Fibre Vector Bend Sensor Based on Femtosecond Laser-Written Integrated Optical Coupler with Bragg Gratings

Optical fibre bend and displacement sensors for hardware and structural health monitoring benefit from compact size and high sensitivity, but often resolve directionality only in-plane [1], [2], require non-standard multi-core fibre as a coupler [2], [3], or cannot be cascaded [1], [2]. Here, we demonstrate a fully vectorial bend sensor based on an optical coupler with a fibre Bragg grating (FBG) in each arm (Fig. 1(a)), fabricated by femtosecond-laser writing [4] in coreless fibre. Bend-induced strain alters the coupling ratio, thus varying the relative power of each gratings' reflected signal, while thermal effects can be compensated by differential FBG measurements. The sensor benefits from simple single-step fabrication, connectivity to standard SMF-28 fibre, higher sensitivity than multicore fibre methods (by routing the coupling section closer to the fibre edge), and is cascadable for quasi distributed sensing.