Numerical and experimental analysis of the birefringence of large air fraction slightly unsymmetrical holey fibres

TL;DR

This study combines numerical simulations and experiments to analyze how slight geometrical imperfections in large air-fraction holey fibres cause significant birefringence, with results showing minimal influence from internal stress.

Contribution

It provides the first detailed numerical and experimental analysis linking small imperfections to high birefringence in large air-fraction holey fibres.

Findings

High birefringence induced by minor imperfections

Spectral birefringence laws similar to elliptical core fibres

Inner stress has negligible effect on birefringence

Abstract

Careful numerical computations show that very slight geometrical imperfections of the cross-section of actual large air-fraction holey fibres (d/{\Lambda} > 0.6) may induce surprisingly high birefringence, corresponding to beat lengths as short as few millimeters. The spectral variations of this birefringence obeys laws similar to those of elliptical core Hi-Bi holey fibres with low air-fraction. For all the tested fibres, the group birefringence numerically deduced from the only shape birefringence is in good agreement with the measured one that does not varies when strongly heating the fibres. These computations and measurements show that the contribution of possible inner stress to the birefringence is negligible.