Research on Temperature and Refractive Index Sensing Characteristics Based on Coreless-Multimode-Coreless Optical Fibers

提出了一种无芯–多模–无芯光纤结构的温度和折射率双参量测量传感器，并对传感器进行理论分析和实验研究。此传感器将多模光纤(Multi-Mode Fiber, MMF)和无芯光纤(Coreless Fiber, CLF)同轴熔接，构成无芯–多模–无芯的光纤结构，在连接的结构两端熔接单模光纤(Single-Mode Fiber, SMF)作为输入和输出光纤。当光从单模光纤进入到无芯光纤时，由于没有纤芯的限制，光束发生衍射扩散，形成较大的光场，扩展后的光场进入多模光纤，与多模光纤中的多个传播模式耦合，这些模式在多模光纤中传播时，由于光程差会相互干涉，形成干涉光谱，最终模式干涉后的光场在第二段无芯光纤中传播后进入输出单模光纤。当外界温度发生变化时，干涉光谱的干涉波谷会发生偏移，选取2个不同的干涉波谷作为特征波长，进行实验分析。实验结果表明，波长在1554.6 nm和1567.9 nm附近的干涉波谷均发生红移，相应的温度灵敏度分别为75.9 pm/℃和71.7 pm/℃，折射率灵敏度分别为−3.23 nm/RIU和−4.66 nm/RIU。该传感器结构简单，易于制造和集成。In this paper, a coreless-multimode-coreless fiber structure with dual parameters of temperature and refractive index measurement sensor was proposed, and the sensor was theoretically analyzed and experimentally studied. This sensor splices multi-mode fiber (MMF) and coreless fiber (CLF) coaxially to form a coreless-multimode-coreless fiber structure, and splices single-mode fiber (SMF) at both ends of the connected structure as input and output fibers. When the light enters the coreless fiber from the single-mode fiber, due to the limitation of the core, the beam undergoes diffraction diffusion to form a larger light field, and the expanded light field enters the multi-mode fiber and is coupled with multiple propagation modes in the multi-mode fiber, when these modes propagate in the multi-mode fiber, they will interfere with each other due to the difference in optical path to form an interference spectrum, and the light field after the final mode interference in the second section of the coreless fiber propagates and enters the output single-mode fiber. When the external temperature changes, the interference troughs of the interference spectrum will be shifted, and two different interference troughs will be selected as the characteristic wavelengths for experimental analysis. The experimental results show that the interference troughs around 1554.6 nm and 1567.9 nm are redshifted, and the corresponding temperature sensitivities are 75.9 pm/°C and 71.7 pm/°C, respectively, and the refractive index sensitivities are −3.23 nm/RIU and −4.66 nm/RIU, respectively. The sensor is simple in structure and easy to manufacture and integrate.