Enhancement of Refractive Index Sensitivity by Bending a Core-Offset In-Line Fiber Mach–Zehnder Interferometer

We proposed a bent core-offset in-line fiber Mach–Zehnder interferometer for refractive index (RI) sensing. It was fabricated by the splicing method and consists of a standard single-mode fiber sandwiched between two core-offset joints, with a core-offset distance of about 6 $\mu \text{m}$ . The RI sensing performance of this so-obtained interferometer was evaluated and compared with that of straight ones. Straight interferometers exhibited a relatively low sensitivity (15.326 nm/RIU), inadequate for RI measurements. However, this sensitivity was greatly improved by simply bending them into U-shaped probes. The RI sensitivity was enhanced as the curvature radius was reduced; the highest sensitivity (358.039 nm/RIU) was achieved with an interferometer having a length of 30 mm and a bend radius of 16.42 mm. Hence, the experimentally confirmed high sensitivity of our proposed sensor, combined with its simple and compact structure, easy fabrication, and low cost, makes it a promising candidate for RI sensing applications.