Theory for mode coupling in perturbed fibers.

Based on a systematic approach to expand non-ideal eigenmodes as superposition of ideal eigenmodes, we develop a theoretical formalism to study mode coupling in perturbed fibers. The formalism explicitly yields the maximum energy-conversion ratios among ideal eigenmodes and the corresponding propagation lengths to reach the maximum conversion, and errors inherent to the underlying perturbative assumption can be estimated straightforwardly. Moreover, the formalism provides a simple way to divide the modes into smaller approximately-decoupled subsets, thus substantially simplifying the theoretical analysis and more directly catching the physical essence. By employing the formalism to analyze a ring fiber under perturbations of ellipticity and misalignment, we find that stability of the ideal eigenmodes is sensitive to the specific perturbation form, i.e., with some modes strongly affected by the ellipticity but not the misalignment, or vice versa. Furthermore, it is revealed that the EH 11 and HE 31 modes are strongly coupled under perturbation of ellipticity despite the relatively large effective-refractive-index difference, which suggests possible inadequacy of a widely adopted criterion against the coupling of EH ν − 1 , p and HE ν + 1 , p modes. • A new theory for mode coupling in perturbed fibers is proposed. • The theory is employed to study OAM-mode coupling in a ring fiber. • The stability of OAM-modes is found to be sensitive to perturbation types. • A widely-adopted criterion on mode coupling is found to be potentially inadequate. [ABSTRACT FROM AUTHOR]