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OPTICAL FIBER HUMIDITY SENSOR BASED ON A TAPERED FIBER WITH HYDROXYETHYLCELLULOSE/POLYVINY- LIDENEFLUORIDE COMPOSITE Asiah Lokman, 1 Somayeh Nodehi, 2 M. Batumalay, 1 H. Arof, 1 H. Ahmad, 2 and Sulaiman Wadi Harun 1,2 1 Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia; Corresponding author: swharun@um.edu.my 2 Photonics Research Centre, University of Malaya, Kuala Lumpur 50603, Malaysia Received 25 May 2013 ABSTRACT: A simple relative humidity (RH) sensor is demonstrated using a tapered fiber with an hydrogel coating. Hydroxyethylcellulose/ polyvinylidenefluoride polymers are used to form the hydrogel coating of the tapered fiber as they are sensitive to moisture and thus the humid- ity of the atmosphere. Changes in humidity level alter the refractive index of the fiber coating and this condition leads to variation in optical output power. A difference of up to 0.89 dB of the transmitted optical power is observed when RH changes from 50 to 80%. The proposed sen- sor has a sensitivity of about 0.0228 dB/%RH with a slope linearity of more than 99.91%. In summary, the hydrogel coating acts as an inner cladding whose refractive index decreases with the rise in humidity and thus allows more light to be transmitted in humid state. V C 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:380–382, 2014; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.28091 Key words: fiber optic sensor; tapered optical fiber; humidity sensor; relative humidity; hydroxyethylcellulose/polyvinylidenefluoride 1. INTRODUCTION Relative humidity (RH) of the air is defined as the ratio of the water vapor in the atmosphere to the saturation value. Common methods used to measure RH employ resistive, capacitive, and hygrometric sensors [1]. Among these methods, the ones utilizing conventional electrical sensors suffer from several drawbacks such as high cost and need for maintenance. Another disadvant- age is they are unsuitable to be used in hazardous or explosive environments or places where electromagnetic interference immunity is required. Optical fiber sensors are not affected by these limitations. They also offer the possibility of multiplexing a large number of different sensors (temperature, displacement, pressure, pH value, humidity, high magnetic field, and accelera- tion) into the same optical fiber, thus reducing the need for multi- ple cabling required in traditional electronic sensing [1]. To date, a wide range of RH sensing techniques based on optical fibers have been reported including the ones using long period gratings [2], fiber Bragg gratings [3], side polished fibers [4], plastic opti- cal fibers [5], and surface plasmon resonance [6]. Tapered optical fibers have attracted much interest as they exhibit a proportionally large evanescent field that travel along the cladding and can be manipulated for various sensing appli- cations [7, 8]. Consequently, the travelling wave characteristics of tapered fibers are more sensitive to the ambience of its sur- rounding. Recently, many evanescent wave-based sensors have been proposed and demonstrated for humidity measurement. For instance, Muto et al. demonstrated humidity sensors which are based on reversible absorption of water (H 2 O) from the ambient atmosphere into a porous thin-film interferometer that sits on the tapered fiber [5]. The water absorbed from the ambience changes the refractive index of the thin films and subsequently transforms the lossy fiber into a light guide. Humidity sensing was also demonstrated using a tapered fibre with agarose gel [9]. In this article, we present an innovative RH sensor based on tapered fiber coated with a polymer blend of hydroxyethylcellu- lose/polyvinylidenefluoride (HEC/PVDF) composite. The com- posite coating changes its optical properties in response to the change in RH of its surrounding. The measurement is based on intensity modulation technique where the output power of the transmitted light is investigated for changes in RH. The pro- posed RH sensor is simple to operate, easy to fabricate, low in cost but high in sensitivity, and has a large operational dynamic range. 2. PREPARATION OF THE SENSOR PROBE AND EXPERIMENTAL SETUP Preliminary research has reported a variety of sensing materials, such as polyimide, crystal violet, porous silica xerogel [10], 380 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 2, February 2014 DOI 10.1002/mop