Distributed Measurement of Hollow-Core Fibre Gas Filling and Venting via Optical Time-Domain Reflectometry

The gas pressure and composition within a hollow core optical fibre (HCF) are important across diverse application scenarios. For example, it has been shown that the gas pressure inside HCFs immediately after fabrication is sub-atmospheric [1]. When such an ‘as-drawn’ HCF is exposed to ambient conditions, the air penetrates the core and cladding holes at different rates (due to their different cross-sectional areas), temporarily creating a pressure difference and therefore a transient, gas-induced differential refractive index (GDRI) [2] between the core and cladding holes that modifies the fibre's optical properties. A transient GDRI also occurs when an HCF is intentionally pressurised with gas [2]. GDRI can therefore impact HCF based gas-sensing and purging (unwanted gas species removal), and is an important consideration in as-drawn fibre characterisation. So far this effect was studied via integrating techniques, i.e. measuring the change of the overall fibre transmission over time, without tracing the process along the fibre. However, to fully map the gas flow dynamics and enable comprehensive comparison with gas flow modelling further characterisation techniques are needed. Here, we present a distributed method of observing gas flow dynamics in HCFs via optical time-domain reflectometry (OTDR).