Ray Optics Model for Light Attenuation in U-Bent Fiber Optic Sensors

Geometrically modified optical fibers have been widely exploited for realizing refractive index (RI) sensors. In particular, U-bent fiber optic sensors (FOS) have been reported to be efficient for chemical and biosensing applications. Experimental investigations show that the U-bent FOS are highly sensitive, when the bend diameter is less than 5 times the fiber core diameter. However, the phenomenon behind high RI sensitivity is yet to be fully understood. So far, geometric effects alone have been considered to significantly contribute to medium-sensitive refractive losses. In this work, a simple ray optics model is developed to evaluate the light propagation and attenuation in a U-bent FOS. These numerical simulations point to a significant contribution from modified core RI profile as result of bend induced inhomogeneity in the material towards high sensitivity of acutely bent probes.