Low-Loss Orbital Angular Momentum Ring-Core Fiber: Design, Fabrication and Characterization

We design, fabricate, and characterize two types of low-loss weakly guiding ring-core fibers (RCFs) supporting orbital angular momentum (OAM) modes. The ring core and cladding have a relative refractive index difference of 1%. The RCFs feature largely separated OAM mode groups (MGs), where the first MG only contains two fundamental modes and the other MGs each have four OAM modes. The effective refractive index difference (Δneff) between all MGs in the type-1 RCF is larger than 1.5 × 10
^-3
. For the type-2 RCF, except the first two MGs, the Δneff between the other MGs is also larger than 1.0 × 10
^-3
. These enable relatively low-level inter-group crosstalk and facilitate multiple-input multiple-output (MIMO) simplified OAM multiplexing transmission systems, where small-scale MIMO assisted intra-group OAM multiplexing and MIMO-free inter-group OAM multiplexing can be considered to take full use of all OAM modes in RCFs. The propagation loss of all supported OAM modes in the fabricated RCFs is measured to be less than 0.25 dB/km. We study the modal effective area and anti-bending performance of the guided OAM modes in RCFs. The effective refractive index, group velocity dispersion, differential group delay, crosstalk, and stability of RCFs are all characterized showing favorable performance. Moreover, using the fabricated two types of low-loss RCFs and adopting 8-ary quadrature amplitude modulation (8-QAM) signals, we also demonstrate data-carrying two OAM mode group multiplexing transmission over 25.32 km type-1 RCF and 50 km type-2 RCF free of MIMO. The demonstrations show a great potential to use low-loss RCFs in long-distance fiber-optic OAM multiplexing communications.