Flexible photonic sensors: Investigation of an approach based on ratiometric power in few-mode waveguides for bending measurement

A novel approach to monitor the degree of bending via flexible photonics devices, using ratiometric power variation in a few-mode optical waveguide, is proposed. To demonstrate its feasibility, a sensor exploiting a Bragg grating, approximately aligned to the neutral axis, is designed, fabricated and characterized. The reduced thickness of the proposed planar photonic sensor is uniquely limited by optical confinement requirements, enabling for an ultra-thin and highly flexible planar device, based on all-glass platform. Finite Element Method, Beam Propagation Method and Coupled Mode Theory are employed in the design to model the electromagnetic and mechanical phenomena occurring during the three-point bending test. The experiment demonstrates that the planar device withstands tight curvature without mechanical failure. The device shows that by increasing the bending the reflected power from the fundamental mode decreases but the reflected power from the higher order mode increases. The measured ratiometric power sensitivity versus displacement is K_(P_R )=-0.78 dB/mm with negligible variation over a 40 °C thermal range. Therefore, exploiting both the Bragg wavelength shift and the mode optical power change, the proposed sensor can be employed for multiparameter sensing purposes, e.g. simultaneous temperature and curvature monitoring.