The Evanescent Mode Coupling phenomenon between two identical fibres has been studied in devices made with three different techniques. All show the coupling constant K to be linearly related to wavelength λ. This is explained in a simple model. In one of them, the BiCore Fibre (BCF), the effects of loading, temperature, bending and twisting are discussed. The BCF is intrinsically insensitive to temperature, bending is a most useful tuning mechanism and loading leads to polarization-dependent sensor effects. Through the wavelength dependence the BCE is shown to act as a distributed sensor. The BCF can be a polarization splitter for a particular length, but twisting allows any length to be used to that effect. In combination with other fibre-optic components the BCF can be used to advantage in all-fibre optical instruments. A reflection polarimeter is shown as an example.