Model of a fiber-optic evanescent-wave fluorescence sensor

A fiber-optic cure sensor based on evanescent-wave fluorescence spectroscop y has been designed to probe the interphase region of glass-reinforced comp osites. The size of the interphase fan vary significantly, depending on the exact nature of the fiber surface treatment and its interaction with the s urrounding resin matrix. m this study, a model based on mode coupling theor y is developed to determine whether the fiber provides information about th e bulk resin or the interphase between the fiber and the matrix. Model pred ictions include the fraction of fluorescence signal collected as a function of radial distance from the optical fiber surface. In all cases the refrac tive 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 penetrat ion depths of the excitation modes are analyzed as a function of the mode g roup number, as well as the root number, for two resin refractive indices. The dependence of the fluorescence signal on selected excitation and fluore scence modes is also investigated. Finally, we examine the role of coupling optics by analyzing the relationship between penetration depth and the ang le between the excitation beam and fiber axis.