Reciprocal Inverse Graded Index Fiber (R-IGIF) for OAM-SDM Networks

In this article, we propose, design, and numerically assess a novel parametric refractive index profile for optical fiber, which we refer to as reciprocal-inverse graded index fiber (R-IGIF). The R-IGIF features a shape parameter $\alpha$ that controls the refractive index behavior (i.e. concave vs convex). We performed a systematic scanning of parameter' effects; using finite element method in Comsol Multiphysics, in order to design R-IGIFs that handle robust orbital angular momentum (OAM) channels (i.e. low radial modes). The designed fibers possess high inter-channels separation ($\Delta n_{eff} \geq 1\times$10⁻⁴) that outperforms standard step index fiber and inverse parabolic graded index fiber; hence, reduces channels crosstalk. Numerical simulations show that the supported OAM modes are of high purity ($\geq\! 99.9\%$), enabling low-level intrinsic crosstalk ($\leq\! -30\,dB$). Further investigations considered metrics over the C and L ITU-T bands including the cut-offs wavelengths, chromatic dispersion, differential group delay, effective area, and nonlinearity coefficient. Results show potential capability to achieve stable data channels transmission. Further assessments have been carried out under practical condition (i.e. bending) by evaluating the $\Delta n_{eff}$, the intra-mode walk-offs, and the confinement loss. The findings show great resilience to bending effects. Therefore, based on these deep physical investigations of modes carrying data, the R-IGIF could find application in next generation space division multiplexing SDM-Networks.