Analysis of the dependence of the guided-mode field distribution on the silica bridges in hollow-core Bragg fibers

Abstract

The guiding properties of fabricated air-silica Bragg fibers with different geometric characteristics have been numerically investigated through a modal solver based on the finite element method. The method has been used to compute the dispersion curves, the loss spectra and the field distribution of the modes sustained by the Bragg fibers under investigation. In particular, the silica bridge influence on the fundamental mode has been analyzed, by considering structures with different cross sections, that is an ideal Bragg fiber, without the silica nonosupports, a squared air-hole one and, finally, a rounded air-hole one, which better describes the real fiber transverse section. Results have shown the presence of anti-crossing points in the effective index curves associated with the transition of the guided mode to a surface mode. Moreover, it has been verified that these surface modes are responsible of the loss peaks in the fiber transmission spectra, also experimentally measured. Surface modes are mainly localized in the regions of the cladding where the bridge supports join the cladding rings, forming silica islands where the field can focuses.