基于模式干涉理论和布拉格光纤光栅的传感特性, 提出了一种单模-少模光纤布拉格光栅(FBG)-单模结构的组合传感器。 利用光纤熔接机将一定长度的少模光纤(FMF)熔接在两段单模光纤(SMF)中间构成SFS结构干涉仪, 再在FMF上刻制FBG, 通过光谱仪得到模式干涉与耦合共同作用后的传输光谱。 首先分析了组合传感器的传感原理, 由于外界环境的改变会引起FMF中纤芯模式有效折射率的改变, 从而导致SFS结构的干涉光谱和FBG的波长发生移动, 因此可以通过检测组合传感器传输光谱的波长漂移量, 实现对待测参数的测量。 然后仿真了FMF长度对干涉光谱的影响, FMF越长, 干涉光谱越明显, 自由光谱范围FSR越小, 为了便于观测组合传感器的整体光谱, 最终选择长度为110 mm的FMF进行传感实验。 实验所用FMF可稳定传输LP01, LP11, LP21和LP02四种模式, 通过对比分析不同模式间的干涉和耦合, 确定此组合传感器的干涉光谱是由LP01-LP11干涉形成, FBG透射谱是由LP02-LP02, LP11-LP11, LP01-LP02和LP01-LP01耦合形成。 最后进行温度和折射率的传感实验, 结果表明, 随着外界温度的升高, SFS结构的干涉光谱发生明显的蓝移, 而FBG透射谱发生红移, 其温度灵敏度分别为-62.04和10.87 pm·℃-1, 线性度良好; 将FMF包层直径腐蚀至22 μm, 在1.366~1.455的折射率变化范围内, 组合传感器的传输光谱并未发生明显的移动, 灵敏度最大仅为3.933 nm·RIU-1。 该传感器利用干涉峰和谐振峰同时监测外界的变化, 提高了检测准确度, 减小测量过程中出现的偶然误差, 结构简单新颖、 灵敏度高、 易于制备, 且FBG的4个谐振峰具有很强的传感一致性, 使得传感变得更加灵活方便。

Based on the theory of mode interference and the sensing characteristics of fiber Bragg grating, a combined sensor composed of a single modefiber (SMF)-few mode fiber Bragg grating (FBG)-SMF structure is proposed. The SFS structure interferometer is constructed by fusing a certain length of few mode fiber (FMF)between two SMFs with afiber fusionsplicer, and then FBG is etched on FMF. The transmission spectrum is obtained by the optical spectrum analyzer after the interaction of mode interference and coupling. Firstly, the sensing principle is analyzed. Since the change of environment will cause the effective refractive indexchange of the core mode in FMF, which will cause the wavelength shift of interference spectrum and FBG, the measured parameters can be realized by detecting the wavelength shift of transmission spectrum. Then the effects of FMF length on interference spectrum are simulated. The longer FMF is, the more obvious the interference spectrum is and the smaller the free spectrum range is. In order to observe the transmission spectrum of the combined sensor, the length of FMF is chosen as 110 mm for sensing experiment. FMF can stably transmit four modes with LP01, LP11, LP21 and LP02. By comparing and analyzing the interference and coupling between different modes, it is determined that the interference spectrum is formed by LP01-LP11, and the transmission spectrum of FBG is formed by LP02-LP02, LP11-LP11, LP01-LP02 and LP01-LP01. Finally, the temperature and refractive index sensing experiments are carried out. The results show that the interference spectrum of SFS structure appears obvious blue shift and the transmission spectrum of FBG appears red shift with the increase of temperature. Their temperature response sensitivities are -62.04 and 10.87 pm·℃-1 respectively with good linearity. When the cladding of FMF is corroded to 22 μm, there is no obvious shift phenomenon in the transmission spectrum within therange of 1.366~1.455 andthe maximum sensitivity is only 3.933 nm·RIU-1. The interference peak and transmission peak are used to monitor the environment changes at the same time, which improves the detection accuracy and reduces the accidental errors. The structure has the advantages of novel structure, high sensitivity, easy preparation, and the four resonance peaks of FBG have strong sensing consistency, which makes the sensing more flexible and convenient.