Modal characteristics analysis of a doubly clad optical fiber with semi-weakly guiding approximation

Modified characteristic equations for doubly clad multimode fibers were rigorously derived under semi-weakly guiding approximation in this article. The analytical expressions of the power flows distributed in the three regions of fiber cross section are also given. Different from previous works, results in this paper elucidate that, other than the inner cladding, the refractive indices also hold a non-ignorable influence on the modal power distribution and mode behaviors. By comparing the results of two models, this article demonstrates that the singly clad fiber model may not be suitable for analyses of common doubly clad fibers.