Point-by-point inscribed sapphire parallel fiber Bragg gratings in a fully multimode system for multiplexed high-temperature sensing

We study the point-by-point inscription of sapphire parallel fiber Bragg gratings (sapphire pFBGs) in a fully multi-mode system. A parallel FBG is shown to be critical in enabling detectable and reliable high-order grating sig-nals. The impacts of modal volume, spatial coherence, and grating location on reflectivity are examined. Three cas-caded seventh-order pFBGs are fabricated in one sapphire fiber for wavelength multiplexed temperature sensing. Using a low-cost, fully multimode 850-nm interrogator, reliable measurement up to 1500 degrees C is demonstrated. (c) 2022 Optica Publishing Group