Non-destructive characterization of nested and double nested antiresonant nodeless fiber microstructure geometry

Antiresonant hollow-core fibers (HCFs) are rapidly establishing themselves as a promising technology with the potential to overcome the limitations faced by conventional solid-core silica fibers. The optical properties and performance of these fibers depend critically on the precise control and uniformity of their delicate glass microstructure at all points along the length of the fiber. Their fabrication is complicated by the inability to monitor this microstructure without cutting into the fiber and viewing a sample under a microscope during the fiber draw. Here we show that a non-destructive interferometric technique using side-illumination of the fiber and first demonstrated for simple tubular fibers can be used to measure the diameters of all nested capillary elements of two promising HCF designs: the nested and double-nested antiresonant nodeless fiber (NANF and DNANF, respectively) with accuracy comparable to a microscope measurement. We analyze the complexities enabled by the presence of multiple nested capillaries in the structure and present techniques to overcome them. These measurements, carried out on a small (similar to 50 cm) length of fiber, require less than 60s to collect and process the data for all capillaries. We also show how we can use this technique to detect defects in the fiber, making it a potential candidate for real-time in-situ monitoring of NANF and DNANF structures during fabrication.