We propose a method based on the white-light interference technique for measuring the Verdet constant of a polarization-maintaining air-core photonic bandgap fiber (PM-PBF). The experimental results show that the Verdet constant of the PM-PBF is ~3.3 mrad/T/m for the broadband light with a spectral width of ~38 nm and a mean wavelength of ~1550 nm, which is ~124 times less than that of a conventional stress-induced birefringent fibers called PANDA fibers (~0.41 rad/T/m for the same broad-spectrum light). The results indicate that the nonreciprocal error induced by the Faraday effect in a fiber optic gyroscope (FOG) made of the PM-PBF is theoretically ~25 times less than that of a conventional FOG made of the PANDA fiber when other conditions, such as the fiber twist, fiber coil area, and so on, are the same.

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.

Second-order backscattering (SOBS) error is an important source of bias error in closed-loop hollow-core photonic crystal fiber (HC-PCF) gyroscopes, but its generation mechanism and measurement method have not been fully clarified. We establish a SOBS-error model and analyze its dependence on the square-wave and step-wave modulation phases introduced through the Y-branch waveguide, as well as on environmental phase fluctuations. Simulations show that the SOBS error exhibits two periods of variation as the step-wave phase varies from 0 to π. We further propose a dual-fiber-coil interference method for experimentally measuring the SOBS error. For a 260-m-long, 140-mm-diameter HC-PCF coil, the measured SOBS-error amplitude is 0.453 °/h, providing an experimental reference for the SOBS-error magnitude under the tested conditions.