4. Photonic Bandgap Fibers

Photonic bandgap (PBG) fibers guide light by a mechanism distinct from both antiresonance and total internal reflection: a two-dimensional periodic lattice in the cladding opens a photonic bandgap, a range of effective indices for which light cannot propagate transversely through the lattice. A defect at the center of the lattice, formed by omitting or altering some of the periodic elements, then supports modes whose effective indices fall inside the gap, confining them even though the defect’s own index need not exceed its surroundings. We now examine leaky modes of such fibers with the PBG class, starting with a six-ring lattice of high-index strands and then a tighter, single-ring lattice of high-index rods.