光纤折射率传感器广泛应用于各种复杂环境的监测。设计了一种基于少模光纤（few-mode fiber，FMF）–无芯光纤（coreless fiber，CLF）–FMF结构的高灵敏度折射率传感器。该传感器由2小段FMF之间熔接1段减薄的CLF组成马赫-增德尔干涉仪（Mach–Zehnder interference，MZI），测量外界折射率，利用光纤布拉格光栅（fiber Bragg grating，FBG）进行温度补偿。MZI干涉光谱中的谐振波谷同时受折射率和温度影响，FBG只受温度的影响。利用MZI和FBG的折射率和温度灵敏度系数构建灵敏度矩阵，实现折射率和温度的同步测量。实验结果表明，MZI折射率灵敏度为345.66 nm/RIU，温度灵敏度为0.0134 nm/℃；FBG的温度灵敏度为0.0104 nm/℃。

传统波动光学法测量样品折射率所采用的探测方法主要是强度探测和波长探测。波长的最佳检测器件为干涉型光谱仪，干涉型光谱仪是通过光功率计测量信号强度、分析条纹数目变化及所对应的光程差，进而计算出信号光波长，因此，其本质仍是以强度探测为基础的。然而，利用强度探测干涉信号，分辨率受经典衍射极限限制，很难得到进一步提升。为了解决这一瓶颈问题，本文提出利用奇偶探测突破经典分辨率的极限限制，实现超分辨率折射率测量。根据量子探测与估计理论，推导了奇偶探测和强度探测折射率测量信号及其灵敏度表达式，并进行了数值对比分析。同时，研究了损耗对系统输出信号分辨率和灵敏度的影响。数值结果表明：奇偶探测分辨率是强度探测的 ${\text{2{\text{π}}}}\sqrt {{N}} $倍，实现了超分辨率折射率测量，最佳灵敏度达到了折射率测量散弹噪声极限 ${\lambda / {\left( {2{\text{π}} l\sqrt N } \right)}}$，损耗降低了信号的分辨率和灵敏度，除极大损耗和极低光子数外，奇偶探测信号分辨率始终优于强度探测。最后，从探测手段本身出发分析了奇偶探测超分辨率折射率测量的物理本质。

Three-dimensional (3D) refractive index (RI) distribution is important to reveal the object’s inner structure. We implemented terahertz (THz) diffraction tomography with a continuous-wave single-frequency THz source for measuring 3D RI maps. The off-axis holographic interference configuration was employed to obtain the quantitative scattered field of the object under each rotation angle. The 3D reconstruction algorithm adopted the filtered backpropagation method, which can theoretically calculate the exact scattering potential from the measured scattered field. Based on the Rytov approximation, the 3D RI distribution of polystyrene foam spheres was achieved with high fidelity, which verified the feasibility of the proposed method.