提出了一种基于光纤布拉格光栅嵌入单模-多模纤芯-单模(single-mode-multimode fiber core-single mode, SMS)光纤结构的湿度传感器。 当环境湿度变化时, SMS光纤结构的干涉光谱会发生漂移, 而光纤布拉格光栅对湿度不敏感, 其纤芯基模保持不变。 因此利用SMS光纤结构对环境湿度的敏感性去调制光纤布拉格光栅纤芯基模, 通过检测光纤布拉格光栅纤芯基模的反射能量变化就可以实现湿度测量。 数值模拟了SMS光纤结构的内部光场分布规律, 理论计算了不同环境折射率时, 多模纤芯的长度、 直径对SMS光纤结构输出能量耦合系数的影响。 理论模拟表明, 随着环境折射率变化, SMS光纤结构中传输的纤芯基模的输出能量耦合系数会发生变化。 同时制作了传感器样品并对其进行了传感实验研究, 实验结果表明多模纤芯长35 mm、 纤芯直径为85 μm的传感器在45%～95%RH湿度变化范围内, 湿度灵敏度为0.06 dBm·(%RH)-1。 在20～80 ℃温度范围内, 传感器的温度灵敏度为0.008 nm·℃-1, 温度所带来的湿度测量误差为0.047%RH·℃-1。 传感器具有制作简单、 灵敏度高、 反射式能量检测等优点, 在湿度测量领域有一定的应用价值。

A fiber humidity sensor based on Fiber-Bragg Grating (FBG) sandwiched in single-mode-multimode fiber core-single mode (SMS) fiber structure is proposed and demonstrated. When the surrounding humidity changes, the central wavelength of FBG remains unchanged for it is insensitive to humidity, while the interference spectrum of SMS fiber structure will shift for it is sensitive to the surrounding humidity. Hence, the shift of the SMS fiber structure interference spectrum with humidity could modulate the FBG core mode. Through measuring the reflected power of the FBG core mode the detection of humidity can be realized. The beam propagation of the SMS fiber structure with different lengths of multimode fiber core (MMFC), diameters of MMFC, and surrounding refractive indices are theoretically simulated with beam propagation method. Theoretical simulation indicates that the output core mode power coefficients shift with surrounding humidity of the SMS fiber structure. Experimental results show that the sensor has a linear response to humidity with enhanced sensitivity of 0.06 dBm·(%RH)-1 in the humidity range of 45%~95%RH with length of 35 mm and diameter of 85 μm. The temperature effect of the sensor is also discussed, the temperature sensitivity is 0.008 nm·℃-1 in the temperature range of 20~80 ℃ and the measurement error of temperature is 0.047% RH·℃-1. Such cost-effective, high sensitive, and reflective power detection based optical fiber humidity sensor could be used in humidity sensing applications.