应用有限差分光束传播法和平面波展开法分别计算得到光子晶体光纤的基模有效折射率和包层有效折射率,结合阶跃折射率光纤的模色散方程解出该结构的等效芯半径.通过对不同结构下光子晶体光纤的研究,找到其等效芯半径随结构参数的变化规律,并根据计算结果将此规律以近似公式的形式给出.该分析和结果可为利用有效折射率法分析光子晶体光纤时等效芯半径的选取提供一定的理论依据.

The effective index of fundamental mode and effective cladding index for index-guiding photonic crystal fibers with triangular lattice are calculated by using finite-difference beam propagation method and plane wave expansion method, respectively. Then the equivalent core radius for photonic crystal fibers is obtained through putting them into the mode dispersion equation for step index fibers. Variational regularity of the normalized equivalent radius(r norm=r/Λ) with normalized frequency(Λ/λ) is found out for different photonic crystal fiber parameters. Finally, the approximate formula for r norm as a function of d/Λ and Λ/λ, suitable to 2≤Λ/λ≤10 and 0.2≤d/Λ≤0.7, is given out.