中文摘要：

直接从电磁场的普遍原理共轭互易定理出发，针对无损折射率双缓变光纤，得到了双缓变光纤模耦合方程，推导出了互耦合系数、自耦合系数、干涉耦合系数和传播常数之间关系。在此基础上，对模耦合方程进行了求解，得到了相同双缓变光纤模耦合方程的近似解析解，讨论了耦合系数对光功率耦合的影响。

英文摘要：

For dual-tapered fibers with non-absorbing refractive index, the mode-coupling equation for dual-tapered fiber coupler is derived, and the relationship among cross-coupling coefficients, self-coupling coefficients, interference-coupling coefficients and propagation constants is deduced by using the conjugated reciprocity theorem. Based on this relationship, the approximate solution to mode-coupling equation of dual-tapered fiber coupler with the identical propagation constant is solved. The analytical expression for the coupling optical power is obtained. Impacts of these coupling coefficients on the coupling optical power are discussed.