Development of large core hollow core photonic bandgap fibres for telecommunications applications

Hollow core photonic bandgap fibres (HC-PBGFs) are emerging as contenders in the race to identify the best solution for next-generation high capacity transmission systems, owing to their low nonlinearity, ultimate-low signal latency, an ideal match to thulium-doped fibre amplifiers and to the promise to deliver ultralow loss, potentially comparable or even below the level of state-of-the-art single mode fibres. Recently, substantial progress in the development of HC-PBGFs has been reported; however, for these fibres to stand a realistic chance of competing with solid transmission fibres, the key challenges of loss reduction over a sufficiently wide transmission bandwidth, as well as the ability to produce fibres in multi-hundred kilometre lengths must be addressed. A predicted route to reduce loss is by increasing the core size, e.g. from 19-cell to 37-cell. In this work we review recent progress in the development of HC-PBGFs based on a 37-cell core geometry, including the realization of fibres with low loss, wide bandwidth, low modal crosstalk and low bending loss, which have paved the way to data transmission experiments using mode division multiplexing. We also investigate the issue of longitudinal uniformity in HC-PBGFs and report a systematic study of defects observed in these fibres - a first step towards their removal and thus towards the demonstration of multi-kilometre lengths of defect free fibres.