Highly Dispersive Germanium-Doped Coupled Ring-Core Fiber for Vortex Modes

In this work, we propose and design a germanium-doped highly dispersive coupled ring-core fiber (HD-CRF) with two concentric high-index rings for vortex modes. By using a full-vector finite-element method (FEM), we show that the most dispersive wavelength and its corresponding dispersion value of the proposed CRF can be adjusted and optimized by properly selecting the fiber geometrical parameters. The OAM
_1,1
mode supported in the designed CRF can achieve an extremely negative chromatic dispersion (CD) as low as −86,288 ps/(nm·km) at 1535.4 nm. The dispersion properties of the others vortex modes (TE
_0,1
and TM
_0,1
) further exhibit that our proposed CRF is promising for various CD-related applications of the complex vortex modes in the optical fiber. Furthermore, by varying the mole fraction of GeO
₂
and fiber structure parameters, the most dispersive wavelength of the OAM
_1,1
mode can be flexibly controlled over a 449.8-nm wide spectral range around −37,126 ps/(nm·km) with <2.8% CD variation.