Simultaneous measurements of refractive index and temperature by non-uniform thinned fiber Bragg gratings

Abstract

Abstract: In this work, the use of non uniform thinned Fiber Bragg Gratings (ThFBGs) for the simultaneous measurements of temperature and refractive index is proposed. The proposed technique relies on the use of partial thinned FBG: along half of the grating length the cladding layer is partially or totally removed, while the remaining part is unchanged. The perturbation leads to a wavelength-splitting of the spectral response of the unperturbed structure in two separate peaks in dependence of the outer refractive index and the local temperature. The peak at lower wavelengths corresponds to the etched region and is dependent on the outer refractive index and the local temperature, while the peak at longer wavelength is related to the original structure and thus is dependent only on the local temperature. The simultaneous measurements of the Bragg wavelengths related to the two regions allow the accurate measurements of the refractive index and the temperature by using a single sensing element, simultaneously. Here, wet chemical etching in a buffered hydrofluoric acid (HF) solution was used for sensor fabrication. Experimental characterization for a 7.6 m etched cladding sensor is presented. The same approach can be used as key element for the development of sensors array for chemical sensing in practical applications.