Mode Coupling Effects in Ring-Core Fibers for Space-Division Multiplexing Systems

An optical fiber with weak mode coupling is desirable for future ultrahigh capacity space-division multiplexing (SDM) systems because mode coupling in an optical fiber results in extrinsic loss of the fiber and crosstalk between guided optical modes. To study the feasibility of a ring-core fiber (RCF) for SDM systems, in this paper, we investigate the mode coupling in the RCF supporting five or seven guided mode groups (MGs) at a wavelength of 1550 nm. For this purpose, the coupled mode/power theory with identified spatial power spectrum of random perturbations of fiber axis is used to estimate the bend loss/crosstalk of the RCF due to microbending. It is shown that based on the identified parameters for the spatial power spectrum in the 5/7-MG RCF, the estimated bend loss/crosstalk of the RCF agrees well with experimental results. In addition, the impact of the gradient parameter α and refractive index contrast Δ of the fiber refractive index profile on bend loss and crosstalk of the RCF is explored. Simulation results indicate that the Δ instead of the α significantly affects bend loss and crosstalk of the RCF. The magnitude improvement in bend loss by increasing the Δ is dependent on the spatial power spectrum.