Polyimide-coated fiber Bragg grating for relative humidity sensing

Yao Lin,Yu Wu,Huijuan Wu,Yuan Gong

doi:10.1007/s13320-014-0218-8

Yao Lin, Yu Wu + Show 2 more

Open Access

https://doi.org/10.1007/s13320-014-0218-8

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Abstract

A fiber-optic humidity sensor has been fabricated by coating a moisture sensitive polymer film to the fiber Bragg grating (FBG). The Bragg wavelength of the polyimide-coated FBG changes while it is exposed to different humidity conditions due to the volume expansion of the polyimide coating. The characteristics of sensors, including sensitivity, temporal response, and hysteresis, were improved by controlling the coating thickness and the degree of imidization during the thermal curing process of the polyimide. In the relative humidity (RH) condition ranging from 11.3% RH to 97.3% RH, the sensitivity of the sensor was about 13.5 pm/% RH with measurement uncertainty of ±1.5% RH.

Highlights

With the development of industry and agriculture, the requirements for humidity control and sensing in the areas of aerospace, defense, and power industries are increasingly urgent, especially when the key performance such as the environmental adaptability, measurement range, long-term stability, and measurement accuracy are required to be higher
Some experiments have verified the excellent linear relationship between the relative humidity and Bragg wavelength shift of the fiber Bragg grating (FBG) coated with the moisture sensitive material, e.g., PI [10]
Are the changes in temperature and relative humidity, rH is the thickness of the polyimide coating, and rF is the radius of the fiber

Summary

Introduction

With the development of industry and agriculture, the requirements for humidity control and sensing in the areas of aerospace, defense, and power industries are increasingly urgent, especially when the key performance such as the environmental adaptability, measurement range, long-term stability, and measurement accuracy are required to be higher. Compared to conventional capacitive and resistive humidity sensors, the FBG coated with polyimide (PI) used for humidity sensing has unique advantages in terms of high temperature resistance, corrosion resistance, high measurement accuracy, miniaturization, etc. The fiber Bragg gratings coated with various sensitive materials have been applied to measure different kinds of samples such as hydrogen, and for ultrasonic [4, 6] and humidity sensing [8, 9]. The temperature and humidity responses of polyimide-coated FBG humidity sensors have been studied by Yeo and other researchers [10,11,12,13]. Yao LIN et al.: Polyimide-Coated Fiber Bragg Grating for Relative Humidity Sensing humidity sensor was modified, and the fabrication parameters such as the coating thickness and heating temperature were optimized. The sensing performance of the polyimide-coated FBG humidity sensor was evaluated in detail

Sensing principle

Moisture-sensitive material

Time response

Humidity sensitivity

Comprehensive performance

Conclusions