sensors Review

Optical Fiber Grating Hydrogen Sensors: A Review Jixiang Dai 1 , Li Zhu 1 , Gaopeng Wang 1 , Feng Xiang 1 , Yuhuan Qin 1 , Min Wang 1 and Minghong Yang 1,2, * 1

2

*

National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, Wuhan 430070, China; [email protected] (J.D.); [email protected] (L.Z.); [email protected] (G.W.); [email protected] (F.X.); [email protected] (Y.Q.); [email protected] (M.W.) Key Laboratory of Fiber Optic Sensing Technology and Information Processing, Ministry of Education, Wuhan University of Technology, Wuhan 430070, China Correspondence: [email protected]

Academic Editors: Christophe Caucheteur and Tuan Guo Received: 15 December 2016; Accepted: 28 February 2017; Published: 12 March 2017

Abstract: In terms of hydrogen sensing and detection, optical fiber hydrogen sensors have been a research issue due to their intrinsic safety and good anti-electromagnetic interference. Among these sensors, hydrogen sensors consisting of fiber grating coated with sensitive materials have attracted intensive research interests due to their good reliability and distributed measurements. This review paper mainly focuses on optical fiber hydrogen sensors associated with fiber gratings and various materials. Their configurations and sensing performances proposed by different groups worldwide are reviewed, compared and discussed in this paper. Meanwhile, the challenges for fiber grating hydrogen sensors are also addressed. Keywords: hydrogen sensor; optical fiber grating; sensitive materials

1. Introduction With aggravated air pollution and increasing fossil fuel consumption, exploiting clean and renewable energy is urgent for humankind. Hydrogen, which is an ideal candidate as a future energy source, has attracted much research interest [1–3] due to its recyclable and non-polluting characteristics. However, hydrogen is dangerous due to its high diffusivity and flammability, and its explosive limit covers a wide range of H2 -air mixtures (4%–75% (v/v)). Commercial electrochemical hydrogen sensors still have the potential for explosion due to potential electric sparks. Optical fiber hydrogen sensors can be intrinsically safe as they employ optical signals as a sensing medium, and as such they have attracted much research interest. Several kinds of optical fiber hydrogen sensors, such as evanescent sensors [4–6], micro-mirror sensors [7,8], interference sensors [9], surface plasmon resonance (SPR) sensors [10], acoustic resonator sensors [11], and fiber grating sensors [12–35], have been proposed in recent years. Compared to other optical fiber hydrogen sensors, hydrogen sensors based on fiber grating are more suitable for distributed measurement and temperature compensation because of their wavelength multiplexing capability. Several review articles [36–39] related to optical hydrogen sensors have been published. These papers have summarized optical fiber hydrogen sensors from different perspectives. In this paper, a review paper about optical fiber grating hydrogen sensors is presented from a distinct perspective. Since hydrogen molecules cannot be detected by the spectral absorption method, utilizing materials that can react with hydrogen is an essential method for hydrogen sensing technology. Hydrogen-sensitive materials are indispensable for preparing hydrogen sensing probes. According to classification of hydrogen-sensitive materials, two types of fiber grating hydrogen sensors have been

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reported. The first kind of fiber grating hydrogen sensor is based on Pd [12–20] or Pd alloys [21–25], [21–25], which can produce volume expansion or optical constant change during hydrogen which can produce volume expansion or optical constant change during hydrogen response. The other response. The other type is based on Pt-loaded WO3 coatings [40–44] or other oxide materials [45] type is based on Pt-loaded WO3 coatings [40–44] or other oxide materials [45] undergoing an undergoing an exothermic or gasochromic reaction [46,47] in a hydrogen atmosphere. The exothermic or gasochromic reaction [46,47] in a hydrogen atmosphere. The configurations and configurations and performances of these sensors are presented in the following paragraphs. performances of these sensors are presented in the following paragraphs.

2. Optical Hydrogen Sensor Sensor Based Based on on Pd Pd and and Pd Pd Alloy Alloy 2. Optical Fiber Fiber Grating Grating Hydrogen In 1999, 1999, the first Pd-based Pd-based fiber fiber Bragg Bragg grating grating (FBG) (FBG) hydrogen hydrogen sensor sensor was was proposed proposed by by In the first Sutapun et with cladding of Sutapun et al. al. [12]. [12]. The Thetypical typicalstructure structureofofthe thesensor sensorisisshown shownininFigure Figure1.1.FBG FBG with cladding 35 μm was evaporated with 560 nm of pure Pd film for hydrogen characterization. The sensor of 35 µm was evaporated with 560 nm of pure Pd film for hydrogen characterization. The sensor showed linear linear wavelength wavelength shift shift when when the the hydrogen hydrogen concentration concentration (in (in N N22 atmosphere) from showed atmosphere) ranged ranged from 0.3% (v/v) to 1.8%. When the hydrogen concentration was over 1.8%, the hydrogen sensor 0.3% (v/v) to 1.8%. When the hydrogen concentration was over 1.8%, the hydrogen sensor deteriorated deteriorated and becameThe irreversible. Thefor main for thisisphenomenon is due to the poor and became irreversible. main reason thisreason phenomenon due to the poor stability of pure stability pure Pd film. caused by Pd film is about three times Pd film. of Additionally, theAdditionally, stress causedthe bystress Pd film is about three times greater than thatgreater of bulkthan Pd that of bulk Pd when the hydrogen concentration is between 0.5% and 1.4% at room temperature, when the hydrogen concentration is between 0.5% and 1.4% at room temperature, which is a very which is a very interesting phenomenon. interesting phenomenon.

Figure 1. 1. Typical Typical structure structure of of aa fiber fiber Bragg Bragg grating grating (FBG) (FBG) hydrogen hydrogen sensor Figure sensor [12]. [12].

During the hydrogen sensor based on on a Pd tube. By the same same year, year,Tang Tangetetal. al.[13] [13]reported reporteda aFBG FBG hydrogen sensor based a Pd tube. employing a Pd tube (with different thickness) By employing a Pd tube (with different thickness)asasthe thehydrogen-sensitive hydrogen-sensitivematerial, material, the the sensitivity sensitivity of FBG hydrogen hydrogensensor sensorwas wasincreased. increased.However, However, response time of the sensor (under H2Nin2 the FBG thethe response time of the sensor (under 4% 4% H2 in N 2 atmosphere) was more than 200 min at room temperature. The influence of operating atmosphere) was more than 200 min at room temperature. The influence of operating temperature on temperature onsensors FBG hydrogen sensors was in also inthe thistemperature paper. When the temperature FBG hydrogen was also investigated thisinvestigated paper. When was between 23 ◦ C ◦ C, the23 was 45 between °C and 45 °C, the hydrogen theshift. largest wavelength shift. However, and hydrogen sensor displayed thesensor largestdisplayed wavelength However, the response time of ◦ the sensor response time of the sensor was still too long. By increasing the operating temperature to 95 can °C, was still too long. By increasing the operating temperature to 95 C, the response time theless response be lessthe than 2 min. However, the sensitivity of be thesacrificed hydrogen sensor be be than 2 time min. can However, sensitivity of the hydrogen sensor will due to thewill lower sacrificed due to the lower hydrogen absorption capability of the Pd tubes at higher temperature. hydrogen absorption capability of the Pd tubes at higher temperature. The purging process under N2 The purging process under 2 and air was studied in this paper, andobserved quicker degassing rate was and air was also studied inN this paper, andalso quicker degassing rate was in air. This result observed air. This also result thatinfluence atmosphere has significant influence on the implies thatinatmosphere hasimplies significant on thealso performance of the sensor. performance of the sensor. A Pd-based FBG hydrogen sensor was developed by Zalvidea et al. [14] in 2004. Pd film was A on Pd-based hydrogen sensor developed Zalvidea al. [14] in 2004. Pd film coated taperedFBG single mode fiber, andwas FBG was used by to obtain theetoptical power reflected bywas the coated on tapered mode fiber, and FBG was used to obtain thewith optical power reflected byand the sensing probe. Thissingle configuration enables double interaction of light the deposited Pd film, sensing probe. This configuration enables double interaction of light with the deposited Pd film, and therefore the sensitivity of the sensor can be increased. therefore the sensitivity of thecoated sensor with can be5 increased. In 2007, a 25-µm FBG nm of Pd was proposed for hydrogen detection by In 2007, a 25-μm FBG coated with 5 Pd was for hydrogen by Aleixandre et al. [15]. The response time ofnm theofsensor wasproposed reduced within 10 min.detection Compared Aleixandre et al. [15]. the Therepeatability response time of the sensor reduced within 10However, min. Compared to the to the previous work, of the sensor waswas obviously improved. the hydrogen previous work,athe repeatability of the sensor obviously improved. the hydrogen sensor showed longer response time due to thewas superficial oxidation of Pd However, film. sensor a longer response time to the superficial oxidation of Pdmethod film. was utilized by Toshowed improve the performance of due hydrogen sensors, optical heating performance of into hydrogen sensors,fiber, optical heatingby method was utilized by Buric BuricTo etimprove al. [16]. the FBG was written double-clad followed sputtering glue metal and et al.or[16]. was into double-clad fiber, by sputtering metal 150 or 500 nm 150 500FBG nm of Pdwritten film. With heating power of followed 560 mW (910 nm diodeglue laser), theand hydrogen sensor of Pd film. With heating power of 560 mW (910 nm diode laser), the hydrogen sensor showed a showed a quick response rate (less than 10 s). Meanwhile, the hydrogen response was repeatable, quick response rate (less than 10 s). Meanwhile, the hydrogen response was repeatable, and there was little hysteresis at room temperature and lower temperature of −50 °C. This work exhibited an

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◦ C. This effective prepare atPd-based FBG hydrogen with temperature a quick response and work high and thereapproach was little to hysteresis room temperature and lower of −50 rate sensitivity, especially low temperatures. In Pd-based 2009, further improvement was achieved by inscribing exhibited an effectiveatapproach to prepare FBG hydrogen with a quick response rate FBG in 1-cm-high attenuation fibers and then splicing with single-mode fiber [17]. The heating and high sensitivity, especially at low temperatures. In 2009, further improvement was achieved efficiency was FBG greatly improved by this technique, enabled the detection of 1% hydrogen at by inscribing in 1-cm-high attenuation fiberswhich and then splicing with single-mode fiber [17]. −150 °C. Thisefficiency method can ability due to the lowerthe optical powerofloss The heating wasprovide greatly multiplexing improved bysensing this technique, which enabled detection 1% ◦ C. This only at theatBragg section and theprovide spicingmultiplexing junction. Moreover, was hysteretic effect hydrogen −150grating method can sensing there ability duelittle to the lower optical even when thickness of thegrating Pd film section was between 150spicing and 500junction. nm. power loss the only at the Bragg and the Moreover, there was little Another way to increase the sensitivity of FBG hydrogen sensor is using by an FBG with a hysteretic effect even when the thickness of the Pd film was between 150 and 500 nm. smaller diameter. shown in 2, theofsensing probe was fabricated 150 nm Pd Another way As to increase theFigure sensitivity FBG hydrogen sensor is usingby bydepositing an FBG with a smaller on tapered with of 50 μm [18]. head was ablePd toon detect H2 diameter. AsFBG shown in diameter Figure 2, the sensing probeThe wasproposed fabricatedsensing by depositing 150 nm tapered concentration in the range of 0.1%–1% (v/v) in N 2 , and sensitivity of 81.8 pm was attained at room FBG with diameter of 50 µm [18]. The proposed sensing head was able to detect H2 concentration temperature. However, film stillsensitivity has the possibility to was peelattained off due at to room its poor adhesion in the range of 0.1%–1%pure (v/v) Pd in N of 81.8 pm temperature. 2 , and towards fiber. However,optical pure Pd film still has the possibility to peel off due to its poor adhesion towards optical fiber.

Figure 2. 2. FBG FBGhydrogen hydrogensensor sensor based based on on aa tapered tapered single-mode single-mode fiber fiber [18]. [18]. Figure

The pooradhesion adhesionbetween between thick optical fiberbecan be improved by aadding a The poor thick Pd Pd filmfilm and and optical fiber can improved by adding polymer polymer coating as an intermediate layer [19,20]. The characterization of these hydrogen sensors was coating as an intermediate layer [19,20]. The characterization of these hydrogen sensors was performed performed in power transformer oil. By sputtering 20 560 nm Ti film on FBG with in power transformer oil. By sputtering 20 nm Ti and nmand Pd 560 filmnm on Pd FBG coated withcoated polyimide polyimide layer, the sensitivity of the sensor can reach to 13.5 ppm/pm when set in transformer oil. layer, the sensitivity of the sensor can reach to 13.5 ppm/pm when set in transformer oil. The polymer The polymer layer can successfully prevent the blistering of Pd film, resulting in a significant layer can successfully prevent the blistering of Pd film, resulting in a significant improvement of the improvement reproducibility of sensors. Pd-basedThe hydrogen sensors. alternative to improve reproducibilityofofthe Pd-based hydrogen alternative wayThe to improve the way stability of FBG the stability of FBG hydrogen sensors is adopting Pd alloys. In 2012, Dai et al. [21] proposed FBG hydrogen sensors is adopting Pd alloys. In 2012, Dai et al. [21] proposed an FBG hydrogenan sensor hydrogen sensor based on Pd91Ni9 composite film. FBG with a diameter of 17 μm was prepared by based on Pd 91 Ni9 composite film. FBG with a diameter of 17 µm was prepared by removing fiber removing fiber cladding HF solution. Then, Pd91Ni 9 composite film was deposited on the side-face cladding in HF solution. in Then, Pd91 Ni9 composite film was deposited on the side-face of etched FBG. of etched FBG. X-ray powder diffraction (XRD) results proved Pd 91Ni9 composite film had good X-ray powder diffraction (XRD) results proved Pd91 Ni9 composite film had good structural stability ◦ structural stability during hydrogen response. At room temperature of 23 °C, the sensor showed an during hydrogen response. At room temperature of 23 C, the sensor showed an approximately approximately 15 pm wavelength shift towards 1% hydrogen in air, and the response time no 15 pm wavelength shift towards 1% hydrogen in air, and the response time was no more thanwas 5 min. more than 5 min. Moreover, the hydrogen sensor displayed good repeatability. In the next year, Moreover, the hydrogen sensor displayed good repeatability. In the next year, Pd76 Ag24 composite Pd 76Ag24 composite film was employed for hydrogen sensing [22]. Although the FBG (diameter of film was employed for hydrogen sensing [22]. Although the FBG (diameter of 20.6 µm) hydrogen 20.6 μm) hydrogen showedthe good repeatability, theabout sensitivity was only pm/1% sensor showed goodsensor repeatability, sensitivity was only 10 pm/1% (v/v) about in air. 10 Two years (v/v) later, in Two years later, the sensitivity of the hydrogen sensor greatly polypropylene increased by employing theair. sensitivity of the hydrogen sensor was greatly increased bywas employing sheet as a polypropylene sheet a flexible substrate [23]. shifts of were 37 pm under 1% hydrogen flexible substrate [23].asWavelength shifts of 37 pmWavelength under 1% hydrogen observed in this paper; were observed in this paper; the better sensitivity of the sensor was mainly due to the much lower the better sensitivity of the sensor was mainly due to the much lower Young’s modulus of the polymer Young’s modulusthe of hydrogen the polymer sheet. hydrogen sensor has good sheet. Although sensor stillAlthough has good the response towards 4% still hydrogen, the response response towards 4% hydrogen, the response time nearly doubled due to the oxidation of the composite film time nearly doubled due to the oxidation of the composite film after setting in air for six months. after setting in air for six months. The oxidation of hydrogen sensitive film not only has a negative The oxidation of hydrogen sensitive film not only has a negative effect on the response rate, but also effect on theitsresponse rate, but also canTherefore, reduce itsthe hydrogen responsibility. Therefore, thesensor detection can reduce hydrogen responsibility. detection threshold of the hydrogen can threshold of the hydrogen sensor can be increased due to this passive effect. be increased due to this passive effect. In some special facilities, there is little oxygen and an FBG hydrogen sensor based on a Pd alloy has better potential applications. In 2015, FBG hydrogen sensor based on Pd/Ag composite film

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In some special facilities, there is little oxygen and an FBG hydrogen sensor based on a Pd alloy has better potential applications. hydrogen sensor composite film displayed better stability than pureInPd2015, film FBG [24] in transformer oil. based When on thePd/Ag operating temperature displayed better20 stability than film [24] inhydrogen transformer oil. When thelower operating temperature increased from °C to 80 °C,pure the Pd Pd/Ag-based sensor showed sensitivity and a ◦ C to 80 ◦ C, the Pd/Ag-based hydrogen sensor showed lower sensitivity and a increased from 20rate. quicker response A similar comparison was conducted in 2016 [25]. An FBG hydrogen sensor quicker rate. A42 similar comparison was conducted in 2016 [25]. An FBG hydrogen sensor based onresponse Pd and Pd 58Cr was reported for hydrogen concentration monitoring in the same facilities. based on Pd and Pd was reported forlayer, hydrogen concentration monitoring in thecoated same facilities. After depositing TiO as42an intermediate the hydrogen-sensitive films were on FBG. 582Cr After TiO2 asnearly an intermediate layer, the hydrogen-sensitive films were coatedincreased on FBG. These depositing sensors displayed 15 pm wavelength shifts when the hydrogen concentration These displayed nearly pm wavelength shifts when concentration increased to 650 sensors ppm. The relatively low 15 sensitivity can be attributed tothe thehydrogen employment of standard FBG in to 650 ppm.The The Pd-based relatively low sensitivity can beresponse attributedrate to the of standard FBG this this work. sensor had quicker andemployment higher sensitivity, while Pdin58Cr 42 work. The Pd-based sensor had quicker response rate and higher sensitivity, while Pd Cr showed showed the capability to detect higher concentration hydrogen. And these experimental 58 42 results the capability detect higher concentration hydrogen. these experimental results proved that proved that Pdto 52Cr 48 can enhance the dissolved hydrogenAnd detection range in transformer oil. Pd52 Cr can enhance the dissolved hydrogen detection range in transformer oil. Another type FBG hydrogen sensor, which was based on optical constant change of Pd film 48 FBG hydrogen sensor, which on optical of aPd film whenAnother exposedtype to hydrogen, was demonstrated bywas Tienbased et al. [26] in 2008.constant FBG waschange glued on silicon when exposed to hydrogen, demonstrated al. Thus, [26] inthe 2008. FBG was glued onofa FBG silicon V groove and polished withwas residual diameterby of Tien 62.9 et μm. central wavelength is V groove to and polished residual diameter of 62.9 µm.AThus, thePdcentral wavelength of FBG is sensitive the changewith of the ambient refractive index. 20-nm film was deposited on the sensitive the change of thetoambient index. A 20-nm film was deposited on the polished polished to FBG. In contrast the redrefractive wavelength shift of thePdsensor based on mechanical stress FBG. contrast the red wavelength shift of thedevice sensordisplayed based onblue mechanical stress caused by causedInby volumetoexpansion, the proposed sensing wavelength shifts during volume expansion, the proposed sensing device displayed blue wavelength shifts during the hydrogen the hydrogen response process. The main reason for this phenomenon is that the influence of response Theindex main reason for this is thatstress. the influence of decreased decreasedprocess. refractive is greater thanphenomenon that of imposed Since the FBG was refractive fixed on index is greater than that imposed stress. Since the FBG was fixedThus, on v-groove, change ofindex grating v-groove, the change of of grating period was greatly suppressed. effectivethe refractive of period was greatly Thus, effective index of Braggof grating can leading be remarkably Bragg grating can suppressed. be remarkably reduced by refractive the volume expansion Pd film, to the reduced expansion of Pd film, leading to the decrease of the central wavelength. decreaseby of the volume central wavelength. In 2009, Schroeder et al. [27] proposed a side-polished FBG hydrogen sensor based on evanescent-interaction of Pd film. Two Two Bragg Bragg gratings gratings with with lengths lengths of of 1.5 1.5 mm mm and and 2 mm were evanescent-interaction of cemented in in the the groove grooveof ofglass glassblock. block.The TheFBG FBG(1.5 (1.5 mm) was polished and coated with 50 nm mm) was polished and coated with 50 nm Pd Pd (in (in Figure 3a). In contrast to the above work [26], the transverse magnetic (TM) mode of emitted light Figure 3a). In contrast to the above work [26], the transverse magnetic (TM) mode of emitted was used response toward to used for for hydrogen hydrogencharacterization. characterization.The Thehydrogen hydrogensensor sensorshowed showeda nonlinear a nonlinear response toward hydrogen from 0.1% to 5% (v/v) in Ar ThisThis phenomenon waswas more obvious during the to hydrogen from 0.1% to 5% (v/v) in atmosphere. Ar atmosphere. phenomenon more obvious during exposure of 1% 2%,2%, which could be due to α-ß phase transition of Pd TheThe response timetime for the exposure of and 1% and which could be due to α-ß phase transition of film. Pd film. response 1% was about 2 min,2 while forthat 4% hydrogen was about 30 about s. Although the proposed for hydrogen 1% hydrogen was about min, that while for 4% hydrogen was 30 s. Although the device wasdevice much more complex, a higher sensitivity cansensitivity be obtained bybe thisobtained work. Another of proposed was much more complex, a higher can by thistype work. FBG hydrogen this principle reported in 2015 A multimode fiber A (62.5/125 µm)fiber was Another type ofbased FBG on hydrogen basedwas on this principle was[28]. reported in 2015 [28]. multimode spliced toμm) single mode fiber,toand thenmode tapered with the aid tapered of a 2-mm flame. A 193-nm excimer laser (62.5/125 was spliced single fiber, and then with the aid of a 2-mm flame. A was utilized to write 3-mm grating in the tapered multimode fiber, and 15 nm of Pd was deposited 193-nm excimer laser was utilized to write 3-mm grating in the tapered multimode fiber, and 15 nm on thewas grating sectionon asthe a sensitive layer (inasFigure 3b). The sensor (diameter of 3.3 µm) of Pd deposited grating section a sensitive layerproposed (in Figure 3b). The proposed sensor displayed 5% −1.08 hydrogen (in nitrogen atmosphere), a response time of with 60 s at (diameter − of1.08 3.3 nm μm)towards displayed nm towards 5% hydrogen (in with nitrogen atmosphere), a room temperature. response time of 60 s at room temperature.

Figure 3. (a) Side-polished FBG (SPFBG) hydrogen sensor fixed on quartz substrate [27]; Figure 3. (a) Side-polished FBG (SPFBG) hydrogen sensor fixed on quartz substrate [27]; (b) Micro-FBG (b) Micro-FBG (MFBG) hydrogen based on tapered fiber [28]. (MFBG) hydrogen sensor based onsensor tapered multimode fibermultimode [28].

Long period fiber grating (LPFG) is sensitive to external refractive index change without the side-polishing process. Therefore, it can be coated with hydrogen sensitive coating for hydrogen detection. The typical structure of the sensor is illustrated in Figure 4. In 2006, Trouillet et al. [29] compared the performance of FBG and LPFG coated with 50 nm Pd film. When the hydrogen

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Long period fiber grating (LPFG) is sensitive to external refractive index change without the side-polishing process. Therefore, it can be coated with hydrogen sensitive coating for hydrogen Sensors 2017, 17, 577typical structure of the sensor is illustrated in Figure 4. In 2006, Trouillet et al.5 of 15 detection. The [29] compared the performance of FBG and LPFG coated with 50 nm Pd film. When the hydrogen concentration concentration was was set set at at 4%, 4%, the the wavelength wavelength shifts shifts of of FBG FBG and and LPFG LPFG were were approximately approximately 14 14 pm pm and and 77 nm (fundamental mode) respectively. The wavelength shift of the latter is nearly 500 times that of nm (fundamental mode) respectively. The wavelength shift of the latter is nearly 500 times that the former. After two years, the hydrogen-sensitive performance of LPFG coated with 70 nm of Pd of the former. After two years, the hydrogen-sensitive performance of LPFG coated with 70 nm of was investigated by by Wei et et al.al.[30]. Pd was investigated Wei [30].The Theresonance resonancewavelength wavelengthofofLPFG LPFG decreased decreased obviously obviously as as ◦ ◦ hydrogen concentration increased from 0% to 16% (in He atmosphere) at 30 °C, 100 °C, 150 ◦°C and hydrogen concentration increased from 0% to 16% (in He atmosphere) at 30 C, 100 C, 150 C and ◦ C, and 200 AtAt anan operating temperature of 200 °C, and the theresponse responsetime timeatatall alltemperatures temperatureswas wasless lessthan than7070s. s. operating temperature 30 °C,◦ C, thethe hydrogen response absorption of 30 hydrogen responseofofthe thesensor sensorwas wasespecially especially remarkable. remarkable. The The hydrogen hydrogen absorption capability of Pd decreased significantly as temperature increased, leading to a lower change the capability of Pd decreased significantly as temperature increased, leading to a lower change in in the refractive at higher higher temperatures. temperatures. At operating temperature temperature of of 30 30 ◦°C, the sensor sensor refractive index index of of Pd Pd film film at At the the operating C, the exhibited excellent sensitivity during hydrogen response, shifting more than 4 nm towards 4% exhibited excellent sensitivity during hydrogen response, shifting more than 4 nm towards 4% H H22.. These Pd-coated LPFG and MFBG hydrogen sensors showed excellent sensitivity during hydrogen These Pd-coated LPFG and MFBG hydrogen sensors showed excellent sensitivity during hydrogen response, but aa little little hysterics hysterics effect effect still still can can be be observed observed due due to to the the phase phase transition transition of of Pd Pd film. film. response, but

Figure grating (LPFG) (LPFG) hydrogen hydrogen sensor sensor [29]. [29]. Figure 4. 4. Typical Typical structure structure of of aa long-period long-period fiber fiber grating

Side-polished an interesting interesting structure structure that that is is intrinsically intrinsically sensitive sensitive to to Side-polished FBG FBG (SPFBG) (SPFBG) has has an curvature improve thethe sensitivity of FBG hydrogen sensor. SPFBG was curvature [48], [48], which whichcan canbe beutilized utilizedtoto improve sensitivity of FBG hydrogen sensor. SPFBG prepared by polishing common FBG with a motor-driven polishing wheel [49]. By sputtering the was prepared by polishing common FBG with a motor-driven polishing wheel [49]. By sputtering WO -Pd composite film on side-polished FBG [31], the sensitivity and repeatability can be greatly the 3WO 3 -Pd composite film on side-polished FBG [31], the sensitivity and repeatability can be improved. greatly improved. The typicalstructure structureof of SPFBG hydrogen sensor is shown in 5. Figure 5. Compared to The typical thethe SPFBG hydrogen sensor is shown in Figure Compared to standard standard FBG,can SPFBG can increase the sensitivity of the sensor more100%. than 100%. Similar results FBG, SPFBG increase the sensitivity of the sensor more than Similar results werewere also also proposed by Jiang et al. [32]. In power transformer oil, SPFBG (with residual thickness of 20-μm proposed by Jiang et al. [32]. In power transformer oil, SPFBG (with residual thickness of 20-µm cladding) coated with with 560 560 nm nm of ofPd/Ag Pd/Ag composite composite film film exhibited exhibited aasensitivity sensitivityof of0.477 0.477pm/(µL/L), pm/(μL/L), cladding) coated which was was11.4 11.4 times higher thanof that of the FBG common FBGsensor. hydrogen sensor.improvements Subsequent which times higher than that the common hydrogen Subsequent improvements were reported in 2016 [33], and the hydrogen sensor showed a linear response when were reported in 2016 [33], and the hydrogen sensor showed a linear response when exposed to exposed to different concentrations of hydrogen at room temperature. All these results prove the different concentrations of hydrogen at room temperature. All these results prove the FBG hydrogen FBG hydrogen has great potential to monitor hydrogenin concentration in a power transformer. sensor has greatsensor potential to monitor hydrogen concentration a power transformer. Recently, a novel technique to enhance the sensitivity FBG hydrogen was presented by Karanja et al. [34]. The cross section schematic diagram of the sensing probe is shown in Figure 6a. Fiber Bragg gratings were micro-machined by femtosecond laser to form microgrooves along the fiber, followed by sputtering 520 nm of Pd/Ag composite film on their grating section (Figure 6b). At room temperature (25 ◦ C), the maximum sensitivity of the micro-machined FBG was increased more than 350% when compared with the standard FBG, which can be attributed to the larger surface area of fiber coated with Pd/Ag composite film. A similar phenomenon was observed when the ambient temperature was increased to 35 ◦ C. Notably, the sensitivity the hydrogen sensor can be increased more 10 times at the higher operating temperature. For instance, the wavelength shift of 75-mW micro-machined FBG coated with 520 nm of Pd/Ag composite film was only 47 pm towards

Figure 5. Typical structure of SPFBG hydrogen sensor [31]. Recently, a novel technique to enhance the sensitivity FBG hydrogen was presented by Karanja et al. [34]. The cross section schematic diagram of the sensing probe is shown in Figure 6a.

WO3-Pd composite film on side-polished FBG [31], the sensitivity and repeatability can be greatly improved. The typical structure of the SPFBG hydrogen sensor is shown in Figure 5. Compared to standard FBG, SPFBG can increase the sensitivity of the sensor more than 100%. Similar results were also proposed by Jiang et al. [32]. In power transformer oil, SPFBG (with residual thickness of 20-μm Sensors 2017, 17, 577 6 of 15 cladding) coated with 560 nm of Pd/Ag composite film exhibited a sensitivity of 0.477 pm/(μL/L), which was 11.4 times higher than that of the common FBG hydrogen sensor. Subsequent ◦ C this improvements reported 2016 [33],was andincreased the hydrogen sensor a linear response when 4% hydrogen atwere 25 ◦ C, while atin35 to nearly 700showed pm. More attention should be exposed to different concentrations of hydrogen at room these results the concentrated on this phenomenon, which demonstrated thattemperature. much higherAll sensitivity can beprove achieved FBG hydrogenthe sensor has great potentialof tohydrogen-sensitive monitor hydrogen concentration in a power transformer. by increasing operating temperature film. Sensors 2017, 17, 577 Sensors 2017, 17, 577

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fiber, fiber, followed followed by by sputtering sputtering 520 520 nm nm of of Pd/Ag Pd/Ag composite composite film film on on their their grating grating section section (Figure (Figure 6b). 6b). At At room temperature (25 °C), the maximum sensitivity of the micro-machined FBG was increased more room temperature (25 °C), the maximum sensitivity of the micro-machined FBG was increased more than than 350% 350% when when compared compared with with the the standard standard FBG, FBG, which which can can be be attributed attributed to to the the larger larger surface surface area area of fiber coated with Pd/Ag composite film. A similar phenomenon was observed when the ambient of fiber coated with Pd/Ag composite film. A similar phenomenon was observed when the ambient temperature temperature was was increased increased to to 35 35 °C. °C. Notably, Notably, the the sensitivity sensitivity the the hydrogen hydrogen sensor sensor can can be be increased increased more 10 times at the higher operating temperature. For instance, the wavelength shift more 10 times at the higher operating temperature. For instance, the wavelength shift of of 75-mW 75-mW micro-machined FBG coated with 520 nm of Pd/Ag composite film was only 47 pm towards micro-machined FBG coated with 520 nm of Pd/Ag composite film was only 47 pm towards 4% 4% hydrogen hydrogen at at 25 25 °C, °C, while while at at 35 35 °C °C this this was was increased increased to to nearly nearly 700 700 pm. pm. More More attention attention should should be be FFigure igure 5. Typical structure hydrogen sensor sensor [31]. [31]. of SPFBG hydrogen concentrated concentrated on on this this phenomenon, phenomenon, which which demonstrated demonstrated that that much much higher higher sensitivity sensitivity can can be be achieved achieved by by increasing increasing the the operating operating temperature temperature of of hydrogen-sensitive hydrogen-sensitive film. film. Recently, a novel technique to enhance the sensitivity FBG hydrogen was presented by Karanja et al. [34]. The cross section schematic diagram of the sensing probe is shown in Figure 6a. Fiber Bragg gratings were micro-machined by femtosecond laser to form microgrooves along the

Figure 6.6.(a) (a) Configuration of micro-machined FBG hydrogen (b) SEM of Figure Configuration of micro-machined FBG hydrogen [34]; sensor (b) SEM[34]; of micro-machined Figure 6. (a) Configuration of micro-machined FBG sensor hydrogen sensor [34]; (b) SEM of micro-machined FBG coated with Pd/Ag composite film [34]. FBG coated with Pd/Ag composite film [34]. micro-machined FBG coated with Pd/Ag composite film [34].

In the following year, FBG with spiral groove sputtered with Pd/Ag film (in Figure 7), which In the the following following year, year,FBG FBGwith withaaaspiral spiralgroove groovesputtered sputteredwith withPd/Ag Pd/Ag film film (in (in Figure Figure 7), 7), which which In was proposed by the same group, was used for hydrogen detection [35]. At room temperature, wasproposed proposedbybythethe same group, for hydrogen detection [35]. Attemperature, room temperature, was same group, waswas usedused for hydrogen detection [35]. At room double double spiral FBG 51.5 pm/% to in is doublemicro-structured spiral micro-structured micro-structured FBG exhibits exhibits 51.5 responding pm/% responding responding to hydrogen hydrogen in air, air, which which is 7.5 7.5 spiral FBG exhibits 51.5 pm/% to hydrogen in air, which is 7.5 times times that of FBG. In the sensor can 0.2% hydrogen in times higher higher of unprocessed unprocessed FBG. In addition, addition, the hydrogen hydrogen sensor can detect detect hydrogen in higher that ofthat unprocessed FBG. In addition, the hydrogen sensor can detect 0.2%0.2% hydrogen in air. air. These results indicate the sensitivity of the sensor can be greatly improved by increasing the area air. These results indicate the sensitivity of sensor the sensor be greatly improved by increasing the area These results indicate the sensitivity of the can can be greatly improved by increasing the area for for deposition. for film film deposition. film deposition.

Figure 7. Configuration of double spiral FBG hydrogen sensor [35]. Figure 7. 7. Configuration Configuration of of double double spiral spiral FBG FBG hydrogen hydrogen sensor sensor[35]. [35]. Figure

3. 3. Optical Optical Fiber Fiber Grating Grating Hydrogen Hydrogen Sensor Sensor Based Based on on Exothermic Exothermic Reaction Reaction and and Gasochromic Gasochromic Effect Effect of Sensitive Materials of Sensitive Materials At At present present the the fastest fastest response response FBG FBG hydrogen hydrogen sensor sensor is is based based on on Pt-loaded Pt-loaded WO WO33 undergoing undergoing an an exothermic exothermic reaction reaction in in hydrogen hydrogen atmosphere atmosphere [36,37]. [36,37]. The The chemical chemical reaction reaction can can be be expressed expressed as as the the following following equation equation [40]: [40]: Pt Pt WO3 x  xH2 O WO WO33++xH xH22    WO3x  xH2O

x

Pt

(1) (1)

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3. Optical Fiber Grating Hydrogen Sensor Based on Exothermic Reaction and Gasochromic Effect of Sensitive Materials At present the fastest response FBG hydrogen sensor is based on Pt-loaded WO3 undergoing an exothermic reaction in hydrogen atmosphere [36,37]. The chemical reaction can be expressed as the following equation [40]: Pt

WO3 + xH2 → WO3−x + xH2 O

(1)

x Pt WO3−x + O2 → WO3 2

(2)

WO3 can react with hydrogen drastically by utilizing Pt as a catalyst, which can release energy to the surrounding environment. By measuring the temperature change caused by the exothermic reaction, the hydrogen concentration can be calculated. When there is no hydrogen, WO3−x can be oxidized to form WO3 in air. Therefore, the hydrogen sensor based on these reactions can be used for repeated measurements. In 2007, Caucheteur et al. [40] proposed the first FBG hydrogen sensor based on this principle. FBGs with different lengths (0.5–4 cm) were used for hydrogen sensing. FBGs with a length of 4 cm had the lowest detecting threshold of 1% hydrogen in air, which could be due to the more optical power coupled to the cladding for initiating the exothermic reaction. Higher ambient humidity and lower ambient temperature will increase the threshold of hydrogen sensor. With the optical heating assisted by a 4-cm-long FBG, the hydrogen sensor can detect 1% hydrogen in air. Further improvements (different LPFGs activating the sensing layer) were reported in 2008 [41]. The sensing probe consisting of 1-cm FBG and 15-dB LPFG displayed greater than 6-nm wavelength shifts when the hydrogen concentration was 4% in volume ratio. The sensor can detect 0.6% hydrogen in air at 25 ◦ C, while the threshold of sensing probe without LPFG was 2.8%. The proposed sensor with best responsibility can detect hydrogen as low as 1.5% even at −50 ◦ C. Although the reported FBG hydrogen sensor [40,41] has better sensitivity and response rate, its low concentration hydrogen responsibility should be further improved. The energy released by the exothermic reaction should be controlled at proper value so as to ensure its intrinsic safety. An FBG hydrogen sensor based on Pt-loaded WO3 material was developed and its sensing characteristics were developed in 2012 by Yang et al. [42]. The performance of the hydrogen sensitive material was enhanced by optimizing its annealing temperature and constitutes. With the continuous effort, the molar ratio of Pt and WO3 of hydrogen-sensitive coating was optimized at 1:5 and the annealing temperature was set at 315 ◦ C [43]. Besides, FBG was fixed on glass substrate with higher thermal expansion coefficient so as to increase its temperature sensitivity. Additionally, the glass substrate has a groove for depositing Pt-loaded WO3 coatings, which can prevent the shedding of the hydrogen-sensitive coating. The typical structure of sensing probe is shown in Figure 8. At room temperature of 25 ◦ C, FBG hydrogen sensor with best sensitivity has a 448-pm wavelength shift towards 0.8% hydrogen, and the threshold of the FBG hydrogen sensor can reach to 200 ppm in air. Ambient humidity had little effect on its performance. FBG hydrogen sensors showed good selectivity (no response to CO and CH4 ) and repeatability during hydrogen response. Since other reducing gases cannot penetrate the Pt catalyst and react with WO3 , the Pt-loaded WO3 coating has good selectivity towards hydrogen at room temperature. The performance of the sensor at different temperatures was studied in the following work [44]. The hydrogen sensitivity will decrease at lower temperature. However, the hydrogen sensor still can detect hydrogen as low as 400 ppm at 0 ◦ C. Moreover, it is possible to compensate the interference of ambient temperature by using reference FBG. Therefore, FBG hydrogen sensor based on Pt-loaded WO3 coating is very promising for distributed hydrogen leakage monitoring in air.

reducing gases cannot penetrate the Pt catalyst and react with WO3, the Pt-loaded WO3 coating has good selectivity towards hydrogen at room temperature. The performance of the sensor at different temperatures was studied in the following work [44]. The hydrogen sensitivity will decrease at lower temperature. However, the hydrogen sensor still can detect hydrogen as low as 400 ppm at 0 °C. Moreover, it is possible to compensate the interference of ambient temperature by using reference Therefore, FBG hydrogen sensor based on Pt-loaded WO3 coating is very promising for8 of 15 SensorsFBG. 2017, 17, 577 distributed hydrogen leakage monitoring in air.

Figure 8. Configuration FBGhydrogen hydrogen sensor sensor coated WO 3 coating [43]. Figure 8. Configuration of of FBG coatedwith withPt-loaded Pt-loaded WO 3 coating [43].

In 2015, Masuzawa et al. [45] developed the fiber grating sensor based on exothermic8 of reaction. Sensors 2017, 17, 577 15 Pt/SiO2 , Pt/WO3 , Pt/Fe2 O3 , Pt/ZnO, Pt/SnO2 and Pt/Al2 O3 were fabricated for preparing FBG 2015, Masuzawa et al. [45] exposure developedin theswitching fiber grating sensor based on exothermic hydrogenIn sensors. During hydrogen atmosphere of dry and humidreaction. air, Pt/SiO2 Pt/SiO 2 , Pt/WO 3 , Pt/Fe 2 O 3 , Pt/ZnO, Pt/SnO 2 and Pt/Al 2 O 3 were fabricated for preparing FBG showed the better stability than other materials. This work proposed the new material for hydrogen hydrogen sensors. During hydrogen exposure in switching atmosphere of dry and humid air, sensing. It is worth noting that the stability of the sensor should be further studied due to the obvious Pt/SiO2 showed the better stability than other materials. This work proposed the new material for different thermal expansion coefficient of Pt and SiO2 . hydrogen sensing. It is worth noting that the stability of the sensor should be further studied due to WO3 film has a good gasochromic effect in hydrogen atmosphere when Pd [50] or Pt [51] is the obvious different thermal expansion coefficient of Pt and SiO2. employedWO as3 catalyst. A single-mode fiber inscribed with high-low reflective Bragg gratings was film has a good gasochromic effect in hydrogen atmosphere when Pd [50] or Pt [51] is deposited withasWO hydrogen sensing [42]. Although thegratings performance 3 -Pd-Pt employed catalyst. A composite single-modefilm fiberfor inscribed with high-low reflective Bragg was of sensor is still poor with the employment of a super-luminescent light emitting diode (SLED) deposited with WO3-Pd-Pt composite film for hydrogen sensing [42]. Although the performance of light source (80 µW) and an with Optical Analyzer (AQ6370B, YOKOGAWA, Japan), light it proves sensor is still poor the Spectrum employment of a super-luminescent light emittingTokyo, diode (SLED) source (80 μW) and anfor Optical Spectrum sensor. Analyzer (AQ6370B, YOKOGAWA, Japan), it the good anti-interference the hydrogen The performance of sensorTokyo, is greatly improved proves theconstitutes good anti-interference for the hydrogen sensor. The performance sensor is greatlyFBGs by optimizing and the thickness of the hydrogen-sensitive film asofwell as writing improved by optimizing constitutes andwavelength the thicknessofofthe theamplified hydrogen-sensitive filmemission as well as(ASE) (in single mode-fiber) especially at the flat spontaneous writing FBGs (in single mode-fiber) especially at the flat wavelength of the amplified spontaneous light source [47]. As displayed in Figure 9, the reference and sensing single are transmitted through emission (ASE) light source [47]. As displayed in Figure 9, the reference and sensing single are the same single mode fiber and are detected by same FBG demodulator. Thus, the single noise ratio transmitted through the same single mode fiber and are detected by same FBG demodulator. Thus, can bethe considerably enhanced, and the hydrogen sensor can detect as low as 50 ppm hydrogen in air at single noise ratio can be considerably enhanced, and the hydrogen sensor can detect as low as ◦ an ambient temperature of 25 Severaltemperature methods, such of hydrogen-sensitive 50 ppm hydrogen in air at anC.ambient of 25 as °C.optimization Several methods, such as optimizationfilms, utilization of a more stable ASE light source, and special wavelengths (as sensing intensity) as well as of hydrogen-sensitive films, utilization of a more stable ASE light source, and special wavelengths high reflective can be improveFBG thecan performance oftothe hydrogen sensor. Thisofwork (as sensingFBG intensity) as employed well as hightoreflective be employed improve the performance the hydrogen sensor.for This work proposes a new method for preparing fiber optic hydrogen sensor proposes a new method preparing fiber optic hydrogen sensor with greatly enhanced performance, with greatly performance, especially for low concentration hydrogen detection. especially for lowenhanced concentration hydrogen detection.

Figure 9. Configuration of high-low tipcoated coatedwith with WO Figure 9. Configuration of high-lowBragg Bragggrating grating with with tip WO 3-Pd 2Pt-Pt film film [47]. [47]. 3 -Pd 2 Pt-Pt

In 2016, the long-term stability of Pt-loaded WO3 coating was studied by Zhong et al. [52]. With the irradiation of ultraviolet light, the water photolysis effect of Pt-loaded WO3 coating can be introduced into the hydrogen sensing process. The experimental tests demonstrate that the hydrogen sensor under UV irradiation exhibits good stability over 3 months. This paper proposes a novel method to enhance the stability of hydrogen sensor based on Pt-loaded WO3 coating.

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In 2016, the long-term stability of Pt-loaded WO3 coating was studied by Zhong et al. [52]. With the irradiation of ultraviolet light, the water photolysis effect of Pt-loaded WO3 coating can be introduced into the hydrogen sensing process. The experimental tests demonstrate that the hydrogen sensor under UV irradiation exhibits good stability over 3 months. This paper proposes a novel method to enhance the stability of hydrogen sensor based on Pt-loaded WO3 coating. 4. Challenges for Fiber Grating Hydrogen Sensor Tables 1 and 2 list the typical optical fiber grating sensors based on different hydrogen sensitive materials, and their configurations and performances are displayed in these two tables. Meanwhile, some important experimental results of these sensors are shown in the following two tables. Table 1. Comparison of fiber grating hydrogen sensors based on Pd and Pd alloys. Publication Date, Author, Reference

Configuration of Sensing Head, Carrying Gas

Concentration Range, Sensitivity or Wavelength Shift, Response Time, Operating Temperature

Some Important Experimental Results

1999, Sutapun, [12]

560 nm Pd + etched FBG (35 µm), N2

0.3%–1.8%, 22 pm/%, -, room temperature

Poor reversibility (H2 more than 1.8% (v/v)).

1999, Tang, [13]

33 µm Pd tube + FBG, N2

4%, 1.4 nm, more than 200 min, 23 ◦ C; 4%, about 2 min, 0.6 nm, 95 ◦ C

Ambient temperature can affect sensitivity and response rate of the sensor

2006, Trouillet, [29]

50 nm Pd + LPFG and FBG, N2

4%, 14 pm (FBG), −7 nm (LPFG), less than 2 min, room temperature

LPFG has much better sensitivity

2007, Aleixandre, [15]

5 nm Pd + etched FBG (25 µm), N2

0.3%–3.0%, 8.4 pm/% (v/v), about 10 min, room temperature

Longer response time after several months due to superficial oxidation

2008, Wei, [30]

70 nm Pd + LPFG, He

4%, −4.3 nm, less than 70 s, 30 ◦ C

Best sensitivity at 30 ◦ C, quicker response rate and lower sensitivity at higher temperatures

2009, Buric, [17]

450 nm Pd + FBG inscribed in 1 cm high attenuation fiber, N2

1%–10%, about 37–280 pm, −150 ◦ C (under 1.17-W laser heating)

Greater heating power enables higher sensitivity

2013, Silva, [18]

150 nm Pd + 6 mm FBG inscribed in tapered single-mode fiber SMF (50 µm), N2

0.1%–1%, 81.8 pm/%, about 2 min, room temperature

Better sensitivity, without splicing different optical fibers

2014, Dai, [23]

110 nm Pd91 Ni9 + 20 µm FBG fixed on polymer substrate, air

0.5%–4%, about 37 pm/%, 5–6 min, 25 ◦ C

Enhanced sensitivity, superficial oxidation

2015, Jiang, [32]

560 nm Pd/Ag + SPFBG, in transformer oil

100–700 µL/L, 0.477 pm/(µL/L), within 4 h at room temperature, within 1 h at 60 ◦ C

Sensitivity of SPFBG is 11.4 times higher than that of common FBG

2015, Yu, [28]

15 nm Pd + 3.3 µm MFBG, N2

−1.08 nm wavelength shift 5%, 60 s, room temperature

Blue wavelength shift, nonlinear response

2015, Karanja, [34]

10 nm Ni + 520 nm Pd75 Ag25 + FBG with microgrooves, air

1%–4%, 5–48 pm, in air, about 60 s, 25 ◦ C; 1%–4%, 50–550 pm, about 50 s, 35 ◦ C

Greatly improved sensitivity at higher temperature

From the above review and discussion, it can be concluded that the performance of fiber grating hydrogen sensors has a close relationship with the hydrogen-sensitive material, configuration of sensing probe and the optical system. The key factor for the fiber grating hydrogen sensor is to prepare stable hydrogen-sensitive material, which can be effectively integrated with the fiber grating. The hydrogen sensors based on pure Pd film easily suffer from the fatal fracture of Pd film caused by its α-β phase transition [12]. By alloying Pd with other metals such as Au [5,6,53], Ni [21,23], Ag [22,24], Cr [25], Mg [54], Y [55], and Pt [56], the structural stability of hydrogen sensitive films can be improved. Due to significant thermal expansion coefficient gap between the silica and Pd-based hydrogen sensitive film [12], an intermediate layer with good adhesion towards silica fiber, such as polyimide [19,20], Ni [21,23], Cr [22] and TiO2 [25], should be used to enhance the stability of these hydrogen sensors.

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Table 2. Comparison of hydrogen sensor based on thermal reaction and gasochromic effect. Publication Date, Author, Reference

Configuration of Sensing Head, Carrying Gas

Concentration Range, Sensitivity or Wavelength Shift, Response Time, Operating Temperature

Some Important Experiment Results

2008, Caucheteur, [41]

FBG + LPFG, Pt-loaded WO3 coating, air

0.6%–4%, 1.2–8 nm, 4 s, 25 ◦ C (FBG + 15 dB LPFG)

Humidity and temperature affect threshold; more active energy enable better responsibility

2014, Dai, [43]

Temperature sensitive + FBG Pt-loaded WO3 coating, air

0.02%–0.8%, more than 448 pm; about 2 min, 25 ◦ C.

315 ◦ C annealed Pt:WO3 = 1:5 has best sensitivity, threshold of 200 ppm at 25 ◦ C

2015, Masuzawa, [45]

FBG coated with Pt/SiO2 , Pt/WO3 , Pt/Fe2 O3 , Pt/ZnO, Pt/SnO2 and Pt/Al2 O3 , air and N2

Pt/SiO2 : 0.1%–1% (in air), about 20–480 pm, less than 20 s, room temperature

Pt/SiO2 shows better stability than other materials, poor responsibility in N2

2015, Yang, [47]

High-low reflective FBG with tip coated with WO3 -Pd2 Pt-Pt film, air

50–23,900 ppm, 10–30 ppm; 20 s, 25 ◦ C

Better sensitivity at low concentration hydrogen, threshold of 50 ppm

2016, Zhong, [52]

Pt-loaded WO3 coating + FBG, air

1500–20,000 ppm, about 43.5 ppm; 55–80 s, room temperature

Improved stability with ultraviolet irradiation

Several other factors, such as operating temperature, ambient humidity and atmosphere, will also affect the performance of the hydrogen sensor. The operating temperature can influence the diffusion rate of hydrogen molecules, hydrogen absorption capability of sensitive materials, and thus affect the sensitivity, response rate and detection threshold of the sensor. Pd tubes [13], Pd [30] and Pd/Ag [34] film display better hydrogen absorption at 23 ◦ C, 30 ◦ C, and 35 ◦ C respectively. When the operating temperatures are increased to 95 ◦ C or 100 ◦ C, hydrogen sensors based on Pd tube and Pd film have quicker response rate and lower sensitivity. The sensor can work normally at room temperature and may not respond to the same concentration hydrogen at low temperatures [16,17]. As for some special facilities, such as liquid hydrogen tanks, the ambient temperature can decrease below 0 ◦ C when hydrogen leaks. Therefore, fiber grating hydrogen sensors, which can give an accurate alarm of leakage at low temperatures, is very crucial to these facilities. The hydrogen sensor with laser heating [17], which can give a quick response toward 1% H2 at −150 ◦ C, may be more suitable for these facilities. The operating temperature also affects the performance of an FBG hydrogen sensor based on Pt-loaded WO3 coating [40,41,44]. At lower temperature, the proposed FBG hydrogen sensors show the higher threshold [40,41] and poorer sensitivity [44]. Utilizing light heating [16,17] or electric heating device [34], the sensitivity and response rate of the hydrogen sensor can be greatly improved. A similar phenomenon can be observed in the gasochromic effect of Pd/WO3 [8]. However, it is not practical for application with utilization of the electric heating device. However, it can be efficient heating equipment for studying the performance of hydrogen sensor at different temperatures. Therefore, employing optical heating system is a promising method to improve the performance of optical hydrogen sensor. Since the performance of the sensor will change at different temperatures, it is necessary to introduce a self-compensation device to keep the hydrogen sensing materials working at the optimized temperature for accurate hydrogen concentration monitoring. Compared to other optical hydrogen sensors, the hydrogen sensors based on fiber grating have a unique advantage to achieve this goal due to their wavelength multiplexing capability. In 2016, two MFBGs were sputtered with Ni film [57], and the sensing MFBG was further deposited with Pd/Ni composite film. This sensor can provide temperature compensation when the sensing probe is heated by heating (980 nm) laser, and the sensing MFBG has obvious wavelength shift when hydrogen is injected in the gas room. The sensing FBG shows better sensitivity and quicker response rate when it is heated by the laser. Although the sensor needs further improvement for application, it demonstrates that a controllable optical heating scheme is feasible. The second challenge for the optical hydrogen sensor is the interference of the humidity. Most of potential application of hydrogen monitoring is in air. Therefore, developing a hydrogen sensor with good anti-humidity interference is very meaningful. Owing to the water absorption of

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hydrogen-sensitive material, the sensor based on Pt-loaded WO3 coating cannot detect hydrogen below 0.6% under high humidity atmosphere [41]. When exposed to high concentration hydrogen, ambient humidity has little effect on the performance of sensor, which can be attributed to the self-heating effect [58,59] of the sensitive layer. The detection threshold can be greatly reduced by optimizing constitutes of the hydrogen sensitive material [43]. However, the responsibility of these hydrogen may not response to low concentration hydrogen due to the absorption of water molecular when set in air for several weeks. Similar phenomenon can be seen in the gasochromic reaction of WO3 /Pt [60]. After heating the hydrogen sensitive film at higher temperature, the performance of hydrogen sensor can be recovered. More recently, the sensor stability can be significantly enhanced by ultraviolet irradiation [52], which provides a novel method to overcome the interference of humidity. As for optical fiber hydrogen sensors based on Pd and Pd alloys, the influence of humidity will be much more obvious. In 2008, the hydrogen sensing performance of Pd/Au was studied under room temperature and at different humidity levels [6]. The base line and sensing signal was greatly interrupted by ambient humidity. Since the prepared Pd and Pd alloy are dense and uniform, the surfaces of these films are easily covered with water under a high-humidity atmosphere, resulting in a great deterioration of performance of the hydrogen sensor. By adding polymer layer as protective layer [61,62], the anti-humidity interference of hydrogen sensor can be improved due to their better hydrophobic property. The alternative technique to reduce the humidity interference is heating the sensing material under a proper temperature [6,63], which requires the development of a more reliable optical heating system to meet the demand of application. The third negative factor of Pd and Pd alloy-based hydrogen sensors is the superficial oxidation of hydrogen sensitive film [14,23]. Freshly prepared sensing probes usually have a quicker response rate and better responsibility to low concentration hydrogen. However, the responsibility will deteriorate after several weeks, especially for low concentration hydrogen. Although Pd is a noble metal, there are several nanometres of PdO on the surface of hydrogen-sensitive film after several months [15,23]. The superficial oxidation film can reduce number of active sites available for the hydrogen, leading to the degradation of the hydrogen sensor. Two methods can be employed to solve this problem. The first method is to prepare Pd/Pt as a hydrogen-sensitive film, which may improve the diffusion rate of hydrogen atom as Pd/Pt can be a highly efficient catalyst material [64]. In addition, depositing Pt [46,47,51,58,59], Au [63] or Pd/Pt [56,64,65] as protective layer may inhibit the oxidization of hydrogen sensitive film, which can ensure the durability of hydrogen-sensitive film. 5. Conclusions In summary, fiber grating hydrogen sensing systems involve the design of a sensing probe, preparation of hydrogen-sensitive films and acquisition of a sensing signal. Each process is crucial to ensure the performance of the hydrogen sensor. Resolution comparisons of the Pd-based and WO3 -based hydrogen sensors are shown in Figure 10, and different resolutions (LPFG, Optical Spectrum Analyzer, 20 pm; FBG, FBG demodulator, 1 pm) were used during the calculating process. WO3 -based hydrogen sensors show higher sensitivity than Pd-based hydrogen sensor (except [13] and [32]). Pd-based hydrogen sensors are more suitable for application in oxygen-free environments, such as in transformer oil [24,25], nuclear natural gas pipelines [62] and waste repositories [66]. WO3 -based sensors have better performance in air (as shown in Figure 10), so they can be developed for monitoring hydrogen concentration leakage in air. Some obvious improvements of this field were proposed in the last few years. For example, the sensitivity of Pd-based hydrogen sensors [28,32–34] was greatly increased, and some of them [32,33] have great potential application in transformer oil. Moreover, the threshold of WO3 -based fiber grating hydrogen sensor can be reduced to 50 ppm in air at room temperature [47]. Nevertheless, controllable optical heating device can be employed to improve the stability and sensitivity of the sensor [57], which makes it more feasible for accurate hydrogen concentration monitoring. However, preparing fiber grating hydrogen sensor that can be used in a wide range of occasions is still a great challenge. It is much more feasible to develop hydrogen sensor

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based on different principles for special applications. Meanwhile, it is necessary to consider the above mentioned factors for accurate detection of hydrogen in air. At last, fiber grating sensors are still very Sensors 2017, 17, 12 of 15 promising for577 hydrogen detection due to their unique characteristics.

Comparison of of the the resolution resolution of of fiber fiber grating grating hydrogen hydrogen sensors sensors based based on on Pd-based Pd-based and Figure 10. Comparison -based hydrogen hydrogen sensitive sensitive materials materials.. WO33-based

Acknowledgments: This work is financially supported by the Project of National Natural Science Foundation of China, NSFC (Number: 51402228, 61575151, 61505150, Project Natural of National Natural Science This work is financially supported by the 62190311), Project of National Science Foundation of Acknowledgments: China, NSFCof(Number: 51402228, 61575151, 61505150, 62190311), Project of National Natural Science Foundation Foundation Hubei Provincial Government (Number: 2014CFB260, 2014CFC1138), the Creative Group Project of Hubei (Number: 2014CFB260, 2014CFC1138), the Creative Project of Hubei of Hubei Provincial ProvincialGovernment Natural Science Foundation (Number: 2015CFA016), and theGroup Excellent Dissertation Provincial Natural Science Foundation (Number: 2015CFA016), and the Excellent Dissertation Cultivation Funds Cultivation Funds of Wuhan University of Technology (2016-YS-082). of Wuhan University of Technology (2016-YS-082). Conflicts of Interest: Interest: The The authors authors declare declare no no conflict conflict of of interest. interest. Conflicts of

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