Polarization evolution in single-ring antiresonant hollow-core fibers.

Understanding polarization in waveguides is of fundamental importance for any photonic device and is particularly relevant within the scope of fiber optics. Here, we investigate the dependence of the geometry-induced polarization behavior of single-ring antiresonant hollow-core fibers on various parameters from the experimental perspective, showing that structural deviations from an ideal polygonal shape impose birefringence and polarization-dependent loss, confirmed by a toy model. The minimal output ellipticity was found at the wavelength of lowest loss near the center of the transmission band, whereas birefringence substantially increases toward the resonances. The analysis that qualitatively also applies to other kinds of hollow-core fibers showed that maximizing the amount of linearly polarized light at the fiber output demands both operating at the wavelength of lowest loss, as well as carefully choosing the relative orientation of input polarization. This should correspond to the situation in which the difference of the core extent along the two corresponding orthogonal polarization directions is minimal. Due to their practical relevance, we expect our findings to be very important in fields such as nonlinear photonics or metrology.