受拉丝工艺条件的限制, 双芯光纤的纤芯形状与位置常常有一定的变化，这将会对两芯之间的耦合特性产生影响。在给定相同纤芯面积的条件下，计算分析了三种双(圆、椭、卵)芯光纤的耦合长度随纤芯距离、纤芯形状之间的变化关系。在1550 nm波长下，计算发现，双圆芯光纤比双椭芯光纤在更近的纤芯距离处，其耦合长度开始呈指数增长。计算分析了双(圆、椭、卵)芯光纤的耦合长度随波长的变化关系，发现在相同的工作波长下，双圆芯光纤的耦合长度最长，双卵芯光纤的耦合长度次之，双椭圆芯光纤的耦合长度最短。

Limited by the manufacture condition, the cores′ shape and pitch in dual-core fiber often have some deviation which may affect the coupling characteristics between the two cores. For the same given cores′ area in three kinds of dual (circular, elliptical, egg-shaped) core fiber, the relation of the coupling length with the cores′ shape and pitch is calculated. At 1550 nm wavelength, it is found that the coupling length of dual-circular-core fiber grows exponentially at a shorter pitch than dual-elliptical-core fiber. The relation of coupling length with wavelength is examined for these dual-core fibers with same cores′ pitch and area. It is found the coupling length of dual-circular-core fiber is the longest and dual-elliptical-core fiber is the shortest while dual-egg-shaped-core fiber is in the middle.