  We consider an air-silica honeycomb lattice and demonstrate a new approach to
the formation of a core defect. Typically, a high or low-index core is formed
by adding a high-index region or an additional air-hole (or other low-index
material) to the lattice, but here we discuss how a core defect can be formed
by manipulating the cladding region rather than the core region itself.
Germanium-doping of the honeycomb lattice has recently been suggested for the
formation of a photonic band-gap guiding silica-core and here we experimentally
demonstrate how an index-guiding silica-core can be formed by fluorine-doping
of the honeycomb lattice.
