Abstract Humidity is an important parameter in many industry fields and accurate humidity measurement is of great significance to improve production efficiency and ensure product quality. This paper presents a sensing model and scheme for utilizing thermoplastic polyimide (PI) coated fiber Bragg grating (FBG). The humidity sensitive PI layer was firstly prepared and coated on the FBG. The humidity calibration experiment was designed to verify the sensor performance based the saturated salt solution method. The experimental results show this FBG humidity sensor can work steady in the relative humidity (RH) condition ranging from 11%RH to 83% RH, with the sensitivity of the sensor less than 2 pm/% RH and measurement uncertainty of ±1% RH in the temperature range of 30 °C to 70 °C. In order to solve the problem that the PI-coated FBG is both sensitive to the humidity and temperature (cross sensitivity problem), a novel method of temperature compensation has been proposed. The method uses the character that the coefficients of expansion of the PI-coated layer for humidity and temperature are linearly superimposed and obtains the temperature compensation coefficient by linear interpolation of the function relationship between the measured temperature and the FBG central wavelength shift. The analysis of experimental data shows that within the temperature variation range of 20 °C, this temperature compensation method can effectively guarantee the average wavelength errors within 0.03 nm without reference grating. In this way, it can extend the FBG humidity measurement under environmental temperature up to 90 °C making it possible for application in the measurement the humidity in the high temperature environment.

中文翻译：

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一种基于聚酰亚胺涂层光纤布拉格光栅的光纤传感器，用于测量相对湿度

摘要 湿度是许多工业领域的重要参数，准确的湿度测量对于提高生产效率、保证产品质量具有重要意义。本文提出了一种利用热塑性聚酰亚胺 (PI) 涂层光纤布拉格光栅 (FBG) 的传感模型和方案。首先制备湿敏PI层并涂覆在FBG上。湿度校准实验旨在验证基于饱和盐溶液方法的传感器性能。实验结果表明，该FBG湿度传感器可在11%RH~83%RH的相对湿度（RH）条件下稳定工作，传感器灵敏度小于2pm/%RH，测量不确定度为±1%RH在 30°C 至 70°C 的温度范围内。为了解决涂有PI的FBG对湿度和温度都敏感（交叉敏感问题）的问题，提出了一种新的温度补偿方法。该方法利用PI包覆层的膨胀系数对湿度和温度的线性叠加特性，通过对被测温度与FBG中心波长偏移的函数关系进行线性插值得到温度补偿系数。实验数据分析表明，在20°C的温度变化范围内，这种温度补偿方法可以有效保证无参考光栅的平均波长误差在0.03 nm以内。这样，