Mid-Infrared Low-Loss Large-NA GRIN Fiber via Periodic-layered Chalcogenide Glasses

Given the limitations of traditional methods in fabricating mid-infrared gradient-refractive-index (GRIN) fibers with large numerical apertures (NA) at the micrometer scale, this paper proposes a sub-wavelength multilayer periodic structure (MPS) (<λ/n) to achieve the desired GRIN distribution. The proposed fiber structure is designed based on Maxwell-Garnett's effective medium theory (EMT) and fabricated using an innovative isolation-extrusion technique. These fibers not only exhibit a large numerical aperture (NA = 1.448), but also enable the precise control of refractive index gradient profiles. Furthermore, this study explores the intrinsic relationship between the gradient refractive index distribution and the optical field transmission characteristics in the mid-infrared range.