We study the phenomenon of deliberate inter-modal interactions in a specially designed index guided Microstructured Optical Fiber (MOF) by exploiting multipole expansion method (White et al. 2002). The MOF is designed in such a way that the first layer of holes is judiciously filled with a material having refractive index slightly greater than the background material or core and remaining holes are filled with air. Accordingly, we find an interesting phenomenon of mode crossing between the fundamental mode and a targeted defect mode while tuning the wavelength. Exploring this transition wavelength of the mode crossing, we propose a design of a fiber optic sensor for refractive index measurement (Silva et al. 2014) with enhanced sensitivity. © 2017 Springer-Verlag GmbH Germany, part of Springer Nature