Transmission spectra of a double-clad fibre structure under bending

We have studied a fibre-optic structure whose operation relies on conversion of core and cladding modes that are coupled across the interface between two fibres differing in refractive index profile. The structure contains a section of an SM630 double-clad, small-core, single-mode fibre inserted between two SMF-28 standard fibres. We have measured the transmission spectrum of the structure when the SM630 fibre was bent and analysed the mode structure of the double-clad fibre and the origin of dips in its transmission spectrum. The resonance dips have been found to shift to longer wavelengths with increasing fibre curvature. We have evaluated the shift as a function of the length of the inserted fibre, its bend direction and the nature of the input fibre.