Study on the properties of the normalized reflectivity of fiber Bragg grating based on evanescent wave

Based on the optical waveguide theory and the FBG mode coupling theory, a rigorous analysis of the reflectivity of the fiber Bragg grating (FBG) with different diameters were presented. In conditions of cladding etched and core etched, the power attenuation properties of the FBG based on evanescent wave were analyzed. The normalized reflectivity of the FBG, which was the function of the surrounding refractive index (SRI) and the diameter of the FBG, was derived. The theoretical results exhibited that the normalized reflectivity become smaller as the increase of the SRI, and the deeper the FBG was etched, the smaller the normalized reflectivity was. Thus, we can make the conclusion that both the increase of the SRI (small than the cladding refractive index) and the etched depth can reduce the intensity of light propagating in the fiber and enhance the interaction between the evanescent wave and the surrounding medium, leading the sensitivity of the sensor to increase. In addition, theoretical simulation showed good consistency with the experimental characterization. The proposed study of the normalized reflectivity properties of FBG based on evanescent wave is significant in improving the sensitivity of FBG.