Distributed measurement and modified Navier-Stokes model of gas pressure profile evolution in hollow-core antiresonant fibres

Recent progress in reducing the loss of hollow-core fibres (HCFs) makes them great candidates for many fibre applications. However, as the fibre's optical properties depend on the gas pressure and composition within the core and cladding holes, it is essential to understand the gas dynamics at play when the fibres are pressurised, vented or evacuated. Here, we investigate the gas flow dynamics along the core of an HCF with a more complex microstructure design, as is typical of recent state-of-the-art HCFs. We use a novel distributed technique based on optical time-domain reflectometry (OTDR). This technique enables measurement of the evolution of the pressure distribution within the hollow core during the gas-filling process over long fibre lengths. Using these results, we show that the pressure distribution inside the HCF can be simulated using simplified Navier-Stokes equations and approximating the fibre core as a simple cylindrical tube of $\sim$ 0.7 times the fibre core diameter.