基于少模光纤的多维复用高频谱效率光传输机理研究

With the expansion of optical network scale, system capacity, various service, flexible applications, and diversified demands, it is quite challenging to seek novel ultra-high speed optical transmission mechanism for next-generation ultra-high speed, ultra-long distance, ultra-high capacity (3U) optical network. In this project, we innovatively propose a multi-dimension multiplexing based high spectral efficiency (SE) optical transmission mechanism using the few-mode fiber. Those various modes within the few-mode fiber can be treated as multiple independent transmission channels, while the proposed mode-division multiplexing (MDM) structure is compatible with the current dense wavelength-division multiplexing (DWDM) and polarization-division multiplexing (PDM) technique. Thus the proposed transmission mechanism can satisfy the ever-increasing bandwidth demand and drive the optical communication systems to higher capacities. This project will carry out a comprehensive investigation from the design and fabrication of the few-mode fiber, coherent receiver to multi-dimension multiplexing optical transmission experiment. The deliverable results include a new few-mode fiber with low mode coupling, large differential group delay (DGD) and large effective area, which can simultaneously support four-mode transmission without impairments due to fiber nonlinearity. Using the few-mode fiber, one mode channel is used to transmit multi-dimension multiplexing based high-speed optical signal, while another mode channel is used to carry the continue wave (CW) local oscillator (LO) signal. Then, we can implement non-LO coherent receiver and subsequently costly tunable laser source with narrow linewidth can be saved. Next, using optic multi-input multi-output (MIMO) technique and spectral shaping theory, we can equalize the crosstalk among mode channels and transmission impairments among signals in those modes. Finally, we can experimentally demonstrate four-wavelength multi-dimension multiplexing 100Gpbs quadrature phase shift keying (QPSK) transmission over 100km few-mode fiber, within only 25GHz wavelength spacing. The SE of our transmission system can reach as high as 16 bit/s/Hz.

随着光通信网络规模不断扩展、容量快速增长、业务日益丰富、应用愈加灵活，寻求新型高效大容量光传输机理已成为未来光网络面临的重大挑战。本项目创新性地提出一种基于少模光纤的多维复用高频谱效率光传输机制，利用不同模式作为相互独立的信道传输多路信号，同时兼容密集波分复用和偏振复用技术，提升光传输的系统容量和频谱效率。本项目从少模光纤设计与制备、相干接收到多维复用光传输系统实验三个方面展开研究，研制能够支持传输包括基模在内的4个模式，具有低模间耦合、大群时延和有效面积的少模光纤。利用少模光纤，一路模式信道承载多维复用高速信号，另一路模式信道传输本振光，实现多维复用高速信号非本振相干接收。利用光学MIMO技术和频谱整形理论均衡模式信道串扰和信号传输损伤，最终在25GHz波长信道间隔条件下，实现4波长多维复用100Gbps QPSK信号在100公里少模光纤上无误码光传输，频谱效率达到16bit/s/Hz。

以解决超高速光传输的多维复用问题为核心，研究光传输中模式复用基础理论与关键技术，实现基于少模光纤包含密集波分复用、偏振复用、模式复用的多维复用光传输系统。在基金支持下，研究内容涉及模分复用器的设计和性能测试、传输用少模光纤的设计和特性测试、模分复用光传输系统的实验探索等方面。首先，研制出能够支持包括基模在内的三个模式，模式间具有较大的有效折射率差值保证较低的模间耦合，同时具有较大群时延的椭圆芯少模光纤，并发现这种特种少模光纤在模式操控，光纤传感，短距互联上的新应用。开发了支持偏振复用信号的基于硅基液晶的模分复用器，发现硅基液晶具有寄生的强度相关损耗。利用两模光纤，实现多维复用高速信号非本振相干接收方案，从而避免在接收端使用昂贵的窄线宽可调激光器。开展了12个空间和偏振模式的复用传输实验，高级编码调制TCM-8PSK在模分复用系统中可以表现出较好的编码增益，相比QPSK调制在硬判决阈值处可获得1.93 dB的OSNR增益。传输结果表明，在长距离光传输系统中模式相关损耗直接影响到系统的稳定性和误码率，使用少模光放大器时需要对各模式提供相同的增益。

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