Design of weakly guiding Bragg fibres for chromatic dispersion shifting towards short wavelengths

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Abstract
The waveguiding in Bragg optical fibres is based on a radial photonic bandgap effect due to their periodic cladding. Even with a weak index contrast, these fibres can be used to strongly bind the electromagnetic field in the core. With an adequate index profile, the resulting field localization allows us to shift the zero-chromatic-dispersion wavelength of the unique mode below 1.3 µm, i.e. into a spectral range where nonlinear phenomena such as solitons or continuum generation become possible, in contrast to standard monomode fibres. A preliminary analysis, performed with the planar Bragg waveguide for simplicity, gives conditions to obtain a fast decrease of the field distribution in the periodic area. These conditions are then used to develop a simple method for determining an adequate geometry of weak-guiding fibres to achieve dispersion shifting. An approximate formula is given for index transition positions in the case of step index profiles. Examples of such profile determinations are presented. The possibility of reducing the zero-dispersion wavelength to 0.8 µm by means of a realistic index profile is proven. The validity of the method is confirmed by first measurements on an experimental Bragg fibre.