Few-Mode Fiber Gratings for Optical Sensing: From Mode Coupling Mechanisms to Vector and Dual-Parameter Sensing

Few-mode fiber (FMF) grating sensing exploits the mode-dependent responses of multiple spatial modes to external perturbations, enabling vector and dual-parameter measurements. This paper reviews the research progress of FMF grating sensors from the perspectives of fundamental mechanisms, fabrication techniques, signal demodulation, and representative sensing applications. First, the mode propagation characteristics, coupled mode theory, and phase matching conditions of FMF gratings are discussed. Then, the fabrication strategies of FMF gratings are summarized, followed by an analysis of signal demodulation methods. Particular attention is given to vector sensing and dual-parameter sensing. Finally, the remaining challenges related to mode excitation stability, spectral resonance overlap, fabrication repeatability, packaging reliability, and long-term calibration are discussed, and future opportunities in mode engineering, multidimensional demodulation, and intelligent sensing systems are outlined.