Modification of cladding-mode fields upon fibre loading with H2 and its influence on the spectral characteristics of long-period gratings

The influence of diffusion of molecular hydrogen used to write fibre gratings on the spectral characteristics of the gratings is discussed in detail. It is shown that the additional waveguide structure formed by the concentration profile of dissolved molecular hydrogen modifies the spatial distribution of the field of fibre cladding modes, resulting in a considerable change in the coupling coefficient of long-period gratings. This modification is manifested most strongly for cladding modes with low radial indices.