要提高七芯光子晶体光纤激光器的输出功率，必须要增大七芯光子晶体光纤的有效模面积，并且需要纤芯间具有较强的耦合作用，以保证各纤芯输出的光束保持同相位。为了更直观的分析七芯光子晶体光纤的有效模场面积和纤芯间耦合强度的关系，根据七芯光子晶体光纤7个超模的特征，给出了七芯光子晶体光纤耦合长度的一种计算方法。利用多极法和有限差分光束传播法分析了七芯光子晶体光纤的结构对有效模场面积和耦合长度的影响。通过优化七芯光子晶体光纤的结构，设计了一种外层空气孔较大，内层空气孔较小的七芯光子晶体光纤，其有效模场面积高达3703 μm2 ，耦合长度仅为13310 μm。

To raise output power of the sevencore photonic crystal fiber laser, the effective mode area of sevencore photonic crystal fiber must be increased, and strong coupling among cores is needed to ensure the same phase output. In order to analyse the relationship between effective mode area and coupling strength among cores more intuitively, according to characteristics of the intensity distribution of the sevencore photonic crystal fiber supermode, a calculation method of coupling length of sevencore photonic crystal fiber is giren. The effect of structure of sevencore photonic crystal fiber on the effective mode area and coupling length is analyzed with multipolar method and finite difference beam propagation method. Through continuous optimization of the structural parameters, a sevencore photonic crystal fiber with large outer layer air holes and small inner layer air holes is designed. Its effective mode area is 3703 μm2 and coupling length is 13310 μm.