4702 IEEE SENSORS JOURNAL, VOL. 13, NO. 12, DECEMBER 2013 Tapered Plastic Optical Fiber Coated With HEC/PVDF for Measurement of Relative Humidity Malathy Batumalay, Asiah Lokman, Fauzan Ahmad, Hamzah Arof, Harith Ahmad, and Sulaiman Wadi Harun Abstract—A simple humidity sensor is proposed and demon- strated using a tapered plastic optical fiber (POF) as a probe. Its operation is based on intensity modulation technique using a tapered POF probe coated with a polymer blend of hydroxyethyl- cellulose/polyvinylidenefluoride (HEC/PVDF) composite that acts as the humidity sensitive cladding. The sensor is fabricated using an etching method and has a waist diameter of 0.45 mm and tapering length of 10 mm. As the relative humidity varies from 50% to 85%, the output voltage of the sensor increases linearly from 0.32 to 1.25 mV. The HEC/PVDF composite-coated sensor exhibits a sensitivity of 0.023 mV/% with a slope linearity of > 99.65%. The sensitivity of HEC/PVDF composite-coated cladding toward humidity stems from its ability to swell as humidity increases in the atmosphere resulting in a drop in its refractive index below that of the core and thus allowing more light to be transmitted through the tapered fiber. Index Terms—Fiber optic sensor, tapered plastic optical fiber, humidity sensor, relative humidity (RH), hydroxyethylcellu- lose/polyvinylidenefluoride (HEC/PVDF). I. I NTRODUCTION O PTICAL fibers have been used for various applications ranging from transmission medium, which covers a wide spectral range and sensors. Research in fiber sensor devel- opment is expanding and many new applications have been introduced such as in sensing and monitoring humidity, gases, vapours, chemical substances, biosignals, medical and control processes, industrial automation and others [1]–[6]. Recently, tapered optical fibers have attracted much interest especially for sensing applications [2]. An optical fiber becomes more sensitive to its surrounding when it is tapered due to the enhanced power of its evanescent wave (EW) in the cladding. Interest in tapered multimode plastic fiber has also increased Manuscript received February 8, 2013; revised March 7, 2013; accepted March 24, 2013. Date of publication July 3, 2013; date of current ver- sion October 4, 2013. This work was supported in part by the MOHE High Impact Research under Grant D000009-16001. The associate editor coordinating the review of this paper and approving it for publication was Dr. Anna Grazia Mignani. M. Batumalay, A. Lokman, and S. W. Harun are with the Department of Electrical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail: malathy.batumalay@newinti.edu.my; lokman_asiah@yahoo.com; swharun@um.edu.my). F. Ahmad is with the Department of Electrical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia, and also with the Department of Electrical Engineering, Universiti Teknologi Malaysia, Kuala Lumpur 54100, Malaysia (e-mail: fauzan.se@gmail.com). H. Arof and H. Ahmad are with the Photonics Research Center, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail: ahamzah@um.edu.my; harith@yahoo.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2013.2272329 because of the fiber’s superior mechanical strength and ease of manufacturing. Recently, many EW 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. The water absorbed changes the refrac- tive 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 [10]. Corres et al. demonstrated a similar humidity sensor based on nanostructured films, which were deposited onto tapered fibres using the ionic self-assembled monolayer (ISAM) deposition technique [11], [12]. In another scheme, a side-polished optical fibre with a humidity sensitive overlay is used to construct a humidity sensor. Gaston et al. [13], [14] proposed a humidity sensor based on a single mode, side-polished fibre with a PVA overlay. Such a sensor was created by means of polishing the flat surface parallel to the fibre axis in order to remove the cladding. Side polishing can be realised by first immobilising the optical fibre in a rigid material, forming a rectangular block with the fibre extending out from the two end faces of the block orthogonal to the fibre axis. The advantage of this scheme is that the sensing element can be fabricated using inexpensive components and a variety of coating materials can be deposited onto the flat surface of the fibre block. However, the fabrication procedure is very time consuming, dependent upon the design of the fibre block and has limited exposed interaction length. As mentioned earlier, tapered optical fibers have gained much popularity in various sensing applications [15]–[17] due to its high sensitivity. Compared to silica based fiber, plastic optical fibers (POFs) possess several advantages such as ease of handling, mechanical strength, disposability and easy mass production of components and system [2]. In this paper, tapered POF is coated by a polymer blend of hydroxyethyl- cellulose/polyvinylidenefluoride (HEC/PVDF) composite to sense the change in relative humidity. The coating of the tapered fiber changes its optical properties in response to an external stimulus. The measurement is based on intensity mod- ulation technique where the output voltage of the transmitted light is investigated for changes in relative humidity. II. EXPERIMENT For the preparation of HEC/PVDF, 1 g of PVDF pow- der (Mw = 275,000) was dissolved in 120 ml dimethyl 1530-437X © 2013 IEEE