Principal Modes of Multimode Fibers Resisting Fiber Bending

We demonstrate that multimode fibers possess curved principal modes (CPMs) that can withstand significant fiber bending. These CPMs, derived from an extension of the Wigner-Smith operator, exhibit excellent bending resistance even in arbitrarily shaped fibers.

1. Introduction

There has been renewed interest in multimode fibers (MMFs) in various fields (e.g., optical communications [1], mode-locked lasers [2], [3], endoscopy [4]) due to their larger mode area, especially the additional spatial degrees of freedom, compared to single-mode fibers. MMFs have multiple eigenmodes, and for ideal straight, weakly-guided MMFs, the eigenmodes of linearly polarized (LP) modes are widely adopted in classic fiber optics [5]. However, when propagating in a curved MMF, these LP modes couple with each other [6]. The strength of mode coupling depends on the degree of curvature, thus changes in the bending states of MMFs will alter the light transmission. In the practical use of MMFs, the bending of MMFs is usually unavoidable, which significantly limits their applications.