Linearizing the mode transition in LPFG sensors with metal and dielectric thin films

Long Period Fiber Gratings (LPFGs) offer a versatile platform for sensing applications due to their ability to couple core and cladding modes through periodic refractive index modulation, which makes the device sensitive to changes in the surrounding refractive index (SRI). The sensitivity can be enhanced through mechanisms such as the dispersion turning point and mode transition. However, the performance is non-linear. To avoid this, here we explore the less-examined relationship between mode transition and lossy mode resonances (LMRs), both of which occur within the same thin-film thickness range. By depositing a gold layer followed by a high refractive index TiO₂ layer, we obtained a highly sensitive and linear performance, while aiming at minimizing the bandwidth of the attenuation bands. Two LPFGs with different modulation indices were fabricated and their spectral response to varying TiO₂ thicknesses was monitored. The results demonstrated that LPFGs can achieve a sensitivity to the thin film thickness variation of 9 nm/nm, similar to LMR-based sensors, and a linear evolution as a function of thickness, with the possibility to improve the bandwidth. This improvement could boost the applicability of LPFGs in domains such as environmental sensing and biosensing, with the potential for further improvement through thin film refinement and optimization of LPFG parameters.