Approximate model for analyzing band structures of single-ring hollow-core anti-resonant fibers.

Precise knowledge of modal behavior is of essential importance for understanding light guidance, particularly in hollow-core fibers. Here we present a semi-analytical model that allows determination of bands formed in revolver-type anti-resonant hollow-core fibers. The approach is independent of the actual arrangement of the anti-resonant elements, does not enforce artificial lattice arrangements and allows determination of the effective indices of modes of preselected order. The simulations show two classes of modes: (i) low-order modes exhibiting effective indices with moderate slopes and (ii) a high number of high-order modes with very strong effective index dispersion, forming a quasi-continuum of modes. It is shown that the mode density scales with the square of the normalized frequency, being to some extent similar to the behavior of multimode fibers.

Light guidance in photonic band gap guiding dual-ring light cages implemented by direct laser writing.

Efficient waveguiding inside low refractive index media is of key importance for a great variety of applications that demand strong light-matter interaction on small geometric footprints. Here, we demonstrate efficient light guidance in single-defect dual-ring light cages over millimeter distances that are integrated on silicon chips via direct laser writing. The cages consist of two rings of high aspect-ratio polymer strands (length 5 mm, aspect ratio >1000) hexagonally arranged around a hollow core. Clear-core mode formation via the photonic band gap effect is observed, with the experiments showing pronounced transmission bands with fringe and polarization contrasts of >20 dB and >15 dB, respectively. Numerical simulations confirm our experiments and reveal the dual-ring arrangement to be the optimal geometry within the light cage concept. Particularly, the side-wise access to the core regions and the chip integration makes the light cage concept attractive for a great number of fields such as bioanalytics or quantum technology.