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

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 respectively, during the transmission process, the LP 01 mode in the left side core will be coupled and converted to the LP 21 mode light in the central core gradually. Similarly, the LP 01 mode of the right side core is transitioned to 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 to central core can be completed at the output end simultaneously, so as to realize 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


超模理论下的模式复用模型
结合超模理论研究发现， 当在光子晶体光纤中的两个纤芯间发生光耦合并伴随能量 转移时，这两个纤芯会在光传播过程中激发出超模 [19] ，即一种在两个纤芯中同时存在光 在研究时，将在 Z 等于零处旁芯中的输入总能量作为 in P ，在确定的传输长度(即器件 长度)处的中心纤芯中的总能量作为 out P 来进行计算。模式转换效率也是衡量模分复用 器性能的一个重要参数，其表达式 [22] 如下 Design and research of a broadband mode division multiplexer based on three core photonic crystal fiber * Wang Xiaokai 1) Li Jianshe 1) † Li Shuguang 1) Guo Ying 1) Meng Xiaojian 1) Wang Guorui 2) Wang Luyao 1) Li Zenghui 1) Zhao Yuanyuan 1) Ding Yuxin 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 respectively, during the transmission process, the LP 01 mode in the left side core will be coupled and converted to the LP 21 mode light in the central core gradually.Similarly, the LP 01 mode of the right side core is transitioned to 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 to central core can be completed at the output end simultaneously, so as to realize 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 this paper, 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 analyze and calculate.The results show that at wavelength band from 1.49μm to 1.63μm, the maximum insertion loss of the device is 0.72dB, and the lowest insertion loss is 0.543dB at 1.55μm, which is far lower than the general evaluation standard of 1dB insertion loss.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.

图 1 Fig. 1 .
Fig. 1.(a) Profile structure of three core PCF mode division multiplexer ;(b) Section structure of central core waveguide designed by split ;(c) Section structure of side core waveguide designed by split

Fig. 2 .
Fig. 2. The relationship between effective refractive index of LP21 and LP31 modes in central core, effective

Fig. 3 .
Fig. 3. (a) The relationship between the effective refractive index of the side core mode and the wavelength under different d1 conditions; (b) The relationship between the effective refractive index of the side core mode and the wavelength under different refractive index difference of the doped rod

Fig. 4 .
Fig. 4. (a) The relationship between the effective refractive index of the side core fundamental mode and the transmission wavelength under different d1 and d2 conditions; (b) The relationship between the effective refractive index of the side core fundamental mode and the refractive index difference of the doped rod at 1.55μm wavelength; (c) Phase matching occurs between the basic mode of the side core and