  In this paper, to accurately characterize the low inter-mode coupling of the
weakly-coupled few mode fibers (FMFs), we propose a modified inter-mode
coupling characterization method based on swept-wavelength interferometry
measurement, in which an integral calculation approach is used to eliminate
significant sources of error that may lead to underestimation of the power
coupling coefficient. Using the proposed characterization method, a
low-crosstalk ring-core fiber (RCF) with low mode dependent loss (MDL) and with
single span length up to 100 km is experimentally measured to have low power
coupling coefficients between high-order orbital angular momentum (OAM) mode
groups of below -30 dB/km over C band. The measured low coupling coefficients
based on the proposed method are verified by the direct system power
measurements, proving the feasibility and reliability of the proposed
inter-mode coupling characterization method.
