A Review of Optical Fibre Sensors Operating in the Visible Region: Fundamentals, Technological Developments and Biomedical Applications

Optical fiber sensors have experienced remarkable development due to their advantages in sensitivity, immunity to electromagnetic interference, compact size, and remote monitoring capability. Among them, devices operating in the visible spectral region have gained particular attention because of their low cost, ease of implementation, and potential for biomedical, chemical, and environmental applications. This review presents the fundamentals and recent advances in visible-light-based optical fiber sensors, covering the main interferometric configurations, including Michelson, Mach–Zehnder, Fabry–Pérot, and Sagnac interferometers. In addition, different sensing platforms, such as single-mode, multimode, plastic, and photonic optical fibers, are analyzed, highlighting their characteristics and advantages for specific applications. Various sensing mechanisms are reviewed, including optical absorption, fluorescence, surface plasmon resonance, and evanescent field sensing. Furthermore, applications focused on the measurement of pH, glucose, pressure, humidity, and other parameters of biomedical and environmental interest are discussed. The studies analyzed demonstrate that the combination of optical fibers with functional materials and spectroscopic techniques in the visible region enables the development of highly sensitive, selective, and real-time monitoring sensors. Finally, the current challenges and future perspectives of this technology for scientific and industrial applications are presented.