All-fiber vector bending sensor based on a four-core fiber

This paper presents a Mach-Zehnder interferometer (MZI)-based vector bending sensor employing a four-core fiber and experimentally demonstrates its sensing performance. The sensor is fabricated by fusion splicing 1 mm long multimode fiber (MMF) segments to both ends of the four-core fiber, forming an interferometric structure with a well-defined resonance dip for curvature measurement. Due to the asymmetric arrangement of the fiber cores, the effective refractive index distribution changes with the bending orientation, thereby enabling vector bending sensing. Experimental results demonstrate that, within the curvature range of 0.497-0.642m^-1, the maximum bending sensitivities along the two principal sensing axes reach 91.342 and 78.716nm/m^-1, respectively, while the corresponding minimum sensitivities are 19.290 and 21.218nm/m^-1. The sensor exhibits minimum sensitivity when the bending direction is perpendicular to the core-array axis and maximum sensitivity when it is parallel to the core-array axis. As the bending orientation varies from 0° to 360°, the sensitivity exhibits a periodic angular response. In addition, the temperature sensitivity of the sensor is measured to be 0.077 nm/°C. The proposed sensor features a simple fabrication process, good directional discrimination capability, and good repeatability, making it a promising candidate for vector bending sensing applications.