This study proposes the development of a dual-wavelength optical fiber sensor (DWOFS) that integrates two optical fiber structures in a multimode transmission line to measure the refractive index and temperature of a liquid concurrently. One structure is based on a refractive index sensor that utilizes surface plasmon resonance, comprising a 5 mm long single-mode fiber (SMF) section coated with chromium/gold (Cr/Au) films. The secondary structure employs a multimode interferometer with a 29 mm long no-core fiber (NCF) section covered with a thick layer of polydimethylsiloxane (PDMS) to measure temperature. The measurements obtained reveal two distinct drops in the transmission spectrum at approximately 600 nm and 1550 nm, respectively, enabling precise measurement of the two parameters. The sensor demonstrates a high degree of sensitivity to both refractive index and temperature, spanning the visible ($2770.30\mathrm{n}\mathrm{m}/\mathrm{R}\mathrm{I}\mathrm{U}$) and infrared $(-0.178\mathrm{n}\mathrm{m}/°\mathrm{C}$) regions of the spectra, respectively. Furthermore, the thermo-optical coefficient for water $({-0.9928\times 10}^{-4}\mathrm{R}\mathrm{I}\mathrm{U}/°\mathrm{C})$ was estimated. The proposed sensor offers a compact solution for the simultaneous measurement of refractive index and temperature in liquid samples for a variety of applications, including biological, environmental, and healthcare research.

This paper presents a combined numerical and experimental investigation of the transmission interference spectra of hetero-core optical fiber structures, supported by a Python-based repository for data analysis and simulation. The hetero-core device is fabricated using two multimode optical fiber segments and a coreless fiber section, employing a simple and highly reproducible methodology. Compared to other optical fiber or interferometric sensors, the MCM design offers superior fabrication simplicity, enhanced performance metrics, greater structural robustness, reduced losses, and cost-effective manufacturing. The refractometric response of the device was evaluated as a function of the coreless fiber section length, which varied from 21 mm to 60 mm. The refractive index (RI) of the external medium ranged from 1.0000 to values between 1.2971 and 1.3912. The results show that the wavelength corresponding to the transmission minimum shifts toward longer wavelengths, with a sensitivity ranging from 4.32 nm/RIU to 276.87 nm/RIU. The semi-analytical model proposed in this work considers only the two modes in the coreless fiber section with the highest overlap integrals, facilitating the prediction of the wavelength position of minimum transmittance and its dependence on both the coreless section length and the surrounding RI. Consequently, this model can serve either as a semi-analytical tool for designing multimode–coreless–multimode fiber-based sensors or as a complementary computational method for determining the RI of a liquid sample surrounding the fiber-optic structure.