Abstract
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.
