Highly Sensitive Graded Index PCF-Based Refractometric SPR Sensor Offering Enhanced FOM

In this proposed work, a novel graded index photonic crystal fiber (GI-PCF) with gold-TiO₂ layered coating is presented as a surface plasmon resonance (SPR)-based refractive index sensor for a wide range of refractive indices. Present fiber is characterized by two types of air holes: one having gradually increasing radius and other having constant radius. The performance of the proposed sensor is tested on wavelength sensitivity (WS), amplitude sensitivity (AS), sensor resolution, and figure of merit (FOM). Detailed modal analysis of the designed SPR sensor is analyzed by the finite element method. Structural design optimization has been analyzed with respect to various different structural parameters, such as the radius of different air holes, the inherent grading of holes, and the thickness of gold and TiO₂ layers. Our detailed analysis summarized the effectiveness of graded design over conventional design. The calculation result shows a high WS of 22,600 nm/RIU. Apart from high WS, an enhanced AS of -1410 RIU⁻¹ is also observed, which is a key highlight of our PCF sensor. Further, the sensor’s resolution is also obtained having a magnitude of 4.43 × 10⁻⁶ RIU and FOM is 403.57 RIU⁻¹. Thus, the GI-PCF sensor proves its excellent ability to measure both wavelength and amplitude sensitivity with excellent FOM for a wide range of refractive indices. Such a sensor can find its application in various sensing areas, i.e., biomedical, chemical, and industrial field. Apart from promising numerical values, the PCF structure is also possible to fabricate using currently existing fabrication techniques.