Orthogonal Bragg Gratings Excited by Single-Mode Reflection Peaks in Multi-Mode No-Core Fibers Inscribed with Femtosecond Lasers for Vector Bend Sensing

In this Letter, we propose the use of a femtosecond laser (FL) plane-by-plane (Pl-b-Pl) method to achieve narrowband single-mode reflection peak excitation directly in multi-mode no-core fibers (NCFs). The effects of different grating offsets on grating reflectivity were verified. We inscribed orthogonal fiber Bragg gratings with an offset distance of 15 μ m in multi-mode no-core fibers, and the bending sensitivities of the two gratings can reach −2.79 and −2.93 dB/m −1 . This structure can be used as a two-dimensional vector bending sensor, especially for detecting very small bends.