A humidity sensor utilizing multimode interference coreless silica fiber

Abstract Functional film-based humidity optical fiber sensors often face issues such as complexity, high cost, coating degradation, and dependence on uniform coatings, affecting their reliability. This research presents a coating-free relative humidity (RH) sensor using coreless silica fiber (CSF) in an SMF-CSF-SMF multimode interference (MMI) structure. Humidity changes alter the effective refractive index around the CSF, shifting the interference pattern in the C-band. Simulations modeled RH by varying the surrounding refractive index from 1.000 (0% RH) to 1.333 (100% RH), yielding a sensitivity of 1.319 × 10 −9 RIU with strong linearity (R 2 = 0.9468). Sensitivity increased with reduced CSF diameter, and optimal performance was achieved at the 4th self-image length. Experiments using 125 μm CSF showed the 4th self-image sensor had the highest sensitivity, with a 1 dB power change from 50% to 100% RH. This CSF-based sensor is simple, temperature-insensitive, and reliable without using functional films.