Comparative study of guided modes in hollow core Bragg fibers with binary and ternary photonic bandgap claddings

We comparatively investigate the modal properties of guided modes in hollow core Bragg fibers (HC-BFs) with the binary and ternary photonic bandgap claddings by using a full-vector finite element method. The influences of structural changes from the binary periodic cladding (BPC) to the ternary periodic cladding (TPC) on the dispersion relations, confinement losses, and modal field distributions of the lowest five guided modes are thoroughly discussed under different core diameters. The contrasts in bandwidths for the guided modes between both types of HC-BFs strongly depend on their distinct frequency cutoff characteristics. The bandwidth contrast for a given guided mode in large-core HC-BFs is different from that in small-core HC-BFs. The confinement loss of the fundamental HE sub(11) mode in the TPC-based HC-BF is much lower than that in the BPC-based HC-BF, which is essentially attributed to the stronger modal field confinement by the TPC. The differences in minimum confinement losses of the HE sub(11) mode between both types of HC-BFs decrease with decreased core diameters. In contrast, the changes of cladding structures have very limited influence on the confinement loss of the TE sub(01) mode in both the large-core and small-core HC-BFs. Moreover, we explore the reasons behind the evolutions of the minimum loss contrasts for the HE sub(11) and TE sub(01) modes between both types of HC-BFs with the core diameter, by quasi-quantificationally analyzing the influences of reflection characteristics of the transverse electric (TE) and transverse magnetic (TM) waves on the confinement loss.