Advances in Optical Fiber Sensors Based on Multimode Interference (MMI): A Review

In recent years, optical fiber sensors based on multimode interference (MMI) have attracted increasing interest and developed into various sensors used in many practical applications. This review presents MMI-based fiber sensors with a specific focus on the probe structures, measurement methods, and sensing properties of different structures. The fundamentals of MMI-based fiber sensors are briefly described. Further, five main categories of SMS structure-based fiber sensors are reviewed, including conventional SMS fiber sensors, MMI-based sensors with no-core fiber (NCF), MMI-based sensors with etched/polished fiber, MMI-based sensors with tapered fiber, and cascades of MMI-based fiber sensors. Besides, their probe structures, sensing properties, practical fields, and measurement sensitivities are summarized. Finally, the current challenges and technique outlook of the MMI-based fiber sensors are pointed out, which gives the potential solutions for the existing problems and future development. This work indicates that the MMI-based fiber sensors bring the possibilities of applying multi-mode fiber (MMF) in different measurement fields with high sensitivities, easy fabrication, and low cost. With the appearance and development of new SMS fiber sensors, it will contribute significant value to scientific research and industrial applications.