A Photonic Crystal Fiber (PCF) based Surface Plasmon Resonance (SPR) sensor is an optical sensing device that combines the guiding properties of PCFs with the plasmonic resonance effect at a metal–dielectric interface. In such sensors, a thin layer of noble metal (usually gold or silver) is coated inside or on the surface of the PCF. When light propagates through the fiber, the evanescent field interacts with free electrons on the metal surface, exciting surface plasmons. This resonance condition is highly sensitive to changes in the refractive index of the surrounding medium. The advantages of PCF-based SPR sensors include high sensitivity, compact size, tunable design flexibility, and the ability to detect a wide range of biological and chemical analytes. By adjusting the fiber geometry (air-hole size, pitch, and arrangement) and metal layer thickness, the sensing performance—such as resonance wavelength, confinement loss, and resolution—can be optimized. In addition, PCF-based SPR sensors are widely used for biosensing, environmental monitoring, and chemical detection, offering rapid, label-free, and real-time detection capabilities