一种基于三芯光子晶体光纤的宽带模分复用器的设计与研究

Design and research of a broadband mode-division multiplexer based on three-core photonic crystal fiber

本文提出了一种基于非对称三芯光子晶体光纤的宽带模分复用器.该器件主要是由位于光纤中心的可提供基模和高阶模传输的中心纤芯和分别位于中心纤芯两侧的可提供基模传输的2个旁芯构成.根据光耦合理论,在输入端对3个纤芯分别输入LP_(01)模式的光,在传输过程中左旁芯的LP_(01)模式的光将逐步向中心纤芯耦合并转换为LP_(21)模式传输,而右旁芯中的LP_(01)模式的光则逐步耦合并转换为中心纤芯中的LP_(31)模式来传输.通过对光纤结构的优化设计和光纤长度的选择,使得在输出端同时完成旁芯LP_(01)模向中心纤芯LP_(21)和LP_(31)模的最佳转换,从而实现LP_(01)、LP_(21)和LP_(31)3种模式的光在中心纤芯中的复用.反之,若将该器件的输出端用作输入端则可以实现中心纤芯中3种模式的光向3个纤芯的解复用.本文利用有限元法和光束传播法进行了优化设计和仿真,并将光耦合理论与超模理论相结合进行了分析计算,结果表明在1.49—1.63μm的波段下,该器件插入损耗最高为0.72 dB,在中心波长1.55μm处器件插入损耗为最低值0.543 dB,远低于大家普遍采用的1 dB插入损耗的评价标准.较低的插入损耗也为级联型多芯光子晶体光纤模分复用器设计提供了可能.与现有的模分复用方案相比,该器件的集成性更高,受外界影响更小,与多芯空分复用光纤搭配使用,可以更好地提高模式转换效率和模式纯度,降低耦合复杂度,拓展通信容量.

A broadband mode-division multiplexer based on asymmetric three-core photonic crystal fiber is proposed in this paper.The device is mainly composed of a central core,which can provide the transmission of fundamental mode and higher-order mode,and two side cores providing fundamental mode transmission.According to the theory of optical coupling,the LP_(01) mode light is input to the three fiber cores at the initial port separately,and in the transmission process the LP_(01) mode on the left side core will be coupled and converted into the LP_(21) mode light in the central core gradually.Similarly,the LP_(01) mode of the right side core is transformed into the LP_(31) mode of the center core.By optimizing the structural design and selecting the length of optical fiber,the best conversion from side core into central core can be completed at the output end simultaneously,thereby realizing the multiplexing of LP_(01),LP_(21) and LP_(31) modes in the central core.In the opposite direction,if the output end of the device is used as the initial port,the demultiplexing of three modes of light from the central core to the three cores can be realized.In thiswork,the finite element method and beam propagation method are used to optimize the simulation,and the optical coupling theory and supermode theory are combined to conduct analysis and calculation.The results show that at wavelength band from 1.49μm to 1.63μm,the maximum insertion loss of the device is 0.72 dB,and the lowest insertion loss is 0.543 dB at 1.55μm,which is far lower than the general evaluation standard of 1 dB insertion loss.The low insertion loss also makes it possible to design cascaded multi-core photonic-crystal-fiber mode-division multiplexer.Compared with the existing mode-division multiplexing scheme,the device is more integrated and less affected by the external environment.When it is used with multi-core space division multiplexing fiber,it can better improve the mode-conversion efficiency and mode purity,reduce the coupling complexity and expand the communication capacity.