A novel single-polarization single-mode hollow-core photonic bandgap fiber with thin slab waveguide (TSW) was designed and simulated. Single-polarization guidance is achieved by the high loss of a polarized fundamental mode (FM) induced by mode coupling with a higher-order TE/TM mode of TSW and low loss of another polarized FM. We achieve a polarization loss ratio ∼ 46.9 dB, birefringence Δn ∼ 2.4 × 10⁻⁴, loss ∼ 35.9 dB/km and minimum higher-order mode extinction ratio > 15 dB. Moreover, the performance could be maintained when the guidance wavelength λ = 1.44 ∼ 1.56 µm and bending radius r_c > 9 mm. The proposed model will be suitable for application as resonator sensing paths of miniaturized resonator fiber optic gyroscopes, high-performance interferometers, fiber lasers, frequency metrology, quantum communications, and laser-based gas sensing, etc.

Hollow-core nested anti-resonant nodeless fibers (HC-NANFs) exhibit great performance in low loss and large bandwidth. Large core sizes are usually used to reduce confinement losses, but meanwhile, bring side effects such as high bending and coupling losses. This study proposes a small-core HC-NANF with a relatively low confinement loss. Semi-circular tubes (SCTs) are added to constitute the core boundary and reduce the fiber-core radius (R). Double NANFs tubes and single-ring tubes are added inside the SCTs to reduce loss. Simulation results show that the optimized structure with R of 5 µm has confinement loss and total loss of 0.687 dB/km and 4.27 dB/km at 1.55 µm, respectively. The bending loss is less than 10 dB/km at 1.4 ∼ 1.6 µm with a bending radius of 10 mm. The direct coupling loss with standard single mode fiber is greatly reduced to ∼ 0.125 dB compared to other HC-NANFs. The modified structure of HC-NANFs also shows a large bandwidth, effective single-mode operation, potentially high birefringence performance, and remarkable robustness of the optimized structure parameters, making it suitable for short-haul applications in laser-based gas sensing, miniaturized fiber sensing, etc.

A hollow-core antiresonant fiber (HC-ARF) with nested supporting rings (NSRs) is designed and simulated. The HC-ARF with NSRs has advantages and benefits of low loss, large bandwidth, simple structure and a well bending characteristic, in which confinement loss (CL) is ∼ 0.15 dB/km @ 1.55 µm and the bandwidth is ∼ 220 nm @ CL < 1 dB/km. The bending loss (BL) is lower than ∼ 1 dB/km @ bend radius r_c > 24 mm at 1.55 µm. Therefore, the HC-ARF with NSRs has potential applications of data transmission, sensing, high power delivery and so on.

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.

Hollow-core photonic bandgap fibers (HC-PBFs) demonstrate exceptional performance with high damage threshold, low nonlinearity, low thermal sensitivity and ultralow bending loss. However, the spatial efficiency of HC-PBFs is constrained by the absence of the multi-core structure. In this study, we present a detailed proposal and design of multi-core HC-PBFs for the first time. The structure model of multi-core HC-PBFs are constructed and the structure parameters are optimized by the loss, mode, coupling, bending characteristics based on offset-core, scaling-core, and dual-core three structure models. Finally, three kinds of multi-core HC-PBFs structures including homogeneous weakly-coupled, heterogeneous weakly-coupled, and strongly-coupled are designed with comprehensive structure and performance parameters. Simulation results indicate low loss, quasi single-mode, low crosstalk, negligible bending loss, and remarkable multi-core guiding performance. The introduction of a multi-core structure expands the potential applications of HC-PBFs in fiber communications and long-haul miniaturized fiber optic gyroscopes, high power delivery, fiber sensing, etc.

Hollow-core antiresonant fibers (HC-ARFs) have surpassed the minimum transmission loss of traditional silica single-mode fibers. However, this breakthrough comes at the cost of heightened structural complexity, elevated fabrication challenges. Further loss reduction demands innovative structural strategies. In this work, we propose azimuthal-confinement-enhanced HC-ARFs by integrating curved boundaries and radial walls to amplify both boundary curvature effect (BCE) and azimuthal confinement effect (ACE). Three models—optimized radial spokes, single antiresonant silica layer, and grapefruit-shaped structures—are designed and categorized into ideal (no silica jacket layer) and practical (with silica jacket layer) types. Numerical simulations reveal that optimizing dimensionless parameters f_θ and f_r, corresponding to BCE and ACE respectively, achieves a 3–4 order-of-magnitude reduction in confinement loss (CL) for ideal-type fibers (minimum CL: 2.57 × 10⁻⁶ dB/m) and a 16-fold reduction for practical-type fibers compared to conventional designs. Critical tolerances for wavelength, silica wall thickness, and core radius are identified, ensuring robust low-loss operation. Additionally, the optimized structures exhibit enhanced minimum higher-order mode extinction ratio while maintaining bending loss and material absorption loss comparable to traditional HC-ARFs. This work establishes a paradigm for low-structural-complexity, high-performance low-loss HC-ARFs beyond nested-tube architectures, advancing their applicability across near-infrared to terahertz regimes.