Lossy mode resonance-based highly sensitive fiber optic refractive index sensor using the bilayer of FTO/HfO 2 for operation in the visible region

A lossy mode resonance (LMR) based fiber optic refractive index (RI) sensor utilizing a layer of fluorine-doped tin oxide (FTO) over the unclad core of the fiber is theoretically analyzed. To enhance its sensitivity, the thickness of the FTO layer is optimized. For further sensitivity enhancement, an over-layer of two different materials, TiO2 and HfO2, is used. The sensitivity of FTO/TiO2 and FTO/HfO2 bilayer based LMR sensors increases with the increase in the thickness of the over-layer for a fixed thickness of the bilayer. Further, the sensitivity of the sensor utilizing the FTO/HfO2 bilayer is found to be more than the sensitivity of the FTO/TiO2 bilayer sensor. To keep the operating spectral range of the sensor in the visible region, the total thickness of the bilayer is adjusted. It is found that a total bilayer thickness of 30 nm with 90% thickness of HfO2 gives the maximum sensitivity of 4400 nm/RIU for 1.33 RI of the sensing medium, which is more than three times the sensitivity of the FTO coated LMR sensor. The sensitivity of the proposed LMR sensor utilizing the FTO/HfO2 bilayer is compared with the other LMR sensors reported in the literature, and it is found that the proposed sensor possesses the highest sensitivity. The other advantage of the proposed sensor is that it works in the visible region, which reduces the cost of the sensor.