A long-standing challenge in drawing microstructured fibers with a uniformity matching that of standard fibers, has been how to monitor the internal microstructure in real time during the draw. We developed a simple non-destructive optical method for measuring the diameter of individual capillaries in single-ring hollow-core fibres. Based on recording the spectrum scattered from whispering gallery modes excited in the capillary walls, the technique is highly robust, allowing real-time measurement of fiber structure during the draw with sub-micron accuracy. [Frosz (2019)].
The same technique is also ideal for non-destructively measuring the microstructure along a tapered hollow-core fibre. [Pennetta (2019)]
Finite-difference time-domain (FDTD) simulation of how light travelling through the cross-section (only one half shown) of a fibre excites whispering gallery modes in the galleries. It is seen how the light exiting the capillaries exits in a well-defined direction.