Optimized Tapered Optical Fiber for Ethanol (C2H5OH) Concentration

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 32, NO. 9, MAY 1, 2014

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Optimized Tapered Optical Fiber for Ethanol (C2H5OH) Concentration Sensing Hang-Zhou Yang, Xue-Guang Qiao, M. Mahmood Ali, Md. Rajibul Islam, and Kok-Sing Lim, Member, IEEE

Abstract—An optimized study of biconical tapered multi-mode plastic optical fiber sensor for concentration sensing of ethanol (C2 H5 OH) is presented. The sensitivity is enhanced through V-number matching as well as by optimizing the taper radius and taper length. The ray-tracing method is used to analyze the evanescent wave penetration depth (EWPD). The theoretical analysis and experimental results are used to optimize the taper ratio and taper length for the achievement of high EWPD and high sensitivity. The analysis indicates that the sensitivity of tapered fiber sensor can be improved by decreasing the taper ratio with simultaneous increase in the taper length. The highest sensitivity of 1.527 mV/% is achieved from the tapered fiber with a taper ratio of 0.27 and taper length of 8 cm. The proposed parametric optimized tapered fiber sensor can detect the change in concentration of C2 H5OH as small as 6.55 × 10−3 . Index Terms—Ethanol concentration, evanescent wave penetration depth, tapered fiber sensor, V-number.

I. INTRODUCTION APERED fiber is well known for its property to generate Evanescent Waves (EWs) and has potential applications as distributed and remote sensing and detection of many physical parameters [1]–[13]. Light waves propagating through an optical fiber can be seen as the sum of two components: the guided waves in the core region and the EWs in the cladding region which have exponentially decaying behavior [2]. The effect of evanescent waves can be enhanced significantly by modifying the cladding radius. One of the standard ways to modify the cladding radius is to make the fiber in tapered shape. Generally, there are two geometrical structures of tapered fiber named as distal end tapers and biconical tapers. The magnitude change of the EW field is detected by measuring the output power of the

T

Manuscript received December 17, 2013; revised February 17, 2014; accepted March 9, 2014. Date of publication March 10, 2014; date of current version April 10, 2014. This work was supported by the National Natural Science Foundation of China (Nos. 60727004, 61077060, 61205080, 61235005), National High Technology Research and Development Program 863 (Nos. 2007AA03Z413, 2009AA06Z203), Ministry of Education Project of Science and Technology Innovation (No. Z08119), Ministry of Science and Technology Project of International Cooperation (No.2008CR1063), Shaanxi Province Project of Science and Technology Innovation (Nos. 2009ZKC01-19, 2008ZDGC-14), University of Malaya HIR Grant (UM.C/625/1/HIR/181, a/c no: J-21001-73860), and FRGS (FP002–2013B). H.-Z. Yang and X.-G. Qiao are with the College of Physics, Northwest University, Xi’an, Shaanxi 710069, China (e-mail: [email protected]; [email protected]). M. M. Ali, Md. R. Islam, and K.-S. Lim are with the Photonics Research Centre and Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail: [email protected]; [email protected]; [email protected]). 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/JLT.2014.2311175

tapered based fiber sensors [2]. Two major setbacks in the design of distal end taper fiber have been identified in comparison to biconical taper fiber. First, only a small amount of power is available in the EWs of the fiber [3]. Second, in signal acquisition, the collected optical power by the tapered fiber is low due to the poor ambient-fiber coupling efficiency for the EWs [4], [5]. Whereas, the biconical tapered fiber overcomes aforementioned limitations as availability of optical power and coupling of EWs unlike distal end taper fiber. Therefore stronger excitations and higher level of EWs power acquisition at the output end of biconical tapered fiber can be achieved and is preferred in the design of most sensing applications. As the EWs decay exponentially with the radius of the tapered fiber so the strength of the evanescent field strongly depends upon the several parameters of tapered fiber such as refractive index of core, refractive index of the aqueous surrounding mediums, the core radius and operation wavelength. The theoretical investigation of tapered fiber sensors depends upon the application for which these are being used. Many researchers have done a lot of research on tapered fiber sensors in different perspectives by relating them with applications. Singh et al. [6] proposed a gas sensor based on attenuated total internal reflection technique, in which the sensor is constructed from a tapered multimode step-index fiber with porous cladding. In their study, the theoretical results indicate that the geometries structure of the tapered fiber, such as, taper ratio, and taper profile influence the sensor performance including the sensitivity, response time and the minimum detectable concentration of the gas, also it can be seen that tapered fiber with exponential-linear profile provides a faster response time, lower detectable concentrations than other two taper profiles. Corres et al. [7] proposed a humidity sensor based on tapered fiber. In their work, the coating sensitive to humidity, named as polymetric nano-assembled composites, was deposited on the tapered region. Several parameters and their influence to the sensor performance have been studied i.e., the thickness of sensitive material, light sources and dimensions of the taper. Most recently, a polymer blend of hydroxyethylcellulose/polyvinylidenefluoride (HEC/PVDF) composite sensitive to humidity was coated on the cladding of tapered plastic optical fiber for sensing ambient humidity [8]. D. King et al. [9] presented the method for ethanol concentration sensing using three-sensor multipoint optical fiber, in this study they used multiple U-shape fiber probe sensors for generating EWs. As multiple U-shape fiber probe sensors make complex structure so biconical tapered fiber is proposed to make it more simple and efficient. In the previous work, such as ref. [4] only the V-number theory was used for analyte RI sensing. However, the differences

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is defined as [6]

L0 for z ≤ 2 (1) where r(z ) = (rm − r0 ) L2z0 + r0 is the radius distribution of the linear fiber taper in the propagation direction of z and L0 is the length of taper. r(z ) − r0 = Taper ratio = r0

Fig. 1. The model diagram for ray propagation inside the core of biconical tapered fiber.

between RI sensitivity and the geometries of biconical tapered fibers cannot be sufficiently reflected by V-number theory. In this paper, in addition to the V-number theory, a ray-tracing theory is introduced, which demonstrates its potential ability to trace the ray propagation along the tapered region. It can provide information on how the propagation ray is perturbed by the different geometries of tapered fibers and the ambient RI and temperature; therefore, the RI sensitivity can be improved from the optimized geometries of the tapered fiber. The plastic multimode fiber is biconically tapered to improve EWPD significantly, for detecting the concentration of C2 H5 OH. The tapered fiber based sensors that employ intensity modulation technique for RI sensing normally inherit many advantages, such as low cost and easy installation, compared with the FBG and tapered single mode fiber techniques. An multimode plastic fiber is chosen in this study because of its typical inheritance properties, such as better signal coupling, larger core radii, higher numerical aperture, and larger thermal-optic coefficient, compared to those of optical glass fiber. The geometry of tapered fiber is optimized for the achievement of high EWPD and a sufficient amount of power can be transmitted to create the EWs. In the experiment, a number of biconical tapered fibers have been fabricated with different length, radii and taper angle of structure and in particular for investigating the concentration of C2 H5 OH and in the result optimization has been carried out. In Section II, the brief theoretical analysis and parametric optimization for getting higher EWFD and sensitivity of tapered fiber sensor is discussed. In Section III, the experimental results with brief discussions are presented. Also, the optimized design of tapered fiber sensor has been proposed in this section. The conclusions have been drawn as in Section IV. II. MATHEMATICAL ANALYSIS OF PROPOSED TECHNIQUE In this section, the brief mathematical analysis of tapered fiber has been carried out. Fig. 1 depicts ray propagation along the tapered fiber. The z-axis is the core axis of taper fiber has an origin (z = 0) at the centre of the taper structure. Also z ≤ L20 this means that the total length of taper is L0 . The ray propagation from taper length results the radius of tapered fiber is decreasing from r0 to rm along the taper region. The profile of biconical tapered fiber is considered to be linear and the ray-tracing method is used for theoretical investigation of different parameters of sensor i.e., V-number matching, and evanescent wave penetration depth. Also taper ratio is another important parameter which is considered in optimization and is

rm − r0 r0

2z L0

A. V-Number Matching The number of modes guided by optical fiber is governed by the V-number i.e., [3] 2πr0 n2co − n2cl (2) v= λ where λ is the wavelength of transmitting light, r0 is the radius of fiber core, nco and ncl are the refractive index of fiber core and cladding, respectively. The number of propagation modes is approximately equals to V 2 /2. Removal of cladding results to increase V-number and eventually the number of modes is increased in the uncladded region as the cladding is replaced by air or solutions which have lower RI than that of silica glass. Due to this mismatch of V-number, the transmitted light from the uncladded region to the cladded region may undergo attenuation in intensity due to the reduction of supported number of modes in the cladded region. This is a common V-number mismatch phenomenon encountered in tapered multimode fiber and therefore the following optimization and matching procedures are required [4]. The V-number of the uncladded fiber is given by 2πr(z ) 2 nco (T ) − n2aq (T ) (3) v(z ) = λ where nco (T) is the refractive index of fiber core in a funcdn c o co tion of temperature nco (T ) = nco + dn dT . The term dT is the dn thermo-optic coefficient of fiber core, naq (T ) = naq + dTa q is the refractive index of surrounded material change with dn temperature, dTa q is the thermo-optic coefficient of surrounded material. Also, the term rm in the expression of r(z ) , is the minima of taper radius which avoids loss by V-number mismatching, so rm can be written as, n2co − n2cl . (4) rm = r0 n2co (T ) − n2aq (T ) Thereby, in the condition of V-number matching and optimum radius of taper, the taper angle α and the taper length Lo for the fiber core radius r0 can be defined as [3],

2 − n2 n 2r 0 co cl α = tan−1 1− Lo n2co (T ) − n2aq (T ) Lo =

2(r0 − rm ) . tan(α)

(5)

Fig. 2 shows the relationship between the optimum taper radii and RI of different solutions. In this figure, it can be observed that tapered fiber with smaller radius is required for low index

YANG et al.: OPTIMIZED TAPERED OPTICAL FIBER FOR ETHANOL (C2 H5 OH) CONCENTRATION SENSING

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The issue of intensity variation of modes has been taken into account in the sense that lowest order mode has greatest intensity after optimization of taper structure. From Fig. 1, it is obvious that the propagation ray in the cladding region has an initial angle θ0 = sin−1 (sin θ/nco ) with the fiber axis, where the angle θ is the light incident angle at the input cross-section face of tapered fiber. In the cladding region, the temperature-induced index change for the core and cladding are the same since they are made of the same material PMMA. As the ray reached the taper region, it makes a variant angle θz with the fiber axis, which varies with the geometry of tapered fiber. The relationship between the θ0 and θz is given by [3], Fig. 2.

The optimum taper radius for various surrounded solution RI.

r0 sin(θ0 ) = rz sin(θz ).

(6)

By combining Eqs. (4) and (6), the following expression for propagation angle θz can be derived as, ⎛ ⎞ ⎜ θz = sin−1 ⎜ ⎝

Fig. 3. Optimum the taper length for different solutions corresponding to the setting of various taper angles.

solution to avoid large V-number mismatch. The analytical results are based on the parameters given for plastic optical fiber: ro = 500 μm, nco = 1.492 and ncl = 1.402. However, the V-number mismatch is a critical factor in terms of limiting signal coupling, reduction the taper radius that is not so sufficient to optimize the EW acquisition. Therefore, to optimize the EWs, the optimum taper radius should be combined with its optimum length which is shown in the Fig. 3. The refractive indices for air, methanol, water, 2-propanol 1-propanol used in Fig. 3 are 1.3284, 1.333, 1.3617, 1.3772, and 1.3856, respectively. It is the optimum taper length for the aliphatic alcohol by adjusting value of taper angle. The results in this figure indicate that the higher the refractive index of solution, the shorter the lengths of tapered fiber as well as the lower taper angle gives the longer length of taper. B. Evanescent Wave Penetration Depth In the propagation of guided modes of light, the lowest order mode has the close-fit confinement of the field and hence the weakest EW field. Tapering operation can be used to modify the refractive index of the guiding material and light guiding properties in the fiber. This is the reason we have used the ray trace model for the achievement of optimized taper structure.

dp (z) = 2π

θ nco (T )2 cos2 sin−1 2 z n c o rsin m Lo

r0

2z n c o Lo

sin θ n 2c o −n 2c l n 2c o (T )−n 2a q (T )

− 1 + nco

⎟ ⎟ ⎠

(7)

where θin is the incidence ray angle with the normal to the fiber core and surrounding medium interface. θin is a variant with the variation of z; hence, the θin (z) can be arrived from the triangle in Fig. 3 which has the relationship between the taper angle α and ray lunched angle θz , π (8) θin (z) = − θz − α. 2 The penetration depth is a parameter to describe the distance of evanescent field extends beyond the core surrounding mediums interface in the uncladded region. It is given by [10] dp =

λ . 2π n2co sin2 θin − n2aq

(9)

By substituting the Eq. (8) into Eq. (9), the expression for dp is written as (10), shown at the bottom of the page. From Eq. (10), we can found that to achieve a maximum EWPD, the parameters of taper ratio, length and laser lunched angle should be optimized. Given the liquid refractive index and temperature T , an optimum taper radius and length can be designed for the highest EWPD based on the Eqs. (4), (5) and (10). Furthermore, small change of surrounding temperature causes a small change in refractive indices of fiber core and surrounding medium which eventually affects the output of sensors. Fig. 4 shows that the EWPD is decreasing with the increase of the taper ratio until it reaches its minimum value when taper ratio = 1, where the fiber is uniform or non-taper. The maximum EWPD of 0.18 μm is obtained for a taper ratio of 0.1 in combination with the ray launch angle of 0.1 rad. This figure also shows that the increasing of the ray launched angle the higher λ

−1 +n c o

−

tan−1

2r 0 Lo

1−

rm r0

. − naq

(T )2

(10)

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Fig. 6.

Experiment setup for proposed tapered fiber sensing technique.

III. EXPERIMENTAL RESULTS AND DISCUSSION

Fig. 4. Variation of EWPD in a function of taper ratio and initial ray launched angles, (ambient liquid is ethanol, temperature is 25 ◦ C, and taper length is 1 cm).

Fig. 5. EWPD of taper length along the various half taper regions in different initial ray launched angles, (ambient liquid is ethanol, temperature is 25 ◦ C and taper ratio is 0.1).

EWPD can be excited in the taper region. The ray launched angle of zero evoked the smallest EWPD, which indicates that the ray is parallel emitted with the fiber z-axis. The change of ray launch angle will excites different EWPD which influences the performance of the sensor. The relationship between EWPD with different taper lengths and ray launch angles is also investigated. Four tapered fibers with half taper lengths of 1, 1.5, 2 cm, and 2.5 cm are used in the simulations. As shown in Fig. 5, EWPD increases with increasing taper length, especially in the region of larger initial ray launched angle. For various length of biconical tapered fiber, its highest EWPD is obtained at their each half region. In this figure, the highest EWPD is produced from taper length of 5 cm combined with the ray launched angle of 0.1 rad. The EWPD is affected by the ray launched angle in agreement with aforementioned discussion that large ray launched angle yields higher EWPD.

A linear tapered fiber is fabricated from a multi-mode plastic optical fiber (POF) by removing the cladding using chemical etching technique and careful scraped by abrasive paper. The POF (Thorlabs) has a cladding radius of 500 μm, core radius of 490 μm, refractive indices of 1.492 and 1.402 for the core and cladding, respectively. The fiber core is made of acrylic polymer PMMA (polymethyl-methacrylate) and is cladded with a thin layer of fluorine polymer (cladding) to reduce the refractive index than that of fiber core. To taper the fiber, acetone solution is applied on the surface of the fiber by using cotton buds. Some outer portion of POF is dissolved to form a layer of milky white material. Distilled water is used to neutralize the reaction between the fiber and acetone. The milky white surface is carefully removed by using sandpaper with grits size of 320. This process is repeated until specific taper length and radius are achieved. Lastly, the tapered fiber is cleaned by using the 2-propanol and distilled water. Fig. 6 shows the schematic diagram of the experimental setup. A He-Ne laser with a wavelength of 594 nm, average power of 3.0 mW, beam diameter of 0.75 mm and beam divergence of 0.92 mRad is coupled to the tapered plaster fiber through its fiber facet at an angle of 0.1 rad. This is to protect the laser source from the reflected light from the fiber facet. Between the laser source and fiber facet, the petri dish is used to contain the solutions under test while the sensing element is immersed and located at the center of petri dish. A photo detector is located at end of the fiber to convert the received light power into voltage. The input light is modulated at a frequency of 100 Hz and synchronous with the output signal from the photodetector through a lockin amplifier (SR-510) to enhance the SNR of the signal. [14], [15]. The sensitivity of lock-in amplifier and the data-acquisition system was set at 10 μV. The ethanol solution and distilled water are pre-stored in the room for 1 day to achieve temperature equilibrium. During the experiment, the room temperature is kept at 25 ◦ C and the alignment of tray and detector should lie in the same horizontal axis to ensure there is no bending applied on the fiber. Fig. 7(a)–(c) shows the output responses of the tapered fiber sensors with the same taper length of 4 cm but different taper ratios of 0.27, 0.53, and 0.80 in the detection of three different concentrations of ethanol. The proposed sensors with taper ratios of 0.27 and 0.53 give distinct output response to solutions of different ethanol concentrations and the results are as shown in Fig. 7(a) and (b), however, taper ratio of 0.80 does not give any distinctive output response to the solutions of different


Fig. 7. The output responses of tapered fiber sensors and its sensitivities for same taper length L but different taper ratios R. There ethanol concentrations, 10%, 25%, and 50% are tested. (a) L = 4 cm, R = 0.27, (b) L = 4 cm, R = 0.53, (c) L = 4 cm, R = 0.80, and (d) sensitivity.

concentrations. This can be attributed to the V-number mismatching between the sensing portion and cladding portion at taper ratio of 0.8. Based on the Eq. (3), to avoid the V-number mismatching, the largest taper ratio i.e., 0.76 is used to distinguish the difference for various ethanol concentrations. The sensitivities of three sensing elements are shown in Fig. 7(d). The sensor with taper ratio of 0.27 has a highest sensitivity of 1.302 mV/%, in which a sensitivity of 0.512 mV/% is obtained for the ratio of 0.8. The trends in this figure show that the higher sensitivity can be achieved by the sensor with a smaller taper ratio as it produces the largest EWPD. The performance of the sensor with different taper lengths is shown in Fig. 8. Similar to the analysis for Fig. 7 and Fig. 8(d)

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Fig. 8. The responses of tapered fiber sensors and its sensitivities based on same taper ratio R, but different taper length L. Three different ethanol concentrations- 10%, 25%, and 50% are tested. (a) L = 2 cm, R = 0.48, (b) L = 5 cm, R = 0.48, (c) L = 8 cm, R = 0.48, and (d) sensitivity performance.

depicts the sensitivity performance of the sensors based on the output response in Fig. 8(a)–(c). The highest sensitivity of 1.432 mV/% is produced from taper length of 8 cm which is ∼2.16 times higher than 2 cm. Taper length is not one of the parameters in the consideration of V-number mismatch, nevertheless the result in Fig. 5 indicates that it is crucial in the optimization of EWPD. The longer is the taper length, the higher is the EWPD which yields higher sensitivity to the solution concentration. From the results shown in Fig. 7 and Fig. 8, it can be seen that the higher sensitivity of tapered fiber is obtained from the lower taper ratio and longer taper length. Hence, a tapered fiber with taper ratio of 0.27, taper length of 8 cm is fabricated and

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Fig. 9. The response of tapered fiber sensor (a) L = 8 cm, R = 0.27, and (b) its sensitivity for ethanol concentrations of 10%, 25% and 50%.

maintain the temperature of water. The results in Fig. 10 show that at higher temperature, the lower output power from the sensor is observed. Basically, there are two explanations for the degradation in sensor output power. As we know the plastic fiber core is manufactured from polymethylmethacrylate (PMMA) and the cladding is from perfluorinated PMMA. After the removal of the cladding, the core is exposed to the surrounding solution directly which may have strong OH− absorption in the visible light range, so the attenuation rate through the proposed sensor is altered significantly by the degree of moisture [16]. This type of power loss in POF is categorized as intrinsic loss. Besides, extrinsic losses may be the second constituent to the power loss in tapered fiber where a 1% deviation of the fiber core diameter can yield power losses as high as 20% [17] while the refractive index of fiber core is the function of temperature. This can also be explained from the molecule theory—the energy is deposited into the material when light is turned into heat which ten increases the temperature of the material. The rise in temperature can cause the structural deformation in molecules or break up these molecules into smaller ones. This leads to decreasing in the output intensity of the tapered fiber. Through optimization, the proposed sensor has exhibited many advantages in terms of sensitivity, repeatability and accuracy for ethanol concentration measurement. Similar approach can applied for other applications for instance liquid level sensing [18]. For the applications that involve higher temperature, plastic optical fiber may not be the suitable component due to its low melting temperature. Further investigations on other types of fibers are required. IV. CONCLUSION

Fig. 10.

The sensor output response to distil water at different temperature.

the same experiment is performed on it. Its results are shown in Fig. 9(a), sensitivity is shown in (b). A sensitivity of 1.527 mV/% is obtained by this tapered fiber sensor. We can see that this value is the highest in all of these aforementioned tapered fiber sensors if comparison the Fig. 7(d), Fig. 8(d) and Fig. 9(b). As such, to achieve a higher sensitivity for the tapered fiber sensor a lower taper ratio should be combined with a longer taper length. The ethanol concentration change as small as of 6.55 × 10−3 can be detected by using this sensor. Also, the experiments in Fig. 9 were repeated many times by rinsing the tapered sensing part using distilled water and then immersed it into each analyte in turn. The results finding that the proposed sensor has a stable sensitivity in the range of 1.5272–1.5276 mV/%. Besides, the experimental results show that the proposed fiber sensor has a fast response time in the detection of ethanol concentration. In our observation, the sensor output intensity takes ∼5 s in average to come to its steady state as shown in Figs. 7, 8 and 9. Moreover, in the other experiment, the sensor performance in water under the influence of three different temperatures, 25 ◦ C, 30 ◦ C, and 35 ◦ C, has been investigated and the results are shown in Fig. 10. A tapered fiber with radius of 240 μm, length of 2 cm is used in investigation. A digital hotplate is used to heat and

In the design of tapered plastic fiber sensor, the sensitivity of the sensor has been improved by optimizing the geometrical parameters of the sensor namely taper radius, length and temperature. V-number matching is considered in the optimization to yield high EWPD and subsequently the sensitivity. In this investigation, from theoretical and experimental results, it has been shown that higher sensitivity can be achieved by employing low taper ratio and long taper length of tapered fiber. The highest sensitivity i.e., 1.527 mV/% can be attained for C2 H5 OH concentration, based on the taper ratio of 0.27 and the taper length of 8 cm. It has sensing resolution of 6.55 × 10−3 . The average response time has been investigated for sensor as 5 s in the experiment. REFERENCES [1] A. Leung, P. M. Shankar, and R. Mutharasan, “A review of fiber optical biosensors,” Sens. Actuator B, vol. 125, pp. 688–703, 2007. [2] W. Love, L. Button, and R. Slovacek, “Optical characteristics of fiber optic evanescent wave sensors,” in Biosensors With Fiber Optics, D. L. Wise and L. B. Wingard, Eds., Totowa, NJ, USA; Humana Press, 1991, vol. 139. [3] A. W. Snyder and J. D. Love, Optical Waveguide Theory. New York, NY, USA: Chapman & Hall, 1983, p. 109. [4] J. P. Golden, P. A. George, Y. R. Sina, and S. L. Frances, “An evanescent wave biosensor—Part II: Fluorescent signal acquisition from tapered fiber optics probes,” IEEE Trans. Biomed. Eng., vol. 41, no. 6, pp. 585–591, Jun. 1994.


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[5] R. B. Thompson and L. Kondracki, “Sensitivity enhancement of evanescent wave–excited fiber optic fluorescence sensors, time resolved laser spectroscopy in biochemistry, II,” Proc. SPIE, vol. 1204, pp. 35–41, 1995. [6] C. D. Singh, Y. Shibata, and M. Ogita, “A theoretical study of tapered, porous clad optical fibers for detection of gases,” Sens. Actuator B, vol. 92, pp. 44–48, 2003. [7] M. C. Jesus, J. A. Francisco, and R. M. Ignacio, “Design of humidity sensors based on tapered optical fibers,” J. Lightw. Technol., vol. 24, no. 11, pp. 4329–4336, Nov. 2006. [8] M. Batumalay, A. Lokman, F. Ahmad, H. Arof, H. Ahmad, and S. W. Harun, “Tapered plastic optical fiber coated with HEC/PVDF for measurement of relative humidity,” IEEE Sens. J., vol. 13, no. 12, pp. 4702–4705, Dec. 2013. [9] D. King, W. B. Lyons, C. Flanagan, and E. Lewis, “Interpreting complex data from a three-sensor multipoint optical fiber ethanol concentration sensor system using artificial neural network pattern recognition,” Meas. Sci. Technol., vol. 15, pp. 1560–1567, 2004. [10] N. Nath and S. Anand, “Evanescent wave fiber optic fluorosensor: Effect of tapering configuration on the signal acquisition,” Opt. Eng., vol. 37, pp. 220–228, 1998. [11] H. Golnabi and R. Jafari, “Design and performance of an optical fiber sensor based on light leakage,” Rev. Sci. Instrum., vol. 77, pp. 1–3, 2006. [12] H. Golnabi, “Surface profiling using a double-fiber optical design,” Opt. Las. Eng., vol. 48, pp. 421–426, 2010. [13] H. Golnabi and P. Azimi, “Design and performance of a plastic optical fiber leakage sensor,” Opt. Las. Technol., vol. 39, pp. 1346–1350, 2007. [14] H. Z. Yang, K. S. Lim, and S. W. H. Ahmad, “Enhanced bundled fiber displacement sensor based on concave mirror,” Sens. Actuator A, vol. 162, pp. 8–12, 2010. [15] H. Z. Yang, S. W. Harun, H. Arof, and H. Ahmad, “Environment independent liquid level sensing based on fiber-optic displacement sensors,” Microw. Opt. Techn. Lett., vol. 53, pp. 2451–2453, 2011. [16] M. Naritomi, “CYTOP amorphous fluoropolymers for low loss POF,” POF Asia Pacific Forum, Tokyo, Japan, 1996. [17] J. Zubia and J. Arrue, “Plastic optical fibers: An introduction to their technological processes and applications,” Opt. Fiber Technol., vol. 7, pp. 101–140, 2001. [18] H. Golnabi, M. Bahar, M. Razani, M. Abrishami, and A. Asadpour, “Design and operation of an evanescent optical fiber sensor,” Opt. Las. Eng., vol. 45, no. 1, pp. 12–18, 2007.

Xue-Guang Qiao received the B.Sc. degree in physics from Xi’an Jiaotong University, Xi’an, China, in 1982, and the Ph.D. degree in optics from the Xi’an Institute of Optics and Precision Mechanics of CAS, Xi’an, in 1998. From 1999 to 2000, he was at the Massachusetts Institute of Technology, USA, as a visiting scholar. He is currently a Professor at the College of Physics, Northwest University, Xi’an. His research interests includes photonics technology, fiber communication and sensing, fiber logging in oil and gas fields, geophysical prospecting and oil and gas pipeline inspection. He is coauthor of over 160 publications in journals and conference proceedings and is the coinventor on seven invention patents.

Hang-Zhou Yang received the B.Eng. degree in telecommunication engineering from Air Force Engineering University, China, in 2005, and the Ph.D. degree in photonic research from the University of Malaya, in 2012. He is currently an Assistant Professor at College of Physics, Northwest University, China. His current research interests include fiber optic sensing and communication technologies, and micro and nanophotonics.

Kok-Sing Lim received the B.E. degree from the Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Malaysia, in 2008, and the Ph.D. degree from the Photonics Research Centre, Department of Physics at the same university in 2012. He is currently a Senior Lecturer at the Photonics Research Centre, University of Malaya, and his present research interests include optics of microfibre resonators, fiber Bragg grating sensors and fibre lasers.

M. Mahmood Ali received the B.S. and M.S. degrees in electronic engineering from the University College of Engineering and Technology, The Islamia University of Bahawaplur, Pakistan, and Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi, Pakistan, in 2009 and 2012, respectively. He is currently working toward the Ph.D. degree in photonic engineering under Bright Spark Fellowship from Photonics Research Centre, University of Malaya, Kuala Lumpur, Malaysia. He is the Member of Pakistan Engineering Council (PEC), Member of National Academy of Young Scientists (NAYS), Pakistan. His research interests include fiber Bragg grating sensors and their applications, fiber lasers, nonlinear fiber optics, advanced digital signal and image processing, wave propagation in biisotropic media, advance electromagnetic field theory, and microwave engineering.

Md. Rajibul Islam was born in Nilphamari, Bangladesh. He received the B.C.A (Bachelor of Computer Applications) degree from Indira Gandhi National Open University, New Delhi, India, in 2004, and the M.Sc degree in information technology from Multimedia University, Melaka, Malaysia, in 2010. He is currently working as a Research Assistant with the Photonics Research Centre, University of Malaya, Kuala Lumpur, Malaysia. His current research interests include photonics, fiber Bragg grating sensors, acousto-optic sensors, fiber optic vibration sensors, and structural health monitoring.