# 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.