Experimental Assessment of the Accuracy of an Advanced Photonic-Bandgap-Fiber Model

From scanning-electron microscope images of the cross section of a photonic-bandgap fiber (NKT Photonics´ HC-1550-02) we developed a realistic model for its permittivity profile that includes all observable structural deformations in the core and in the first two rows of cladding holes. Using this more accurate index profile in our C++ full-vectorial finite-difference mode solver, we numerically studied this fiber´s modal dispersion, along with the intensity profile, group index spectrum, and group-velocity dispersion spectrum of its fundamental mode. Comparisons between these predictions and their experimental counterparts measured in the fiber show good quantitative agreement for all these characteristics. On the other hand, when these structural deformations are purposely not included in the permittivity profile, the predicted and measured characteristics generally poorly match. The study demonstrates that first, accurate simultaneous predictions of several key modal characteristics of hollow-core fibers can be obtained numerically, and that although small, the aforementioned index-profile perturbations must be included in order to obtain sufficient accuracy.