Simultaneous Measurement for Displacement and Temperature Using Fiber Bragg Grating Cladding Mode Based on Core Diameter Mismatch

A novel fiber Bragg grating (FBG) sensing configuration for simultaneous measurements of temperature and displacement based on a core diameter mismatch is proposed and experimentally demonstrated. The configuration is formed by a short section of thin-core fiber (TCF) followed with a single-mode fiber (SMF) tip inscribed with an FBG. The TCF excites cladding modes into downstream SMF via the mismatch core splicing interface, and the low-order cladding modes can be reflected back to the fiber core via the FBG in which the recoupling efficiency is highly dependent on fiber bending. Experimental results show that the recoupled first-order odd mode (LP₁₃) shows an extremely high sensitivity to fiber bending. The simultaneous measurements, including power-referenced for displacement and wavelength-referenced for temperature, have been achieved via selective cladding modes monitoring.