Model of a Fiber-Optic Evanescent-Wave Fluorescence Sensor

Abstract

A fiber-optic cure sensor based on evanescent-wave fluorescence spectroscopy has been designed to probe the interphase region of glass-reinforced composites. The size of the interphase can vary significantly, depending on the exact nature of the fiber surface treatment and its interaction with the surrounding resin matrix. In this study, a model based on mode coupling theory is developed to determine whether the fiber provides information about the bulk resin or the interphase between the fiber and the matrix. Model predictions include the fraction of fluorescence signal collected as a function of radial distance from the optical fiber surface. In all cases the refractive index of the fiber is 1.617, the value measured for the fiber used in experiments, while the refractive index of the resin varies from 1.56 to 1.615, values representative of curing epoxy resin. In addition, the penetration depths of the excitation modes are analyzed as a function of the mode group number, as well as the root number, for two resin refractive indices. The dependence of the fluorescence signal on selected excitation and fluorescence modes is also investigated. Finally, we examine the role of coupling optics by analyzing the relationship between penetration depth and the angle between the excitation beam and fiber axis.