Fig. 6. Octave SPSM in a continuous Dirac-vortex fibre with a finite vortex size.

a Fibre structure with 16 cladding periods in radius. The colour wheel represents the phase and amplitude of the generalised Kekulé modulation. b Complete fibre dispersion in absolute frequency. First topological mode (red line) and higher-order doublet modes (green dotted line) in the first topological bandgap, as well as a second topological mode (blue line) in the second topological bandgap at higher frequency. The mode profiles ($\widehat{\mathit{z}}\cdot \mathrm{Re}\left[{\mathit{E}}^{*}\times \mathit{H}\right]$) of the two topological modes are shown in the insets, circled with different colours for clarity. c Fibre dispersion in frequency relative to the original nodal-line frequency. d Confinement losses of the guided modes. e Dispersion parameter and effective area of the first topological mode. f Bending loss of the first topological mode at λ = 1550 nm