
the frequency separation of the surface-state intersections with the fundamental
guided mode) was also decreased.
All hybrid modes of a circular dielectric waveguide, including the fundamen-
tal HE
11
mode, have two orthogonal polarizations that travel at the same speed
(i.e., are degenerate), but at different speeds when the circular symmetry is
broken by imperfections. This phenomenon produces birefringence and polar-
ization mode dispersion (PMD). A very similar phenomenon occurs for PCFs,
but the analysis of the symmetry is much more complicated because of the
sixfold symmetry of the fiber structure. In particular, it might seem impossible
that one ...