Theoretical and experimental study of refractive index sensors based on etched fiber Bragg gratings

In this work, a theoretical and experimental study of the effect of the etching process on the properties of fiber Bragg gratings (FBGs) used as refractive index sensors is presented. The theoretical study addresses the dependence of the effective refractive index on the cladding thickness of the etched FBG. The results of this study show that as the cladding thickness is reduced, the effective refractive index decreases exponentially. Based on this result, a simple analytic expression between the effective refractive index and the radius of the etched FBG was developed. Thus, the radius of the FBG after the etching process can be determined directly from the shift of the Bragg wavelength, without the need of extensive numerical simulations. Also, the sensitivity of the FBG sensor in characteristic environments of practical interest has been calculated as a function of the fiber radius, and found to be described by simple analytic functions. The time evolution of the Bragg wavelength during the etching process has also been investigated, both experimentally and theoretically. By means of detailed theoretical analysis of the experimental results, the heating and the etching rate of the FBG was calculated. Finally, the etched FBGs have been used as liquid level sensors for water and oil, having refractive indices lower and higher than the fiber core respectively. The analysis of the experimental results was based on the shift of the diffracted wavelength in the case of water and on the reduction of the reflected power in the case of oil. (C) 2014 Elsevier B.V. All rights reserved.