Nonuniform thinned fiber Bragg gratings for simultaneous refractive index and temperature measurements

In this work, a novel method for simultaneous measurements of temperature and refractive index employing fiber Bragg gratings is presented. The principle of operation relies on the use of a single nonuniform thinned grating, where the cladding layer is partially or completely removed only in a part of the grating. The perturbation leads to a wavelength-splitting of the spectral response of the unperturbed structure in two separate peaks. The first one, corresponding to the thinned region, is sensitive to the local temperature and the surrounding refractive index changes, while the other one, related to the unperturbed part, would respond only to thermal changes. Here, wet chemical etching in a buffered hydrofluoric acid solution was used for sensor fabrication. Experimental characterization of the sensitivities to the external refractive index and temperature, varying in the range 1.33-1.45 and 15 degrees C-50 degrees C, respectively, is presented for a 7.6-mu m etched cladding sensor. The same approach can be efficiently used as key element for the development of sensors array for chemical sensing in practical applications.