Design trade-offs and optimization of radially single-mode and azimuthally multimode ring-core fibers for mode-division multiplexing systems

This paper investigates the design trade-offs of radially single-mode and azimuthally multimode (RSMAM) ring-core fibers (RCFs), thereby revealing a more efficient use of the weakly coupled linearly polarized (LP) modes in mode-division multiplexing (MDM) systems. The influences of the increasing number of LP modes on the main propagation properties (i.e., effective index difference, effective area Aeff, and macro-bending sensitivity at a wavelength of 1550 nm) and the key limiting factors for such an increase are numerically described. Based on 1) the design criteria of weakly coupled few-mode fibers described in [P. Sillard, J. Lightw. Technol.32, 2824 (2014)] and 2) an assumption that the refractive index contrast is ≤1% (for facilitating the fiber manufacturing), we point out that the step-index 4-LP-mode RSMAM RCF appears feasible, while RSMAM RCFs with a higher number of LP modes are still primarily limited by the oversized Aeff and undesired macro-bending sensitivities. Finally, in order to provide a better compatibility with weakly coupled MDM systems, we present improved designs for the 3- and 4-LP-mode RSMAM RCFs given in [M. Kasahara, J. Lightw. Technol.32, 1337 (2014)Y. Jung, J. Lightw. Technol.35, 1363 (2017)] by taking into account the spatial information densities and macro-bending sensitivities.