Quasi-Lossy Mode Resonances in Long Period Fiber Gratings: Achieving Linear Wavelength Shifts with Metallic-Dielectric Thin Films

Long-period fiber gratings (LPFGs) are an important structure in the field of optical fiber sensors, recognized for their ability to couple light between core and cladding modes, enabling sensitivity to the surrounding medium. This characteristic has facilitated their application in biosensing, chemical sensing, and environmental monitoring. Over the years, several strategies have been employed to enhance the sensitivity of LPFGs, including cladding diameter reduction, operation at dispersion turning points (DTPs), and the mode transition phenomenon induced by thin films. However, challenges such as non-linear spectral shifts have limited their practical implementation. In this work, we present a detailed experimental and numerical investigation of LPFGs modified with a gold thin film positioned between the cladding and a high-refractive-index TiO₂ layer. This configuration demonstrates improved linearity in the wavelength shifts observed at DTPs and during mode transitions. Experimental results reveal that increasing the gold layer thickness from 15 nm to 30 nm significantly enhances linearity, reducing the quadratic fitting parameter by nearly 50-fold while maintaining high sensitivity. These findings provide a pathway toward more robust and precise LPFG-based sensors with improved spectral response towards practical applications.