Effect of Surrounding Refractive Index Changes on Amplitude Response in MMI-Based Fiber Optic Sensors

This study investigates the influence of surrounding refractive index (SRI) variations on the amplitude response of multimode interference (MMI)-based fiber optic sensors. Numerical simulations were conducted using a multimode fiber with a core index of 1.4446 and a cladding index of 1.4271, operating at a wavelength of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.55 \mu ~\mathrm{m}$</tex>. The SRI was varied from 1.30 to 1.44 to emulate real-world sensing environments. Results show a consistent decline in self-imaging amplitude with increasing SRI, attributed to reduced refractive index contrast and enhanced mode leakage. Higher-order selfimaging positions demonstrated increased sensitivity, with significant amplitude attenuation near the core index range (1.42-1.44). The maximum calculated sensitivity was <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{6. 0 1 \% / R I U}$</tex> at the fourth self-imaging position, indicating strong potential for high-resolution refractive index sensing. These findings highlight the nonlinear amplitude response in high-SRI regimes and offer insights for optimizing MMI sensor design for enhanced sensitivity and extended dynamic range in applications such as biomedical diagnostics, chemical sensing, and environmental monitoring.