Grating spectra in air-silica microstructure fibers

We report detailed spectral characterization of optical gratings written in different air-silica microstructured optical fibers. Our data reveal the propagation of higher order leaky modes in fibers with periodically spaced air-holes (photonic crystal fibers). As the air-hole diameter increases (i.e., mode ceases to penetrate the inner holes) spectra reveal cladding modes defined by the inner cladding only