Self temperature referenced refractive index sensor by non-uniform thinned fiber Bragg gratings

In this work, the use of non-uniform thinned fiber Bragg gratings (ThFBGs) for self temperature compensated refractive index measurements is proposed. The multi-parameter sensor consists of a standard Bragg grating where the cladding layer is partially or totally removed along half of the grating length. The perturbation leads to a wavelength-splitting of the spectral response in two separate peaks: the peak at lower wavelengths corresponds to the thinned region and is dependent on the outer refractive index and the local temperature, while the peak at longer wavelength would respond only to thermal changes. The simultaneous measurements of the Bragg wavelengths related to the different grating regions allow the accurate measurements of the refractive index and the temperature by using a single sensing element. Here, a simple and cost effective fabrication technique based on wet chemical etching in a buffered hydrofluoric acid (HF) solution and a proper package were used to realize the sensing probe. Experimental characterization for a 8 μm thinned cladding sensor is reported in terms of thermal and refractive index sensitivities. To prove the sensor capability to be used as self temperature referenced refractometer, sugar concentration measurements have been carried out in non-isothermal conditions.