“…In fact, various types of FBGs, such as regenerated FBGs, 10,11) microstructure FBGs, 12) and tilted FBGs, 13) have been developed for hightemperature, refractive index, strain, pressure, acoustic wave, and other applications. [14][15][16][17] When fiber gratings are inscribed in single-mode fibers (SMFs), they reflect only a single wavelength of light, known as the Bragg wavelength λ = 2 • n eff • Λ, where n eff represents the effective refractive index of the fiber core and Λ denotes the grating pitch. In contrast, when FBGs are inscribed in multimode fibers (MMFs), they exhibit multiple reflection peaks around the fundamental Bragg peak due to the coupling between different linearly polarized (LP) modes.…”