Demonstration of true-eigenmode propagation in few-mode fibers by selective LP mode excitation and near-field observation

Abstract

We observed the near-field patterns of light output from few-mode fibers (FMFs) in which an LP mode was selectively excited. It was confirmed from the variation of the intensity profile with the wavelength that the true eigenmodes of a circular core fiber are guided in single-core step-index and graded-index FMFs. On the other hand, we discovered a new phenomenon that LP₁₁ modes propagate as eigenmodes oriented along a specific axis in 4-LP mode 12-core FMF. In addition, we confirmed that this phenomenon is not due to the elliptical deformation of the core by observing the NFP and calculating the eigenmodes in an elliptical core using elliptical cylindrical coordinates and the Mathieu function.