Fiber-optic Fabry–Perot sensors and their demodulation techniques for extreme environment monitoring

Long-term online monitoring in extreme environments (e.g., high temperature, high pressure, liquid corrosion, strong electromagnetic radiation, confined narrow spaces, etc.) poses severe challenges to the stability and reliability of sensors. Optical fiber sensors, which employ optical signals as carriers, leverage inherent advantages such as immunity to electromagnetic interference, corrosion resistance, and electrical insulation, making them a critical technological route for extreme environment sensing. Among these, fiber-optic Fabry–Perot (F–P) sensors, operating by measuring changes in the interference cavity length, feature a simple structure, ease of miniaturization, high sensitivity, and high dynamic response. They are capable of detecting nanoscale displacement, vibration, and other minute signals and can be extended to measure multiple parameters (e.g., temperature, pressure, acceleration, etc.) via external sensitive structures. Consequently, they have become a research hotspot in high-precision optical fiber sensing. This paper systematically reviews fiber-optic F–P sensing technology. It first elaborates on the multi-beam interference mechanism and cavity length demodulation methods. It then analyzes the development history of sensing structure designs for physical quantities such as displacement, temperature, pressure, acceleration, and vibration, with particular emphasis on packaging techniques and demodulation strategies suitable for high temperature, high pressure, liquid media, and strong electromagnetic environments. This review introduces the engineering practices of such sensors in aerospace, oil and gas extraction, nuclear reactor monitoring, and other related fields. Finally, it discusses the progress in developing new-material optical fibers (e.g., sapphire fiber) and integrated probe assemblies suited for more extreme environments, such as those involving ultra-high temperature and strong irradiation. It explores the feasibility of applying artificial intelligence to adaptive decoupling under multi-field coupling, as well as miniaturized networking schemes based on integrated on-chip spectrometers. Furthermore, it envisions a trend toward the large-scale deployment of standardized and engineered measurement systems. This work provides a systematic reference for in-depth research and engineering applications of fiber-optic F-P sensing technology in extreme environments.