Numerous methods have been reported to further enhance the sensitivity of U-shaped fiber optic sensors towards external refractive index variation. This paper introduces a variant U-shaped fiber structure, which is inspired by the U-shaped and twisted fiber configurations. The U-shaped passive-active fiber arrangement consists of two fibers placed in a concentric semicircle manner and is expected to yield higher sensitivity than single U-shaped structure due to the multiple modulations at the boundaries of the fibers. This configuration is also beneficial in terms of its simple fabrication process (i.e. no twisting) and compact-size. Ray tracing analysis was performed, and the simulated data corroborates with the experimental data. Moreover, the sensor sensitivity is enhanced by a factor of around 1.55, 1.48 and 2.90 for bend ratios of 6, 12 and 17, respectively. In addition, the sensor demonstrated good sensing stability, which eliminates the intense signal fluctuation problem caused by the inconsistent gap between the fibers. Ultimately, the U-shaped passive-active fiber provides a promising route towards developing a low-cost, portable, and sensitive sensor, and allows the incorporation of a mixture of transducer coatings for further improvement in the sensing performance.
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