Significant progress has been made in recent years in driving down the loss of hollow core fibres (HCFs), such as photonic bandgap fibres (PBGF), antiresonant nodeless fibres (ANF), and nested antiresonant nodeless fibres (NANF) towards that of their conventional solid single mode counterparts. In order to achieve the ultimate performance of HCFs, an accurate control of the dimensions and symmetry of the internal constituent elements is key in both the assembled preforms and drawn canes and fibres. Non-destructive optical methods to monitor and assess the internal features of HCFs over the entire fabrication process are therefore of great interest. Previously, morphology dependent resonances  and whispering gallery modes (WGMs) , excited by side illumination of the fibre, have been used to enable the precise measurement of cladding diameter variations with an accuracy greater than that of competing interferometric approaches. Motivated by this earlier work we have started studies into the opportunities provided by side illumination of HCF canes and fibres being drawn, and report some of our initial observations, which we hope will ultimately provide useful information on the internal features of HCFs.
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