Influence of Geometrical Parameters of Nonlinear Optical Fibers on Their Optical Properties ^†

The properties of specialty optical fiber technology are determined by many aspects, such as the choice of material from which optical fibers are made, the refractive index profile, or the optical fiber manufacturing method. Typical optical fibers are made from ultrapure silicon dioxide (SiO₂), called fused silica, in a process called chemical vapor deposition (CVD). The differences between refractive indexes are most often obtained by doping silica glass with selected inorganic compounds, mainly germanium dioxide (GeO₂), which increases the refractive index, and fluorine, which lowers it accordingly. The proper design of the dopant profile in the optical fiber core and in the layer surrounding the core is crucial for nonlinear optical fibers with shaped dispersion characteristics. Such optical fibers can be used for the generation of nonlinear phenomena, such as supercontinuum generation (broadband light source) or soliton self-frequency shift. As part of the research, structures of nonlinear optical fibers with flattened normal dispersion in the near-infrared range were designed through the use of numerical simulations in COMSOL Multiphysics. The theoretical chromatic dispersion characteristics and dependence of the effective mode field area on the wavelength were obtained from the theoretical structures. Based on the designed optical fiber structures, a series of nonlinear optical fibers were produced, which were characterized by a high concentration of GeO₂ in the core and the presence of a fluorine-doped layer around the core. The influence of geometrical parameters, e.g., the width of the fluorine-doped layer (ratio of the radius of the fluorosilicate layer to the radius of the core), and the imperfections resulting from the technological aspects on the optical properties of manufactured optical fibers, with a particular emphasis on chromatic dispersion and the effective mode field area, was determined experimentally. Theoretical optical fiber models, along with their calculated properties (chromatic dispersion and effective mode field area), were compared with real measurements.