Resonator fiber optic gyroscope (RFOG) performance has hitherto been limited by nonlinearity, modal impurity, and backscattering in the sensing fibers. The use of hollow-core fiber (HCF) effectively reduces nonlinearity, but the complex interplay among glass and air-guided modes in conventional HCF technologies can severely exacerbate RFOG instability. By employing high-performance nested anti-resonant nodeless fiber, we demonstrate long-term stability in a hollow-fiber RFOG of 0.05 deg/h, nearing the levels required for civil aircraft navigation. This represents a improvement over any prior hollow-core RFOG and a factor of over any prior result at integration times longer than 1 h.

Today’s lowest-loss hollow core fibers are based on antiresonance guidance. They have been shown both theoretically and experimentally to have very low levels of backscattering arising from the fiber structure – 45 dB below that of traditional optical fibers with a solid silica glass core. This makes their longitudinal characterization using conventional reflectometric techniques very challenging. However, it was recently estimated that when filled with air, their backscattering coefficient increases to about 30 dB below that of standard solid core fibers. This level should be measurable with commercially available high performance optical time domain reflectometers (OTDR). Here we demonstrate – for the first time to the best of our knowledge – the measurement of backscattering from the air inside a hollow core fiber. We show that the characterization of multi-km long hollow core fibers with 15 m spatial resolution is possible using a commercial OTDR instrument. To benefit from its full dynamic range, we strongly suppress the 4% back-reflections that ordinarily occur at the OTDR’s standard fiber output when directly-connected to a hollow core fiber. Furthermore, low coupling loss into the hollow core fiber (0.3 dB in our experiment) also helps to maximize the achievable OTDR signal-to-noise ratio. This approach enables distributed characterization and fault-finding in low-loss hollow core fibers, a topic of increasing importance as these fibers are now starting to be installed in commercial optical communication networks.

Hollow-core fibers (HCFs) are a potentially transformative fiber technology, where light is confined within a hollow core surrounded by a cladding composed of air holes defined by glass membranes. Dramatic reductions in the minimum losses achieved in a HCF are driving forward their application in low-latency data transmission and ultra-high-power delivery, and maximizing their performance is of increasing interest. Here, we demonstrate that introducing an extremely small gas-induced differential refractive index (GDRI) between the gas within the core and cladding regions of a HCF enables dramatic changes to a HCF’s optical properties, including loss, bend loss, and modality. Within this work, we focus on a tubular HCF and demonstrate through experiment and simulations that the confinement loss of this fiber can be reduced by a factor of 5 using a differential pressure of only 6.7 bar. Understanding GDRI is critical for applications where the gas content within the fiber is actively controlled. Moreover, GDRI provides a new means to control the optical properties of a HCF post-fabrication, opening up new areas of design space and providing a tool to tailor and enhance the optical performance of even state-of-the-art HCFs.

Coupled-mode theory is a powerful tool to understand and control the effects of deployment and fabrication imperfections on optical waveguides. Although it provides many advantages compared to the finite element method, it still lacks the ability to treat geometric and material perturbations when they act simultaneously on the waveguide. This work fills this gap, providing a novel framework for a unified treatment of geometric and material perturbations in the coupled-mode analysis. The proposed approach consists of, first, applying the theory of transformation optics to convert geometric deformation into material perturbations and, second, studying the obtained waveguide by using a custom-developed coupled-mode theory able to deal with perturbations of both the permittivity and the permeability tensor. The framework is applied to three examples: a solid-core fiber affected by intrinsic perturbations, a bent solid-core fiber, and an elliptical hollow-core fiber. Results are validated against simulations based on the finite element method and compared with the standard coupled-mode theory most suitable for each specific example; they show that the proposed unified coupled-mode theory performs consistently better than standard theories, confirming it as a general and accurate tool for the design and analysis of optical waveguides.