在弱渐变假定下,研究了抛物型渐变光纤传导模式的截止频率对折射率参量的依赖关系。采用矢量场法,得到了在弱导条件下传导模式的特征方程和截止频率的近似计算公式。引入了介电比γ,它是由光纤三个折射率参量所确定的常数。在弱导近似的条件下,归一化截止频率仅与γ这一个变量有关,任一渐变抛物型光纤的实际截止频率可通过γ而由归一化截止频率简单求得,从而找到了渐变光纤的实际截止频率与归一化截止频率间的联系。给出了归一化截止频率随γ的变化曲线,归一化截止频率随γ的增加而单调上升,但HElm模截止频率的上升规律与其他模式不同。

Under the assumption of weakly graded, the dependences of cut-off frequency of guided modes in graded parabolic index optical fibers on refractive indexes of fiber are investigated. The eigenvalue equation and approximate formula for cut-off frequencies of guided modes are obtained by vector-field-method. A parameter γ, called “permittivity ratio”, is defined, which is a constant determined by three refractive indexes of a fiber. It is shown that the normalized cut-off frequencies depend on γ merely. Actual cut-off frequencies of a parabolic index fiber can be obtained with normalized cut-off frequencies by γ simply, and consequently the definite relations between actual frequencies and normalized ones of the graded index fibers are obtained. The curves of normalized cut-off frequencies varying with γ for several modes are displayed. It is shown that the cut-off frequencies increase with γ monotonously, and the rule of increase of HElm modes is different from that of other modes.