Drill-and-Draw: Microstructured Hollow-Core Optical Fibres from Laser-Drilled Fused Silica Preforms

Hollow-core fibres are currently undergoing a revolution, most recently demonstrated with losses reaching below those of the best telecom step-index fibres [1]. The light is guided in the core surrounded by anti -resonant cladding elements. These are typically circular, as the preform (from which the fibre is drawn in multiple steps) is made by stacking round glass capillaries together into the desired structure, hence the technique is termed stack-and-draw. To reduce the losses, capillaries can be nested within each other by fusing them to the jacket. However, several studies indicate that circular cladding elements are not necessarily ideal, and have explored elliptical, or even stadium-shaped cladding elements [2], [3]. Such complex structures cannot be made by the stack-and-draw technique. Laser assisted etching of preforms was recently demonstrated for making a non-stackable solid-core structure [4], but the method is currently limited to making 6 mm thick preform discs, of which 50 were welded on top of each other to achieve a preform long enough for fibre drawing; the welding led to problems with fluctuating outer diameter, preform weakening, and bubble formation around the welded regions. Another approach is inverse laser drilling, which has been used to make solid-core preforms of 80 mm length [5], limited by the accumulation of dust particles inside the relatively small air-holes (< 1 mm in the preform) during the drilling.