Theoretical Model and Design Considerations of U-Shaped Fiber Optic Sensors: A Review

A wide range of geometrical deformations of fiber optic sensors (FOSs) have been explored to improve their performance. In particular, fiber optics bent into a U-shape present various advantages over other approaches and considerably improved performance in terms of sensitivity. The improvement in sensitivity can be understood as a result of the enhanced penetration depth of the evanescent field and higher evanescent power in the cladding or surrounding medium. Consequently, a larger portion of the propagating light experiences changes due to variations in the local refractive index (RI). In view of their significant benefits, U-shaped FOSs deserve a thorough investigation. This review outlines the theoretical model and design considerations along with the fabrication process of U-shaped FOSs. In addition, this paper aims to provide researchers with guidelines on the factors to consider in designing and optimizing U-shaped FOSs.