IEEE SENSORS JOURNAL, VOL. 12, NO. 5, MAY 2012 1361

Fiber Optic Displacement Sensor forTemperature Measurement

Husna Abdul Rahman, Sulaiman Wadi Harun, Norazlina Saidin, Moh. Yasin, and Harith Ahmad

Abstract—A simple design of a temperature sensor is proposedand demonstrated using a fiber optic displacement sensor basedon an intensity modulation technique. The proposed sensor uses aplastic optical fiber (POF)-based coupler as a probe in conjunctionwith a flat surface aluminum rod as a target. The aluminum rod isplaced within the linear range of the sensor’s displacement curve,which is from 0 to 1400 m. The sensor is capable of measuringthe temperature of an aluminum rod ranging from 42 C to 90 Cwith a measured sensitivity of 0.0044�� C, with a linearity ofmore than 98% and a resolution of 2.4 C. The proposed sensoralso shows a high degree of stability and good repeatability. Thesimplicity of design, accuracy, flexible dynamic range, and the lowcost of fabrication are favorable attributes of the sensor and bene-ficial for real-field applications.

Index Terms—Fiber optic, fiber optic displacement sensor, tem-perature fiber sensor.

I. INTRODUCTION

F IBER optic sensors enable measurements of a variety ofparameters in conditions where other sensor technologies

fail or are simply unsuitable [1]–[3]. This type of sensing de-vices has intrinsic advantages, including resistance to electro-magnetic interference, non-electrical conductivity, passive mea-surements, small size and low weight, and compatibility withoptical fiber technology. One of the important applications ap-propriate for fiber sensors is in the area of temperature analysis.To date, various types of fiber optic temperature sensors have

Manuscript received March 05, 2011; revised August 01, 2011; accepted Oc-tober 06, 2011. Date of publication October 17, 2011; date of current versionApril 13, 2012. This work was supported by the University of Malaya underPPP Grant PV033/2011A and HIR-MOHE Grant D0000009-16001. The asso-ciate editor coordinating the review of this manuscript and approving it for pub-lication was Prof. Istvan Barsony.

H. A. Rahman is with the Department of Electrical Engineering, Faculty ofEngineering, University of Malaya, 50603 Kuala Lumpur, Malaysia, with thePhotonics Research Centre, Department of Physics, Faculty of Science, Uni-versity of Malaya, 50603 Kuala Lumpur, Malaysia, and also with the Facultyof Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 ShahAlam, Malaysia.

S. W. Harun and N. Saidin are with the Department of Electrical Engineering,Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia,and also with the Photonics Research Centre, Department of Physics, Facultyof Science, University of Malaya, 50603 Kuala Lumpur, Malaysia (e-mail:swharun@um.edu.my).

M. Yasin is with the Photonics Research Centre, Department of Physics, Fac-ulty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia, and alsowith the Department of Physics, Faculty of Science and Technology, AirlanggaUniversity, Surabaya 60115, Indonesia (e-mail: myasin@um.edu.my).

H. Ahmad is with the Photonics Research Centre, Department of Physics,Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2011.2172409

been reported in the literature such as fiber interferometric [4],fluorescence based fiber optic temperature sensors [5], and op-tical scatterring [6]. However, the first type of sensors are ratherexpensive to produce and complicated to implement on-site [7].The second type often require high cost lasers and detection sys-tems [8] while the third type must use materials that can interactwith a beam of monochromatic radiation in order to produce fre-quency shifts in the scattered photons. Furthermore, this methodhas limited temperature range and resolution [9].

Noncontact displacement detection method using fiber opticlever technique was originally proposed by Frank and Kissinger[10] and its geometrical analysis was later done by Cook andHamm [11]. Later on, theoretical analysis of the displacementsensor based on geometrical and Gaussian approach were de-veloped [12]. Plastic optical fibers (POFs) have widespread usein the transmission and processing of optical signals in opticalfiber communication system compatible with the Internet. POFsalso have potential applications in WDM systems, power split-ters and couplers, amplifiers, sensors, scramblers, integrated op-tical devices, frequency up-conversion, and etc. [13], [14]. Re-cently, an intensity modulated fiber optic displacement sensorshave been demonstrated to be efficient for different applications[15], [16]. They are relatively inexpensive, easy to fabricate andsuitable for employment in harsh environments. In this paper,a rugged, low cost and very efficient fiber optic displacementsensor (FODS) is proposed for the measurement of temperature.The proposed sensor’s operation is based on the intensity modu-lation technique using a POF-based coupler with 50:50 splittingratio as a probe and an aluminum rod with a linear thermal ex-pansion as a target.

II. EXPERIMENTAL SETUP

The proposed temperature sensor is schematically shown inFig. 1. The sensor is essentially a fiber optic displacement sensorwith a 3 dB multimode fiber coupler as a probe. A 594 nm He-Nebeam is modulated by a chopper before it is launched into port1 of the coupler. Light travels to port 3 and is scattered when itexits the fiber end. It is then reflected by the top surface of analuminium rod with 0.5 cm diameter and 7 cm length. The port3 probe is held in position about 1 mm perpendicular to the topsurface of the aluminium rod so that the reflected light can beeasily launched back into the same port. The collected light issent to port 2 by the 3 dB coupler and measured by a siliconphoto-detector. The signal from the photo-detector is convertedto voltage and is measured by a lock-in amplifier. The outputresult from the lock-in amplifier is then connected to a computer

1530-437X/$26.00 © 2011 IEEE

1362 IEEE SENSORS JOURNAL, VOL. 12, NO. 5, MAY 2012

Fig. 1. Experimental setup for the proposed temperature sensor using a POF-based coupler.

Fig. 2. Variation of output voltage against the displacement.

through a RS232 port interface and the signals are processedusing Delphi software.

In the calibration stage, the static displacement of the probeis achieved by mounting it on a micrometer translation stage,which is rigidly attached to a vibration free table. Firstly, at zeropoint where the aluminum rod and the probe are in close contact,the output from port 2 is measured. Then the aluminum rod ismoved away from the probe in 50 m steps and at each position,under vibrationless condition, the output voltage is recorded. Agraph of displacement (gap) against output voltage is drawn anda linear range on the graph is identified. A position at the centerof the linear range is chosen and the gap between the probe andthe aluminum is fixed at the chosen displacement point. Thenan experiment is carried out where the aluminum is fixed ontoa hotplate for heating purpose. A thermocouple placed at theupper region of the aluminum rod is used to display and monitorthe temperature of the aluminum rod. The thermocouple has aresolution of 1 and is able to measure the temperature withina range of to 1300 . The heat of the aluminum rodis controlled by varying the heat intensity produced by the hot-plate ranging from the room temperature (25 ) to a maximumtemperature of 90 . Further increase in the temperature may

TABLE ITHE PERFORMANCE OF FIBER OPTIC DISPLACEMENT SENSOR

damage the plastic-based coupler that is being used in this ex-periment. However, the maximum temperature can be increasedwith the use of a heat resistant fiber probe.

III. RESULTS AND DISCUSSIONS

Fig. 2 shows the result of the calibration at room temperaturewhere the output voltage is measured against the gap betweenthe probe and the end surface of the aluminum rod. The fiberreceives the maximum reflected light when the gap is zero, andthus the measured intensity of the reflected light is maximum atthis position as indicated in the figure. Moreover, the measuredintensity of the reflected light decreases almost linearly as thedistance or gap increases especially for close distance. Theoret-ically, the distance and the reflected power vary according to theinverse square law and the ratio between the reflected power andthe transmitted power is given by

(1)

where , , , , and are the reflected power, transmittedpower, core diameter, axial displacement and fiber’s acceptanceangle, respectively. In this experiment, the core diameter andacceptance angle of the fiber are 960 m and 30 , respectively.The characteristic of the displacement curve is summarized inTable I where the sensitivity is obtained at 0.0005 m andthe slope shows a good linearity of more than 99% within thedisplacement range of 0 m to 1400 m. The displacementsensor is observed to be very stable with the measurement errorof less than 0.8%.

RAHMAN et al.: FIBER OPTIC DISPLACEMENT SENSOR FOR TEMPERATURE MEASUREMENT 1363

TABLE IITHE PERFORMANCE OF THE TEMPERATURE SENSOR USING FODS

Fig. 3. Linear relationship between the output voltage and aluminum rod tem-perature. Inset shows the stability characteristics of the sensor at three differenttemperatures: 25 �, 60 � and 90 �.

Fig. 3 shows the linear function of the output signal againstthe aluminum rod displacement for two different runs. The twodifferent runs were taken because repeatibility of results is cru-cial in the operation of any sensor system. In the experiment, thegap between the aluminum rod and fiber tip of port 3 is fixedat 1 mm, which is within the linear range of the displacementresponse without the temperature effect. The temperature of thealuminum rod is then increased from 25 to 90 , resulting inan output signal ranging from 1.00 mV to 1.20 mV. The outputsignal starts to significantly increase with the rise in rod tem-perature, and at 42 and forward a linear function is formeduntil the rod temperature reaches the maximum temperature of90 . It is observed that the maximum difference between thetwo runs is about 0.04 mV, which is small compared to the fullrange of 1.20 mV. The inset of Fig. 3 shows the stability of thesensor at three different temperatures. The temperature sensoris observed to be very stable for all three temperature settings.For instance, the measurement error of less than 0.8% was ob-tained at 90 . The output voltage is recorded for 200 secondsand the standard deviation obtained at 25 , 60 and 90are 0.5%, 0.5% and 0.8% respectively. The performance of thetemperature sensor is summarized in Table II where the sensi-tivity of the linear function for the first run is 0.0044with 98% linearity whereas the sensitivity of the linear functionfor the second run is 0.0041 mV/ with 96% linearity.

The equation for the linear thermal expansion of a metal rodis given as follows:

(2)

where is the rod length variation, is the initial length,is the linear thermal coefficient of rod (for Aluminum:

), and is the temperature change. Basedon this equation, the linear relationship between the outputvoltage and displacement is obtained for the aluminum rodtemperature change, which is similar to the trend producedby the initial displacement sensor without any temperatureeffects. The sensitivity of the sensor with temperature effects ishigher (0.0028 m) than the latter (0.0005 m) eventhough the initial gap between the aluminum rod and fiber tipof port 3 is fixed at 1 mm, which is within the linear range ofthe displacement response without temperature effect. This isbecause as temperature increase, the reflectivity of aluminumwill also increase [17], hence sensitivity increases when highertemperature is applied to the aluminum rod while maintainingthe linearity of the sensor output. The possible sources of errorin the sensor operation can be due to light source fluctuation,stray light and possible mechanical vibrations. To reduce theseeffects a well-regulated power supply is used for the yellowHe-Ne laser and this minimizes the fluctuation of source inten-sity. The sensor fixture is also designed so that the stray lightcannot interfere with the source light and room light does nothave any effect on the output voltage. To reduce the mechanicalvibrations, the experimental set-up is arranged on a vibrationfree table.

IV. CONCLUSIONS

An extrinsic fiber-optic temperature sensor is demonstratedusing a multimode POF coupler and an aluminum rod to mea-sure temperatures ranging from 42 to 90 . At room tem-perature, the displacement curve of the sensor has a sensitivityof 0.0005 m and a linearity of more than 99% within ameasurement range between 0 to 1400 m. By placing the alu-minum rod within the linear range, the measured output signalis observed to be a linear function of the aluminum rod temper-ature with a sensitivity of 0.0044 , a linearity of morethan 98% and a resolution of 2.4 . The dynamic range of oper-ation, high degree of sensitivity, stability and good repeatabilityof the system are major advantages of this design compared toother temperature sensing designs. Furthermore, the simplicityof the design and the low cost of the fabrication make it attrac-tive for real field application.

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Husna Abdul Rahman received the B. Eng (Hons)from Multimedia University, Malaysia, in 2002, andthe M.Sc. degree in mobile communication systemsfrom the University of Surrey, Surrey, U.K., in 2005.She is currently working toward the Ph.D. degree inphotonics at the University of Malaya, Malaysia.

She is also a Lecturer with the Faculty of Elec-trical Engineering, MARA University of Technology(UiTM), Malaysia.

Sulaiman Wadi Harun received the B.E. degreein electrical and electronics system engineeringfrom Nagaoka University of Technology, Nagaoka,Japan in 1996, and the M.Sc. and Ph.D. degrees inphotonics from the University of Malaya, Malaysia,in 2001 and 2004, respectively.

Currently, he is a Full Professor with the Faculty ofEngineering, University of Malaya. His research in-terests include fiber optic active and passive devices.

Norazlina Saidin received the B.Eng. (Hons) and M.Eng. degree in telecom-munication engineering from the University of Malaya, Malaysia, in 2005 and2009, respectively, where she is currently working toward the Ph.D. degree inphotonics.

She is also a Lecturer at Kuliyyah of Engineering, International Islamic Uni-versity, Malaysia.

Moh. Yasin received the B.Sc. degree from Air-langga University, Indonesia, in 1990, and the M.Sc.and Ph.D. degrees from Gadjah Mada University,Indonesia, in 1999 and 2010, respectively.

Currently, he is an Associate Professor with theFaculty of Science and Technology, Airlangga Uni-versity, Indonesia. His research interests include fiberoptic communications and sensors.

Harith Ahmad received the first class degree in physics from the University ofMalaya, Malaysia, in 1979, and the M.Sc. degree in high voltage technology andPh.D. degree in laser technology from the University of Wales, Cardiff, U.K.,in 1980 and 1983, respectively.

He is currently a Professor of photonics with the University of Malaya,Malaysia.