MCF can be used for many applications such as efficient fluorescence signal collection, spectral filters and wavelength division multiplexing. Changing the optical or MCF geometrical parameters will influence the effective refractive index of the coupled modes and therefore will lead to variations in the coupling coefficient and the overall response of the coupled system. Thanks to the underlying dependence of the coupling coefficient on the external parameters which make the MCF appropriate for sensing applications. Low Xt is necessary for present transmission technology.