Fiber optic sensors represent an attractive alternative in chemical, bio-chemical and medical applications. Their success can be retrieved in their peculiar properties such as: electromagnetic interference immunity, fast response, high sensitivity, and small size. In this context, Fiber Bragg Gratings (FBGs) play a key role in applications like measurements of temperature and strain. The mechanism of FBGs is related to the dependence between the characteristic wavelength reflected by the FBG and the effective index of the modes propagating inside the fibers. This property can be exploited to engineer a new and inexpensive class of FBG devices for measuring refractive index of solutions. By reducing the cladding thickness of the single mode fiber, where the FBG is inscribed, the structure becomes three layers and the modes guidance properties become more and more dependent on the external environment, including the refractive index of the solution to measure. In this work, an FBG has been etched by a solution of HF acid and immersed in different solutions of water and sucrose. Results show a strong multi-modal behavior induced by the guidance properties of the three-layer system. The reflected spectra, characterized by a large band whose width, are strictly dependent on the sucrose concentration in solution. As the sucrose increases, the refractive index of the solution increases. The bandwidth reduces, showing a wavelength shift toward longer wavelength. While the wavelength shift is not so relevant the variation of the bandwidth is significant, suggesting an effective interrogation method based on wavelet signal processing.