Hole-assisted three-core fiber directional coupler operated in reflection mode for vector bending measurement

We demonstrate a vector bending sensor operated in reflection mode based on a hole-assisted three-core fiber (HATCF) coupler. The sensor is built by splicing a piece of HATCF with a gold film-coated end to a single-mode fiber (SMF). The gold film on the center core end is etched with femtosecond laser and only the lights transmitted in the two suspended cores of the HATCF are reflected. The center core couples with the two suspended cores at different wavelengths, respectively. The bending direction and curvature are identified by monitoring the wavelength of the two coupled peaks. The maximum curvature sensitivity of the sensor is 15.146 nm/m-1. The temperature sensitivity is less than - 58.8 pm/degrees C. The vector bending sensor operated in reflection mode has the advantages of a compact structure and simple transmission line, which is conducive to the miniaturization and integration of optical fiber bending sensors.