Fig. 1. Dual-core PBGFs with core separations D_c of (a) 2Λ and (b) √3Λ.

Fig. 2. Photonic DOS plotted against normalized frequency kΛ and effective index n_eff for the cladding of Fig. 1. The bandgaps (DOS = 0) are in red, with non-zero DOS in the bands represented on a grayscale (light for high DOS and dark for low DOS). Also marked are the modes of the rods from which the bands are formed [19].

Fig. 3. Effective indices of the y-polarized even and odd supermodes in the (a) second and (b) third bandgaps, for both core separations D_c .

Fig. 4. Coupling length L_c against frequency in both bandgaps, for both core separations and polarizations of light.

Fig. 5. The dominant E_y field distribution for the y-polarized core-guided mode in a single-core PBGFs near the four band edges (a) kΛ=36, (b) kΛ=44, (c) kΛ=56, and (d) kΛ=64, over a quarter area (the rest is given by reflection symmetry). Field amplitude is represented by the scale on the right; to show the field distribution in the rods, the field in the guiding core is saturated. The labels 2Λ and √3Λ indicate a rod that could be removed to form the respective dual-core structures of Fig. 1, together with the corresponding lines of symmetry. The inset in each figure is the field of the LP _lm rod mode forming the adjacent band edge, as would appear in the rod on which the symmetry axes have been marked in the main figure. The rod mode is therefore oriented to be symmetric about the line from the centre of the PBGF’s core.

Fig. 6. Field distributions in the single-core PBGF along the line joining the center of the core and the centre of the rod that would be omitted to form a D_c = 2Λ structure, for frequencies in the middle of the two bandgaps. (The fields vary only in detail across each bandgap.) Also marked is where the second core would lie.

Fig. 7. (a). The refractive index profile of the high index Ge-doped rod. Optical micrograph of the fibers for (b) D_c = 2Λ, (c) D_c = √3Λ. The outer diameter of the fibers was 210 μm.

Fig. 8. (a). Transmission spectra of D_c = 2Λ fiber in the 2^nd and 3^rd bandgap. (b) Normalized intensity of (a) in the 2^nd bandgap. Fiber length is 185 mm.

Fig. 9. Coupling length as a function of normalized frequency in the fibers shown in Fig. 6. Solid curves represent the numerical results and dots represent the experimental results.