Low-Coherence Interferometric Fiber Optic Sensor for Humidity Monitoring Based on Nafion® Thin Film.

Erwin Maciak

doi:10.3390/s19030629

Abstract

The main aim of this work was the design and development simple fiber optic Fabry-Perot interferometer (FPI) sensor devices for relative humidity (RH) sensing with emphasis on high sensitivity and good stability. The RH fiber FPI sensor is fabricated by coating the end of a cleaved standard multi-mode (MM) fiber with hydrophilic Nafion® sensing film. The Nafion® thin film acts as an active resonance cavity of the low-coherence interferometric sensing structure. The fringe pattern, which is caused by interfering light beam in the Nafion® thin film will shift as the RH changes because the water molecules will swell the Nafion® film and thus change optical pathlength of the sensing structure. The operating principle of a FPI sensor based on the adsorption and desorption of water vapour in the Nafion® and the limitations of this sensor type are discussed in this work. The fiber optic hygrometer was tested in the visible (400–900 nm) region of spectra for measurement of relative humidity (RH) in the range of 5.5–80% at room temperature (RT) in air. The fiber optic humidity sensor has a very short response time (t90 = 5–80 s) and a fast regeneration time (t10 = 5–12 s) as good as commercial sensors.

Highlights

Advanced technology, which is a powerful force in the development of civilization, requires finding effective sensor techniques for measuring the concentrations of various substances
The problems connected with monitoring the concentration of various gas analytes have become important due to environmental protection needs
The results from this study show optical humidity sensing combined with relatively cheap optical components and instruments that were developed for use in the visible region (350–780 nm)

Summary

Introduction

Advanced technology, which is a powerful force in the development of civilization, requires finding effective sensor techniques for measuring the concentrations of various substances. High-tech sensor applications, including detection, measurement and control systems for pollutants and gas emissions, are extremely important for an ecologically sustainable development. Information about the humidity level present in an ambient environment or in industry processes is one of the most important. Humidity measurements determine the amount of water vapor present in a gas that can be a mixture, such as air. The importance of the humidity measurements in science and industry arises from the fact that many important natural and synthetic materials change their characteristics with moisture

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