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常用的光纤氢敏材料主要有两类,即钯(Pd)基氢敏材料(Palladium based hydrogen sensitive film, PHF)和以三氧化钨(WO3)为代表的金属氧化物氢敏材料(Tungsten trioxide hydrogen sensitive material, THF)[14]。
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Pd基氢敏材料(PHF)包括纯Pd、Pd合金等,基于Pd在吸氢和释氢过程中的物理和光学特性变化实现氢气的敏感。一般情况下,Pd的性质较为稳定,但当其所在环境存在氢(H2)时,氢分子会进入Pd金属内部并与之组成氢化物,反过来氢化物本身的氢离子也会扩散至Pd金属表面并合成氢分子。这是一种可逆反应,化学方程式可表示为[15]:
$$ 2{\text{Pd}} + x{{\text{H}}_2} \leftrightarrow 2{\text{Pd}}{{\text{H}}_x} $$ (1) 式中:x表示形成氢化物中H和Pd的原子个数比。Pd金属在吸氢、释氢的过程中,晶格常数会发生变化,引起其折射率和体积变化(形变)[16]。根据Pd、H元素的原子比不同,靶金属在吸氢后形成Pd的氢化物存在α相和β相,随着氢分压和温度的变化,还会出现α、β相的转变,这会导致氢敏材料劣化,严重影响感器的响应重复性和使用寿命。为了避免纯Pd材料的相变问题[15, 17-18],一般在纯Pd中加入银(Ag)、镱(Y)、镍(Ni)、金(Au)等金属或二氧化钛(TiO2)等金属氧化物形成Pd合金。加入Ag不但可以有效抑制相变、提高材料寿命,同时还可以增大氢的渗透率,加快响应时间[19-20];Y的原子半径比Pd大,形成PdY合金后,晶格变大,更利于氢的吸附[21];加入Au在抑制相变的同时,可增强激发表面等离子体波的强度[22];Ni会降低氢渗透率,但检测阈值较高[23]。其中,基于Pd/Ag合金的光纤氢气传感技术研究最活跃、最全面。
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WO3氢敏材料(THF)主体为三氧化钨,WO3是正八面体晶体,暴露在氢气环境中时会发生颜色、折射率变化,同时伴随热量的交换,表现为材料温度的变化[24-25]。这些反应一般需要在Pd/Pt催化下进行,变色和褪色反应可用化学方程式描述[25]:
$$ {\text{W}}{{\text{O}}_{\text{3}}} + x{{\text{H}}_2}\xrightarrow{{{\text{Pd/Pt}}}}{\text{W}}{{\text{O}}_{{\text{3}} - x}} + x{{\text{H}}_2}{\text{O}}\; (变色) $$ (2) $$ {\text{W}}{{\text{O}}_{{\text{3}} - x}} + {x \mathord{\left/ {\vphantom {x 2}} \right. } 2}{{\text{O}}_2}\xrightarrow{{{\text{Pd/Pt}}}}{\text{W}}{{\text{O}}_{\text{3}}}\; (褪色) $$ (3) 研究表明,WO3除能与氢气发生反应外,还对其他气体(如一氧化碳、二氧化氮及乙醇等)敏感[26-27],这种情况会影响其测量准确性,此外,从以上反应方程式还可以看出,基于WO3的氢气传感往往还需要O2参加,对应用环境有特殊的要求。
一些金属氧化物,如ZnO[28]、 Ta2O5[29]、SiO2[30]、TiO2[31]、MnO2[32]等,也可用于氢气传感。研究表明,这些材料的敏感机理和特性均与WO3相似。
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综合PHF及THF与氢气的作用特点,用于光纤氢气传感的主要调制参数有3个,即形变、折射率和温度,适用的主要光纤传感方案和解调技术如表1所列。目前光纤氢气传感技术的主要研究内容集中在如何提高灵敏度、响应时间及测量范围等方面[14, 33-34],也有少部分研究关注了传感器的环境适应性[35-37],微小型化是传感头优化设计的主要方向[38-39]。
表 1 光纤氢气传感器分类
Table 1. Classification of optical fiber hydrogen sensors
Modulation scheme Hydrogen sensitive material Optical path scheme Modulation parameters Ref. Strain Pd-based Optical fiber interferometer Intensity/Phase [40-41] Fabry-Perot interferometer Intensity/Phase [38, 42-43] Fiber Bragg grating Wavelength [44-46] Refractive index Pd-based and WO3 Micro mirror Intensity [47-48] Evanescent wave Intensity [49-50] Surface plasma resonance Intensity/Wavelength [34, 51] Optical fiber interferometer Intensity/Phase [33, 52] Fabry-Perot interferometer Phase/Wavelength [53] Temperature WO3 Fiber Bragg grating Wavelength [54-55] Optical fiber interferometer Phase [35, 56] -
应变调制基于PHF在吸氢和释氢过程中产生应变的特性实现氢气传感。附在光纤(或与光纤相连的结构)上的Pd基氢敏材料吸释氢产生的沿光纤轴向应变会改变光纤中传输光的相位、波长等参数,通过解调波长、相位的变化实现氢气浓度检测。早期基于应变调制的光纤氢气传感器结构一般为光纤干涉仪[40],随着光纤光栅技术的发展,采用光栅实现氢敏材料应变传感也被广泛研究[57]。
1984年M.A.Butler[40]采用马赫-曾德尔(Mach-Zehnder, M-Z)干涉仪基于应变调制原理实现了氢气传感。如图2所示,在信号臂的单模光纤外表面上蒸镀一层Pd膜,通过测量两臂之间相位差的变化实现氢气浓度测量。
光纤法布里-珀罗(Fabry-Perot, F-P)干涉仪对于腔长的变化非常敏感,1994年Zeakes等人首次将F-P干涉仪的用于光纤氢气传感器[42]。如图3 (a)所示,将单模光纤(SMF)与多模光纤(MMF)插入侧面镀有Pd膜的石英玻璃管中形成F-P腔,实现氢气浓度传感。随着微加工技术的发展,在单根光纤上制作微F-P腔、在侧面或端面镀PHF实现氢气传感成为主流方案,典型传感部分结构如图3 (b)及(c)所示,氢敏薄膜厚度一般为nm量级,响应时间较快,能达到秒量级[58-59]。
图 3 (a)典型的应变调制F-P干涉氢气传感器示意图[42];(b)侧面[58]及(c)端面镀Pd膜的F-P干涉结构示意图[59]。PHF:钯基氢敏膜
Figure 3. (a) Schematic of strain modulation F-P interference hydrogen sensor[42]; Schematic of F-P interference structure with Pd film (b) on the side[58] and (c) on the end[59]. PHF:Palladium based hydrogen sensitive film
2020年Liao等人[39]采用飞秒激光光刻技术在单模光纤端面打印了聚合物微悬臂梁,光纤端面与悬臂梁之间形成F-P腔,如图4所示,在悬臂梁外端镀Pd膜。在0~4.5%的范围内,灵敏度达到1778 pm/%,响应时间低至13.5 s。
采用光纤光栅(Fiber Bragg Grating, FBG)可以直接测量应变,1999年Sutapun等人首次报道了基于光纤光栅的氢气传感器[45],如图5 (a),在布拉格光栅的包层外涂覆了560 nm的纯Pd膜,在0.3%~1.8%的范围内,灵敏度为19.5 pm/%。为了提高FBG氢气传感器的灵敏度,基本的思路是减薄光纤包层厚度,加工技术主要为侧面抛磨[57]、腐蚀[60]、拉锥[61]和刻槽[46, 62],如图5 (b)~(e)所示,最高灵敏度达到81.8 pm/%[61]。
基于PHF的轴向应变调制,采用光纤干涉仪、光纤F-P和光纤光栅都可实现大范围、高灵敏氢气浓度测量,采用微加工技术,可以获得微小尺寸的传感头。但是,上述方案对温度均敏感,部分结构对压力也敏感,如何抑制交叉干扰是这种氢气传感器实用化需要克服的关键问题。
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镀在光纤侧面的PHF在吸释氢时还会引起光纤径向的应变,引起光纤双折射参数发生变化,通过测量其双折射的变化可实现氢气浓度的传感[41]。其传感结构如图6 (a)所示,将Pd/Ag膜镀在保偏光子晶体光纤(PM-PCF)侧面,接入混合Sagnac干涉仪,通过Sagnac干涉仪光谱变化确定PM-PCF双折射变化,实现氢气浓度的传感。
如图6 (b)所示,当在PM-PCF(长度为L0)侧面LPd长的区域镀有Pd/Ag膜时,波长偏移量∆λ与吸释氢引起的双折射变化量∆BPd的关系可表示为:
$$ \Delta \lambda = \frac{{{\lambda _m}{L_{\rm Pd}} \cdot \Delta {B_{\rm Pd}}}}{{{B_0}{L_0} + {B_{\rm Pd}}{L_{\rm Pd}}}} $$ (4) 式中:B0、L0分别为所用光纤的固有双折射和长度;BPd为镀膜产生的附加双折射;LPd为光纤镀钯氢敏膜部分的长度;ΔBPd为Pd吸氢后镀膜部分光纤的双折射的变化量;λm是工作波长。当整段双折射光纤都镀上氢敏膜时,即LPd= L0时,公式(4)可改写为:
$$ \Delta \lambda = \frac{{{\lambda _m} \cdot \Delta {B_{\rm Pd}}}}{{{B_0} + {B_{\rm Pd}}}} $$ (5) 从公式(5)中可以看出,当镀膜区域覆盖整根双折射光纤时,氢气敏感系数与光纤的长度无关。基于这个特点,可以制作微小尺寸的本征型光纤氢传感器。笔者所在课题组在PM-PCF表面镀了100 nm厚的Pd/Ag膜,实现了温度无关的高灵敏氢气传感,在0~4%的氢气的浓度范围内,最大灵敏度能达到1310 pm/%。2018年进一步优化光路,发明了一种反射式光纤氢气传感结构[63],抑制了温度交叉干扰,并能实现狭小空间的高灵敏度氢气传感。利用该反射式结构,采用双段保偏光纤结构可实现氢气与工作环境温度同时测量和补偿。
基于PHF径向应变的双折射调制光纤氢气传感器灵敏度高,响应时间快,解调方便,可实现狭小空间的氢气浓度检测,具有较高的实用化潜力。
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折射率调制基于PHF及THF吸释氢引起折射率变化的特性实现氢气传感。沉积在光纤侧面或端面的氢敏材料折射率变化会使光纤中传输光的强度、波长等参数发生变化,通过测量参数的变化实现氢气浓度传感。基于折射率调制的光纤氢气传感技术主要有微反射镜[47]、倏逝场[49]、表面等离子体共振(Surface Plasmon Resonance, SPR)[51]、光纤干涉型[52]及F-P腔型[58]等。
1991年M.A.Butler在光纤的端面蒸镀10 nm的钯膜实现氢气传感[47],如图7 (a)所示。为抑制光源功率波动带来的误差,一般采用差分光路结构[48, 64],如图7 (b)、(c)所示。
基于光纤的倏逝场对外界介质折射率变化的敏感特性,1999年Tabib-Azar等人在多模裸光纤外沉积厚为10~20 nm的Pd膜(见图8 (a)),通过测量传输光强的变化,在室温条件下实现0.2%~0.6%氢气浓度探测[49]。为增强敏感性,同样可采用去除部分包层[65]和光纤拉锥[66]等方法。2019年Cao等人[50]采用无掩模反应离子蚀刻技术将D型光纤表面加工成锯齿状,如图8 (b)所示,增加了镀膜的表面积,提高了灵敏度。
SPR是一种高灵敏度的基于折射率变化的传感技术。Bevenot等人[51]于2001年首次将SPR用于光纤氢气传感,通过测量光强变化实现了0.8%~100%氢气浓度的测量。2013年Perrotto等人通过测量SPR信号的峰值波长的变化,实现氢气测量,灵敏度达到1000 pm/%[67]。典型的SPR光纤氢气传感的传感部分结构可由图9 (a)示意。近年,基于SPR的氢气传感技术研究较多,X.Wang等人[68]在同一根光纤的两个位置分别进行了刻蚀镀膜,形成了双通道的氢气传感器。2016年H.Yan等人在多模光纤的端面制作Au-WO3-Pd复合膜光栅,如图9 (b)所示,获得了极高的氢气灵敏度,当氢浓度从0%变为4%时,共振波长平移了28.1 nm,灵敏度约为7500 pm/%[69]。
近年,基于光纤内包层和芯模的干涉被用于氢气传感,2010年Kim等人[52]采用电弧放电技术,在光纤中形成M-Z干涉结构实现氢气传感,如图10 (a)所示,Pd吸氢后会引起包层模折射率的变化,引起干涉光谱的变化,在氢气浓度为4%时,灵敏度为100 pm/%。如图10 (b)~(e)所示,光纤内模式干涉一般通过光纤错位熔接[70]、拉锥[33]、塌陷熔接[71]和写入长周期光纤(Long Period Grating, LPG)[72-73]等方式实现包层模激发、包层模和芯模耦合干涉。这种结构具有尺寸微小(典型长度量级为cm)、灵敏度高等特点,报道的最大的灵敏度为880 pm/%[73],最大测量范围为0~5%[33]。
可以看出,基于PHF及THF折射率调制的光纤氢气传感器具有灵敏度高、环境适应性强和尺寸微小的特性。其中基于SPR的氢气传感技术具备较高的应用潜力,基于干涉测量的方案依然存在温度误差大的问题。
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温度调制基于THF在吸释氢过程中产生的温度变化实现氢气传感。因此很多光纤温度传感结构都可基于这个调制机理实现氢气传感。如图11所示,Caucheteur在2007年用镀Pt/WO3的光纤光栅,实现最低浓度为1%的氢气检测[54];2021年Du等人[35]在光纤表面采用激光微加工微腔并沉积Pt/WO3粉末形成M-Z干涉仪,实现0.1%~0.8%浓度氢气浓度测量,灵敏度最大为−1.94868×106 pm/%;低浓度氢气与WO3材料的反应热较少,导致检测阈值较大。目前报道的最低氢气浓度检测值为0.02 %[55, 74]。由于环境温度变化直接影响传感的精度,需要设置单独的温度测量单元[74]。另外,这类传感器工作还需要O2参与,应用范围受限。
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近年来香港理工大学靳伟课题组提出了一种基于受激拉曼色散的光纤氢气传感技术。其基本原理如图12 (a)所示,在大芯径的空芯光纤中同时输入两束激光,能量较大的作为泵浦光,其频率为ωpump,能量较小的设为探测光,其频率为ωprobe。当泵浦光与探测光的频率差Δωdiff与氢气分子的拉曼频移ω0相匹配时,氢气分子从基态跃迁至激发态,如图12 (b)所示,氢气的复折射率变化可以表示为:
$$ {n_R}\left( {\Delta {\omega _{{\text{diff}}}}} \right) = - {\text{i}}\frac{{\upsilon {g_0}{I_{{\text{pump}}}}C}}{{2{\omega _{{\text{probe}}}}\left[ {1 - {{2{\text{i}}\left( {{\omega _0} - \Delta {\omega _{{\text{diff}}}}} \right)} \mathord{\left/ {\vphantom {{2{\text{i}}\left( {{\omega _0} - \Delta {\omega _{{\text{diff}}}}} \right)} {{\Gamma _R}}}} \right. } {{\varGamma _{\rm R}}}}} \right]}} $$ (6) 式中:g0为氢气的拉曼增益系数;υ为光速;C为氢气浓度;ΓR/2π是拉曼增益光谱的半峰全宽;Ipump为泵浦光的强度。从公式(6)中可以看出,受激拉曼色散与氢气浓度C呈比例关系,通过测量折射率的变化可以实现氢气浓度的测量。采用M-Z干涉仪实现对受激拉曼色散引起的相位调制进行测量,氢气浓度探测的下限已达到0.0025%[75]。基于受激拉曼色散的氢气传感技术可实现氢气浓度直接测量,探测下限低,响应时间快,具有独特的优势。
Optical fiber hydrogen sensing technology (Invited)
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摘要: 光纤氢气传感器采用光纤作为传光或者传感的介质,基于氢敏材料的相关理化特性实现氢气检测,具有本质安全、稳定性好、体积小、质量轻和易组网等优良特性,是目前氢气传感和光纤传感领域的研究热点。文中首先介绍了典型氢敏材料的作用机理及特点,然后依据氢敏材料的调制机理,综述了几类典型的光纤氢气传感技术及基于受激拉曼增益或色散的新型光纤氢气传感技术,最后从传感器关键工艺及环境适应性方面分析了目前光纤氢气传感器实用化需要解决的问题,并对未来的研究方向进行了展望。Abstract: Optical fiber hydrogen sensors uses optical fiber as the medium of light transmission or sensing, and realizes hydrogen detection based on the physical and chemical properties of hydrogen sensitive materials. It has excellent characteristics such as intrinsic safety, strong stability, small volume light weight and good reusability, which makes it become one of the research hotspots in the field of hydrogen sensing and optical fiber sensing. The action mechanism and characteristics of typical hydrogen sensitive materials were introduced. Several typical optical fiber hydrogen sensor technologies and new progress according to the modulation mechanism of hydrogen sensitive materials were reviewed, a new optical fiber hydrogen sensor technology based on stimulated Raman gain or dispersion was introduced, and finally the problems to be solved in the practical application of optical fiber hydrogen sensor from the aspects of the key technology and environmental adaptability were analyzed, as well as the future research direction was prospected.
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图 3 (a)典型的应变调制F-P干涉氢气传感器示意图[42];(b)侧面[58]及(c)端面镀Pd膜的F-P干涉结构示意图[59]。PHF:钯基氢敏膜
Figure 3. (a) Schematic of strain modulation F-P interference hydrogen sensor[42]; Schematic of F-P interference structure with Pd film (b) on the side[58] and (c) on the end[59]. PHF:Palladium based hydrogen sensitive film
表 1 光纤氢气传感器分类
Table 1. Classification of optical fiber hydrogen sensors
Modulation scheme Hydrogen sensitive material Optical path scheme Modulation parameters Ref. Strain Pd-based Optical fiber interferometer Intensity/Phase [40-41] Fabry-Perot interferometer Intensity/Phase [38, 42-43] Fiber Bragg grating Wavelength [44-46] Refractive index Pd-based and WO3 Micro mirror Intensity [47-48] Evanescent wave Intensity [49-50] Surface plasma resonance Intensity/Wavelength [34, 51] Optical fiber interferometer Intensity/Phase [33, 52] Fabry-Perot interferometer Phase/Wavelength [53] Temperature WO3 Fiber Bragg grating Wavelength [54-55] Optical fiber interferometer Phase [35, 56] -
[1] Watson J, Ihokura K, Coles G. The tin dioxide gas sensor [J]. Measurement Science and Technology, 1993, 4(7): 711-719. doi: 10.1088/0957-0233/4/7/001 [2] Buttner W J, Post M B, Burgess R, et al. An overview of hydrogen safety sensors and requirements [J]. International Journal of Hydrogen Energy, 2011, 36(3): 2462-2470. doi: 10.1016/j.ijhydene.2010.04.176 [3] Devine J C. A progress report: Cleaning up TMI [J]. IEEE Spectrum, 1981, 18: 44-49. [4] Blandford E D, Ahn J. Examining the nuclear accident at fukushima daiichi [J]. Elements, 2012, 8(3): 189-194. doi: 10.2113/gselements.8.3.189 [5] Han C H, Hong D W, Kim I J, et al. Synthesis of Pd or Pt/titanate nanotube and its application to catalytic type hydrogen gas sensor [J]. Sensors and Actuators B: Chemical, 2007, 128(1): 320-325. doi: 10.1016/j.snb.2007.06.025 [6] Han C H, Hong D U, Gwak J, et al. A planar catalytic combustion sensor using nano-crystalline F-doped SnO2 as a supporting material for hydrogen detection [J]. Korean Journal of Chemical Engineering, 2007, 24(6): 927-931. doi: 10.1007/s11814-007-0099-2 [7] Diener P G, Obermeier E. Heat-conduction microsensor based on silicon technology for the analysis of two- and three-component gas mixtures [J]. Sensors and Actuators B: Chemical, 1993, 13(1-3): 345-347. doi: 10.1016/0925-4005(93)85397-S [8] Simon I, Arndt M. Thermal and gas-sensing properties of a micromachined thermal conductivity sensor for the detection of hydrogen in automotive applications [J]. Sensors and Actuators A Physical, 2002, 97: 104-108. [9] Sakthivel M. A portable limiting current solid-state electrochemical diffusion hole type hydrogen sensor device for biomass fuel reactors: Engineering aspect [J]. International Journal of Hydrogen Energy, 2008, 33(2): 905-911. doi: 10.1016/j.ijhydene.2007.10.048 [10] Nikolova V, Nikolov I, Andreev P, et al. Tungsten carbide-based electrochemical sensors for hydrogen determination in gas mixtures [J]. Journal of Applied Electrochemistry, 2000, 30(6): 705-710. doi: 10.1023/A:1003813210270 [11] Han C H, Han S D, Singh I, et al. Micro-bead of nano-crystalline F-doped SnO2 as a sensitive hydrogen gas sensor [J]. Sensors and Actuators B: Chemical, 2005, 109(2): 264-269. doi: 10.1016/j.snb.2004.12.115 [12] Shukla S, Zhang P, Cho H J, et al. Room temperature hydrogen response kinetics of nano–micro-integrated doped tin oxide sensor [J]. Sensors and Actuators B: Chemical, 2007, 120(2): 573-583. doi: 10.1016/j.snb.2006.03.010 [13] Wang G, Dai J, Yang M. Fiber-optic hydrogen sensors: A review [J]. IEEE Sensors Journal, 2020, 21(11): 12706-12718. [14] Zhang Y N, Peng H, Qian X, et al. Recent advancements in optical fiber hydrogen sensors [J]. Sensors and Actuators B Chemical, 2017, 244(2017): 393-416. [15] Wicke E, Brodowsky H, Züchner H. Hydrogen in palladium and palladium alloys [J]. Metal Finishing, 1996, 95(2): 73-155. [16] Butler M A, Ginley D S. Hydrogen sensing with palladium‐coated optical fibers [J]. Journal of Applied Physics, 1988, 64(7): 3706-3712. doi: 10.1063/1.341414 [17] Armgarth M, Nylander C. Blister formation in Pd gate MIS hydrogen sensors [J]. IEEE Electron Device Letters, 1983, 3(12): 384-386. [18] Kalli K, Othonos A, Christofides C. Characterization of reflectivity inversion, α- and β-phase transitions and nanostructure formation in hydrogen activated thin Pd films on silicon based substrates [J]. Journal of Applied Physics, 2002, 91(6): 3829-3840. doi: 10.1063/1.1417992 [19] Cui L J, Chen Y P, Gang Z. An optical fiber hydrogen sensor with Pd/Ag film [J]. Optoelectronics Letters, 2009, 5(3): 220-223. doi: 10.1007/s11801-009-9040-8 [20] Fang Y, Duan F, Zhang M, et al. Pd-Ag film coated LPG for hydrogen sensing [C]//SPIE, 2012, 8409: 840935. [21] Liu Y, Chen Y P, Song H, et al. Characteristics of an optical fiber hydrogen gas sensor based on a palladium and yttrium alloy thin film [J]. IEEE Sensors Journal, 2013, 13(7): 2699-2704. doi: 10.1109/JSEN.2013.2258904 [22] Zhao Z, Carpenter M A, Xia H, et al. All-optical hydrogen sensor based on a high alloy content palladium thin film [J]. Sensors and Actuators B: Chemical, 2006, 113(1): 532-538. doi: 10.1016/j.snb.2005.03.070 [23] Hughes R C, Schubert W K. Thin films of Pd/Ni alloys for detection of high hydrogen concentrations [J]. Journal of Applied Physics, 1992, 71(1): 542-544. doi: 10.1063/1.350646 [24] Dai J, Zhu L, Wang G, et al. Optical fiber grating hydrogen sensors: A review [J]. Sensors, 2017, 17(3): 577. doi: 10.3390/s17030577 [25] Xu B, Zhao C L, Yang F, et al. Sagnac interferometer hydrogen sensor based on panda fiber with Pt-loaded WO3/SiO2 coating [J]. Optics Letters, 2016, 41(7): 1594-1597. doi: 10.1364/OL.41.001594 [26] Park S, Kim H, Jin C, et al. Enhanced CO gas sensing properties of Pt-functionalized WO3 nanorods [J]. Thermochimica Acta, 2012, 542: 69-73. doi: 10.1016/j.tca.2011.12.002 [27] Zeng W, Dong C, Miao B, et al. Preparation, characterization and gas sensing properties of sub-micron porous WO3 spheres [J]. Materials Letters, 2014, 117: 41-44. doi: 10.1016/j.matlet.2013.11.080 [28] Tabassum R, Gupta B D. Surface plasmon resonance-based fiber-optic hydrogen gas sensor utilizing palladium supported zinc oxide multilayers and their nanocomposite [J]. Applied Optics, 2015, 54(5): 1032-1040. doi: 10.1364/AO.54.001032 [29] Hosoki A, Nishiyama M, Igawa H, et al. A hydrogen curing effect on surface plasmon resonance fiber optic hydrogen sensors using an annealed Au/Ta2O5/Pd multi-layers film [J]. Optics Express, 2014, 22(15): 18556-18563. doi: 10.1364/OE.22.018556 [30] Downes F, Taylor C M. Theoretical investigation of a multi-channel optical fiber surface plasmon resonance hydrogen sensor [J]. Optics Communications, 2021, 490: 126916. doi: 10.1016/j.optcom.2021.126916 [31] Yan A, Chen R, Zaghloul M, et al. Sapphire fiber optical hydrogen sensors for high-temperature environments [J]. IEEE Photonics Technology Letters, 2015, 28(1): 47-50. [32] Yahya N A M, Hamid M R Y, Ibrahim S A, et al. H2 sensor based on tapered optical fiber coated with MnO2 nanostructures [J]. Sensors and Actuators B: Chemical, 2017, 246: 421-427. doi: 10.1016/j.snb.2017.02.084 [33] Yu Z P, Jin L, Sun L P, et al. Highly sensitive fiber taper interferometric hydrogen sensors [J]. IEEE Photonics Journal, 2016, 8(1): 6800309. [34] Mikami M, Komatsu D, Hosoki A, et al. Quick response hydrogen LSPR sensor based on hetero-core fiber structure with Palladium nano-particles [J]. Optics Express, 2020, 29(1): 48-58. doi: 10.1364/OE.412789 [35] Du B, He J, Yang M, et al. Highly sensitive hydrogen sensor based on in-fiber Mach-Zehnder interferometer with polymer infiltration and Pt-loaded WO3 coating [J]. Optics Express, 2021, 29(3): 4147-4158. doi: 10.1364/OE.417424 [36] Xu B, Zhao F P, Wang D, et al. Tip hydrogen sensor based on liquid filled in-fiber Fabry-Perot interferometer with Pt-loaded WO3 coating [J]. Measurement Science and Technology, 2020, 31: 125107. doi: 10.1088/1361-6501/ab7e6a [37] Fisser M, Badcock R A, Teal P D, et al. High-sensitivity fiber-optic sensor for hydrogen detection in gas and transformer oil [J]. Sensors Journal, IEEE, 2019, 19(9): 3348-3357. doi: 10.1109/JSEN.2019.2891523 [38] Ma J, Zhou Y, Bai X, et al. High-sensitivity and fast-response fiber-tip Fabry-Perot hydrogen sensor with suspended palladium-decorated graphene [J]. Nanoscale, 2019, 11(34): 15821-15827. doi: 10.1039/C9NR04274A [39] Xiong C, Zhou J, Liao C, et al. Fiber-tip polymer microcantilever for fast and highly sensitive hydrogen measurement [J]. ACS Applied Materials and Interfaces, 2020, 12: 33163-33172. doi: 10.1021/acsami.0c06179 [40] Butler M A. Optical fiber hydrogen sensor [J]. Applied Physics Letters, 1984, 45(10): 1007. doi: 10.1063/1.95060 [41] Yang Y, Yang F, Wang H, et al. Temperature-insensitive hydrogen sensor with polarization-maintaining photonic crystal fiber-based sagnac interferometer [J]. Journal of Lightwave Technology, 2015, 33(12): 2566-2571. doi: 10.1109/JLT.2014.2375362 [42] Zeakes J S, Murphy K A, Elshabini-Riad A, et al. Modified extrinsic Fabry-Perot interferometric hydrogen gas sensor [C]//IEEE Lasers and Electro-optics Society Meeting, 1994: 235-236. [43] Yang Z, Zhang M, Liao Y, et al. Extrinsic Fabry-Perot interferometric optical fiber hydrogen detection system [J]. Applied Optics, 2010, 49(15): 2736-2740. doi: 10.1364/AO.49.002736 [44] Karanja J M, Dai Y, Zhou X, et al. Micro-structured femtosecond laser assisted FBG hydrogen sensor [J]. Optics Express, 2015, 23(24): 31034. doi: 10.1364/OE.23.031034 [45] Sutapun B, Tabib Azar M, Kazemi A. Pd-coated elastooptic fiber optic Bragg grating sensors for multiplexed hydrogen sensing [J]. Sensors and Actuators B: Chemical, 1999, 60(1): 27-34. doi: 10.1016/S0925-4005(99)00240-3 [46] Xian Z, Dai Y, Karanja J M, et al. Microstructured FBG hydrogen sensor based on Pt-loaded WO3 [J]. Optics Express, 2017, 25(8): 8777. doi: 10.1364/OE.25.008777 [47] Butler M A. Fiber optic sensor for hydrogen concentrations near the explosive limit [J]. Journal of the Electrochemical Society, 1991, 138(9): L46. doi: 10.1149/1.2086073 [48] Liu Y, Chen Y P, Song H, et al. Modeling analysis and experimental study on the optical fiber hydrogen sensor based on Pd-Y alloy thin film [J]. Review of Scientific Instruments, 2012, 83(7): 075001. doi: 10.1063/1.4731725 [49] Tabib Azar M, Sutapun B, Petrick R, et al. Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interactions [C]//Proceedings of SPIE, 1998, 56(1): 158-163. [50] Cao R, Wu J, Liang G, et al. Functionalized PdAu alloy on nanocones fabricated on optical fibers for hydrogen sensing [J]. IEEE Sensors Journal, 2019, 20(4): 1922-1927. [51] Bévenot X, Trouillet A, Veillas C, et al. Surface plasmon resonance hydrogen sensor using an optical fibre [J]. Measurement Science and Technology, 2001, 13(1): 118-124. [52] Kim Y H, Kim M J, Rho B S, et al. Mach-Zehnder interferometric hydrogen sensor based on a single mode fiber having core structure modification at two sections [C]//IEEE Sensors, 2010: 1483-1486. [53] Yu C, Li L, Chen X, et al. Fiber-optic Fabry-Perot hydrogen sensor coated with Pd-Y film [J]. Photonic Sensors, 2015, 5(2): 142-145. doi: 10.1007/s13320-015-0237-0 [54] Caucheteur C, Debliquy M, Lahem D, et al. Catalytic fiber Bragg grating sensor for hydrogen leak detection in air [J]. IEEE Photonics Technology Letters, 2008, 20(2): 96-98. doi: 10.1109/LPT.2007.912557 [55] Dai J, Yang M, Zhi Y, et al. Performance of fiber Bragg grating hydrogen sensor coated with Pt-loaded WO3 coating [J]. Sensors and Actuators B: Chemical, 2014, 190: 657-663. doi: 10.1016/j.snb.2013.08.083 [56] Wang Y, Yang M, Zhang G, et al. Fiber optic hydrogen sensor based on Fabry-Perot interferometer coated with Sol-Gel Pt/WO3 coating [J]. Journal of Lightwave Technology, 2015, 33(12): 2530-2534. doi: 10.1109/JLT.2014.2365183 [57] Liao Y, Dai J, Yang M, et al. Comparison of side-polished fiber Bragg grating hydrogen sensors sputtered with Pd/Ag and Pd/Y composite films [C]//SPIE, 2012, 8421: 842162. [58] Wang M, Yang M, Cheng J, et al. Fabry-Pérot interferometer sensor fabricated by femtosecond laser for hydrogen sensing [J]. IEEE Photonics Technology Letters, 2013, 25(8): 713-716. doi: 10.1109/LPT.2013.2241421 [59] Cao R, Yang Y, Wang M, et al. Multiplexable intrinsic Fabry-Perot interferometric fiber sensors for multipoint hydrogen gas monitoring [J]. Optics Letters, 2020, 45(11): 3163-3166. doi: 10.1364/OL.389433 [60] Dai J X, Yang M H, Yu X, et al. Greatly etched fiber Bragg grating hydrogen sensor with Pd/Ni composite film as sensing material [J]. Sensors and Actuators B: Chemical, 2012, 174: 253-257. doi: 10.1016/j.snb.2012.07.018 [61] Silva S, Coelho L, Almeida J M, et al. H2 sensing based on a Pd-coated tapered-FBG fabricated by DUV femtosecond laser technique [J]. IEEE Photonics Technology Letters, 2013, 25(4): 401-403. doi: 10.1109/LPT.2013.2239985 [62] Meng Z, Dai Y, Xian Z, et al. Femtosecond laser ablated FBG with composite microstructure for hydrogen sensor application [J]. Sensors, 2016, 16(12): 2040. doi: 10.3390/s16122040 [63] 杨远洪, 李慧, 陆林, 等. 一种全保偏反射式氢气浓度检测装置: 中国, 201810522603.6 [P]. 2018-10-30. [64] Park K S, Kim Y H, Eom J B, et al. Compact and multiplexible hydrogen gas sensor assisted by self-referencing technique [J]. Optics Express, 2011, 19(19): 18190-18198. doi: 10.1364/OE.19.018190 [65] Sekimoto S, Nakagawa H, Okazaki S, et al. A fiber-optic evanescent-wave hydrogen gas sensor using palladium- supported tungsten oxide [J]. Sensors and Actuators B, 2000, 66: 142-145. doi: 10.1016/S0925-4005(00)00330-0 [66] Yahya N, Hamid M, Ong B H, et al. H2 gas sensor based on Pd/ZnO nanostructures deposited on tapered optical fiber [J]. IEEE Sensors Journal, 2020, 20(6): 2982-2990. doi: 10.1109/JSEN.2019.2957838 [67] Perrotton C, Westerwaal R J, Javahiraly N, et al. A reliable, sensitive and fast optical fiber hydrogen sensor based on surface plasmon resonance [J]. Optics Express, 2013, 21(1): 382-390. doi: 10.1364/OE.21.000382 [68] Wang X, Tang Y, Zhou C, et al. Theoretical investigation of a dual-channel optical fibre surface plasmon resonance hydrogen sensor based on wavelength modulation [J]. Measurement Science and Technology, 2013, 24(6): 065102. doi: 10.1088/0957-0233/24/6/065102 [69] Yan H, Zhao X, Zhang C, et al. A fast response hydrogen sensor with Pd metallic grating onto a fiber’s end-face [J]. Optics Communications, 2016, 359: 157-161. doi: 10.1016/j.optcom.2015.09.041 [70] Sun Zhiqiang, Liu Zexu, Xiao Yike, et al. Thermal stability of optical fiber metal organic framework based on graphene oxide and nickel and its hydrogen adsorption application [J]. Optics Express, 2018, 26(24): 31648-31656. doi: 10.1364/OE.26.031648 [71] Zhou F, Qiu S J, Luo W, et al. An all-fiber reflective hydrogen sensor based on a photonic crystal fiber in-line interferometer [J]. IEEE Sensors Journal, 2014, 14(4): 1133-1136. doi: 10.1109/JSEN.2013.2293347 [72] Trouillet A, Marin E, Veillas C. Fibre gratings for hydrogen sensing [J]. Measurement Science and Technology, 2006, 17(5): 1124-1128. doi: 10.1088/0957-0233/17/5/S31 [73] Kim Y H, Kim M J, Rho B S, et al. Ultra sensitive fiber-optic hydrogen sensor based on high order cladding mode [J]. IEEE Sensors Journal, 2011, 11(6): 1423-1426. doi: 10.1109/JSEN.2010.2092423 [74] Yang M, Wang G, Dai J, et al. Fiber Bragg grating sensors with Pt-loaded WO3 coatings for hydrogen concentration detection down to 200 ppm [J]. Measurement Science and Technology, 2014, 25(11): 114004. doi: 10.1088/0957-0233/25/11/114004 [75] Bao H, Jin W, Miao Y, et al. Laser-induced dispersion with stimulated Raman scattering in gas-filled optical fiber [J]. Journal of Lightwave Technology, 2019, 37(9): 2120-2125. doi: 10.1109/JLT.2019.2898463 [76] Uemiya S, Matsuda T, Kikuchi E. Hydrogen permeable palladium-silver alloy membrane supported on porous ceramics [J]. Journal of Membrane Science, 1991, 56(3): 315-325. doi: 10.1016/S0376-7388(00)83041-0 [77] Zhao Z, Sevryugina Y, Carpenter M A, et al. All-optical hydrogen-sensing materials based on tailored palladium alloy thin films [J]. Analytical Chemistry, 2004, 76(21): 6321-6326. doi: 10.1021/ac0494883 [78] Ma G M, Jiang J, Li C R, et al. Pd/Ag coated fiber Bragg grating sensor for hydrogen monitoring in power transformers [J]. Review of Scientific Instruments, 2015, 86(4): 226-232. [79] Yuan Z, Ma Y, Qin Y, et al. Improved performance of fiber-optic hydrogen sensor based on Mg-Ti alloys composite thin films [C]//Proceedings of the 18 th International Conference on Optical Communications and Networks, 2019: 978. [80] Hosoki A, Nishiyama M, Sakurai N, et al. Long-term hydrogen detection using a hetero-core optical fiber structure featuring Au/Ta2O5/Pd/Pt multilayer films [J]. IEEE Sensors Journal, 2019, 20(1): 227-233. doi: 10.3390/s20010227 [81] Zhang C, Shen C, Liu X, et al. Pd/Au nanofilms based tilted fiber Bragg grating hydrogen sensor [J]. Optics Communications, 2021, 502(3): 127424. [82] Wang G, Qin Y, Dai J, et al. Performance-enhanced optical fiber hydrogen sensors based on WO3-Pd2Pt-Pt composite film with controlled optical heating [J]. Optical Fiber Technology, 2019, 52: 101979. doi: 10.1016/j.yofte.2019.101979 [83] Ohodnicki P R, Baltrus J P, Brown T D. Pd/SiO2 and AuPd/SiO2 nanocomposite based optical fiber sensors for H2 sensing applications [J]. Sensors and Actuators B: Chemical, 2015, 214(2015): 159-168. [84] Sun C, Ohodnicki P R, Yang Y. Double-layer zeolite nano-blocks and palladium-based nanocomposite fiber optic sensors for selective hydrogen sensing at room temperature [J]. IEEE Sensors Letters, 2017, 1(5): 1-4. [85] Mfa B, Rab A, Pdt B, et al. Optimizing the sensitivity of palladium based hydrogen sensors [J]. Sensors and Actuators B: Chemical, 2018, 259: 10-19. [86] Yang M, Yan S, Zhang D, et al. Using Pd/WO3 composite thin films as sensing materials for optical fiber hydrogen sensors [J]. Sensors and Actuators B: Chemical, 2010, 143(2): 750-753. doi: 10.1016/j.snb.2009.10.017 [87] Li H, Yang Y H, Lu L, et al. Practical reflective birefringent fiber interferometer sensor [J]. Applied Optics, 2019, 58(28): 7862-7867. doi: 10.1364/AO.58.007862 [88] 杨远洪, 宋叔淇, 李慧. 一种用于光纤侧面镀膜的装置: 中国, 201811601099.5 [P]. 2019-03-29. [89] GB/T 28786—2012, 真空技术. 真空镀膜层结合强度测量方法. 胶带粘贴法[S]. 北京: 国家市场监督管理总局, 2012. [90] Yang M, Zhi Y, Dai J, et al. Fiber optic hydrogen sensors with sol–gel WO3 coatings [J]. Sensors and Actuators B: Chemical, 2012, 166-167: 632-636. doi: 10.1016/j.snb.2012.03.026