Evanescent wave absorption in U-bent fiber optic sensors—a ray-optics model for evanescent power and experimental validation

U-bent fiber optic sensors (FOS) with fiber core diameters (2<i>ρ</i>)<1<i>m</i><i>m</i> and bend diameter <10<i>ρ</i> are known to exhibit remarkably high evanescent wave absorption sensitivity. This study proposes, to our knowledge, a novel ray-optics-based model for estimation of the evanescent power availability by considering the often-neglected effects of the reduction of propagation width of the light in the U-bent region as well as its dependence on the bend-induced refractive index inhomogeneity of the fiber core. This model presents a simpler expression for the optical power ratio (<i>P</i>_{<i>c</i><i>l</i><i>a</i><i>d</i>}/<i>P</i>_{<i>t</i><i>o</i><i>t</i><i>a</i><i>l</i>}) as a ratio of the depth of penetration of the evanescent field to the width of propagation of light within the bent fiber core. The model was validated by comparing the numerical simulation results for the U-bent FOS geometries having various combinations of the fiber core and bend diameters with experimental investigations.