长周期光纤光栅表现为前向传播的纤芯基模与同向传播的各阶次包层模式在特定波长时的模式耦合。由纤芯基模、一阶包层模式的场分布推得其功率分布 ,进一步得到两种模式的归一化常量。推导了纤芯基模与一阶各次包层模式 (HE1l/EH1l)的耦合常数。结果表明 ,纤芯基模与HE1l包层模式产生的耦合常数远大于与EH1l包层模式的耦合常数。次数低时 ,耦合常数随包层模式次数的升高而增大 Long-period fiber gratings exhibit the mode coupling of a forward-propagating core fundamental mode and a coherent propagating order cladding mode at a particular wavelength. The power distribution is deduced from the field distribution of core fundamental mode and first order cladding mode, and the normalized constants of the two modes are further obtained. The coupling constants between the fundamental mode and the first order cladding modes (HE1 / EH1l) are deduced. The results show that the coupling constants between the core mode and the HE1l cladding mode are much larger than the coupling constants with the EH1l cladding mode. When the number is low, the coupling constant increases with the number of cladding modes