Experimental and scalar beam propagation analysis of an air-silica microstructure fiber

We study the higher order guided modes in an air-silica microstructure fiber comprising a ring of six large air-holes surrounding a Germanium doped core. We characterize the modes experimentally using an intra-core Bragg grating. The experimentally observed modes are then accurately modeled by beam propagation simulations using an index profile similar to the observed fiber cross section. Theory and experiment confirm the presence of "inner cladding" modes with approximate cylindrical symmetry near the core, similar to conventional cladding modes, but which strongly exhibit the symmetry of the microstructure at large radius. Such modes are useful in fabricating robust tunable grating filters and we show that the Bragg grating is a useful diagnostic to measure their effective indices and intensity profiles.