  The field energy distributions and effective mode areas of silica-based
photonic bandgap fibers with a honeycomb airhole structure in the cladding and
an extra airhole defining the core are investigated. We present a
generalization of the common effective area definition, suitable for the
problem at hand, and compare the results for the photonic bandgap fibers with
those of index-guiding microstructured fibers. While the majority of the field
energy in the honeycomb photonic bandgap fibers is found to reside in the
silica, a substantial fraction (up to ~30%) can be located in the airholes.
This property may show such fibers particularly interesting for sensor
applications, especially those based on nonlinear effects or interaction with
other structures (e.g. Bragg gratings) in the glass.
