Low-Cost Multipoint Liquid-Level Sensor With Plastic ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 16, AUGUST 15, 2014

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Low-Cost Multipoint Liquid-Level Sensor With Plastic Optical Fiber Xiao Lin, Liyong Ren, Yiping Xu, Nana Chen, Haijuan Ju, Jian Liang, Zhengquan He, Enshi Qu, Baowen Hu, and Yulin Li Abstract— A simple and low-cost discrete liquid-level measurement system is present in this letter. It consists of a group of plastic optical fiber segments, which are aligned coaxially and spaced equally. When the spacing between every two adjacent fiber segments is filled with liquid, the light power will be easier to couple from one segment to the next as compared with the situation that the fibers are exposed in air. Based on this point, we design this intensity-based sensor and investigate its working properties theoretically by using the ray-tracing method. The performance of this sensor is demonstrated in detail where different liquids are utilized as specimens. Index Terms— Optical fiber sensor, plastic optical fiber, liquidlevel measurement, ray-tracing.

I. I NTRODUCTION

A

S WE all know, plastic optical fibers (POFs) process a number of advantages, such as immunity to electromagnetic interference, light weight, excellent flexility, low cost, and large numerical aperture, etc. Owing to these merits, POF is widely acted as transducer or signal transmission medium in the measurement of strain, temperature, and refractive index, etc [1]–[3]. The researches about liquid-level measurement have attracted great interest, because it is crucial to flood early warning, fuel-level monitor in tanks and chemical industry, etc. Given the fact that optical fiber sensors have lots of advantages, e.g., high accuracy and compact size, various optical fiber liquid-level sensors are investigated which are based on different operating principles [4]–[9]. Some of these sensors are founded on the frustrated total internal reflection effect, in which a prism or a specially shaped fiber tip is employed as the sensing head [4]. Another kind of all optical fiber liquidlevel sensors is constructed with fiber grating, or operating with multimode interference effect [5], [6]. Comparing with the above two types of sensors, the POF intensity-based liquid-level sensor can achieve continuous gauge or multipoint monitor with a simpler structure and lower cost [7]–[9]. The basic principle of these intensity-based sensors is introducing a continuously or periodically varied loss when the liquid-level keeps changing.

Manuscript received April 25, 2014; revised May 28, 2014; accepted June 2, 2014. Date of publication June 5, 2014; date of current version July 24, 2014. This work was supported by the National Natural Science Foundation of China under Grant 61275149, Grant 51207129, and Grant 61275086. The authors are with the State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2329037

Fig. 1. Diagram of the sensor system used to realize the discrete liquid-level measurement.

In this letter, we propose an intensity-based POF sensor to monitor the liquid-level. This sensor is extremely low-cost, easy to fabricate and simple in structure. It consists of a group of POF segments which are aligned coaxially and spaced equally. When all the gaps between segments are initially exposed in air, the light coupling between every two adjacent segments is weak, and consequently, the output power from the last segment is low. If the liquid-level keeps increasing, more gaps will be filled with liquid gradually. As a result, the light coupling is no longer so weak and the detected power is enhanced accordingly. Based on this principle, we design this sensor with the aid of ray-tracing method. The corresponding experiments are also performed to verify our design where water, sucrose solution and diesel oil are served as liquid measurand. II. S ENSOR C ONFIGURATION AND O PERATING P RINCIPLE As shown in Fig. 1, a group of POF segments, which are utilized as the sensor elements, are aligned coaxially and spaced equally. The beam irradiated from laser diode (LD) couples into a scrambler to achieve the equilibrium mode distribution (EMD), which aims to mitigate the influence of the launch condition. Then, this beam will keep transmitting in the input POF for several meters until it reaches the 1st gap. The length of the POF under the 1st gap is l0 , and the 1st gap is employed as the 1st sensing head. The 1st fiber segment is located between the 1st and 2nd gaps, and is just below liquid-level 2. Likewise, the (n-1)th POF segment is between the (n-1)th and n th gaps, and is below liquid-level n, while the n th gap acts as the n th sensing head. The length of the n th fiber segment and gap are labeled as ln and gn , respectively.

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Fig. 2. (a) Two-dimensional diagram of a single sensing head. (b) Effect of the index of the medium in gaps on the transmission coefficient while three different θfi s are concerned.

After the beam has transmitted through all these fiber segments and gaps, it will be captured by the output POF and finally be detected by power meter. When the liquid-level is rising, more gaps will be filled with the liquid. The light coupling becomes strong and the detected light power is enhanced due to the increase of the refractive index of the medium in the gaps. The detailed analysis is shown as follow. Firstly, let us analyze the simple case for just having one sensing head. As shown in Fig. 2(a), θfi is the propagation angle of a certain ray in the launch POF, θg is the corresponding refraction angle in the gap, and θft is the propagation angle of the captured ray in the receiving POF. With Snell laws, we could find that, when the index of the medium in the gap, n g , is increasing, θg will keep reducing, which means more rays emitted from the launch POF could be captured by the receiving POF. In addition, these captured rays will still transmit with the original propagation angle θfi , namely, θfi = θft . When an unpolarized beam transmits from POF into the gap, the transmission coefficient, T , could be written as sin(2θ f i ) sin(2θg ) 1 ], (1) T = 0.5 [1 + 2 2 cos (θ f i -θg ) sin (θ f i + θg ) where θg =arcsin[n c sin(θfi )/n g ] and n c is the index of POF core. When n g is increasing in the range of 1 to 1.45, T is also augmenting as shown in Fig. 2(b) where three different θfi s are used. This result indicates that more light power from the launch POF could be coupled into the medium of the gap. Similarly, more light power could also be coupled from the gap into the receiving POF. Therefore, it is easy to conclude that more light power could be transported from one POF segment to the next if the gap is filled with a high index medium instead of air. Thus, when an increasing number of gaps are filled with liquid as the result of the rise of the liquid-level, the detected power at the output POF keeps enhancing accordingly. III. E XPERIMENTAL S ETUP AND T HEORETICAL M ODEL A power-tunable LD with the centre wavelength of 660 nm and the wavelength bandwidth of 10 nm is utilized as the light source. The POF used to monitor the liquid-level is the step-index one (Super Eska SK-40, Mitsubishi Rayon Co.). The diameter and the index of the core are 980 μm and 1.49, while the thickness and the index of the cladding are 10 μm and 1.40. Although, in principle, each segment could be arbitrary length, in our experiment all the segments are set at 25 cm as an example. Note that such a simplified setting does


Fig. 3. (a) Structure of a single sensing head in the experiment. (b) Apparatus utilized to fabricate the sensing head. (c) Micrograph of a real sensing head, whose gap length is about 1 mm.

not lose the generality. Before the fabrication of the sensor, the end faces of each POF segment are polished with lapping films. Then, we fix two POF segments onto a two dimension translation stage with jigs, and precisely align the segments under the microscope. When this arrangement of the POF segments is accomplished, we set a PMMA substrate under the POF segments and put a metal stick beside the POF segments to improve the structural strength of the sensing head. After that, the epoxy resin, which has no influence on the physical or chemical characters of the POF [10], is spread onto the POF, the metal stick and the substrate to anchor the POF permanently as shown in Fig. 3(a). Note that the area near the gap should not be covered by resin to keep the gap clean. The corresponding apparatus and the micrograph of the sensing head are shown in Fig. 3(b) and (c). In order to theoretically investigate the performance of the sensor and design the sensing head, a simulation model is established by using the ray-tracing method. Note that such a method is widely used in the POF research [2], [7], [8], [11]. In this letter, a two-dimension (2-D) ray-tracing model is built due to the axial symmetry of the sensor structure. This model is composed of several coaxially aligned imaginary 2-D fibers. The configuration of two adjacent imaginary fibers is similar to that shown in Fig.2(a). The size and index settings are identical to the real POF, namely, the diameter of the core is 980 μm, the thickness of the cladding is 10 μm, and the indices of the core and cladding are 1.49 and 1.40, respectively. As an EMD beam is used in the real system, we need to acquire an imaginary light source under the EMD, initially. Like Arrue demonstrated [12], a special structure that comprises a series of imaginary bent fibers could be employed as a scrambler in the simulation program to obtain the imaginary light source under the EMD. Here, we also use this method and translate the original model reported in our early work [13] into 2D configuration to generate 2D imaginary EMD source. By utilizing this method, when the parallel rays are launched into this imaginary scrambler, the calculated output far-field angular power distribution from this scrambler is obtained and plotted in Fig. 4(a). Meanwhile, the calculated output rays will be deployed as the imaginary EMD source. For comparison, the measured output far-field angular power distribution from the real system is also shown in the same figure. It is evident that the EMD is really achieved in the real system, and the simulated result is coincident with the experimental one.

LIN et al.: LOW-COST MULTIPOINT LIQUID-LEVEL SENSOR

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Fig. 4. (a) Simulated and measured far-field angular power distribution under the EMD condition. (b) For a single sensing head, the effect of the gap length on the coupling coefficient where the sensing head is exposed in air or water.

After the imaginary source is established, the rays will be emitted from it, and keep propagating in the imaginary 2D POF segments and gaps. When the rays propagate through the n th sensing head like that shown in Fig. 2(a), there exist two steps of refractions, namely, the refractions occur at the interface between the launch POF and the gap, as well as the interface between the gap and the receiving POF. With raytracing method, it is simple for us to acquire the geometrical relations between the rays and the two interfaces, meaning that the θfi , θg and θft of each ray could be obtained. Moreover, we notice that some rays escape from the end face of the launch POF cannot be captured by the receiving POF. Therefore, the total transmission coefficient of these lost rays, Tn,i , is set to be 0. The subscript n denotes the n th sensing head and i reprsents the i th ray. On the other hand, other rays will finally be captured by receiving POF after two steps of refractions. and The transmission coefficients at these two steps are Tn,i T "n,i , which can be calculated by Equ. (1), too. Thus, for the n th sensing head, the total transmission coefficient of each T and the total captured power of captured ray is Tn,i = Tn,i n,i the n th POF segment, Pn , is T n,i T n,i Pn−1,i = Tn,i Pn−1,i , (2) Pn = i

i

i th

where Pn−1,i is the power of the ray that exports from the (n-1)th POF segment. Considering that several sensing heads are included in the system, the light power emits from the output POF, Pt , could be calculated by successively using Equ. (2) for N times Pt =

N [ (Tn,i )P0,i ], i

(3)

n=1

where N is the amount of the sensing head and P0,i is the light power of the i th ray of the imaginary EMD source. Since the theoretical model has been completely established, let us use it to determine the gap length, gn , which is predicted to have significant impact on the light coupling of a single sensing head. Here, we use the coupling coefficient, defined as the ratio of the output power of the receiving POF to the incidence power of the launch POF shown in Fig. 2(a), to scale the light coupling strength. During our simulation n g are set to be 1 and 1.33, respectively, when the gap is immersed in air and in water. We theoretically calculate the coupling coefficients with different gn s, as shown in Fig. 4(b). Note that in this figure the experimental results are given, too. And these experimental results verify our above prediction well. Also,

it is conspicuous that the coupling coefficient of water is higher than that of air due to the increase of n g . Meanwhile, one can notice that, no matter the gap is filled with air or water, the coupling coefficient will continue to decline if the gap keeps expanding. Hence, if gn is set to be excessively large, more loss will be introduced. On the other hand, while the liquid-level falls below the sensor head, some residual liquids would exist in the gap due to the strong surface tension if gn is set to extremely small. This problem is detrimental to the response time and repeatability of the sensor. To investigate this problem, we fabricate two single sensor heads whose gn s are 0.5 mm and 1 mm, respectively. By regulating the liquid-level to rise above or fall below the sensor head repeatedly, the output powers are logged. We notice that the response time and repeatability are reasonable for gn = 1 mm, while for gn = 0.5 mm the response time expands a lot (>2 mins). Therefore, for achieving the real-time measurement in our research, we set gn at 1 mm. IV. E XPERIMENT AND S IMULATION R ESULTS Initially, we analyze the performance of the sensor with the theoretical model when three different liquid specimens (water, sucrose solution and diesel oil) are concerned. Note that, in this simulation, the amount of the imaginary sensing head, N, is set to be 6. While water is employed to examine the sensor, n g is set as 1.33, which is identical to the measured value. By using the simulation model, the output powers under different liquidtotal input power from the levels, Pt s, are obtained. As the EMD source is known as P0 = P0,i , the output ratio β, i

which is defined by β = Pt /P0 , could also be calculated. The simulation results of the βs under different liquid-levels are plotted in Fig. 5(a). It is obvious that, as the liquid-level rises, the simulated output ratio increases. When the sucrose solution and diesel oil act as the specimens, the refractive indices of the medium in the gaps are set to be 1.38 and 1.44, respectively. The simulated Pt s and βs are acquired with the aid of the theoretical model. The corresponding results are illustrated in Fig. 5(b) and (c), respectively. The tendencies of all these simulated curves shown in Fig. 5 are identical to each other. This result indicates that, no matter what the liquid is, the output ratio will increase provided that more gaps are filled with liquid. As the theoretical study is completed, for comparison, the experiment will be described and the measured results will be given as follow. In the sensor system, six sensing heads are introduced to achieve the multipoint measurement, and all the POF segments are bonded to a cantilever before the test. A power meter (Labmaster, Coherent) is employed to detect the output light power and transport this power information to the PC for further processing. In our experiment, water, sucrose solution and diesel oil are used as the specimens to examine the sensor. During the experiment, for each liquid, when the liquidlevel rises or falls repeatedly, the output powers are logged continuously. Owing to the fact that gn is large enough and the viscosity of the liquid is not very high, the liquid surface tension is weak and little liquid exists in the gap while the

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that a group of shorter segments are used. If more sensing heads are introduced into the system, it is promised to realize the large-scale measurement. In the real applications, it is better to wrap a filter cloth around the gap to prevent the penetration of the impurity in the liquid. V. C ONCLUSION

Fig. 5. Theoretical predictions and experimental examinations of the performance of the sensor system for three liquid specimen: (a) water, (b) sucrose solution, (c) diesel oil. (d) Theoretical predictions of the performance of the sensors with three different gap lengths.

liquid-level drops. Thus, the response time and repeatability of the sensor are satisfying. As the irradiated power from the scrambler could be measured, for different liquid-levels, the output ratios of the different liquids could be obtained. The corresponding experimental results of the six-head sensor are plotted in Fig. 5(a), (b) and (c), respectively. It should be pointed out that the inherent loss of the whole sensor is about 11 dB. Moreover, it also can be seen that, like the simulation results, no matter what the liquid is, the output ratio keeps increasing with the rise of liquid-level. However, it ought to be pointed that all the output ratios from the experiment are lower than that from simulation, which is caused by the imperfect end face of each sensing head. As we all know, the end face cannot be perfectly smooth, though we have delicately polished it. Owing to this point, comparing with the ideal simulation, more light power would be lost due to the scattering at a slightly rough end face in the experiment. In all, though a moderate difference exists between the theoretical and experimental results, the experiment corroborates the prediction, and the performance of the sensor is proved to be reasonable. The theoretical analysis and experimental examination of the performance of the sensor with the gap length gn =1 mm are accomplished. Then, considering that the gap length is a critical parameter to the performance of the sensor, we will analyze this influence theoretically for the case of water. Hereafter, 10 imaginary sensing heads are used in the theoretical model while gn are set to be 1, 1.5 and 2 mm, respectively. With our model, for these three different gap lengths, the simulated βs under the varied liquid-levels are easily obtained as shown in Fig. 5(d). As the figure illustrates, when the gap length is diminishing, the variation of β between two successive liquid-levels is augmenting accordingly, which benefits the power detection. Moreover, with a small gap length, the precision of this discrete liquid-level sensor is also enhanced. In conclusion, the gap length ought to be set relatively small provided that the liquid surface tension is too weak to deteriorate the response time. From all these theoretical and experimental results, it ought to be remarked that this sensor can achieve the multipoint liquid-level measurement with an extremely simple structure. The resolution of the sensor is 25 cm, which is determined by the length of the POF segment. It could be improved provided

We demonstrate a POF-based multipoint liquid-level sensor in this letter. The operation principle, construction and fabrication of the sensor are described in detail at first. Then, the influence of a critical parameter, named as the gap length, on the sensing performance is investigated theoretically and experimentally. As a result, a gap length of 1 mm is found to be suitable in the experiment when the loss, the response time and the repeatability are addressed. With the aid of raytracing method, we theoretically investigate the performance of such a kind of sensor with six sensing heads for three liquid specimens. And a sensor system is established accordingly. The simulated and measured results are coincident with each other to some degree, which verifies the feasibility of the sensor. Considering that the sensor can achieve real-time measurement and is easy to fabricate, it is promised to be utilized in many fields, such as the flood or tide early warning. R EFERENCES [1] K. Peters, “Polymer optical fiber sensors—A review,” Smart Mater. Struct., vol. 20, no. 1, pp. 013002-1–013002-17, 2011. [2] L. Bilro, N. J. Alberto, L. M. Sá, J. L. Pinto, and R. N. Nogueira, “Analytical analysis of side-polished plastic optical fiber as curvature and refractive index sensor,” J. Lightw. Technol., vol. 29, no. 6, pp. 864–870, Mar. 15, 2011. [3] J. Huang et al., “Polymer optical fiber for large strain measurement based on multimode interference,” Opt. Lett., vol. 37, no. 20, pp. 4308–4310, Oct. 2012. [4] I. K. Ilev and R. W. Waynant, “All-fiber-optic sensor for liquid level measurement,” Rev. Sci. Instrum., vol. 70, no. 5, pp. 2551–2554, May 1999. [5] B. Yun, N. Chen, and Y. Cui, “Highly sensitive liquid-level sensor based on etched fiber Bragg grating,” IEEE Photon. Technol. Lett., vol. 19, no. 21, pp. 1747–1749, Nov. 1, 2007. [6] J. E. Antonio-Lopez, J. J. Sanchez-Mondragon, P. LiKamWa, and D. A. May-Arrioja, “Fiber-optic sensor for liquid level measurement,” Opt. Lett., vol. 36, no. 17, pp. 3425–3427, Sep. 2011. [7] M. Lomer, J. Arrue, C. Jauregui, P. Aiestaran, J. Zubia, and J. M. López-Higuera, “Lateral polishing of bends in plastic optical fibres applied to a multipoint liquid-level measurement sensor,” Sens. Actuators A, Phys., vol. 137, pp. 68–73, Jan. 2007. [8] M. Lomer, A. Quintela, M. López-Amo, J. Zubia, and J. M. López-Higuera, “A quasi-distributed level sensor based on a bent side-polished plastic optical fibre cable,” Meas. Sci. Technol., vol. 18, no. 7, pp. 2261–2267, 2007. [9] K. S. C. Kuang, S. T. Quek, and M. Maalej, “Remote flood monitoring system based on plastic optical fibres and wireless motes,” Sens. Actuators A, Phys., vol. 147, pp. 449–455, Jan. 2008. [10] T. Hamouda, K. Peters, and A. F. M. Seyam, “Effect of resin type on the signal integrity of an embedded perfluorinated polymer optical fiber,” Smart Mater. Struct., vol. 21, no. 5, p. 055023, May, 2012. [11] X. Lin, L. Ren, E. Qu, J. Liang, and H. Ju, “Theoretical and experimental study on nonintrusive light injection via cladding in plastic optical fibers,” J. Lightw. Technol., vol. 31, no. 3, pp. 359–365, Feb. 1, 2013. [12] J. Arrue, G. Aldabaldetreku, G. Durana, J. Zubia, I. Garcés, and F. Jiménez, “Design of mode scramblers for step-index and graded-index plastic optical fibers,” J. Lightw. Technol., vol. 23, no. 3, pp. 1253–1260, Mar. 2005. [13] X. Lin, L. Ren, and J. Liang, “Nondestructive scheme for measuring the attenuation coefficient of polymer optical fiber,” Opt. Lett., vol. 38, no. 4, pp. 528–530, Feb. 2013.