Photonic Crystal Fiber SPR Sensors for Power and Biomedical Applications: A Review

Photonic crystal fiber (PCF) sensors based on surface plasmon resonance (SPR) have matured into a versatile platform, yet the literature remains fragmented: biomedical and power system studies are reviewed in isolation, and most surveys catalog individual designs without explaining why particular structural choices deliver particular performance. This review departs from that catalog style by introducing a unifying, dual-domain analytical framework that treats power-system monitoring and biomedical diagnostics as two ends of a shared refractive-index sensing continuum governed by the same small set of design levers. We organize recent (2023–2026) PCF-SPR research around five such levers geometry exposure, plasmonic-material engineering, two dimensional (2D) functional overlayers, multi core/multi parameter architectures, and data-driven inverse design and we critically benchmark reported sensitivities, resolutions, and trade-offs rather than merely listing them. Particular emphasis is placed on three currently underexplored frontiers: hybrid plasmonic stacks combining gold with graphene, MXene (Ti₃C₂Tₓ), and black phosphorus; machine learning and explainable AI workflows that replace exhaustive finite element sweeps; and multi peak ten hole architectures that enable simultaneous voltage, temperature, magnetic-field, humidity, and refractive-index sensing for smart grid diagnostics. By mapping performance gains against fabrication cost and identifying concrete research gaps, this review offers a decision-oriented guide for designing the next generation of field-deployable, lab on fiber PCF SPR sensors.