Surface plasmon resonance-based ring-core fiber sensor for ultra-low refractive index (gas detection) with high resolution

Surface plasmon resonance (SPR) sensors based on fiber optics have demonstrated strong potential in recent years for detecting refractive index (RI), temperature, and pressure. Among these, RI sensors have primarily focused on sensing aqueous solutions, particularly for biomedical applications. In contrast, gas sensing using similar configurations has received less attention, largely due to the challenge of accurately detecting the low RI range of gases. In this work, we report an SPR-based fiber sensor capable of detecting RI values in the range of 1.00 to 1.10, which encompasses nearly all gases. A practical and simple SPR sensor design is proposed using a ring-core fiber, where a thin gold layer is coated on the surface of the central air hole filled with the gas sample. Simulation results show a linear shift in spectral response with a wavelength sensitivity of 795 nm/RIU and a resolution on the order of 10^-4<i>R</i><i>I</i><i>U</i>, enabling accurate discrimination of gas samples. Furthermore, the sensor exhibits a maximum amplitude sensitivity of approximately 401/RIU, indicating that a small change in the analyte RI (e.g., 0.01 RIU) results in a 40% variation in transmitted intensity, thus supporting cost-effective, intensity-based interrogation methods.