the lattice sites embodiments of this invention use graded index grin inclusions for this purpose step index rods can also be used the grins have an index that is higher than the background glass and support true guided modes when surrounded by the depressed lattice they may also support modes with an effective index lower than the background glass but higher than the sfm and like the core modes these may have an arbitrarily small loss however the effective index of their modes decrease towards that of the sfm with increasing wavelength and crucially any higher order grin mode will be cutoff lose guidance at some wavelength the modes of a grin that is only partially surrounded by the lattice will experience high losses if its effective index is lower than the background glass by choice of the parameters of the lattice the size of the core and the size and index of the grin the core and grin can be arranged to be resonant at some wavelength at this point their modes will hybridize and the optical field becomes delocalized amongst these two parts of the structure the grin inclusions are arranged as a channel from the pcf core to the edge of the lattice so that when the core and.