A Weakly-Guiding Low-Loss and Low-Crosstalk Center-Raised Ring-Core Fiber With Seven OAM Mode Groups Containing High-Order Radial Modes

We design and fabricate a weakly guiding low-loss and low-crosstalk center-raised ring-core fiber (RCF) with a triple-ring core and a fluorine-doped cladding. This fiber supports seven orbital angular momentum (OAM) mode groups (MGs), including two higher-order radial MGs. The employment of a fluorine-doped silica cladding to lower the cladding refractive index reduces the required dopant concentration in the core, which in turn mitigates Rayleigh scattering loss. The unique center-raised index profile, paired with the triple-ring core design, yields an effective refractive index difference (Δneff) of 1.12 × 10−3between adjacent MGs, and 1.48 × 10−3between adjacent radial first-order MGs. Propagation loss of all MGs is measured in the experiment, which ranges from 0.165 to 0.212 dB/km at 1550 nm, and spans 0.158 to 0.227 dB/km over the entire C-band. Characterized via off-axis digital holography, the crosstalk of all MG pairs is maintained at a consistently low level after 8.3 km of transmission. Furthermore, the power coupling coefficients between all radial first-order MGs over a 27.2 km fiber link are precisely measured using swept-wavelength interferometry. These measurements confirm that the fabricated fiber features low level inter-modal crosstalk. The transmission stability and anti-bending performance of the fabricated RCF are further characterized, both of which exhibit excellent performance. These results collectively demonstrate the significant potential of the presented weakly-guiding low-loss and low-crosstalk RCF for high-capacity fiber-optic mode-division multiplexing communications using OAM modes.