Highly Sensitive Temperature Sensors Based on Fiber-Optic ... - MDPI

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Jul 9, 2016 - School of Electronics Engineering, Kyungpook National University, ... temperature sensor has good sensing ability; its sensitivity is ~3.733 mV/˝C. The ... industry, the automotive industry, metal industries, geothermal wells, the ...
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Highly Sensitive Temperature Sensors Based on Fiber-Optic PWM and Capacitance Variation Using Thermochromic Sensing Membrane Md. Rajibur Rahaman Khan and Shin-Won Kang * School of Electronics Engineering, Kyungpook National University, 80 Daehakro, Bukgu, Daegu 41566, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-53-950-6829 (ext. 940-8609); Fax: +82-53-950-7932 Academic Editor: Alexander Star Received: 1 June 2016; Accepted: 6 July 2016; Published: 9 July 2016

Abstract: In this paper, we propose a temperature/thermal sensor that contains a Rhodamine-B sensing membrane. We applied two different sensing methods, namely, fiber-optic pulse width modulation (PWM) and an interdigitated capacitor (IDC)-based temperature sensor to measure the temperature from 5 ˝ C to 100 ˝ C. To the best of our knowledge, the fiber-optic PWM-based temperature sensor is reported for the first time in this study. The proposed fiber-optic PWM temperature sensor has good sensing ability; its sensitivity is ~3.733 mV/˝ C. The designed temperature-sensing system offers stable sensing responses over a wide dynamic range, good reproducibility properties with a relative standard deviation (RSD) of ~0.021, and the capacity for a linear sensing response with a correlation coefficient of R2 « 0.992 over a wide sensing range. In our study, we also developed an IDC temperature sensor that is based on the capacitance variation principle as the IDC sensing element is heated. We compared the performance of the proposed temperature-sensing systems with different fiber-optic temperature sensors (which are based on the fiber-optic wavelength shift method, the long grating fiber-optic Sagnac loop, and probe type fiber-optics) in terms of sensitivity, dynamic range, and linearity. We observed that the proposed sensing systems have better sensing performance than the above-mentioned sensing system. Keywords: temperature sensor; fiber-optic pulse width modulation; Rhodamine-B; interdigitated capacitor; sensing membrane; sensing element; dielectric constant

1. Introduction Temperature sensors are useful (even essential) devices in many areas of our lives. They are primarily used to measure and monitor the temperature driven by several environmental conditions. Temperature sensors have applications in various fields such as medicine/biomedicine, the food and beverage industry [1], agriculture and horticulture [2], industrial processing, and research and development [3]. In the field of medicine [4,5], temperature sensors are used in real time structural health monitoring (SHM), kidney dialysis machines, organ transplant systems, and medical incubators. In the agricultural sector, they are used for monitoring the temperature of plants, soil, and water [6]. In the food and beverage industry temperature sensors are used in fermentation, brewing, meat processing, and the fabrication of storage tanks. Temperature sensors are also used in the petrochemical industry, the automotive industry, metal industries, geothermal wells, the consumer electronics industry, petroleum industries, and in harsh environmental applications [7]. Over the last several decades, many sensors have been designed to detect temperature, such as capacitive [8,9], surface acoustic wave [10,11], carbon nanotube [12,13], Schottky diode [14], BJT-MOSFET pair [15], CMOS [16], surface Plasmon resonance (SPR) [17], and fiber-optic [18] sensors.

Sensors 2016, 16, 1064; doi:10.3390/s16071064

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Recently, fiber-optic temperature sensors have been shown to be excellent candidates because they offer advantages such as low weight, small dimensions, geometrical versatility, remote-sensing capabilities, freedom from electromagnetic interference, and complete electrical isolation. Therefore, they are highly safe and can sense multiple parameters using a single optical fiber with no cross talk. To date, many fiber-optic temperature sensors have been developed, such as the side-polished fiber-optic temperature sensor [19], U-shaped temperature sensor [20], D-type fiber-optic temperature sensor [21], fiber-optic Fabry-Perot temperature sensor [22], and fiber-optic Bragg-grating temperature sensor [23]. However, interdigitated electrode (IDE)-based sensors are in high demand at present. These sensors are used in a broad range of applications; for example, gas sensors [24], biosensors [25,26], pH sensors [27], chemical sensors [28], taste sensor [29,30], glucose sensors [31], humidity sensors [32], pressure sensors [33], and temperature sensors [34]. Mariam-deme Dankoco et al. proposed a flexible, inkjet-printed IDE-based temperature sensor to measure the temperature of the human body [35]. The sensor works on the resistive variation principle under a varied temperature. The construction and principle of operation of the sensor are very simple; it works at a bias voltage of 1 V and has a small size. However, the proposed sensor has a low dynamic range of 20–60 ˝ C and less stable performance. A fiber-optic MEMS temperature sensor was developed by Yixian Ge et al. in 2016 [36]. The operation principle of the sensor is based on optical Fabry-Perot interference and the bimetallic diaphragm effect. Although, their proposed temperature sensor has some advantages, it also has several disadvantages such as a complex fabrication procedure and a low dynamic range of 20–70 ˝ C. Ying Wang et al. proposed an ultrahigh sensitivity photonic crystal fiber temperature sensor in 2011 [37]. In their study, they create several air holes in the photonic crystal fiber; then, they fill one of the air holes with a standard refractive index liquid to create the sensor. The main feature of this sensor is its ultrahigh sensitivity, which is ~54.3 nm/˝ C. However, the proposed temperature sensor also has disadvantages such as a complex fabrication process and a very short dynamic range of approximately 34–35.4 ˝ C. Evanescent field coupling between a single-mode side-polished optical fiber and a polymer waveguide overlay on the side-polished optical fiber was proposed by Alberto Álvarez-Herrero et al. [38]. In this work, the temperature was measured by observing the resonance wavelength shift. Although the temperature sensor offers good sensitivity and linearity, it also has some disadvantages for example low dynamic range (26 ˝ C to 40 ˝ C) and difficulty of obtaining the resonance wavelength. Ruan proposed a Sagnac-loop-based fiber-optic sensor to measure temperature from 20 ˝ C to 50 ˝ C in 2015 [39]. In his work, he fabricated the sensor head by combining a single-mode/multimode/polarization-maintaining fiber with a tilted long-period fiber grating. The proposed temperature has good sensing ability and linearity; however, its temperature-sensing range is low and it has a complex fabrication process. In our experiment, we proposed a side-polished fiber-optic temperature sensor that is based on the principle of fiber-optic pulse width modulation (PWM) [40–42] technique. Using this principle, the received sensing signal pulse width changes as the side-polished fiber-optic temperature-sensing element is heated. A thermochromic dye, namely, Rhodamine-B, was mixed with a polymer and an N,N-dimethylacetamide (DMAC) solution to create a dielectric/thermally sensitive material. Then, this material was deposited on the side-polished fiber-optic device using a spin coater to create a fiber-optic sensing element. In our study, we also prepared an interdigitated capacitor (IDC) temperature sensor that is based on the capacitance variation principle as the IDC sensing element is heated. When the sensing elements (fiber-optic and IDC) are heated, the refractive index and dielectric constant of the sensing membrane change. As a result, the received sensing signal’s pulse width for the case of a fiber-optic sensing element changes. In the case of the IDC temperature-sensing element, the capacitance of the IDC sensing element changes. The proposed temperature sensors have many advantages including low-cost, compactness, ease of fabrication, high sensitivity, highly stable response performance, wide dynamic range, fast response and recovery times, and ease of manufacture (the

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high sensitivity, highly stable response performance, wide dynamic range, fast response and recovery times, and ease of manufacture (the designed circuitry can be prepared from easily obtainable, low-cost electronic components). Finally, we compare the performance of our proposed Sensors 2016, 16, 1064 and IDC temperature sensor with that of other sensors in terms of parameters 3 of 14 fiber-optic PWM (sensitivity, dynamic range, and linearity); we found that our proposed sensors offer better sensing performance than others. designed circuitry can be prepared from easily obtainable, low-cost electronic components). Finally, we compare the performance of our proposed fiber-optic PWM and IDC temperature sensor with that 2. Theory and Working Principle of other sensors in terms of parameters (sensitivity, dynamic range, and linearity); we found that our In oursensors study, offer we applied two different sensingthan principles proposed better sensing performance others.(i.e., fiber-optic PWM and capacitance variation) to prepare different types of temperature sensor to measure the temperature from 2. Theory 5–100 °C. and Working Principle In our study, we applied two different sensing principles (i.e., fiber-optic PWM and capacitance 2.1. Theoryto and Working Principle of the PWM Temperature Sensing System variation) prepare different types of Fiber-Optic temperature sensor to measure the temperature from 5–100 ˝ C. A sensing membrane containing Rhodamine B was deposited on a side-polished optical fiber to 2.1. Theory and Working Principle of the Fiber-Optic PWM Temperature Sensing System form an optical waveguide; the schematic diagram is shown in Figure 1a. When light travels through the side-polished fiber-optic device, aRhodamine fraction of the radiation spreads small distance to create A sensing membrane containing B was deposited on a aside-polished optical fiberan to evanescent field,waveguide; as defined the by [43]: form an optical schematic diagram is shown in Figure 1a. When light travels through the side-polished fiber-optic device, a fraction of the radiation spreads a small distance to create an  z  evanescent field, as defined by [43]: E  z  = E0 exp  (1)  ˆ dP ˙  z E0 expat the E pzqof“light ´ interface of core–cladding and z is the (1) where E 0 is the electric field amplitude dP distance of electric field in the cladding from the interface. d p is the penetration depth, and the where E0 is the electric field amplitude of light at the interface of core–cladding and z is the distance of sensitivity of the fiber-optic sensor depends on the penetration depth. The energy of the evanescent electric field in the cladding from the interface. dp is the penetration depth, and the sensitivity of the field may change because of changes in the refractive index of the overlay waveguide or changes in fiber-optic sensor depends on the penetration depth. The energy of the evanescent field may change the absorption or scattering of light into the overlay waveguide. Mathematically, penetration depth because of changes in the refractive index of the overlay waveguide or changes in the absorption or can be represented by [44]: scattering of light into the overlay waveguide. Mathematically, penetration depth can be represented -0.5 2 by [44]:  λ #  2 ˆ n 2˙2 + ´0.5 (2) dp = λ θ - n2   sin 2 n dp “ 2πn1 sin θ ´ (2)  1    2πn1 n1

where λλand and θ the are wavelength the wavelength the transmitted light the of incidence to the where θ are of theoftransmitted light and theand angle of angle incidence to the normal at normal at the interface, respectively. n is the refractive index of the fiber cladding. n is the the interface, respectively. n1 is the refractive index of the fiber cladding. n2 is the refractive index of 1 2 the material in contact with the in topcontact surfacewith of the refractive index of the material theoverlay. top surface of the overlay.

Figure 1. Schematic diagram of the temperature-sensing elements: (a) Side-polished optical fiber Figure 1. Schematic diagram of the temperature-sensing elements: (a) Side-polished optical fiber with with a sensing membrane; and (b) IDC sensing element. a sensing membrane; and (b) IDC sensing element.

An electrical PWM system generally has two inputs—a pulse input, which is used to input the electrical PWMand system generally haswhich two inputs—a input, usedof to the input the pulseAn into the system; a control input, is used topulse change thewhich pulse is width input pulse into the system; and a control input, which is used to change the pulse width of the input signal without changing the time period of the input signal—and one output, which is used to obtain signal without changing theour time period offiber-optic the input PWM signal—and one usedthrough to obtain the desire pulse width. In proposed system, weoutput, send awhich light is pulse a the desire pulse width. In our proposed fiber-optic PWM system, we send a light pulse through a fiber-optic-based polymer waveguide that contains a thermochromic compound. If the overlay waveguide containing a thermochromic compound is heated, then its optical properties (for example, refractive index) change. As a result, the peak value of the pulse and the fall time change because

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of the light absorption in the waveguide, which, in turn, changes the pulse width of the received sensing signal. The output received pulse width relies on light absorption within the overlay polymer waveguide, which can be taken into account as a pulse control input. This is because the absorption of the light pulse occurs due to the change in the refractive index of the overlay waveguide, which corresponds to the change in temperature. The received light pulse width TH can be written as [40,41]: T

TH “

´ n

γL α

(3)

¯0.5

where T and L are the time period of the light wave and the length of the polished cladding, respectively. γ is the evanescent wave absorption coefficient, α is the phenomenological ion-specific parameter, and n is the refractive index of the overlay waveguide. Therefore, by measuring the pulse width of the received signal, the amount of heat can be determined. The pulse width is proportional to the heat absorbed by the thermochromic-compound-based polymer waveguide, which corresponds to the absorption of the evanescent field into the wave guide. 2.2. Theory and Working Principle of the IDC Temperature Sensing System Figure 1b shows a schematic diagram of an IDC consisting of finger/comb-shaped electrodes covered with the thermochromic-compound-based dielectric or sensing membrane. An AC voltage source is applied to the two terminals of the IDC to generate an electric field; these electric lines of flux penetrate the dielectric or the sensing membrane. The total capacitance of the IDC can be expressed mathematically as follows [30]: $ , ´ ¯0.5 ’ ˆ ˙/ & ε ε K 1 ´ k2 t . 0 r C “ l pN ´ 1q ` 2ε0 εr ’ K pkq S / % 2 -

(4)

where N, l, S, and t are the number of fingers, the length of the IDE, the spacing between two adjacent fingers of the IDE, and the thickness of the electrode, respectively. ε0 is the absolute dielectric constant and εr is the relative dielectric constant of the sensing membrane or medium. K pkq is the elliptic integral of the first kind of modulus k, which can be expressed as: " k “ cos where W is the width of the electrode. Therefore,

ż1

Kpkq “ 0

!`

πW 2 pW ` Sq

*

K pkq ¯)0.5 dt ˘´ 2 2 2 1´t 1´k t

(5)

(6)

Because the proposed sensor is based on the principle of the capacitor, the voltage across the IDC sensing element is defined as: i vC “ C (7) 2πfC where iC , f, andC are the current flowing through the IDC sensing element, the frequency of the applied signal, and the capacitance of the IDC sensing element, respectively. When the IDC-sensing element absorbs heat from any source, its dielectric constant changes, which corresponds to changing both the capacitance and the voltage across the IDC sensing element; this fact can be represented by the following equation: iC (8) ∆vC “ 2πf∆C

changing both the capacitance and the voltage across the IDC sensing element; this fact can be represented by the following equation: v C = Sensors 2016, 16, 1064

iC 2π f C

(8) 5 of 14

3. Experimental Details 3. Experimental Details 3.1. Fabrication of the Side-Polished Optical Fiber Device 3.1. Fabrication of the Side-Polished Optical Fiber Device In our experiment, we prepared a side-polished fiber-optic device. We selected a quartz block of In×our we prepared a side-polished Weblock selected a quartz block of size 25 10 ×experiment, 5 mm and made a V-groove of 160 µm infiber-optic width on device. the quartz using a mechanical size 25 ˆ 10 ˆ 5 mm and made a V-groove of 160 µm in width on the quartz block using a mechanical slicer. Then, we removed the jacket, which had a length of ~20 cm, of this single-mode optical fiber. slicer. Then, jacket, which a length of ~20 cm, of this single-mode optical The The radii of we the removed core andthe cladding of thehad used single-mode optical fiber were 3 µm and fiber. 125 µm, radii of the core and we cladding used single-mode optical fiber were 3 µm 125placed µm, respectively. respectively. Then, bent of thetheremoved jacket with a radius of ~60 cmand and it in the Then, we bent the removed jacket with a radius of ~60 cm and placed it in the V-grooved quartz V-grooved quartz block. Then, we applied epoxy and dried the epoxy to strongly attach the fiber to block. Then, weused applied epoxyand and8000-µm dried thepolishing epoxy to powders strongly attach the fiber to the We the quartz. We l000-µm on polishing pads to quartz. polish the used l000-µm 8000-µm powders on quartz polishing padstotocreate polishathe surface of the cladding surface of the and cladding thatpolishing was attached to the block side-polished fiber-optic that was attached to the quartz block to create a side-polished fiber-optic device. The step-by-step device. The step-by-step fabrication process of the side-polished fiber-optic device is shown in fabrication of the device is shown in Figure 2a–f. The length of the Figure 2a–f. process The length of side-polished the fiber was fiber-optic 1 m. A photograph of a side-polished fiber-optic device is fiber was 1 m. A photograph of a side-polished fiber-optic device is shown in Figure 2g. shown in Figure 2g.

Figure2.2.Step-by-step Step-by-stepfabrication fabricationprocedure procedureof ofthe theside-polished side-polishedfiber-optic fiber-opticdevice: device:(a) (a)Quartz Quartzblock; block; Figure (b) Creating the thethe quartz block; (c) Removing the jacket of the of optical fiber; (d)fiber; Bending (b) theV-groove V-grooveonon quartz block; (c) Removing the jacket the optical (d) the removed-jacketed-portion of the optical fiber and placing it into the V-groove of the quartz block; Bending the removed-jacketed-portion of the optical fiber and placing it into the V-groove of the (e) Using epoxy; (f) Polishing the fiber-optic block; and block; (g) A photograph of the fabricated quartz block; (e) Using epoxy; (f) Polishing thequartz fiber-optic quartz and (g) A photograph of the side-polished fiber-opticfiber-optic device. device. fabricated side-polished

3.2. 3.2.Fabrication Fabricationofofthe theIDE IDE

We We prepared prepared IDEs IDEs to to form an IDC-based temperature-sensing element. To To make make an an IDC IDC temperature-sensing temperature-sensingelement, element,we wefirst firsthad had to to fabricate fabricate an an IDE. IDE. Then, Then, we we deposited deposited Rhodamine-B Rhodamine-B including includingsensing sensingmembrane membraneinto intothe theIDE IDEtotomake makeIDC. IDC.We We prepared prepared an an IDE IDE with with aa thickness thickness of of approximately 2 cm polyimide (PI)(PI) substrate with 40 pairs of fingers. We applied the approximately22 22µm µmon ona a4 4× ˆ 2 cm polyimide substrate with 40 pairs of fingers. We applied vacuum evaporation method to make a thina film Crof and theonPIthe substrate, respectively. The the vacuum evaporation method to make thin of film CrCu andonCu PI substrate, respectively. thicknesses of theofCrthe and films were approximately 10 nm10 and nm,15respectively. The IDE’s The thicknesses CrCu and Cu films were approximately nm15and nm, respectively. The photomask pattern was transferred onto the thin metal film. Then, a chemical etchant was used to IDE’s photomask pattern was transferred onto the thin metal film. Then, a chemical etchant was etch unmasked patternpattern to obtain thin IDE on the PI PI substrate. WeWeapplied usedthe to etch the unmasked to obtain thin fingers IDE fingers on the substrate. appliedaaCu Cu electroplating electroplatingmethod methodto toincrease increasethe thethickness thicknessof ofthe theCu Cuelectrodes. electrodes.Finally, Finally,we wecut cutthe theresidual residualPI PI substrate. substrate.Figure Figure33shows showsthe thevarious variousstages stagesused usedtotofabricate fabricatethe theIDE. IDE.To Tomeasure measurethe theIDE IDEthickness, thickness, the thewidth widthof ofthe the fingers, fingers, and and the the distance distancebetween betweenfingers, fingers,we weused usedaa scanning scanningelectron electron microscope microscope (SEM) (S-4800, Hitachi, Ibaraki, Japan); the measured values were approximately 22 µm, 100 µm, and 100 µm, respectively.

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(SEM) (S-4800, Hitachi, Ibaraki, Japan); the measured values were approximately 22 µm, 100 µm, (SEM) (S-4800, Hitachi, Ibaraki, Japan); the measured values were approximately 22 µm, 1006 µm, Sensors 2016, 16, respectively. 1064 of 14 and 100 µm, and 100 µm, respectively.

Figure 3. Fabrication stages of the IDE. Figure 3. Fabrication stages of the IDE. Figure 3. Fabrication stages of the IDE. 3.3. Fabrication of the Sensing Membrane, Fiber-Optic Sensing Element, and Interdigitated Capacitor 3.3. Fabrication of the Sensing Membrane, Fiber-Optic Sensing Element, and Interdigitated Capacitor 3.3. Fabrication of the of Sensing Membrane, Fiber-Optic Sensingcolor Element, andtoInterdigitated The capability a chemical compound to change owing a change inCapacitor temperature is The capability of a chemical compound to change color owing to a ischange in temperature is calledThe thermochromism; a chemical compound that exhibits this property called a thermochromic capability of a chemical compound to change color owing to a change in temperature is called called thermochromism; a chemical compound sensing that exhibits this property is called a thermochromic dye [45]. To create the temperature-sensitive we used Rhodamine Bdye [46,47] thermochromism; a chemical compound that exhibits thismembrane, property is called a thermochromic [45]. dye [45]. To create the temperature-sensitive sensing membrane, we used Rhodamine B [46,47] thermochromic dye, a polymer, namely, polyvinyl chloride (PVC), and a DMAC solution. The To create the temperature-sensitive sensing membrane, we used Rhodamine B [46,47] thermochromic thermochromic dye,ofa the polymer, namely, polyvinyl chlorideB)(PVC), andinaFigure DMAC4. solution. The molecular structure thermochromic (Rhodamine issolution. shown We structure obtained dye, a polymer, namely, polyvinyl chloridedye (PVC), and a DMAC The molecular molecular from structure of the thermochromic dye (Rhodamine B) is shown in used Figure 4. We obtained chemicals the Sigma-Aldrich Chemical (Seoul, and them without of the thermochromic dye (Rhodamine B) isCorporation shown in Figure 4.Korea) We obtained chemicals from any the chemicals from the Sigma-Aldrich Chemical Corporation (Seoul, Korea) and used them without further purification. First,Corporation 0.020 g of Rhodamine B was dissolved in the DMAC solution. isany an Sigma-Aldrich Chemical (Seoul, Korea) and used them without any furtherDMAC purification. further solution; purification. First, 0.020 g ofnot Rhodamine Bthe wasdye dissolved in the DMAC solution. DMAC is an aprotic therefore, it does react with molecule. Then, we added 0.035 g of PVC First, 0.020 g of Rhodamine B was dissolved in the DMAC solution. DMAC is an aprotic solution; aprotic solution; therefore, B it solution does not and react with the itdye we added 0.035 g of PVC powder to itthe Rhodamine formolecule. 10 we minadded toThen, obtain a temperature-sensitive therefore, does not react with the dye sonicated molecule. Then, 0.035 g of PVC powder to powder to the Rhodamine B solution and sonicated it for 10 min to obtain a temperature-sensitive dielectric solution. Before depositing the sensing we cleaned the side-polished fiber-optic the Rhodamine B solution and sonicated it for 10solution, min to obtain a temperature-sensitive dielectric dielectric solution. Before depositing the sensing solution, we cleaned the side-polished fiber-optic device andBefore IDE with ethanol, the methanol, deionized water, Then, we dried both solution. depositing sensingand solution, we (DI) cleaned therespectively. side-polished fiber-optic device device and IDE with ethanol, and methanol, and deionized (DI) water, respectively. Then, we dried both the side-polished fiber-optic the IDE in N 2 gas. We used a syringe to take 0.25 mL of sensing and IDE with ethanol, methanol, and deionized (DI) water, respectively. Then, we dried both the the side-polished fiber-optic and the IDE in N2 gas. We used a syringe to take 0.25we mLused of sensing solution to deposit the solution fiber-optic the IDE. Then, a spin side-polished fiber-optic and the on IDEthe in side-polished N2 gas. We used a syringeand to take 0.25 mL of sensing solution solution to deposit the solution on the side-polished fiber-optic and the IDE. Then, we used a spin coater to deposit the solution smoothly onfiber-optic the side-polished fiber-optic andused IDEaand to deposit the solution on the side-polished and the IDE. Then, we spindried coaterthe to coater to deposit the solution smoothly on the side-polished fiber-optic and IDE and dried the devices at room temperature tothe obtain fiber-optic and IDCand temperature-sensing elements. The deposit the solution smoothly on side-polished fiber-optic IDE and dried the devices at room devices at fiber-optic room temperature obtain fiber-optic and IDC temperature-sensing The fabricated and IDEtotemperature-sensing elements, after deposition onelements. the sensing temperature to obtain fiber-optic and IDC temperature-sensing elements. The fabricated fiber-optic fabricated fiber-optic and IDE 5a,b, temperature-sensing elements, after deposition on the sensing membrane, are shown in Figure respectively. and IDE temperature-sensing elements, after deposition on the sensing membrane, are shown in membrane, are shown in Figure 5a,b, respectively. In 5a,b, our respectively. experiment, we used the PVC polymer to immobilize the Rhodamine B on the Figure In our experiment, we used the PVC surface polymer to optical immobilize thevery Rhodamine B on the side-polished optical fiber. Since thePVC polished of the fiber is it side-polished is difficult to In our experiment, we used the polymer to immobilize the Rhodamine B low, on the side-polished optical fiber. Since the polished surface of the optical fiber is very low, it is difficult to deposit the Rhodamine B containing PVC polymer membrane on the side-polished optical fiber. optical fiber. Since the polished surface of the optical fiber is very low, it is difficult to deposit the deposit the Rhodamine B containing PVC polymer membrane on the side-polished optical fiber. Therefore, attached aPVC quartz blockmembrane to the side-polished portion of the optical fiber, which Rhodaminewe B containing polymer on the side-polished optical fiber. Therefore, we Therefore, we attached a quartz block to the side-polished portion of the optical fiber, which increased the surface area to allow the temperature sensitive sensing membrane to be deposited attached a quartz block to the side-polished portion of the optical fiber, which increased the surface increased We the used surface allow the temperature sensitive sensing to bethere deposited properly. thearea PVCtopolymer, whose melting point is deposited about 240membrane °C. Therefore, no area to allow the temperature sensitive sensing membrane to be properly. We used theisPVC properly. We used the PVC polymer, whose melting point is about 240 °C. Therefore, there is no ˝ significant effect of temperature on the polymer as well as the quartz block for the proposed sensing polymer, whose melting point is about 240 C. Therefore, there is no significant effect of temperature significant effect of temperature on the polymer as well as the quartz block for the proposed sensing systems. on the polymer as well as the quartz block for the proposed sensing systems. systems.

Figure 4. Molecular Molecular structure structure of the Rhodamine-B thermochromic dye. Figure Figure 4. Molecular structure of the Rhodamine-B thermochromic dye.

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Figure 5. Fabricated temperature-sensing elements after deposition on the sensing membrane: (a) Figure 5. Fabricated temperature-sensing elements after deposition on the sensing membrane: Fiber-optic and (b) IDE. (a) Fiber-optic and (b) IDE.

3.4. Detection Mechanism of the Proposed Temperature Sensing System 3.4. Detection Mechanism of the Proposed Temperature Sensing System Figure 6a shows the experimental setup for the characterization of the fiber-optic PWM Figure 6a shows the experimental setup for the characterization of the fiber-optic PWM temperature-sensing system. The proposed fiber-optic PWM temperature-sensing system consists of temperature-sensing system. The proposed fiber-optic PWM temperature-sensing system consists of three units: a pulse modulation unit, a transducer/temperature-sensing unit, and a signal processing three units: a pulse modulation unit, a transducer/temperature-sensing unit, and a signal processing unit. We have designed the pulse modulation unit and the signal-processing unit using low-cost and unit. We have designed the pulse modulation unit and the signal-processing unit using low-cost and easily obtainable electronic components from local electronics suppliers. The pulse modulation unit easily obtainable electronic components from local electronics suppliers. The pulse modulation unit consists of a square wave generator, which generates a 1-kHz square wave with a 50% duty cycle, a consists of a square wave generator, which generates a 1-kHz square wave with a 50% duty cycle, a buffer amplifier, a laser diode (LD) driver, and an LD, which emits at an 850-nm peak wavelength. buffer amplifier, a laser diode (LD) driver, and an LD, which emits at an 850-nm peak wavelength. The buffer amplifier has low input impedance and high output impedance, which is used to reduce The buffer amplifier has low input impedance and high output impedance, which is used to reduce loading effects. We used an NE555 timer and associated passive electronic components to design the loading effects. We used an NE555 timer and associated passive electronic components to design the square wave generator; it generates a square wave at a frequency of ~2 kHz without a 50% duty square wave generator; it generates a square wave at a frequency of ~2 kHz without a 50% duty cycle. cycle. Then, its output was passed through a T-flip-flop (CD4027, JK flip-flop employed in toggle Then, its output was passed through a T-flip-flop (CD4027, JK flip-flop employed in toggle mode) to mode) to obtain a perfect 1-kHz signal with 50% duty cycle. The output of the flip-flop is connected obtain a perfect 1-kHz signal with 50% duty cycle. The output of the flip-flop is connected to the input to the input of the LD driver circuit via the buffer amplifier to obtain the 1-kHz light pulse. This light of the LD driver circuit via the buffer amplifier to obtain the 1-kHz light pulse. This light pulse is pulse is passed through the side-polished fiber-optic temperature-sensing element and its output is passed through the side-polished fiber-optic temperature-sensing element and its output is connected connected to the signal processing unit. to the signal processing unit. The signal processing unit can be divided into four parts: a light/photo detector (PD), an The signal processing unit can be divided into four parts: a light/photo detector (PD), an amplifier, amplifier, a pulse shaping circuit, and a peak detector. The optical signal from the side-polished a pulse shaping circuit, and a peak detector. The optical signal from the side-polished fiber-optic fiber-optic temperature-sensing element was detected by the photodiode. The output of the PD is temperature-sensing element was detected by the photodiode. The output of the PD is connected to an connected to an operational amplifier for amplification. The op-amp is connected in the current operational amplifier for amplification. The op-amp is connected in the current follower configuration. follower configuration. Then, the output of the amplified signal is fed to the input of the pulse Then, the output of the amplified signal is fed to the input of the pulse shaping circuit. Then, the output shaping circuit. Then, the output signal of the pulse shaping circuit is fed to the input of the peak signal of the pulse shaping circuit is fed to the input of the peak detector. The peak detector is used detector. The peak detector is used to obtain the peak value of the signal. The fiber-optic sensing to obtain the peak value of the signal. The fiber-optic sensing element contains thermochromic dye element contains thermochromic dye including the sensing membrane; therefore, when the sensing including the sensing membrane; therefore, when the sensing membrane is heated, the refractive index membrane is heated, the refractive index of its overlay waveguide changes (as a result the pulse of its overlay waveguide changes (as a result the pulse width of the received sensing signal changing). width of the received sensing signal changing). Therefore, the relative pulse width (ΔTH) is the Therefore, the relative pulse width (∆TH ) is the difference between the reference pulse width and the difference between the reference pulse width and the sensing pulse width for a given temperature. sensing pulse width for a given temperature. The relative pulse width increases as the temperature The relative pulse width increases as the temperature increases. An oscilloscope (OWON, VDS3104, increases. An oscilloscope (OWON, VDS3104, Guangzhou, China) was used to observe both the pulse Guangzhou, China) was used to observe both the pulse width variation due to the change in width variation due to the change in temperature and the output voltage of the peak detector. temperature and the output voltage of the peak detector. Figure 6b shows a schematic diagram of the experimental setup of the proposed IDC Figure 6b shows a schematic diagram of the experimental setup of the proposed IDC temperature-sensing system. The IDC sensing element contains Rhodamine B dielectric material; temperature-sensing system. The IDC sensing element contains Rhodamine B dielectric material; therefore, when the IDC sensing element is heated, its dielectric constant and the capacitance of the therefore, when the IDC sensing element is heated, its dielectric constant and the capacitance of the IDC change (as a result the output voltage of the peak detector changing). The output voltage is IDC change (as a result the output voltage of the peak detector changing). The output voltage is measured using a digital multimeter (DMM) (Keithley, 2002, Cleveland, OH, USA). measured using a digital multimeter (DMM) (Keithley, 2002, Cleveland, OH, USA). In our study, we developed the hot/cold air blower which could provide accurate air temperature In our study, we developed the hot/cold air blower which could provide accurate air from 3–100 ˝ C. We took temperature measurements using the proposed systems from 5–100 ˝ C. To temperature from 3–100 °C. We took temperature measurements using the proposed systems from observe the temperature below 5 ˝ C we put an ice pack on the fiber-optic/IDC temperature sensing 5–100 °C. To observe the temperature below 5 °C we put an ice pack on the fiber-optic/IDC temperature sensing element of the proposed temperature sensing system and measured the ice pack temperature using a commercial thermocouple thermometer (ET-959, Shenzhen, China). The

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element the temperature sensing system and measured the ice pack temperature8 of using Sensors of 2016, 16,proposed 1064 14 a commercial thermocouple thermometer (ET-959, Shenzhen, China). The observed temperature temperature wasthe 0 °C. Then ability we observed the sensing ability ofPWM the proposed wasobserved 0 ˝ C. Then we observed sensing of the proposed fiber-optic and IDC fiber-optic temperature ˝ PWM and IDC temperature sensing system at 0 °C. We found that the proposed systems sensing system at 0 C. We found that the proposed sensing systems also gave asensing response at 0 ˝ C. gave apulse response at 0and °C.the Thecapacitance relative pulse and the capacitance of the proposed fiber-optic Thealso relative width ofwidth the proposed fiber-optic PWM and IDC temperature PWM and IDC temperature sensing system at 0 °C were about 0.06 µs and 1.73 nF, respectively. ˝ sensing system at 0 C were about 0.06 µs and 1.73 nF, respectively. From this experiment, we found this experiment, we found thatsystems the proposed temperature sensing systems is also able thatFrom the proposed temperature sensing is also able to measure the temperature below 5 ˝to C. measure the temperature below 5 °C. There is still a controversy about the innocuousness of Rhodamine B. Therefore, if we want to There is still a controversy about the innocuousness of Rhodamine B. Therefore, if we want to use the proposed sensing system for medical applications, the whole fiber-optic or IDC temperature use the proposed sensing system for medical applications, the whole fiber-optic or IDC temperature sensing element must be covered with a thin layer of medical grade plastic that has a high melting sensing element must be covered with a thin layer of medical grade plastic that has a high melting point. This keeps the Rhodamine B containing the sensing membrane from coming into direct contact point. This keeps the Rhodamine B containing the sensing membrane from coming into direct with the human body. We believe thatbelieve this situation not affect sensing performance contact with the human body. We that thiswill situation willthe nottemperature affect the temperature sensing of the proposed sensing systems. Because when the fiber-optic/IDC sensing elements, which will be performance of the proposed sensing systems. Because when the fiber-optic/IDC sensing elements, covered thecovered medical grade heat from anyheat source, B containing whichwith will be with the plastic, medicalreceives grade plastic, receives fromthe anyRhodamine source, the Rhodamine temperature sensitive membrane also receives heat. As a result, the refractive index of the B containing temperature sensitive membrane also receives heat. As a result, the refractiveRhodamine index of B containing the sensing membrane will change, which will change pulse width, as well as the the Rhodamine B containing the sensing membrane will change, whichthe will change the pulse width, capacitance proposedof fiber-optic PWM and IDCPWM sensing respectively. The proposed as well as of thethe capacitance the proposed fiber-optic andsystem, IDC sensing system, respectively. temperature sensors are practical andare cost effective temperature The proposed temperature sensors practical andsingle-point cost effective single-pointsensors. temperature sensors.

Figure 6. Schematic diagram of the experimental setup of the proposed temperature sensing system: Figure 6. Schematic diagram of the experimental setup of the proposed temperature sensing system: (a) Fiber-optic PWM and (b) IDC. (a) Fiber-optic PWM and (b) IDC.

4. Results and Discussions 4. Results and Discussions Figure 7a,b shows the reference and sensing waveforms at 3 °C for the case of a fiber-optic Figure 7a,b shows the reference sensing waveforms 3 ˝aCreference for the case of a fiber-optic PWM temperature-sensing system;and we consider this value toatbe temperature. FigurePWM 7b temperature-sensing system; we consider value to be a reference Figure 7b shows shows that there was no change of pulsethis width of the sensing signal temperature. with respect to the reference thatsignal. thereHowever, was no change pulse widthfiber-optic of the sensing signal withis respect to the reference once theofside-polished sensing element heated, the pulse width ofsignal. the sensing signal changes, as shown in Figure 7c. The relative pulse width difference at 50 °C was ~8.5

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µs; the result is shown in Figure 7c. This result indicates that the sensing response of our 9designed Sensors 2016, 16, 1064 of 14 signal unit is goodfiber-optic and that sensing it has the abilityisto detectthe small differences in sensing the pulse However,processing once the side-polished element heated, pulse width of the width. µs; the result is indicates thedifference sensing response our ~8.5 designed signal changes, asshown shownininFigure Figure7c. 7c.This Theresult relative pulse that width at 50 ˝ Cofwas µs; the We also prepared an IDC temperature sensor to measure the temperature from 5–100 °C. The signal processing unit is good and that it has the ability to detect small differences in the pulse result is shown in Figure 7c. This result indicates that the sensing response of our designed signal width. capacitance of the IDC sensing element, the capacitive impedance, and the phase shift processing unit is good and that it has the ability to detect small differences in the pulse width. of the also prepared IDC temperature sensor to measure temperature from 5–100 The ˝ C. received signal of the proposed IDCs aresensor functions of the the temperature variation. The°C. waveform WeWe also prepared anan IDC temperature to measure the temperature from 5–100 The capacitance of the IDC sensing element, the capacitive impedance, and the phase shift of the response for phase shifting is shown in Figure 7e; we found that at 3 °C, there is no phase difference; capacitance of the IDC sensing element, the capacitive impedance, and the phase shift of the received received signal of proposed IDCs are functions ofshift the occurs temperature variation. The waveform however, thethetemperature increases, a phase between the received sensingfor and signal of the when proposed IDCs are functions of the temperature variation. The waveform response response for phase shifting is shown in Figure 7e; we found that at 3 °C, there is no phase difference; reference signals. Theinphase difference between received sensing and reference signals phase shifting is shown Figureshift 7e; we found that at 3 ˝ C,the there is no phase difference; however, when at however, when the temperature increases, a phase shift occurs the between the received sensing and 50 °C was ~14.75 ns and was measured using an oscilloscope; results are shown in Figure 7f. The the temperature increases, a phase shift occurs between the received sensing and reference signals. reference signals. The phase shift difference between the received sensing andsystem reference signals at ˝ temperature sensing ability of the proposed fiber-optic PWM sensing under different The50phase shift difference between the received sensing and reference signals at 50 C was ~14.75 °C was ~14.75 ns and was measured using an oscilloscope; the results are shown in Figure 7f. Thens temperature is shown in 8. From figure, found the7f.relative pulse width linearly and was measured usingability anFigure oscilloscope; thethis results arewe shown in that Figure The temperature sensing temperature sensing of the proposed fiber-optic PWM sensing system under different 2 was about 0.998 with a high increases as the temperature increases and its correlation coefficient R ability of the proposed fiber-optic sensing system under temperature is linearly shown in temperature is shown in Figure 8. PWM From this figure, we found thatdifferent the relative pulse width temperature sensing ability. The that sensitivity of the proposed fiber-optic PWM sensing system was Figure 8. From this figure, we found the relative pulse width linearly increases as the temperature increases as the temperature increases and its correlation coefficient R2 was about 0.998 with a high 2 ~178 ns/°C. increases and its correlation R was 0.998 with a high temperature temperature sensing ability.coefficient The sensitivity of about the proposed fiber-optic PWM sensingsensing system ability. was ˝ C. The~178 sensitivity of the proposed fiber-optic PWM sensing system was ~178 ns/ ns/°C.

Figure 7. Waveform response of the proposed temperature sensors: (a) Reference signal of fiber-optic Figure 7. Waveform response of the proposed temperature sensors: (a) Reference signal of fiber-optic Figure 7. sensing Waveform response the proposed temperature sensors: (a) Reference PWM system; (b) of Fiber-optic PWM sensing system before applyingsignal heat; of(c)fiber-optic Fiber-optic PWM sensing system; (b) Fiber-optic PWM sensing system before applying heat; (c) Fiber-optic PWM sensing system; (b)after Fiber-optic PWM sensing system applying (c) Fiber-optic PWM sensing system recording heat response afterbefore applying heat; heat; (d) Reference signalPWM of IDC PWM sensing system after recording heat response after applying heat; (d) Reference signal of IDC sensing system after recording heat response after applying heat; (d) Reference signal of IDC sensing sensing system; (e) IDC sensing system before applying heat; and (f) IDC sensing system after sensing system; (e) IDC sensing system before applying heat; and (f) IDC sensing system after system; (e) IDC applying heat.sensing system before applying heat; and (f) IDC sensing system after applying heat. applying heat.

Figure8. 8. Behavior thethe proposed fiber-optic PWM temperature sensing system under different Figure Behaviorofofof proposed fiber-optic PWM temperature sensing under Figure 8. Behavior the proposed fiber-optic PWM temperature sensing systemsystem under different temperatures. different temperatures. temperatures.

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The the sensing signal and and the the reference reference signal The phase phase shift shift difference difference between between the sensing signal signal at at different different The phase shift difference between the sensing signal and the reference signal at different ˝ C is recorded, temperatures from 5–100 as shown in Figure 9a. We find that the phase shift between temperatures from 5–100 °C is recorded, as shown in Figure 9a. We find that the phase shift between temperatures from 5–100 °C is recorded, as shown in Figure 9a. We find that the phase shift between the The variation variation of of capacitance capacitance with with respect respect to the two two signals signals increases increases linearly linearly with with temperature. temperature. The to the the the two signals increases linearly with temperature. The variation of capacitance with respect to the different temperature is shown in Figure 9b. different temperature is shown in Figure 9b. different temperature is shown in Figure 9b. Figure Figure 10 10 shows shows the the sensing sensing performance performance of of the the proposed proposed temperature-sensing temperature-sensing systems. systems. This This Figure 10 shows the sensing performance of the proposed temperature-sensing systems. This figure In Figure Figure 10, 10, figure shows shows that that as as the the temperature temperature increases increases the the relative relative voltage voltage linearly linearly increases. increases. In figure shows that as the temperature increases the relative voltage linearly increases. In Figure 10, we used linear curve fitting to determine the slope, i.e., the sensitivity of the proposed fiber-optic we used linear curve fitting to determine the slope, i.e., the sensitivity of the proposed fiber-optic we used linear curve fitting to determine the slope, i.e., the sensitivity of the proposed fiber-optic PWM systems. The The sensitivity sensitivity of of the the proposed PWM and and IDC IDC temperature-sensing temperature-sensing systems. proposed fiber-optic fiber-optic PWM PWM and and PWM and IDC temperature-sensing systems. The of the fiber-optic PWM and ˝ C sensitivity ˝ C, proposed IDC temperature-sensing systems was 3.733 mV/ and 2.96 mV/ respectively. From the above IDC temperature-sensing systems was 3.733 mV/°C and 2.96 mV/°C, respectively. From the above IDC temperature-sensing systems was 3.733 mV/°C and 2.96 mV/°C, respectively. From the above described that thethe fiber-optic PWM sensing system offersoffers greatergreater sensingsensing ability described results, results,we wedetermined determined that fiber-optic PWM sensing system described results, we determined that the fiber-optic PWM sensing system offers greater sensing than the IDCthe temperature-sensing system.system. ability than IDC temperature-sensing ability than the IDC temperature-sensing system. In our experiment, we tried to In our experiment, we tried to measure measure the the linearity linearity of of the the proposed proposed fiber-optic fiber-optic PWM PWM and and IDC IDC In our experiment, we tried to measure the linearity ˝of the proposed fiber-optic PWM and IDC temperature sensors over the dynamic range from 5–100 C; the device had good linearity. temperature sensors over the dynamic range from 5–100 °C; the device had good linearity. The The temperature sensors over the dynamic range from 5–100 °C; the device had good linearity. The 2 values correlation for the the proposed proposed fiber-optic fiber-optic PWM and IDC temperature sensors correlation coefficient coefficient (R (R2)) values for PWM and IDC temperature sensors correlation coefficient (R2) values for the proposed fiber-optic PWM and IDC temperature sensors were were approximately approximately 0.992 0.992 and and 0.989, 0.989, respectively. respectively. Figure Figure 11 11 shows shows the the graphical graphical representation representation for for were approximately 0.992 and 0.989, respectively. Figure 11 shows the graphical representation for sensitivity sensitivity and and linearity linearity of of the the proposed proposed fiber-optic fiber-optic PWM PWM and and IDC IDC temperature-sensing temperature-sensingsystem. system. sensitivity and linearity of the proposed fiber-optic PWM and IDC temperature-sensing system.

Figure 9. 9. Behavior of the proposed IDC IDC thermal/temperature thermal/temperature sensing system under different Figure thethe proposed different Figure 9.Behavior Behaviorof of proposed IDC thermal/temperaturesensing sensingsystem systemunder under different temperatures: (a) (a) Phase Phase shift shift and and (b) (b) Capacitance. Capacitance. temperatures: temperatures: (a) Phase shift and (b) Capacitance.

Figure 10. Temperature response of the proposed sensing systems. Figure 10.10. Temperature response of the proposed sensing systems. Figure Temperature response of the proposed sensing systems.

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Figure11. 11. Graphical Graphical representation representation of of the the proposed proposed fiber-optic fiber-optic PWM PWM and and IDC IDC temperature temperature sensing sensing Figure system: (a) (a) Sensitivity Sensitivity and and (b) (b) Linearity. Linearity. system:

Wecreated createdthree three samples of fiber-optic temperature-sensing elements. we We samples of fiber-optic PWM PWM temperature-sensing elements. Then, weThen, observed observed sensing ability of thesesensing fiber-optic sensing at 50 °Cthe to reproducibility determine the ˝ C to determine the sensingthe ability of these fiber-optic elements at 50elements reproducibility performance. We observed that the three devices had almost the same sensing performance. We observed that the three devices had almost the same sensing performance. Therefore, performance. Therefore, the sensing reproducibility; elements have excellent reproducibility; their relative standard the sensing elements have excellent their relative standard deviations (RSDs) were deviations (RSDs) were approximately 0.021. approximately 0.021. Toobserve observe the the repeatability repeatability of of the the proposed proposed fiber-optic fiber-optic PWM PWM temperature temperature sensing sensing system, system, our our To experiment was was repeated repeated three three times times at at different different temperatures. temperatures. The The relative relative pulse pulse width width for for the the experiment measurements was recorded. We found that there was no significant variation of the pulse width at measurements was recorded. We found that there was no significant variation of the pulse width the specified temperature using the proposed fiber-optic PWM temperature sensing system. For at the specified temperature using the proposed fiber-optic PWM temperature sensing system. For example, the the relative relative pulse pulse width width at at 50 50 ˝°C example, C for for three three observations observations were were 8.50 8.50 µs, µs, 8.506 8.506 µs, µs, and and 8.492 8.492 µs. µs. Therefore, we can say that the proposed temperature sensing system has an excellent repeatability Therefore, we can say that the proposed temperature sensing system has an excellent repeatability response.According Accordingtoto experimental observations, the response and recovery times of the response. ourour experimental observations, the response and recovery times of the proposed proposed temperature sensors were 5 s and 6 s, respectively. temperature sensors were 5 s and 6 s, respectively. We compared compared the the performance performance of of the the proposed proposed temperature-sensing temperature-sensing systems systems with with different different We temperature sensors. sensors. The fiber-optic PWM PWM and and IDC IDC temperature temperature temperature The sensitivities sensitivities of of the the proposed proposed fiber-optic sensors were approximately 3.733 mV/°C and 2.96 mV/°C, respectively, whereas the sensitivities of ˝ ˝ sensors were approximately 3.733 mV/ C and 2.96 mV/ C, respectively, whereas the sensitivities of the fiber-optic wavelength shift temperature sensor [38], fiber-optic Sagnac loop sensor [39], and the fiber-optic wavelength shift temperature sensor [38], fiber-optic Sagnac loop sensor [39], and probe type type fiber-optic fiber-optic temperature −1.5´1.5 nm/°C, ˝ C, probe temperature sensor sensor [48] [48] were were approximately approximately−0.97 ´0.97nm/°C, nm/˝ C, nm/and 0.0044 mV/°C, respectively. Moreover, the dynamic ranges of the proposed sensors (fiber-optic ˝ and 0.0044 mV/ C, respectively. Moreover, the dynamic ranges of the proposed sensors (fiber-optic PWMand andIDC), IDC),the the fiber-optic wavelength temperature sensor, the fiber-optic loop PWM fiber-optic wavelength shiftshift temperature sensor, the fiber-optic SagnacSagnac loop sensor, sensor, and the probe type fiber-optic temperature sensor were 5–100 °C, 5–100 °C, 26–40 ˝ ˝ ˝ and the probe type fiber-optic temperature sensor were 5–100 C, 5–100 C, 26–40 C, 20–50 ˝°C, C, 20–50 °C, and 42–90 °C. Therefore, the sensitivities and dynamic of the proposed temperature ˝ C. Therefore, and 42–90 the sensitivities and dynamic ranges ofranges the proposed temperature sensors sensors were better than those of the other above-mentioned temperature sensors. In addition, the were better than those of the other above-mentioned temperature sensors. In addition, the linearity 2 of the proposed fiber-optic PWM as well as IDC linearity i.e. the correlation coefficient R 2 i.e. the correlation coefficient R of the proposed fiber-optic PWM as well as IDC temperature sensing temperature sensingthan system were higher than the fiber-optic wavelength sensor system were higher the fiber-optic wavelength shift temperature sensorshift [38],temperature fiber-optic Sagnac [38], sensor fiber-optic looptype sensor [39], and probe type sensor. fiber-optic temperature sensor. loop [39],Sagnac and probe fiber-optic temperature 5. Conclusions 5. Conclusions

In this highly sensitive, widewide dynamic range temperature sensors sensors that are In this study, study,we weproposed proposed highly sensitive, dynamic range temperature based on fiber-optic PWM and the capacitance variation principle. A well-known thermochromic that are based on fiber-optic PWM and the capacitance variation principle. A well-known dye (i.e., Rhodamine was incorporated into PVC into andPVC a DMAC solution to create the thermochromic dye (i.e.,B) Rhodamine B) was incorporated and a DMAC solution to create temperature-sensitive sensing membrane, which was then deposited on the side-polished single the temperature-sensitive sensing membrane, which was then deposited on the side-polished single mode optical opticalfiber fiber device in IDE the to IDE to prepare fiber-optic IDC temperature-sensing mode device andand in the prepare fiber-optic and IDCand temperature-sensing elements, elements, respectively. The pulse width of the received sensing signal the IDCchange capacitance respectively. The pulse width of the received sensing signal and the IDC and capacitance with change with change in temperature. As a result, the amplitude of the received sensing signal change in temperature. As a result, the amplitude of the received sensing signal changes. The proposed changes. The proposed sensors offer a wide dynamic range with linear sensing performance. The correlation coefficient (R2) values were ~0.992. The sensors have highly stable sensing capabilities,

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sensors offer a wide dynamic range with linear sensing performance. The correlation coefficient (R2 ) values were ~0.992. The sensors have highly stable sensing capabilities, high reproducibility, low fabrication costs, and real-time sensing responses. Moreover, the cost of the electronic components used to design the electronic circuitry is low and they are available at local electronic suppliers. In future studies, we will use temperature-sensing membranes based on different thermochromic compounds to increase the number of sensing elements in the array. We also plan to design a fiber-optic distributed temperature sensor. Acknowledgments: This study was supported by the BK21 Plus project funded by the Ministry of Education, Korea (21A20131600011). Author Contributions: Md. Rajibur Rahaman Khan is the main author of the manuscript and research. He proposed the detection idea, designed the methodologies of the fiber-optic PWM and IDC temperature sensing system, performed the experiments, drew the schematic diagrams, wrote the text of the manuscript and revision of the manuscript. Shin-Won Kang provided insightful comments and suggestions. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11. 12. 13.

14. 15. 16. 17.

Yurish, S.Y. Modern Sensors, Transducers and Sensor Networks; International Frequency Association Publishing: Barcelona, Spain, 2014; pp. 18–19. Chavan, C.H.; Karande, P.V. Wireless Monitoring of Soil Moisture, Temperature & Humidity Using Zigbee in Agriculture. Int. J. Eng. Trends Technol. 2014, 11, 493–497. Gupta, B.D. Fiber Optic Sensors: Principles and Applications; New India Publishing Agency: New Delhi, India, 2006; pp. 7–8. Rajan, G. Optical Fiber Sensors: Advanced Techniques and Applications; CRC Press: Boca Raton, FL, USA, 2014; p. 10. Rao, Y.J.; Webb, D.J.; Jackson, D.A.; Zhang, L.; Bennion, I. In-Fiber Bragg-Grating Temperature Sensor System for Medical Applications. J. Lightwave Technol. 1997, 15, 38–44. Sui, R.; Fisher, D.K.; Barnes, E.M. Soil Moisture and Plant Canopy Temperature Sensing for Irrigation Application in Cotton. J. Agric. Sci. 2012, 12, 93–105. [CrossRef] Kairm, H.; Delfin, D.; Shuvo, M.A.I.; Chavez, L.A.; Garcia, C.R.; Barton, J.H.; Gaytan, S.M.; Cadena, M.A.; Rumpf, R.C.; Wicker, R.B.; et al. Concept and Model of a Metamaterial-Based Passive Wireless Temperature Sensor for Harsh Environment Applications. IEEE Sens. J. 2015, 15, 1445–1452. [CrossRef] Ren, Q.Y.; Wang, L.F.; Huang, J.Q.; Zhang, C.; Huang, Q.A. A Novel Capacitive Temperature Sensor for a Lab-on-a-Chip System. IEEE Sens. Proc. 2014, 436–439. [CrossRef] Cai, C.H.; Qin, M. High-Performance Bulk Silicon Interdigital Capacitive Temperature Sensor based on Graphene Oxide. Electron. Lett. 2013, 49, 488–490. [CrossRef] Reeder, T.M.; Cullen, D.E. Surface-Acoustic-Wave Pressure and Temperature Sensors. IEEE Proc. 1976, 64, 754–756. [CrossRef] Li, B.; Yassine, O.; Kosel, J. A Surface Acoustic Wave Passive and Wireless Sensor for Magnetic Fields, Temperature, and Humidity. IEEE Sens. J. 2015, 15, 453–462. [CrossRef] Karimova, K.S.; Khalida, F.A.; Chania, M.T.S.; Mateena, A.; Hussain, M.A.; Maqbool, A. Carbon Nanotubes Based Flexible Temperature Sensors, Optoelectron. Adv. Mat. 2012, 6, 194–196. Cagatay, E.; Falco, A.; Abdellah, A.; Lugli, P. Carbon Nanotube Based Temperature Sensors Fabricated by Large-Scale Spray Deposition. In Proceedings of the 10th Conference on Ph.D. Research in Microelectronics and Electronics, Grenobel, France, 30 June 2014; pp. 1–4. Ocaya, R.O.; Ghamdi, A.A.; Tantawy, F.E.; Farooq, W.A.; Yakuphanoglu, F. Thermal Sensor based Zinc Oxide Diode for Low Temperature Applications. J. Alloy. Compd. 2016, 674, 277–288. [CrossRef] Wang, R.L.; Yu, C.W.; Yu, C.; Liu, T.H.; Yeh, C.M.; Lin, C.F.; Tsai, H.H.; Juang, Y.Z. Temperature Sensor Using BJT-MOSFET Pair. Electron. Lett. 2012, 48, 1–2. [CrossRef] Souri, K.; Chae, Y.; Makinwa, K.A.A. A CMOS Temperature Sensor with a Voltage-Calibrated Inaccuracy of ˘0.15 ˝ C (3σ) from 55 ˝ C to 125 ˝ C. IEEE. J. Sol.-St. Circ. 2013, 48, 292–301. [CrossRef] Zhao, Y.; Deng, Z.Q.; Hu, H.F. Fiber-Optic SPR Sensor for Temperature Measurement. IEEE Trans. Instrum. Meas. 2015, 64, 3099–3104. [CrossRef]

Sensors 2016, 16, 1064

18. 19. 20. 21. 22. 23.

24. 25.

26. 27. 28. 29. 30.

31. 32.

33. 34.

35. 36. 37. 38. 39. 40.

13 of 14

Pang, F.; Xiang, W.; Guo, H.; Chen, N.; Zeng, X.; Chen, Z.; Wang, T. Special Optical Fiber for Temperature Sensing based on Cladding-Mode Resonance. Opt. Express 2008, 16, 12967–12972. [CrossRef] [PubMed] Senosiain, J.; Diaz, I.; Gaston, A.; Sevilla, J. High Sensitivity Temperature Sensor based on Side-Polished Fiber Optic. IEEE Trans. Instrum. Meas. 2002, 50, 1656–1660. [CrossRef] Zhong, N.; Liao, Q.; Zhu, X.; Zhao, M.; Huang, Y.; Chen, R. Temperature-Independent Polymer Optical Fiber Evanescent Wave Sensor. Sci. Rep. 2015, 5, 1–10. [CrossRef] [PubMed] Sameer, M.C.; Nicolas, A.F.J. Fiber-Optic Temperature Sensor Using Evanescent Fields in D Fibers. IEEE Photo. Tech. Lett. 2005, 17, 2706–2708. Li, X.; Lin, S.; Liang, J.; Zhang, Y.; Oigawa, H.; Ueda, T. Fiber-Optic Temperature Sensor based on Difference of Thermal Expansion Coefficient between Fused Silic and Metallic Materials. IEEE Photo. J. 2012, 4, 155–162. Posey, R.; Vohra, S.T. An Eight-Channel Fiber-Optic Bragg Grating and Stimulated Brillouin Sensor System for Simultaneous Temperature and Strain Measurements. IEEE Photonics Technol. Lett. 1999, 11, 1641–1643. [CrossRef] Kumara, N.; Sahatiyaa, P.; Dubeya, P. Fabrication of CNT based Gas Sensor using Interdigitated Gold Electrodes. Procedia Mater. Sci. 2014, 6, 1976–1980. [CrossRef] Wang, H.; Wu, X.; Dong, P.; Wang, C.; Wang, J.; Liu, Y.; Chen, J. Electrochemical Biosensor based on Interdigitated Electrodes for Determination of Thyroid Stimulating Hormone. Int. J. Electrochem. Sci. 2014, 9, 12–21. Jung, H.W.; Chang, Y.W.; Lee, G.Y.; Cho, S.; Kang, M.J. A Capacitive Biosensor based on An Interdigitated Electrode with Nanoislands. Anal. Chim. Acta 2014, 844, 27–34. [CrossRef] [PubMed] Arshak, K.; Gill, E.; Arshak, A.; Korostynska, O. Investigation of Tin Oxides as Sensing Layers in Conductimetric Interdigitated pH Sensors. Sens. Actuators B Chem. 2007, 127, 42–53. [CrossRef] Korostynska, O.; Mason, A.; Al-Shamma, A.L. Flexible Microwave Sensors for Realtime Analysis of Water Contaminants. J. Electromagn. Wave 2013, 27, 2075–2089. [CrossRef] Khan, M.R.R.; Kang, S.W. Highly Sensitive Multi-Channel IDC Sensor Array for Low Concentration Taste Detection. Sensors 2015, 15, 13201–13221. [CrossRef] [PubMed] Khan, M.R.R.; Khalilian, A.; Kang, S.W. Fast, Highly-Sensitive, and Wide-Dynamic-Range Interdigitated Capacitor Glucose Biosensor using Solvatochromic Dye-Containing Sensing Membrane. Sensors 2016, 16, 265. [CrossRef] [PubMed] Khan, M.R.R.; Khalilian, A.; Kang, S.W. A High Sensitivity IDC-Electronic Tongue using Dielectric/Sensing Membranes with Solvatochromic Dyes. Sensors 2016, 16, 668. [CrossRef] [PubMed] Islam, T.; Nimal, A.T.; Mittal, U.; Sharma, M.U. A Micro Interdigitated Thin Film Metal Oxide Capacitive Sensor for Measuring Moisture in the Range of 175–625 ppm. Sens. Actuators. B Chem. 2015, 221, 357–364. [CrossRef] Arshak, K.; Morris, D.; Arshak, A.; Korostynska, O.; Moorea, E. PVB, PVAc and PS Pressure Sensors with Interdigitated Electrodes. Sens. Actuators. A Phys. 2006, 132, 199–206. [CrossRef] Shih, W.P.; Tsao, L.C.; Lee, C.W.; Cheng, M.Y.; Chang, C.; Yang, Y.J.; Fan, K.C. Flexible Temperature Sensor Array based on A Graphite-Polydimethylsiloxane Composite. Sensors 2014, 14, 23321–23336. [CrossRef] [PubMed] Dankoco, M.D.; Tesfay, G.Y.; Benevent, E.; Bendahan, M. Temperature Sensor Realized by Inkjet Printing Process on Flexible Substrate. Mater. Sci. Eng. 2016, 205, 1–5. [CrossRef] Ge, Y.; Wang, T.; Zhang, J.; Chang, J. Wavelength-Demodulation MEMS Fabry Perot Temperature Sensor based on Bimetallic Diaphragm. Optik 2016, 127, 5040–5043. [CrossRef] Wang, Y.; Yang, M.; Wang, D.N.; Liao, C.R. Selectively in Filtrated Photonic Crystal Fiber with Ultrahigh Temperature Sensitivity. IEEE Photo. Tech. Lett. 2011, 23, 1520–1522. [CrossRef] Álvarez-Herrero, A.; Guerrero, H.; Belenguer, T.; Levy, D. High-Sensitivity Temperature Sensor based on Overlay on Side-Polished Fibers. IEEE Photo. Tech. Lett. 2000, 12, 1043–1045. [CrossRef] Ruan, J. Fiber Temperature Sensor Employed SMP Fiber Structure and a Long Period Fiber Grating based on a Sagnac Loop. Optik 2015, 126, 5044–5046. [CrossRef] Khan, Md.R.R.; Kang, B.H.; Yeom, S.H.; Kwon, D.H.; Kang, S.W. Fiber-Optic Pulse width Modulation Sensor for Low Concentration VOC Gas. Sens. Actuators. B Chem. 2013, 188, 689–696. [CrossRef]

Sensors 2016, 16, 1064

41.

42.

43.

44. 45. 46. 47. 48.

14 of 14

Khan, Md.R.R.; Kang, B.H.; Lee, S.W.; Kim, S.H.; Yeom, S.H.; Lee, S.H.; Kang, S.W. Fiber-Optic Multi-Sensor Array for Detection of Low Concentration Volatile Organic Compounds. Opt. Express 2013, 21, 20119–20129. [CrossRef] [PubMed] Khan, M.R.R.; Kang, S.W. Highly Sensitive Fiber-Optic Volatile Organic Compound Gas Sensor using a Solvatochromic-Dye Containing Polymer Waveguide based on Pulse-Width Modulation Technique. Sens. Lett. 2015, 13, 663–668. [CrossRef] Lee, S.T.; Kumar, P.S.; Unnikrishnan, K.P.; Nampoori1, V.P.N.; Vallabhan, C.P.G.; Sugunan, S.; Radhakrishnan, P. Evanescent Wave Fiber Optic Sensors for Trace Analysis of Fe3+ in Water. Meas. Sci. Technol. 2003, 14, 858–861. [CrossRef] Khan, M.R.R.; Kang, S.W. A High Sensitivity and Wide Dynamic Range Fiber-Optic Sensor for Low-Concentration VOC Gas Detection. Sensors 2014, 14, 23321–23336. [CrossRef] [PubMed] Thermochromism. Available online: https://en.wikipedia.org/wiki/Thermochromism (accessed on 31 May 2016). Hinckley, D.A.; Seybold, P.G.; Borris, D.P. Solvatochromism and Thermochromism of Rhodamine Solutions. Spectrochim. Acta. Mol. Biomol. Spectrosc. 1986, 42, 747–754. [CrossRef] Rosenthal, I.; Peretz, P.; Muszkat, K.A. Thermochromic and Hyperchromic Effects in Rhodamine B Solutions. Phys. Chem. 1979, 83, 350–353. [CrossRef] Rahman, H.A.; Harun, S.W.; Saidin, N.; Yasin, M.; Ahmad, H. Fiber Optic Displacement Sensor for Temperature Measurement. IEEE Sens. J. 2012, 12, 1361–1364. [CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).