提出一种基于单模-多模-单模光纤结构的折射率传感方案，采用波谷检测方法可以提高传感灵敏度。研究发现多模光纤中模式数量对传输结果影响较大。当改变单模光纤和多模光纤的结构参数而保持其归一化频率不变，从而保证多模光纤中模式数量不变时，总能获得规则的传输曲线。增大光纤直径可以有效提高传感灵敏度。当取多模光纤纤芯直径为70 μm时，最高灵敏度达到3.725×105 nm/RIU，检测极限为7×10-7 RIU。当多模光纤实际长度偏离设计长度时，长度偏差对传感灵敏度和检测极限的影响较小,其导致的输出光谱偏移为-0.07 nm/μm，利用此特性可以实现高灵敏度的应力传感。

A sensing method of refractive index based on single-mode-multimode-single-mode fiber structure is proposed. The sensor sensitivity is increased through valley detection method. It shows that number of pattern in the multimode fiber has vital influences on the transmission result. When the structure parameters of single-mode fiber and multimode fiber change and normalized frequency remains, the number of pattern of multimode fiber remains, and the regular transmission curve can be acquired. The sensitivity can be improved effectively with the increase of diameter of multimode fiber. The maximum sensitivity is 3.725×105 nm/RIU and the detection limit is 7×10-7 RIU, when the diameter of multimode fiber is 70 μm. When the actual length of multimode fiber is deviating from the design, the length of the deviation has little effect on the sensor sensitivity and detection limit, the output spectrum shift is -0.07 nm/μm, which means it can be practiced in high sensitive stress sensing.