Thermo-optic characterization of optically similar aqueous solutions using SMS/MMI fiber sensor

Abstract

Abstract Multimode interference (MMI) fiber sensors with singlemode–multimode–singlemode (SMS) structures based on no-core multimode fiber (NC-MMF), have been widely employed for refractive index (RI) sensing of liquid samples due to their simplicity, low cost, and high sensitivity to changes in the immersion medium. In this work, we investigate the temperature-dependent spectral response of an SMS/MMI fiber sensor immersed in aqueous solutions with closely similar RI. Aqueous solutions of tris(hydroxymethyl)aminomethane (Tris), urea, and fructose were selected as model analytes because they exhibit nearly indistinguishable RI properties under conventional room-temperature conditions. The thermo-optic response of the sensor was studied by performing controlled temperature sweeps from 25 °C to 45 °C. Experimental results reveal that, although the three solutions produce comparable concentration-dependent refractometric responses, they exhibit distinct nonlinear temperature-dependent spectral shifts. To explain this behavior, a numerical model based on MMI theory was developed, showing that the observed nonlinearities arise from the intrinsic nonlinear dependence of the MMI peak wavelength on the RI of the immersion medium. Consequently, small thermo-optic variations in the liquid samples are transformed into measurable nonlinear spectral signatures. The results demonstrate that SMS/MMI fiber sensors can provide access to thermo-optic information that is not available through conventional single temperature refractometry. The proposed approach highlights the potential of MMI devices as simple optical platforms for thermo-optic characterization of aqueous media and for the study of temperature-dependent optical properties in liquid samples with closely similar RI.