Compact Fluorine-Doped Multimode Fiber Sensor for Refractometry in the Visible Spectrum

We present the design and experimental validation of a compact refractive index sensor based on a multimode-fluorine-doped-multimode (MFM) fiber structure. The sensor exploits modal interference effects in the visible spectrum by incorporating a FG105LCA multimode fiber segment with a fluorine-doped cladding. By varying the length and diameter of the central fiber segment, we achieve a significant shift in the attenuation peaks, enabling a high-resolution and compact design. Experimental results demonstrate a maximum sensitivity of 137.07 nm/RIU and a sensitivity of the wavelength shift to the refractive index up to 41.66 nm/RIU. Furthermore, a miniaturized reflective configuration using a 15 mm segment is proposed, offering both performance and integration advantages.