为了简化光纤液位传感器的设计与制作工艺, 提出了一种基于纤芯失配模间干涉的在线型光纤迈克耳孙干涉仪, 由单模光纤熔接一段细径光纤构成。单模-细径光纤熔点处充当耦合器, 激发出光纤高阶包层模, 纤芯基模与高阶包层模被细径光纤端面反射后传输至单模光纤产生模间干涉并输出。传感器干涉条纹清晰、对比度高, 对环境液位改变敏感。对细径光纤长度为12 mm的传感器进行了不同溶液液位和温度响应特性的实验研究, 实验结果表明在0~9 mm 的液位变化范围内, 干涉谷波长与液位呈线性关系, 水液位灵敏度为－0.116 nm/mm, 质量分数为4.7%的 NaCl溶液液位灵敏度为－0.129 nm/mm；在20~80 ℃的水温变化范围内, 干涉谷的温度灵敏度为0.038 nm/℃。传感器结构简单、制作简便, 而且成本低廉, 在石油化工等领域具有较好的应用前景。

In order to simplify the design and fabrication of optical fiber liquid level sensor, we propose an in-fiber Michelson interferometer based on fiber-core mismatching intermodal interference, which consists of a single mode fiber (SMF) fusion-spliced with a section of thinned fiber (TF). The fusion point of the SMF-TF acts as a fiber coupler to excite the high-order cladding modes. The core mode and high-order cladding modes are reflected by the end face of the TF and transmitted to SMF to generate intermodal interference. The output interference fringes are clear and in high contrast, which are sensitive to the change of environment liquid level. The sensing characteristics of a sensor sample with 12-mm-long TF to liquid level and temperature are investigated. The experimental results show that the reflection dip wavelength linearly changes for a liquid level variation of 0-9 mm, and the sensitivities to water and NaCl solution with mass faction of 4.7% are －0.116 and －0.129 nm/mm; the temperature sensitivity to water is 0.038 nm/℃ in the temperature range of 20-80 ℃. The sensor has advantages of simple structure, easy fabrication, and low cost, which offers the prospective application in petroleum industrial and chemical industry.