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In this paper, numerical and experimental analysis for high-resolution refractive index measurement based on micro-structured fiber Bragg gratings is presented. The structures investigated rely on a partial and localized etching of the cladding layer along the grating region. The main effect of the perturbation is the increasing of the stopband and the formation of defect states inside the bandgap. The form and the position of the defect state inside the bandgap are strongly dependent on the surrounding refractive index and the etching features. Numerical results reveal the possibility to carry out refractive index measurements by monitoring of allowed wavelength shift within the bandgap or monitoring the reflected power using a narrow bandwidth interrogation system. Finally, the experimental results, in according to the theoretical analysis, are presented.