在未来集成光通讯技术中存在潜在应用。在结构设计和理论仿真的基础上，制备了以金为材质的超表面结构并且进行了涡旋光产生的实验验证。在涡旋光的传输方面，提出了一种环形剖面POF来支持OAM模的传输。与已报道的玻璃环芯光纤相比，这种新型的中空环芯POF（HRC-POF）可以拥有更大的折射率对比， 大幅提高了的neff使得OAM模传输更稳定。除此之外，POF的材料选择也较多， HRC-POF有着加工温度低、制造工艺简单、易切割抛光、成本低的优点。仿真结果表明参数合适的HRC-POF可以实现大的neff（约10-3~10-2），支持30个模式（包含26个OAM模）的传输，且模式纯度均高于99.73%。这意味着这种新型HRC-POF可实现高出传统SI-POF几倍的带宽来解决现有POF带宽被限制在千兆比特的难题。该新型POF将在超高数据容量、短距离POF通信系统中有着潜在的应用前景。