Hybrid photonic bandgap effect in twisted hollow-core photonic bandgap fibers.

A hybrid photonic bandgap effect in twisted hollow-core photonic bandgap fibers (HC-PBFs) is theoretically investigated for the first time, to the best of our knowledge. Due to the topological effect, twisting of the fibers changes the effective refractive index and lifts the degeneracy of the photonic bandgap ranges of the cladding layers. This twist-induced hybrid photonic bandgap effect shifts up the center wavelength and narrows the bandwidth of the transmission spectrum. A quasi-single-mode low-loss transmission is achieved in the twisted 7-cell HC-PBFs with a twisting rate α = 7-8 rad/mm, which has a loss < 30 dB/km and higher-order mode extinction ratio > 15 dB. The twisted HC-PBFs could be suitable for applications such as spectral and mode filters.