Semi-analytical model for coreless fiber-optic refractive index sensors and experimental validation

Abstract 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.