  Geometrically modified fiber optic sensors (FOS), particularly U-bent FOS,
have gained significant attention due to their remarkably high refractive index
(RI) and evanescent wave absorbance (EWA) sensitivity, as well as their
ergonomic design and ease in handling. In this study, we present a theoretical
model for the U-bent FOS probes, to predict the sensor behavior by numerically
simulating the light propagation in an equivalent 2D semi-circular ring using
ray tracing approach. In addition to the effects due to the modification of
geometry, this study presents a thorough investigation of the influence of the
bend-induced material deformation on the nature of light propagation and
refractive losses. We introduce bend ratio (ratio of bend radius to fiber core
radius), to explain the influence of geometry modification and the bend-induced
inhomogeneity in RI (BIRI) of the fiber core on RI sensitivity. The bend ratio
of bent plastic optical fiber sensors falls under one of the four bending
regimes namely, gentle, geometric, saturation and plastic, for which the bend
ratios are less than 35, 25, 17 and 7 respectively. The results also show that
for bend ratios less than 7, BIRI inhomogeneity is responsible for the high RI
sensitivity observed with U-bent probes as opposed to the simple geometric
modification. This study also indicates the existence of an optimum bend ratio
(for a given value of RI of the surrounding medium) where RI sensitivity is
maximum. These findings were validated with previously reported experimental
results.
