Investigation of transmission loss dependence on core surround parameters in 7-cell hollow-core photonic-bandgap fibers

In designing photonic-bandgap fibers, the most effective and well-known approach utilized to suppress surface modes is the T ∼ S / 2 rule, which translates to using a core surround with only half the thickness of the average cladding strut. We investigate transmission loss in a commercial 7-cell hollow-core photonic-bandgap fiber. To reduce transmission loss at 1550 nm, we apply the T ∼ S / 2 rule to suppress the surface modes. While the strut surface modes are suppressed, the rod surface modes (RSMs) are blueshifted to wavelengths near 1550 nm because the round-core diameters of the cladding-layer corners resting against the fiber core are quite small, which leads to the coupling between the fundamental mode and the RSMs. This induces attenuation at 1550 nm, which indicates that the T ∼ S / 2 rule is applicable only within certain limits. By optimizing the round-core diameters of the corners resting against the fiber core, we obtain a structure that is free of surface modes, with the subsequent transmission loss at 1550 nm being ∼10 dB / km lower than that of the original fiber. Moreover, we propose a simple method to modify the round-core diameters to aid in surface-mode suppression.