A Novel Air-Gap Ring-Core Fiber for Low Inter-Core Crosstalk and Enhanced Fiber-Fuse Resistance

Space-division multiplexing (SDM) has emerged as a key solution to meet the growing demands for high-capacity optical communication systems. In this context, ring-core fibers (RCFs) have gained attention due to their unique potential for supporting a large number of orbital angular momentum (OAM) modes [1]. OAM modes exhibit a helical phase front <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\exp(il\phi))$</tex>, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$l$</tex> represents the topological charge and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\phi$</tex> the azimuthal angle [2]. With OAM's theoretically infinite number of orthogonal eigenstates, it offers substantial potential for increasing the capacity of mode-division multiplexing (MDM) systems [3]. This work presents an air-gap fiber with two ring cores with reduced inter-core crosstalk (IC-XT) [4] and improved fiber fuse resistance [5]. It supports 52 OAM modes with IC-XT below -100 dB/100 km, ideal for high-capacity, long-distance space-division multiplexing. The cross-section of the proposed fiber design and its modal analysis, conducted using COMSOL Multiphysics, are shown in Fig. 1. The results confirm the confinement of OAM modes from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\ell=1$</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\ell=7$</tex> with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta \mathrm{n}_{\text{eff}} &gt; 10^{-4}$</tex> ensuring minimal intermodal interference. The simulated IC-XT was below -100dB/100 km for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\ell=5$</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\ell=7$</tex> at a 140 mm bending radius. Based on the designs, we fabricated a two-core RCF. The preform was formed and drawn into a 145 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}$</tex> fiber as shown in Fig 2, though some deformation in the air gap and core attachment occurred. These initial results indicate areas for further optimization in the drawing process to meet our design targets.