Two-dimensional vector bending sensor based on single excessively tilted fiber grating

We introduced a two-dimensional vector-bending sensor using excessively tilted fiber grating (ExTFG) encapsulated in a non-neutral axis position within a flexible cylinder. The asymmetrical structure enabled both TM and TE modes of the ExTFG to achieve the competency of one-dimensional vector bending sensing by cleverly leveraging the orthogonal relationship between these modes to achieve the effect of orthogonal cascading of two one-dimensional sensors. The resonance wavelength changes of the TM and TE modes were monitored under equal excitation states to achieve two-dimensional vector bending sensing, which significantly simplified the manufacturing complexity of the sensor; the longer the off-axis distance, the higher the sensitivity, and the smaller the maximum measurable curvature. Through an optimal parameter designation for the sensor, the results showed that the transmission spectra of TM and TE modes varied significantly depending on the curvature and direction of the sensor bending with a maximum sensitivity of -105 pm/m^-1 and 105.5 pm/m^-1, -70.25 pm/m^-1 and 67.5 pm/m^-1, respectively. Subsequently, the direction and curvature of the vector bending could be reconstructed using the established algorithm with the measured wavelength shifts of TM and TE modes. The average relative error was about 2.02% for the reconstructed direction angle and about 3.61% for the reconstructed curvature. The proposed sensor offered higher sensitivity than those based on FBGs and improved stability compared to those based on optical fiber interferometers. Therefore, it holds promising applications in structural health monitoring in complex environments and intelligent machinery.