Birefringence enhancement in ZEBRA fiber with artificially anisotropic core

We report on development of all-solid single-mode ZEBRA fiber with engineered artificial anisotropy in the core. The fiber has the core in a form of glass slab with the layers of germanium-doped silica interleaved with the layers of fluorine-doped silica. Germanium increases refractive index (RI) of the silica glass, whereas fluorine decreases it, resulting in the RI contrast between the layers as high as Δn~24×10-3. Because the layers thickness was a fraction of operating wavelength (1/3*1550nm), the light propagating in the core experiences different effective RI in the directions perpendicular and parallel to the layers, which we defined as an artificial anisotropy of the core. At the same time average RI of the core is similar to one for SMF-28. We calculated birefringence induced by such a core structure obtaining B_sim=8.5×10-5. The measured birefringence of developed ZEBRA fiber is lower and equals to B=5.6×10-5, however is almost twice higher than for recently reported ZEBRA fiber with the lower RI contrast between the core layers (Δn~12×10-3). Summarizing, the increase of RI contrast of artificially anisotropic core layers is efficient for birefringence enhancement and has no impact on the ZEBRA fiber performance.