A practical hollow-core photonic crystal fiber design suitable for attaining low-loss propagation is analyzed. The geometry involves a number of localized elliptical features positioned on the glass ring that surrounds the air core and separates the core and cladding regions. The size of each feature is tuned so that the composite core-surround geometry is antiresonant within the cladding band gap, thus minimizing the guided mode field intensity both within the fiber material and at material / air interfaces. A birefringent design, which involves a 2-fold symmetric arrangement of the features on the core-surround ring, gives rise to wavelength ranges where the effective index difference between the polarization modes is larger than 10-4. At such high birefringence levels, one of the polarization modes retains favorable field exclusion characteristics, thus enabling low-loss propagation of this polarization channel.
Roberts, P. J., Williams, D. P., Sabert, H., Mangan, B. J., Bird, D. M., Birks, T. A., Knight, J. C., & Russell, P. S. J. (2006). Design of low-loss and highly birefringent hollow-core photonic crystal fiber. Optics Express, 14(16), 7329-7341. https://doi.org/10.1364/OE.14.007329