Fig. 4. Optical properties of the modes guided in twisted coreless PCF.
(A) Upper: Calculated refractive indices (left axis) and propagation losses (right axis) for the left circular (LC) (dashed curves) and right circular (RC) (solid curves) polarized modes, plotted against twist rate α. Lower: Measured (data points) and calculated (curves) MFDeff in a coreless PCF with six hexagonal layers of hollow channels (normalized to the interhole spacing Λ), plotted against α at a wavelength of 818 nm. LC (blue dashed line) and RC (red dashed line) polarized modes gave identical results. The saturation in MFDeff at small α (gray-shaded area) is an artifact of the finite size of the photonic crystal. In a 10-layer structure, it saturates at a higher value (dotted gray curve). Note the highly anomalous inverse relationship between MFDeff and refractive index. (B) Experimental (data points) and theoretical (curve) group velocity dispersion of the twist-induced mode, together with the experimental far-field pattern, for α = 1.26 rad/mm. Note that the calculated dispersion of the fundamental space-filling mode (dashed line) is quite different, confirming that the modes form at the bottom of the passband. (C) Experimental (data points) and theoretical (curve) circular birefringence BC as a function of α at 818 nm (left axis). Normalized intensities of the −3 (dashed) and +3 (dash-dotted) harmonics of the mode with OAM order ℓ in the first ring of the fiber as a function of α (right axis).