An optical fibre with weak mode coupling is desirable for future ultra-high capacity space-division multiplexing (SDM) systems because mode coupling in an optical fibre results in extrinsic loss of the fibre and crosstalk between guided optical modes. To study the feasibility of a ring-core fibre (RCF) for SDM systems, in this paper, we investigate the mode coupling in the RCF supporting 5 or 7 guided mode groups (MGs) at a wavelength of 1550nm. For this purpose, the coupled mode/power theory (CMT/CPT) with identified spatial power spectrum of random perturbations of fibre 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 fibre 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.
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