A W-type weakly-conductive doubly cladding fiber with an inner cladding made of negative-refractive-index material and an external cladding and fiber core made of conventional material was investigated. An exact characteristic equation of oscillation mode was derived by using full vector method and Helmholtz equation without considering the material chromatic dispersion and medium loss. The influence of refractive index and thickness of inner cladding on HE11 mode of this left-handed fiber was discussed respectively. The chromatic dispersion curve and the curve of cut-off frequency versus inner layer thickness and inner layer refractive index were depicted and compared with those of the right-handed fiber HE11 mode. It was shown by analysis that doubly cladding fiber with an inner cladding made of negative-refractive-index material exhibited many different abnormal characteristics from conventional optical fiber, providing a theoretical basis for designing new optical devices.%对内包层为负折射率介质,纤芯和外包层都为常规材料的W型弱导双包层光纤进行研究.在不考虑材料色散和介质损耗的情况下,采用全矢量的方法,由亥姆霍兹方程出发得到严格精确的震荡模式的特征方程.分另讨论了内包层折射率和厚度对该左手材料光纤的HE11模式的影响,画出了该模式不同情况下的色散曲线及截止频率随内包层折射率和厚度变化的曲线,并且和右手材料光纤HE11模式的相应曲线进行比较.分析表明,负折射率光纤具有许多不同于常规光纤的反常特性,为设计新型光纤器件提供理论基础.
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