Compact FPI-Based Fiber Optic Humidity Sensors Functionalized with PMMA/PVA/PEG

The Fabry-Pérot interferometer (FPI) structure has been designed and fabricated through the heterogeneous splicing of single-mode fiber to hollow-core fiber, coupled with precision length cutting. Humidity sensitive materials of polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), and polyethylene glycol (PEG) film have been elaborated via a dip-coating and withdrawal technique, enabling the development of three distinct FPI-based fiber optic humidity sensors. Experimental data revealed that the PMMA-coated FPI sensor demonstrated the lowest sensitivity to humidity variations, while the PEG-functionalized FPI exhibited a sensitivity approximately an order of magnitude higher than that of PMMA. The proposed fiber optic humidity probe features a compact design, simplified fabrication workflow, and robust compatibility with spatially restricted, integration-demanding, or electrically hostile environments unsuitable for conventional sensor deployment.