提出了一种基于无芯光纤的单模多模单模（SMS）结构的折射率光纤传感器并对其进行了理论和实验研究。用一种新颖的无芯光纤来作为SMS结构中的多模波导，避免了常规SMS折射率传感器制作过程中的化学腐蚀所带来的问题，具有易于设计和制作的优点。制作了一支基于无芯光纤的SMS折射率光纤传感器，并用具有不同折射率的蔗糖溶液对其进行了实验测试，在1.356～1.392的折射率范围内得到了431.4 nm/RIU的平均折射率灵敏度，实验结果与模拟结果符合得很好。进一步的理论研究发现，通过减小无芯光纤的直径可以进一步提高传感器对折射率的灵敏度。

A fiber sensor based on the single mode-multimode-single mode (SMS) structure for refractive index (RI) sensing is proposed. A kind of no core fiber is used as the multimode waveguide. Compared with the conventional SMS with an etched multimode fiber, the proposed sensor is easy to design and fabricate. A model RI sensor based on the proposed structure is designed and experimentally tested with sucrose solutions with different RI values. The results show that the sensitivity of the sensor is 431.4 nm/RIU on average in the range of 1.356~1.392. The experimental results agree well with the simulation results. Besides, computer simulation is also performed to systematically analyze the sensor′s characteristics. The results show that the sensitivity of the sensor can be improved by reducing the diameter of the no core fiber.