基于对称双环嵌套管的低损耗弱耦合六模空芯负曲率光纤

Low-loss weak-coupling 6-mode hollow-core negative curvature fiber based on symmetric double-ring nested tube

本文设计了一种具有对称双环嵌套管结构的新型低损耗少模空芯负曲率光纤,该光纤支持LP_(01),LP_(11),LP_(21),LP_(02),LP_(31a),LP_(31b)共6种纤芯模式.所设计的光纤以SiO(_2)作为基底材料,采用特殊的对称双环嵌套结构将包层区域进行划分,能够有效地减小纤芯模式与包层模式的耦合.使用有限元法对该少模空芯负曲率光纤的结构参数进行优化,并分析了纤芯各个模式的限制损耗和弯曲损耗.仿真结果表明,所提出的少模空芯负曲率光纤能够同时支持弱耦合的6种纤芯模式独立传输(相邻模式间的有效折射率差均大于10~(–4),有效地避免了纤芯内模式间的耦合).在400 nm带宽(1.23—1.63μm,覆盖O,E,S,C,L波段)范围内,纤芯中的6个模式均保持低损耗稳定传输.各模式限制损耗在1.4μm处达到最低,其中基模LP_(01)模式的限制损耗最低,为4.3×10^(–7)d B/m.此外,当弯曲半径为7 cm时,各模式在一定工作波长范围内均保持低弯曲损耗传输.公差分析表明,当结构参数偏移±1%时,该少模空芯负曲率光纤仍然可以保持低损耗弱耦合的传输特性.

Few-mode optical fibers have played an increasingly important role in breaking through the transmission capacity limitations of single-mode optical fiber and alleviating the bandwidth crisis in optic fiber communication systems in recent years.Nevertheless,traditional solid core few-mode optical fibers usually suffer optical fiber nonlinearity and mode coupling,leading to mode crosstalk between channels.Hollow core negative curvature fibers(HC-NCF)have attracted widespread attention due to their advantages,such as low latency,low nonlinearity,low dispersion,low transmission loss,and large operating bandwidth.In this work,a novel low-loss few-mode HC-NCF with symmetrically double ring nested tube structure is designed,which supports six core modes including LP_(01),LP_(11),LP_(21),LP_(02),LP_(31a),and LP31b.The designed optical fiber is based on silica dioxide substrate and adopts a unique symmetrical double ring nested cladding structure,which can effectively suppress the coupling between the core mode and the cladding mode.The finite element method(FDE)is used to numerically analyze the properties of the proposed few-mode HC-NCF and optimize the structural parameters of the few-mode HC-NCF.Moreover,the confinement loss and bending loss of all core modes are investigated.The simulation results show that the proposed few-mode HC-NCF can support the independent transmission of six weakly coupled core modes(with the effective refractive index difference greater than 1×10^(–4)between the adjacent core modes,which greatly avoids the coupling between the adjacent modes in the fiber core).In the 400 nm bandwidth(1.23–1.63μm,covering the O,E,S,C,and L bands),all six modes in the fiber core maintain low loss transmission.Moreover,in the range of 1.3–1.63μm,the confinement loss(CL)of LP_(01),LP_(11)and LP_(21)mode are all less than 1×10^(–3)dB/m,and the CL of LP_(02)and LP31b mode are both less than 3×10^(–3)dB/m.The CL of each mode reaches the lowest value at 1.4μm,and the LP_(01)mode has the lowest CL of 4.3×10^(–7)dB/m.In addition,for a bending radius of 7 cm,each mode maintains the low bending loss characteristic in a certain operating wavelength range.In the range of 1.23–1.61μm,the BL of LP_(01)is less than 4.5×10^(–4)dB/m,and the BL of LP_(11)is less than 1.3×10^(–3)dB/m.The tolerance analysis shows that even with the deviation of structural parameters of±1%,the few-mode HC-NCF can still maintain the characteristic of low-loss and weak coupling.The designed few-mode HC-NCF has ultra-low CL and bending-insensitive characteristics while supporting independent transmission of six modes,which will find huge potential applications in future high performance mode division multiplexing systems.