Nondestructive determination of the core size of a hollow-core photonic bandgap fiber using Fabry–Perot interference

In this Letter, we propose a simple and nondestructive method for the determination of the core size of a hollow-core photonic bandgap fiber (HC-PBF) and its axial uniformity based on a Fabry–Perot cavity induced by a pair of opposite silica–air interfaces within the hollow core. The experimental results indicate that the core size test of the HC-PBF has a nanometer-level precision, and its axial uniformity test has an ultimate spatial resolution of tens of microns. The method provides an effective and precise tool for the investigation of the hollow-core size and its longitudinal evolution.