Measurements of birefringence and its sensitivity to hydrostatic pressure and elongation in photonic bandgap hollow core fiber with residual core ellipticity

We investigated polarization properties of the photonic bandgap hollow core fiber operating in the first communication window. Using specially developed interferometric method, we measured dependence of phase and group modal birefringence upon wavelength in a wide spectral range. Our results show that the phase birefringence (Δn) is of the order of 10−5, depends parabolic like upon wavelength, and reaches minimum value in the vicinity of the bandgap center. The group modal birefringence (ΔN) decreases monotonically vs. wavelength from positive to negative value and crosses zero near the bandgap center. Due to high chromatic dispersion of the phase modal birefringence, the absolute value of ΔN is one order of magnitude higher than Δn at the bandgap boundaries. We also determined the susceptibility of the phase and the group modal birefringence to strain and hydrostatic pressure in the whole analyzed spectral range.