I. Introduction
Hollow-core antiresonant fibres (HC-ARFs), such as nested antiresonant nodeless fibres (NANFs) and double nested antiresonant nodeless fibres (DNANFs), are state-of-the-art HCFs which combine the usual benefits of HCFs such as low nonlinearity, low dispersion, high damage threshold, near-vacuum speed of light [1] with a record low loss [2], [3]. These fibres are being deployed in various applications [4], and both control and long-term stability of their optical properties are becoming increasingly important. Here, the gas within the core and cladding regions of the fibre plays an important role.