sensors Article
Tilted Fiber Bragg Grating Sensor Using Chemical Plating of a Palladium Membrane for the Detection of Hydrogen Leakage Jiachen Yu 1 , Zhenlin Wu 1, *, Xin Yang 2 , Xiuyou Han 1 1
2
*
and Mingshan Zhao 1
School of Optoelectronic Engineering and Instrumentation Science, Dalian University of Technology, Dalian 116023, China;
[email protected] (J.Y.);
[email protected] (X.H.);
[email protected] (M.Z.) Department of Electrical and Electronics Engineering, School of Engineering, Cardiff University, Cardiff CF10 3AT, UK;
[email protected] Correspondence:
[email protected]; Tel.: +86-155-666-71099
Received: 21 November 2018; Accepted: 12 December 2018; Published: 18 December 2018
Abstract: A tilted fiber Bragg grating (TFBG) hydrogen sensor coated with a palladium (Pd) membrane by the electroless plating method is proposed in this paper. A uniform layer of Pd metal is fabricated in aqueous solutions by the chemical coating method, which is used as the sensitive element to detect the change of the surrounding refractive index (SRI) caused by hydrogen absorption. The change in SRI causes an unsynchronized change of the cladding modes and the Bragg peak in the TFBG transmission spectrum, thereby eliminating the cross-sensitivity due to membrane expansion and is able to simultaneously monitor the presence of cracks in the pipe, as well as the hydrogen leakage. By subtracting the wavelength shift caused by fiber expansion, the change of SRI, i.e., the information from the H2 level, can be separately obtained. The drifted wavelength is measured for the H2 concentration below the hydrogen explosion limit between 1% and 4%. The chemical-based coating has the advantages of a low cost, a simple operation, and being suitable for coating on long fiber structures. The proposed sensor is able to detect the H2 signal in 5 min at a 1% H2 concentration. The proposed sensor is proved to be able to monitor the hydrogen level without the cross-sensitivity of temperature variation and expansion strains, so could be a good candidate for security applications in industry. Keywords: optical fiber hydrogen sensor; palladium membrane; electroless plating; tilted fiber Bragg grating
1. Introduction Hydrogen is considered an ideal clean energy and efficient fuel. At present, liquid hydrogen has been widely used in the aerospace field. Hydrogen-oxygen fuel cells can also be used to convert driving energy from hydrogen for automobiles. However, due to the unique nature of hydrogen, its storage and pipeline transportation is very difficult and it can be dangerous once it has leaked. At room temperature and standard atmospheric pressure, the explosive concentration of hydrogen can be as low as 4% in volume percentage [1]. Therefore, a safe and effective method for detecting hydrogen is urgently needed. Compared with traditional electronic sensors, optical sensors have many advantages, including electrical passiveness and insensitivity to electromagnetic interference [2]. Moreover, due to the flammable and explosive nature of hydrogen, the use of optical fiber detection can be much safer. Currently, various types of hydrogen sensors based on optical fibers have been proposed, including fiber Bragg gratings (FBG) [3,4], specially treated fibers (side polishing fiber [5]
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and singlemode-multimode-singlemode fiber [6]), and micro-cantilever beam fiber Bragg grating [7], as well as TFBG sensors using the SPR effect [8]. Most of the sensors use a palladium (Pd) metal membrane as the hydrogen sensing element, due to its strong ability to absorb hydrogen [7]. Once the Pd film absorbs hydrogen, the metal lattice experiences an α–β phase transition, which can be detected by various sensing structures. However, these types of hydrogen fiber sensors require special fibers or transform the sensing element into special forms in order to improve the sensitivity, which will introduce great difficulty to the production of the sensors. Taking the FBG hydrogen sensor as an example, Fisser et al. [9] used a layer of Pd membrane with a certain morphology to wrap the FBG, and the FBG spectrum could be changed by the expansion of Pd after hydrogen absorption. In practice, many other factors can also affect the FBG spectrum, such as temperature and changing stress caused by pipe cracking. Therefore, it can be helpful to develop a type of sensor which can detect the tube cracking and the hydrogen leakage at the same time, allowing the real-time monitoring of H2 leakage and pipe safety. Tilted fiber Bragg grating (TFBG) is another type of fiber grating which has been investigated to solve this problem. TFBG is fabricated with the grating plane tilted at a certain angle with respect to the light propagation direction. Compared with FBG, except for a portion of the forward-directed light, which is coupled into the backward-directed core mode, the coupling of the core guided mode to the cladding mode and radiation mode also becomes stronger as the tilted angle increases, resulting in unique spectral characteristics [10]. Therefore, for sensing applications, the FBG sensor may not be able to distinguish the spectral changes caused by the surrounding environment or stress deformation. After the hydrogen absorption by the Pd layer, both the change of the refractive index and the expansion of the film can affect the output spectrum of FBG, which would cause inaccuracy of the sensing results. As TFBG is extremely sensitive to the change of refractive index of the environment, it can be utilized to detect the hydrogen concentration by measuring the change in the refractive index of the Pd membrane. At the same time, the deformation of the grating region due to other factors can also be detected. For the preparation of the thin layer of metal coating, the magnetron sputtering method is commonly used [3,4,8,11–14]. A uniform layer of metal can be formed on the surface of the short fiber by rotating it during the sputtering. However, for long fibers, such as TFBG or FBG, this method can be difficult to apply. In order to simplify the coating process with reduced costs and manufacturing difficulties, a new method of electroless coating is utilized. During the chemical reaction, Pd particles are generated and slowly condense on the surface of the fiber, forming a uniform thin layer of Pd over the grating area [6,15–18]. In this paper, an optical sensor based on TFBG is proposed which uses a chemically coated Pd layer as the sensing material. Through the pretreatment “sensitization and activation” and chemical coating, a dense Pd membrane will grow on the surface of the fiber. Through experimental results combined with calculations, the proposed TFBG sensor can accurately monitor pipe cracks and hydrogen leakage at the same time at different temperatures. 2. Sensing Principle A schematic diagram of TFBG with a tilt angle θ is shown in Figure 1. The grating structure is inscribed in the core of a single mode fiber (SMF). When the optical signal propagating in the fiber core reaches the grating region, a part of the signal will be reflected by the TFBG, forming a backward-propagating cladding mode. Similar to FBG, the transmission spectrum of TFBG also has a Bragg peak. Figure 2 shows the transmission spectrum of an 8◦ TFBG with a series of cladding modes and a Bragg peak.
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Figure 1. A schematic of TFBG with a tilt angle θ inscribed in the core of SMF. Figure SMF. Figure 1. 1. A Aschematic schematic of of TFBG TFBG with with aa tilt tilt angle angle θθ inscribed in the core of SMF.
The 8° angle TFBG allows the transmission spectrum to have sufficiently clear Bragg peak and The modes. 8◦ angle angleThe TFBG allows theof transmission spectrum to have have sufficiently sufficiently clearwavelength Bragg peak peak and and cladding higher order the claddingspectrum mode corresponds to the shorter The 8° TFBG allows the transmission to clear Bragg and cladding modes. The higher orderof of external the cladding cladding mode corresponds corresponds toifthe the shorter wavelength and larger peak shiftsThe for higher the change environment. However,to theshorter order wavelength of the cladding cladding modes. order of the mode and larger peak shifts for the change of external environment. However, if the order of the cladding mode mode too high, can be unstable, which is not conducive to quantitative measurement. larger ispeak shiftsthe forpeak the change of external environment. However, if the order of the cladding is too high, peak becan unstable, which which is not conducive to quantitative measurement. mode is toothe high, thecan peak be unstable, is not conducive to quantitative measurement. 0
Transmission (dB) Transmission (dB)
0 -2 -2 -4 -4 -6 -6 -8 -8 -10 -10 1500
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Wavelength 1540 1560
(nm) 1580 Wavelength (nm) ◦
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Figure2. 2.Transmission Transmission spectrum spectrum of of 8° 8 TFBG TFBG used used in in this this paper. paper. Figure
Figure 2. Transmission spectrum of 8° TFBG used in this paper.
The wavelength of the Bragg peak and the i-th cladding mode of the TFBG spectrum can be The wavelength of the Bragg peak and the i-th cladding mode of the TFBG spectrum can be expressed as [19] expressed as [19] The wavelength of the Bragg peak and the i-th cladding mode of the TFBG spectrum can be δ (1) λ Bragg = 2 × ncore,e f f × expressed as [19] δcos θ λBragg = 2 × ncore , eff × (1) δ θ) × δ cos (ncore,e + niclad,e f f λ = 2 × n × f f (1) core , eff λiclad = Bragg (2) cos θ cos iθ n + n × δ ( core,eff clad ,eff ) of the grating; and n i i (2) where δ is the spacing period of the grating; λclad =θ is the tilt angle i core,e f f and nclad,e ff ncore ,eff cos + nclad ( θ ,eff ) × δ i represent the effective refractive index of the= core and i-th order of cladding mode, respectively. (2) λclad cos θ i cladding in the fiber, their refractive be and affected by ncore,eff nclad whereAsδ the is the spacingmodes periodtransmit of the grating; θ is the tilt effective angle of the grating;index and can , eff i the surrounding refractive index (SRI), a shift oforder theof wavelength cladding mode peaks. nrespectively. where δ isthe theeffective spacing period of index the grating; θ istothe tilt grating;of and core,eff and nclad , eff represent refractive ofleading the core and i-thangle ofthe cladding mode, As the effective refractive index of the fiber core and the period of TFBG will not change with the SRI, As thethe cladding transmit theiri-th effective refractive can be affected by represent effectivemodes refractive indexinofthe thefiber, core and order of claddingindex mode, respectively. ∂n
core,e f f i.e.,surrounding = ∂n∂δ =modes 0. The relationship the effective change of SRI andof wavelength shift can by be the refractive index (SRI),inleading to atheir shift of the wavelength cladding peaks. As the cladding transmit thebetween fiber, refractive index can bemode affected ∂nSRI SRI simplified as Equation (3). As effective refractive the leading fiber core theofperiod of TFBG will not change with the the the surrounding refractiveindex index of (SRI), to and a shift the wavelength of cladding mode peaks. i n δ ∂ncore,eff refractive As the effective index of the fiber core and the period of TFBG will not change with the clad,e f f ∂δ · ·change ∆nSRI of SRI and wavelength shift can (3) ∆λiclad = = = 0 . The relationship SRI, i.e., between the cos θ ∂nSRI ∂∂nncore,eff ∂n∂δ SRI SRI = = 0 . The relationship between the change of SRI and wavelength shift can SRI, i.e., ∂nis ∂nSRI (3). where ∆nSRI change of SRI. be simplified asthe Equation SRI
be simplified as Equation (3).
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When the Pd film absorbs hydrogen, the lattices’ transition from α phase to β phase will cause changes to the refractive index, resulting in a variation of the SRI of the TFBG. The higher the hydrogen concentration is, the faster and larger the SRI of the Pd film changes, which leads to a wavelength drift of the TFBG transmission spectrum. 3. Sensor Fabrication 3.1. Sensitization and Activation (S&A) In this experiment, the SMF-28 corning telecom fiber was loaded in a high pressure hydrogen environment for two weeks. Then TFBGs were inscribed using a 248 nm UV excimer laser and the phase-mask method [20]. The laser pulse was set to 6 mJ/150 Hz and a 15 mm long TFBG was made by a scanning technique. By adjusting the angle and the period of the phase-mask, a TFBG with an 8◦ tilted angle at a 1550 nm working wavelength was prepared. After that, the grating section of the fiber was sonicated in acetone, anhydrous ethanol, and deionized (DI) water for 10 min, respectively. For the cleaning process, the acetone and DI water were used to dissolve the fat-soluble and water-soluble dirt, respectively. Anhydrous ethanol was used to clean up the acetone residue left on the fiber surface. The fiber was then dried in the oven at 60 ◦ C for 20 min. The sensitization and activation (S&A) process was employed to grow Pd particles on the fiber surface. This process requires repeatedly dripping reactive solutions onto the surface of the fiber, which are at low concentrations to maintain the reaction occurring at a very low rate. The process relies on the low concentration, as well as the small amount, of two reactive solutions to make sure that the slowly formed Pd particles can condense onto the surface of the fiber. By repeating this process, eventually, a large amount of Pd nuclei will be formed. These condensation nuclei can facilitate the process of chemical coating. The reaction rate needs to be limited. A faster reaction rate of electroless plating will form larger flocculated Pd particles and form precipitates. The sensitization solution was prepared using 0.30 g SnCl2 ·2H2 O dissolved in 50 mL 36% hydrochloric acid. Activation solution was prepared using 0.02 g PdCl2 dissolved in 50 mL 36% hydrochloric acid. The above chemical reagents were purchased from Liaodong Chemical Reagent Factory of Dalian (China), with the purity of analytical reagent (AR). By using the same method, the TFBG section of the fiber was immersed in two solutions for about 20 s each time, several times. The initial S&A step was completed until sparse Pd particles were homogeneously formed on the fiber surface. The chemical reaction that occurred during the process can be expressed as Equation (4): Sn2+ + Pd2+ = Sn4+ + Pd ↓
(4)
A new sensitization and activation method has been developed in this work to densify the crystallized Pd particles on the fiber, which is referred to as the “drop and dip” (D&D) method. Here, two pipettes are used to mix the solution on the top of the fiber surface. In this way, the reaction can be localized within the solution drops, and therefore, more Pd particles can condense onto the grating area. After several D&D processes, dense Pd particles can be formed on the fiber surface, as shown in Figure 3.
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Figure in microscope. microscope. Figure 3. 3. TFBG TFBG processed processed through through drop drop and and dip dip processes processes in
3.2. Chemical Chemical Coating Coating Process Process 3.2. A dense dense layer layer of of Pd Pd coating coating is is prepared prepared during during this this process process in in aa heated heated water waterbath bathenvironment. environment. A The speed and thethe formation of the Pd Pd filmfilm around the The water water bath bathtemperature temperatureaffects affectsboth boththe thereaction reaction speed and formation of the around fiber. It is found that a higher temperature can facilitate the reaction speed. If the reaction speed is too the fiber. It is found that a higher temperature can facilitate the reaction speed. If the reaction speed slow, Pd the particles cannot form a homogeneous film; however, the speedif isthe toospeed fast, the Pd particles is toothe slow, Pd particles cannot form a homogeneous film; if however, is too fast, the tend to precipitate fast and subside in the solution before beingbefore coatedbeing onto the fiberonto surface. Pd particles tend totoo precipitate too fast and subside in the solution coated the fiber The chemical reaction requires 1 g/L PdCl2 , 50 g/L Na2 EDTA·2H2 O, 260 mL/L 25% ammonia, surface. 10 mL/L hydrochloric, 0.04 mol/L N2 H24, ·50 H2 O. growing on 2top of the nucleus The 36% chemical reaction and requires 1 g/L PdCl g/LBy Na 2EDTA·2H O, 260 mL/L 25% condensed ammonia, in the S&A process, a uniform layer Pd film be2O. prepared. The reaction on hydrazine to 10 mL/L 36% hydrochloric, and 0.04ofmol/L N2can H4·H By growing on top ofdepends the nucleus condensed 2+ , which needs to be performed under an alkaline condition. If Pd particles form too fast, reduce Pd in the S&A process, a uniform layer of Pd film can be prepared. The reaction depends on hydrazine they will agglomerate with each to formunder precipitates that will not grow on particles the fiber. form The role to reduce Pd2+, which needs to beother performed an alkaline condition. If Pd too of Nathey to form coordination with Pd2+ , which reduce the reaction rate, and 2 EDTA fast, willisagglomerate with eachcompounds other to form precipitates thatcan will not grow on the fiber. The 2+ . Because 2+, which the same true for ammonia. The Cl− ofcompounds hydrochloric canPd increase thecan solubility of Pd role of Nais 2EDTA is to form coordination with reduce the reaction rate, 2+. solution. the amount acid isThe small, it will not affectcan theincrease alkalinethe environment the and the sameofishydrochloric true for ammonia. Cl- of hydrochloric solubility ofofPd Because The chemicalofreaction that occurred duringitthe can the be expressed as Equation (5): the amount hydrochloric acid is small, willprocess not affect alkaline environment of the solution.
The chemical reaction that occurred during the process can be expressed as Equation (5): 2Pd2+ + N2 H4 + 4OH− → 2Pd ↓ +N2 ↑ +4H2 O
(5)
2Pd 2+ + N2 H4 + 4OH− → 2Pd ↓ + N2 ↑ +4H2 O (5) For the chemical coating process, Na2 EDTA·2H2 O and ammonia were dissolved in 300 mL DI
2O and ammonia were dissolved in 300 mL DI chemical process, Na2EDTA·2H ◦ C to form waterFor andthe heated to 60coating the alkaline solution. The PdCl2 was then dissolved in hydrochloric water and heated to 60 °C to form the alkaline The PdCl 2 was then dissolved in and added to the above solution. Since the amount solution. of hydrochloric is very small compared with hydrochloric and added to the above solution. Since the amount of hydrochloric is very the amount of ammonia, it will not affect the alkaline environment of the solution. After Pd2+small was compared the amount of N ammonia, it will not affect the alkaline environment of the solution. completelywith dispersed at 60 ◦ C, 2 H4 ·H2 O was slowly added to start the reaction and formed a layer After Pd2+ was completely dispersed at 60 °C, N2H4·H2O was slowly added to start the reaction and of Pd on top of the fiber surface. formed a layer of Pd on top of the fiber surface. In this experiment, an optimal water bath temperature of 60 ◦ C was determined to obtain a In this experiment, an optimal water bath temperature of 60 °C was determined to obtain a high high quality of Pd coating, as shown in Figure 4. It can be found that a layer of homogenous Pd quality of Pd coating, as shown in Figure 4. It can be found that a layer of homogenous Pd coating is coating is formed on the fiber surface with the particle size around 1 µm, see Figure 4b. The scanning formed on the fiber surface with the particle size around 1 μm, see Figure 4b. The scanning electronic electronic microscopic (SEM) image was taken using Nova NanoSEM 450 field emission scanning microscopic (SEM) image was taken using Nova NanoSEM 450 field emission scanning electron electron microscopy (FEI Company, Hillsboro, OR, USA). microscopy (FEI Company, Hillsboro, Oregon, USA).
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Figure images of ofcoated PdPd layer ononthethePd-plated fiber (a)(a)Optical Figure 4.Microscopic Microscopic images coated layer Pd-plated fiber surface.microscope Optical Figure 4.4.Microscopic images of coated Pd layer on the Pd-plated fiber surface. (a)surface. Optical microscope of the fiber surface. (b) SEM imagine showing the uniform Pd particles condensed onon of the fiber surface. (b) SEM imagine uniform Pd particles on condensed of microscope the fiber surface. (b) SEM imagine showing the showing uniform the Pd particles condensed the fiber, thethe fiber, forming thethe PdPd layer. fiber, forming layer. forming the Pd layer.
4. 4. Results and Discussion Results and Discussion Figure 55shows the schematic ofofthe entire experimental setup. TheThe Pd-coated TFBG is placed in a Figure shows thethe schematic the entire experimental setup. Pd-coated TFBG is is placed Figure 5 shows schematic of the entire experimental setup. The Pd-coated TFBG placed gas chamber, which is fastened onto the optical tablet to avoid vibration. The two ends of the gas in in aflow gas flow chamber, which is fastened onto the optical tablet to avoid vibration. The two ends of of a gas flow chamber, which is fastened onto the optical tablet to avoid vibration. The two ends flow chamber are sealed by glue to ensure gas tightness. The mixed gas enters and exits through the thethe gas flow chamber are sealed by glue to ensure gas tightness. The mixed gas enters and exits gas flow chamber are sealed by glue to ensure gas tightness. The mixed gas enters and exits inlet pipe and the outlet pipe, respectively. through the inlet pipe and thethe outlet pipe, respectively. through the inlet pipe and outlet pipe, respectively.
schematic chamber. Figure 5. The of of gasgas flow chamber. Figure 5. The schematic flow chamber.
AA broadband source (BBS) working between 1500 nm and 1620 nm with thethe power of of 1717 dBm dBm broadband source (BBS) working between 1500 nm and 1620 nm with power dBm ◦ C) was used in this experiment. The output spectrum was monitored using an optical spectrum (at 25 (at(at2525°C) was used in this experiment. The output spectrum was monitored using an optical °C) was used in this experiment. The output spectrum was monitored using an optical analyzer (OSA) (AQ6370 model, Yokogawa, Japan) with a minimum wavelength resolutionwavelength ofwavelength 0.05 nm. spectrum analyzer (OSA) (AQ6370 model, Yokogawa, Japan) with spectrum analyzer (OSA) (AQ6370 model, Yokogawa, Japan) witha aminimum minimum The experiment was performed several times at different hydrogen concentrations. The Pd film resolution of 0.05 nm. resolution of 0.05 nm. absorbs hydrogen continuously during the testing until it reaches an equilibrium state. During this The experiment was performed several times at at different hydrogen concentrations. The PdPd film The experiment was performed several times different hydrogen concentrations. The film process, the Pd film expands, causing thethe change ofuntil SRI the grating and the state. peak shift of this the absorbs hydrogen continuously during testing until it reaches anan equilibrium During absorbs hydrogen continuously during testing itofreaches equilibrium state. During this cladding mode. Figure 6expands, shows causing thecausing continuous peak of shift ofof a of certain cladding mode atpeak 4%shift hydrogen process, the PdPd film expands, thethe change SRI thethe grating and thethe peak of of thethe process, the film change of SRI grating and shift atmosphere overFigure 30 min. cladding mode. 6 shows thethe continuous peak shift of of a certain cladding mode at at 4%4% hydrogen cladding mode. Figure 6 shows continuous peak shift a certain cladding mode hydrogen
atmosphere over 3030 min. atmosphere over min.
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Transmission (dB)
-5
2 min 4 min 6 min 8 min 10 min 12 min 14 min 16 min 18 min 20 min 22 min 24 min 26 min 28 min 30 min
-10
-15
-20
1557.4
1557.6
1557.8
1558.0
1558.2
Wavelength (nm) Figure 6. One of the peaks of cladding mode changes with hydrogen absorption.
Figure 6. One of the peaks of cladding mode changes with hydrogen absorption.
From Equations (1)–(3), it can be found that two main factors can cause the peak drift of the outputFrom spectrum, which(1)~(3), are theitrefractive indexthat change the factors Pd filmcan duecause to hydrogen Equations can be found two of main the peakabsorption drift of the and the grating deformation caused by the Pd film expansion. In the experiment, the measurements output spectrum, which are the refractive index change of the Pd film due to hydrogen absorption were carried out under constant temperature conditions, so there is no effect of temperature on the and the grating deformation caused by the Pd film expansion. In the experiment, the measurements amount of spectral drift. The grating deformation due to metal film expansion affects both the were carried out under constant temperature conditions, so there is no effect of temperatureBragg on the peak (∆B) and the cladding mode (∆C), leading to the same wavelength shift. However, the cladding amount of spectral drift. The grating deformation due to metal film expansion affects both the Bragg modes bymode the change of the refractive index of the metal film around the peak can (ΔB)only and be theaffected cladding (ΔC), leading to the same wavelength shift. However, the grating cladding area. The difference of these two drifts (∆C-∆B) only gives the information of the hydrogen-related modes can only be affected by the change of the refractive index of the metal film around the grating changes. Fordifference ∆C-∆B = 0, suggests no change of SRI, meaning noinformation hydrogen leakage be detected. area. The of itthese two drifts (ΔC-ΔB) only gives the of the can hydrogen-related Inchanges. this case,For if ∆B 6 = 0, it means a small crack may occur at the pipe, but has not caused a yet. ΔC-ΔB = 0, it suggests no change of SRI, meaning no hydrogen leakage canleakage be detected. However, the case of ∆C-∆B 6 = 0 indicates that leaked hydrogen can be detected in the atmosphere. In this case, if ΔB ≠ 0, it means a small crack may occur at the pipe, but has not caused a leakage yet. ByHowever, measuringthe thecase ∆B of and ∆C at ≠the same time, both the pipeline cracks the hydrogen can ΔC-ΔB 0 indicates that leaked hydrogen can and be detected in theleakage atmosphere. beBy monitored at the same time. measuring the ΔB and ΔC at the same time, both the pipeline cracks and the hydrogen leakage For the wavelength the output spectrum, it can be found that different orders of the can be monitored at the shift sameoftime. cladding mode have different ∆C-∆B. higherspectrum, order of the cladding modethat hasdifferent a larger wavelength For the wavelength shift of theAoutput it can be found orders of the drift. Figure 7 shows the drifted wavelength (∆C-∆B) for the highest stable order cladding mode cladding mode have different ΔC-ΔB. A higher order of the cladding mode has aoflarger wavelength (atdrift. 1526.20 nm)7 and one ofdrifted a medium order of(ΔC-ΔB) cladding (at 1559.98 at of a 2% hydrogen Figure shows the wavelength formode the highest stablenm) order cladding mode concentration. A roughly 80% increase of the wavelength drift can be detected in the highest order (at 1526.20 nm) and one of a medium order of cladding mode (at 1559.98 nm) at a 2% hydrogen curve compared Awith the medium orderofone 30 min, which gives sensitivity during order the concentration. roughly 80% increase theat wavelength drift can be high detected in the highest measurement. Therefore, in this experiment, the wavelength change of the highest order cladding curve compared with the medium order one at 30 min, which gives high sensitivity during the mode is used for Therefore, sensing purposes. measurement. in this experiment, the wavelength change of the highest order cladding The same TFBG sensor was tested at the same temperature under 1%, 2%, 3%, and 4% hydrogen mode is used for sensing purposes. concentration conditions, respectively. Figure 8 shows the peak drift fit curve of ∆C-∆B over time. The sensor was placed in a ventilated environment for a sufficient time to allow the hydrogen to overflow. It can be seen that the higher the hydrogen concentration, the larger and faster the drift (∆C-∆B) varies. The value of ∆C-∆B gradually rises as the time increases and tends to stabilize after approximately 20 min.
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Figure 7. The fitting curves of ΔC-ΔB for different orders of cladding modes at same H2 concentration.
The same TFBG sensor was tested at the same temperature under 1%, 2%, 3%, and 4% hydrogen concentration conditions, respectively. Figure 8 shows the peak drift fit curve of ΔC-ΔB over time. The sensor was placed in a ventilated environment for a sufficient time to allow the hydrogen to overflow. It can be seen that the higher the hydrogen concentration, the larger and faster the drift (ΔC-ΔB) varies. The value of ΔC-ΔB gradually rises as the time increases and tends to stabilize after approximately 20 min. The relationships between the wavelength drift and the hydrogen concentration for 5 min and 10 min are plotted in Figure 9. Both curves can be fitted as follows: x
y = 0.695 × e 3.410 − 0.631 x 5.281
(6) (7)
=for 3.596 × e orders −of3.448 Figure 7. The curves fitting curves of ΔC-ΔB different of cladding modes at same H2 Figure 7. The fitting of ∆C-∆B for ydifferent orders cladding modes at same H2 concentration. concentration.
The same TFBG sensor was tested at the same temperature under 1%, 2%, 3%, and 4% hydrogen concentration conditions, respectively. Figure 8 shows the peak drift fit curve of ΔC-ΔB over time. The sensor was placed in a ventilated environment for a sufficient time to allow the hydrogen to overflow. It can be seen that the higher the hydrogen concentration, the larger and faster the drift (ΔC-ΔB) varies. The value of ΔC-ΔB gradually rises as the time increases and tends to stabilize after approximately 20 min. The relationships between the wavelength drift and the hydrogen concentration for 5 min and 10 min are plotted in Figure 9. Both curves can be fitted as follows: x
y = 0.695 × e 3.410 − 0.631 x
y = 3.596 × e 5.281 − 3.448
(6) (7)
Figure 8. The wavelength drift (∆C-∆B) at different H2 concentrations with the fitting curves.
The relationships between the wavelength drift and the hydrogen concentration for 5 min and 10 min are plotted in Figure 9. Both curves can be fitted as follows: x
y = 0.695 × e 3.410 − 0.631 x
y = 3.596 × e 5.281 − 3.448
(6) (7)
It can be seen that the peak drift of ∆C-∆B for both curves shows an exponentially increasing trend as the hydrogen concentration grows. The 5 min curve already shows a clear trend with increasing H2 concentration, indicating a short sensing time for the proposed sensor. For the low H2 concentration of 1%, the wavelength drift rises to 1 pm after 10 min, which shows a strong ability for low concentration sensing.
It can be seen that the peak drift of ΔC-ΔB for both curves shows an exponentially increasing trend as the hydrogen concentration grows. The 5 min curve already shows a clear trend with increasing H2 concentration, indicating a short sensing time for the proposed sensor. For the low H2 concentration 1%, the wavelength drift rises to 1 pm after 10 min, which shows a strong ability Sensors 2018, 18,of 4478 9 of 11 for low concentration sensing.
Figure 9. 9. The fitting curve of of ΔC-ΔB in in different hydrogen concentrations forfor 5 min and 1010 min. Figure The fitting curve ∆C-∆B different hydrogen concentrations 5 min and min.
For Fora ahigh highconcentration concentrationand andexposure exposuretime, time,the thepure purePdPdlayer layermay mayhave havea ahydrogen hydrogenbrittle brittle effect, leading toto the fast transition from alpha phase toto beta phase and forming small cracks onon the effect, leading the fast transition from alpha phase beta phase and forming small cracks the membrane overcome this this effect effectisisto membranesurface, surface,which whichcan canaffect affectthe thesensitivity sensitivityof ofthe the sensor. sensor. One way to overcome todope dopegold goldororsilver silverinto intothe thefilm film and form alloys Pd/Au and Pd/Ag.Further Furtherimprovement improvement and form alloys of of Pd/Au and Pd/Ag. ofof the the sensor will include an investigation of the co-deposition of the alloy metal layer using the sensor will include an investigation of the co-deposition of the alloy metal layer using the proposed proposed electroless chemical plating method paper forsensitivity better sensor sensitivity and electroless chemical plating method in this paperinforthis better sensor and reliability. reliability. Compared to the traditional H2 sensors, the sensor proposed in this work has the advantages of Compared to the traditional H2and sensors, proposed in this work the advantages electrical passiveness, a lower cost, beingthe easysensor to fabricate. In addition, it canhas monitor the cracking ofofelectrical passiveness, a lower cost, and being to fabricate. In addition, can monitor thefor an H2 pipeline and detect hydrogen leakage at easy the same time, which makes it ait good candidate cracking of anthe H2pipeline pipelineinand detect hydrogen leakage at the same time, which makes it a good monitoring the industry. candidate for monitoring the pipeline in the industry. 5. Conclusions 5. Conclusions In this paper, a new method for reducing Pd2+ in aqueous solution for plating Pd film on optical 2+ in uniform fibers is proposed, which can form a dense on afor long fiberPd surface easily and In this paper, a new method for reducing Pdand aqueous film solution plating film on optical ◦ inexpensively. By which depositing a Pd membrane onuniform the surface ana8long tilted TFBG, its spectrum can fibers is proposed, can form a dense and filmofon fiber surface easily and have a specific response to different hydrogen concentrations with a high sensitivity. The sensor can inexpensively. By depositing a Pd membrane on the surface of an 8° tilted TFBG, its spectrum can monitor the crack of a pipeline while hydrogen only responding to hydrogen For a 4% have a specific response to different concentrations withbya calculation. high sensitivity. Thehydrogen sensor concentration, calculated wavelength drift by 1 pm in to lesshydrogen than 5 min. the same For situation can monitor thethe crack of a pipeline whilecan only responding by For calculation. a 4%of 1%, the time will be in 10 min. The spectral drift of non-hydrogen-specific detection without calculation hydrogen concentration, the calculated wavelength can drift by 1 pm in less than 5 min. For the will be much larger. thisbe study provides a new method for of coating Pd films on optical same situation of 1%,The theresult time of will in 10 min. The spectral drift non-hydrogen-specific fibers, and will becalculation useful for will hydrogen sensing of TFBG in theofnear detection without be much larger. The result thisfuture. study provides a new method for coating Pd films on optical fibers, and will be useful for hydrogen sensing of TFBG in the near Author Contributions: The manuscript was conceived by all authors. J.Y. and Z.W. performed most of the future. experiments and prepared a draft with X.Y.; X.Y. and X.H. analyzed the data and provided suggestions to the
experiments. The final manuscript was edited by X.H. and M.Z. Funding: Z. Wu would like to acknowledge the support from the Natural Science Foundation of China (NSFC) (61704017) and PetroChina Innovation Foundation (2016D-5007-0603). X. Han acknowledges the support from NSFC (61875028) and Fundamental Research Funds for the Central Universities (DUT18ZD106, DUT18GF102, and DUT18LAB20). X. Yang acknowledges the support from the UK Engineering and Physical Sciences Research Council (EPSRC) (grant reference EP/P002803/1).
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Acknowledgments: Z. Wu would like to acknowledge the inspiring discussion with L. Wang, X. Yin and C. Chen from Henan Shijia Photons Technology Co., Ltd. Conflicts of Interest: The authors declare no conflict of interest.
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