Guiding Properties of Silica/Air Hollow-Core Bragg Fibers

The guiding properties of realistic silica/air hollow-core Bragg fibers have been investigated by calculating the dispersion curves, the confinement loss spectrum and the field distribution of the guided modes through a full-vector modal solver based on the finite element method. In particular, the silica bridge influence on the fundamental mode has been analyzed, by comparing the properties of an ideal structure, without the silica nano-supports, and of two realistic fibers, with squared off and rounded air-holes. Simulation results have demonstrated the presence of anti-crossing points in the dispersion curves, associated to the transition of the fundamental mode into a surface one. It has been shown that surface modes are responsible of the sharp loss peaks, also experimentally measured, which pollute the loss spectrum of the fundamental mode and of the higher-order ones. Then, the influence on the guiding properties of each geometric characteristic in the hollow-core Bragg fiber cross-section has been deeply investigated, thus showing which parameter it is better to change in order to properly modify the loss values or its spectral behaviour. Moreover, in order to improve the loss properties of hollow-core Bragg fibers, the number of silica and air layers in the fiber cladding has been increased, and the layer thickness has been modified. Results have shown that the first change is more effective for the loss reduction, while the second is useful for a spectral shift. Finally, among the different possible applications, the feasibility of a DNA bio-sensor based on a hollow-core Bragg fiber has been demonstrated.