Novel tilted Fiber Bragg Grating based sensors for physical parameter measurement

Abstract

Measurement of physical parameter has perpetually remained a paramount do­main within sensor research. Fiber optic sensors, characterized by attributes such as high stability, cost-effectiveness, ease of fabrication, and immunity to electromagnetic interference, have found widespread utility in the mea­surement of diverse physical parameters. Among these measurements, high-precision assessments of temperature and lateral pressure have furnished foun­dational insights for a plethora of monitoring scenarios. Simultaneously, vector measurements of curvature hold pivotal significance across various sectors, en­compassing applications in industries, structural monitoring, and aerospace, among others. Tilted Fiber Bragg Gratings (TFBGs), evolving from the foundation of fiber Bragg grating technology, have undergone substantial development over the course of several years and have found application across a spectrum of sens­ing contexts. Divergent from conventional fiber Bragg gratings, TFBGs intro­duce an angular offset between the grating plane and the fiber cross-sectional plane, engendering a disparity that facilitates the coupling of light propagat­ing within the fiber core into the cladding region. This phenomenon engenders the creation of a series of resonant modes within the transmission spectrum, thereby manifesting a sensitivity to distinct physical parameters. As these reso­nant modes traverse discrete portions of the optical fiber, they facilitate a sensi­tivity to a range of physical parameter. Notably, the inherent asymmetry of the grating configuration in TFBGs imparts a pronounced polarization sensitivity to the sensor’s operation. In this study, we initially integrate TFBGs into a Sagnac interferometer con­figuration. By leveraging the inherent polarization characteristics of TFBGs, the Sagnac loop is transformed into a high-sensitivity interferometric sensor. This sensor exhibits notable sensitivities to both temperature variations and lateral pressure, rendering it suitable for the concurrent measurement of these two parameters. This integrated sensing platform holds promise for applications demanding accurate and simultaneous monitoring of temperature and lateral pressure effects. However, the high sensitivity of interferometric sensors is often accompanied by reduced mechanical robustness and instability. Consequently, our research direction shifted towards exploring novel structural designs of TFBGs within the fiber itself. To enhance TFBG sensitivity and achieve simultaneous curva­ture measurement with directional calibration, we devised and implemented a symmetric fiber grating sensor (STFBG) based on multiple exposures. This sensor exhibits heightened sensitivity to curvature measurement and, through comprehensive calibration, leverages the sensitivity variation to discern the curvature direction. This innovative approach enhances the reliability and ac­curacy of curvature sensing in optical fiber systems, presenting opportunities for advanced applications in various fields. However, this approach did not inherently enable true vector measurement. To overcome the inherent directional symmetry of the grating, we introduced an asymmetric tilted fiber grating (ATFBG). This innovative strategy allowed us to achieve high-precision curvature vector measurements in both the +y and -y directions, thereby extending the capabilities of curvature sensing beyond a single plane. This advancement opens avenues for comprehensive and accurate characterization of complex curvature profiles in optical fiber-based sensing ap­plications.