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基于保偏光纤马赫曾德干涉结合布拉格光纤光栅的折射率传感器(英文)

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第28卷第11期 2015年11月 传感技术学报 CHINESE JOURNAL OF SENSORS AND ACTUATORS V01.28 No 11 Nov.2015 Refractive Index sensor Based on Polarization—Maintaining Fiber M.Z Interferometer with an FBG ZENG Hongyan ,SHEN Changyu1,2,LU Yahfang ,LIU Huanan , DONG Xinyong ,LI Chenxia (1.Institute ofOptoelectronic Technology,China Jiliang University,Hangzhon 310018,China; 2.Department ofElectronics,Carleton University,Ottawa,OntarioK1S 5B6,Canada) Abstract:In order to measure the refractive index(RI)and temperature simultaneously,a refractive index(RI)sensor based on a Mach—Zehnder interferometer(MZI)with a ifber Bragg grating(FBG)is proposed.The MZI is fabricated by core—offset fusion splicing one section of the polarization.maintaining ifber(PMF)between two single mode fibers (SMFs).A FBG was connected to the output port of the MZI.Part of the MZI interference light was reflected by the FBG.Two independent interference patterns corresponding to fast axis and slow axis modes in the PMF were obtained. and the interference pattern changed with the variation of the RI for the two orthogonal polarization modes.In the RI range of 1.3426 to 1.3692。the sensitivities of一118.38 nm/RIU and一80.02 nm/RIU forfast axis polarization mode and slow axis polarization mode were obtained,respectively.In the RI range of 1.3692 to 1.4191,the sensitivities of 48.3 nm/RIU and 25.75 nm/RIU orf fast axis polarization mode and slow axis polarization mode were obtained,respectively. The FBG was used as the temperature compensation and the RI and temperature can be measured simuhaneously. Key words:optical fiber sensing;refractive index sensor;Mach—Zehnder interferometer,Fiber Bragg grating EEACC:4125;4140 doi:10.39690.issn.1004-1699.2015.11.025 基于保偏光纤马赫曾德干涉结合布拉格 光纤光栅的折射率传感器 曾红燕 ,沈常宇 一,路艳芳 ,刘桦楠 ,董新永 ,李晨霞 (1.中国计量学院光电子研究所,杭州310018;2.卡尔顿大学电子工程系,加拿大渥太华K1S 5B6) 摘 要:为了能同时测量折射率和温度,提出了一种基于保偏光纤马赫曾德干涉仪结合布拉格光纤光栅的折射率传感器。保偏 光纤在两个单模光纤之间通过错位熔接形成马赫曾德干涉仪,布拉格光纤光栅连接在马赫曾德干涉仪的输/ti端用作温度补偿。 实验结果表明保偏光纤快轴和慢轴两个正交模式的干涉谱随着折射率的变化而变化,当折射率范围在1.3426到1.3692时,快轴 和慢轴方向上的灵敏度分别为一118.38 nm/RIU和一80.02 nm/RIU。当折射率在1.3692到1.419l时,快轴和慢轴方向上的灵敏度 分别为48.3 nm/RIU和25.75 nm/RIU。同时通过测量布拉格光纤光栅谐振波长的偏移量,折射率和温度能被同时测量。 关键词:光纤传感;折射率传感器;马赫一曾德干涉仪;布拉格光纤光栅 中图分类号:0433.1 文献标识码:A 文章编号:1004-1699(2015)11—1727—05 taper.Iadicicco used thinned fiber Bragg gratings as high sensitivity refractive index sensor.Shu[12]proposed Optical fiber sensors have attracted people’s at— tention for its high sensitivity and large capacity.Sever— al fiber sensors for the detecting of temperature 一 , sampled fiber Bragg grating for simultaneous refractive index and temperature measurement.Ding used fiber— taper seeded long—period grating pair to formed one kind of highly sensitive refractive-index sensor.Howev— bending[ ,strain 。curvature[6- and refractive index (RI)㈨…have been reported.For instance,Shao㈨pro- posed optica1 refractive~index sensor based on dua1 fi- ber—Bragg gratings interposed with a muhimode—fiber er,grating—based(FBG,LPG)sensors are more respon— 项目来源:National Natural Science Foundation ofChina(61405185) 收稿日期:2015—06—19 修改日期:2015—09—08 1728 传感技术学报 www.ehinatransducers.COITI 第28卷 sive to a larger range of RI,but they require precise SOUrce.Light from the ASE source was launched into the PMF MZI.The polarization state of output light from ASE source is a non—polarized light,and the out— put spectrum was detected with an optical spectrum an- and expensive phase masks and stringent photolitho— graphic procedures or require other fabrication meth- ods.In this letter.a simple PMF MZI structure is pro— posed.The proposed PMF MZI is formed by core-offset fusion splicing one section of the polarization maintain— alyzer(OSA,A06370,Japan),the maximum resolution of the OSA is 20 pm.The fiber holder was used to keep ing fiber(PMF)between two single mode fibers the sensor head free from any strain and bending.The MZI is formed by 2 cm—long PMF between SMFs.Both (SMFs).The polarization maintaining fiber production process is relatively simple.And compared with the ends of the PMF are mismatch fusion spliced(using a similar existing RI sensor,the sensitivity of the pro— posed RI sensors based on PMF M—Z interferometer was improved effectively.The obtained RI sensitivity 0f一1 18_38 nm/RIU and 48.3 nm/RIU at least is 2 times higher than that of double-pass MZI.Further— more,a ifber Bragg grating(FBG)was connected to the output port of the MZI,which can reflect a narrowband of light into the MZI.The resonance wavelength of the PMF MZI is sensitive to RI and temperature,while the Bragg wavelength of the FBG is only sensitive to tem— perature,and the Bragg wavelength of the FBG against temperature is linear relation .In addition.during the RI detecting process,the RI variation is only occurred between the two SMFs.So the FBG’s reflected peak can be used as temperature measurement and cross— sensitivity compensation.The core mode in the PMF has different effective indices corresponding to the two orthogonal polarization modes because of the birefrin. gence of the PMF.Therefore,the interference patterns of the MZI corresponding to the two orthogonal polariza— tion modes have different resonant dip wavelengths.By measuring the resonanee wavelength shifts of interfer— ence pattern,the variation of the surrounding RI and temperature can be detected simuhaneously. 1 EXPERIMENT The experiment device is shown in Fig.1.An am— plified spontaneous emission(ASE)source of 1 450 nm to 1 650 nm wavelength range is used as the light SMF1 PMF SMF2 Fig.1 Schematic diagram of the proposed RI sensor.The inset SHOWS the partially enlarged drawing of the sensing head. commercial fusion splicer(Fujikura FSM一40S))with the lead-in SMF1 and lead—out SMF2,respectively.As the SMF 1 and SMF2 are mismatch fusion spliced with the PMF,the misalignment degree is controllable.To control the degree of the mismatch,during the fusion splicing process,the manual splicing mode of the fu— sion splicer is used.The fiber core offset size is about 4txm in order to ensure the splicing loss is approxi— mately 3dB.And during the manual operation,the mis— match degree can be adjusted by the splicing loss showing on the screen of the fusion splicer.A fiber Bragg grating(FBG)was connected to the output port of the MZI,and the FBG was used as the temperature compensation.After the MZI interference light passed through the FBG,the Bragg wavelength of the FBG was reflected.And after that,the fiber end will reflect Dart of interferenee light back to the MZI.In addition。the polymer coating on the PMF was removed.The core and the cladding diameters of the used SMF are 9/xm and 1 25 m,respectively.The core—offset size of the two fibers(SMF and PMF)is only about 4 m.Com— pare to the diameter of the fiber cladding(125 Ixm). The small core—offset size of the fiber has little influ— ence to the mechanical stability when it has error.The glycerol solutions with different concentrations were used as the test sample.The RI of the test sample was measured with an Abbe refractometer,and the range of the RI value is 1.3426 to 1.4191. When the light through SMF 1 reached the left end of PMF,the part of the light will couple into the cladding mode of the PMF to excite some cladding modes,the other part of the fight will propagate alone the core mode of the PMF.The excited cladding modes were re—coupled to the core mode after propagating through the PMF,the two beams coming from the fiber core and cladding interfered at SMF2.The interference 日p/A ∞II 口H 第11期 曾红燕,沈常宇等:基于保偏光纤马赫曾德干涉结合布拉格光纤光栅的折射率传感器 1729 ∞口君 quI时I10一甚N一 暑10 light forming the interference pattern was detected on the OSA,a PMF MZI interference pattern with an FBG reflected peak is formed as shown in Fig.2.As respect. ed,the interference patterns corresponding to the slow axis and fast axis polarization modes have different res. onant dip wavelength.In Fig.2,九 and are the reso— nant dip wavelength corresponding to the slow axis and fast axis polarization modes,respectively.It can be seen that the interference patterns have a little asym- metry since there are more than two modes involved in the interference patterns.In our experiment,it can be assumed that only one exited cladding mode is domi— nant.In order to examine the assumption,we obtained the spatial frequency spectra of the interference pat・ terns of Fig.2 by using the fast Fourier transform meth- od.As shown in Fig.3.the power is mainly distributed in the core mode and a strong cladding mode. Wavelength/nm Fig.2 Interference fringes variations of the PMF MZ1 with different surrounding RI. 1.O ……1.348 6 O.8 1.408 1 0.6 /Core mode 0.4 0_2 /,Strong cladding mode 一 之 ,.Weak cladding mode 0 0 0.2 0.4 0.6 0.8 1.o Spatial frequency/(1/nm) Fig.3 Spatial frequency spectra of the proposed sensor Other excited higher order cladding modes are weak.The main interference pattern is mainly formed by the interference of the dominant strong cladding mode with the core mode.Other weak cladding modes should also interference with the core mode,and the in. terference between the core mode and the weak clad— ding modes will slightly modulate the main interference pattern.However,the modulation effect is very weak. The light through the polarization--maintaining fi-- bet MZI and the output light intensity of the interfer— ence pattern is decided by, (A)=, + +2、『7 cos( ̄ ) (1) Where Ii and 12 are the intensities of fiber core and cladding.respectively. is the phase difference of between the core and the cladding modes,and the phase difference can be described as, =21T△ mff 从 (2) where A nm。Ⅱis the effective RI difference be— tween the fiber core and mth cladding modes.L is the length of PMF between SMF 1 and the SMF2,A is the center wavelength of the input light.The SMF2 was connected to the right end of the PMF.The effective re— fractive index difference between the core and the ruth cladding modes corresponding to the slow axis and fast axis polarization modes can be described as, △凡m c oreclad—eesneffm (3) 凡core 一ncla eI e eⅡ where the 。mm and cmot:are the effective refrac— tire index diffefence between the core and the excited urth cladding modes and the effective refractive index of the fiber core on the slow axis of the PMF,respee- tively.And Anm。m,and n are the effective refractive index difference between the core and the excited mth cladding modes and the effective refractive index of the fiber cladding off the fast axis of the PMF,respec— tively.n clⅡa is the effective refractive index of the excit- ed cladding modes of the PMF. The resonant dip wavelength satisfies the equa— tion of中 =(2 +1)盯. is the natural number.There— fore,the resonant dip wavelength corresponding to slow axis and fast axis polarization modes,A and A, can be described as, 、 2,f(nc ̄re .—n: ) 一— ■ (4) 2,f(nc ̄rec_。 。lⅡa ) …^,一—— 、 T— And the resonant dip wavelength A D of the polar— ization—maintaining fiber MZI is A。=2丌△ (2k+1) (5) When add glycerol solution to the surface of the MZI,the effective RI of the cladding mode will change with the variation of the surrounding RI between SMF 1 and SMF2.Therefore,the m will decrease with the in一 1730 传感技术学报 www.ehinatransducers.com 第28卷 creasing of the surrounding RI.So the shifts ofthe reso. nant dip wavelength A D as follows, 8AD=2"rr6Am§ 一I{∞ I10—0 controlled container.A po・ was put into a temperature larization controller(PC)is used to adjust the polariza- tion states of the input light in order to obtain a high fringe visibility.The interference pattern changed with 0 efel/(2k+1) where Z is the interaction length of the SenS0r 2 Results and Discussions The experiment was carried out at room tempera— the temperature in the range from 22℃to 80 ̄C. ture(22 ),as shown in Fig.2,the resonant wave— length of the interference patterns changed with the in. creasing of the surrounding refractive index.When the surrounding RI varied from 1.3426 to i.3692.the reso— nant wavelength of the PMF MZI shitfed towards the shorter wavelength due to the inereased effeetive index of cladding modes.When the variation of the surround— ing RI varied from 1.3692 to 1.4191,the shitfs of the resonant dip wavelength decreased gradually,because ∞p,茸【s【Iu uI the surrounding RI is gradually close to the effective in. dex of the cladding mode.Fig.4 shows the different functions of the fast axis polarization mode and the slow axis polarization mode in different refractive index rang of wavelength shitf,and no matter in which region 0f RI,the wavelength shift and the variati0n of the sur. rounding RI is approximate linear relation.At the same time,the peak of interference patterns is satisfied we11 with the theoretical expectation,the Bragg wavelength of FBG almost remains unchanged with the variati0n 0f the surrounding RI as shown in the inset of the Fig.4. 1・34 1.35 1.36 1.37 1.38 1.39 1.40 1.41 1.42 1.43 Refractive index Fig.4 Wavelength shitf of the proposed sens0r as a functi0n of the RI.The inset shows that the Bragg wavelength of the FBG has no shift. The RI experimental measurement was performed in a temperature—controlled environment,and the tem. perature variation was less than±0.1。C.But in practi. cal applications,the surrounding temperature was not invariable.The influence of temperature on the pro- posed RI sensor was also studied.In the temperature measurement of proposed RI sensor,the total MZI part Wavelength/nm Fig.5 Interference fringes variations of the PMF MZ1 with different surrounding temperature. Ⅱp/^lI∽II qH Wavelength/nm Fig.6 Interference fringes variation of the FBG with different surrounding temperature. 3_ 耄z. L 娄 : Temperature/ Fig.7 Wavelength shitf of the proposed sens0r as a function of the temperature. The resonant dip wavelengths of the MZI and FBG shows red shifts as the surrounding temperature increased as shown in Fig.5 and Fig.6,respectively. Fig.7 SHOWS the linear relationship of the proposed sen— sor against temperature.The temperature sensitivities 0f 56.66 pm/ ̄C and 9.28 pm/ ̄C f【】r the PMF MZI and FBG were obtained respectively.Therefore.here the RI and temperature can be measured simuhaneouslv by using follow matrix equation, = AA 。R] kVBCTkMz —kFSGkMzrr nL△ J ]第11期 曾红燕,沈常宇等:基于保偏光纤马赫曾德干涉结合布拉格光纤光栅的折射率传感器 1731 Where AT is the variation of the surrounding tem— perature,An is the variation of surrounding RI.kFBG and kFBGT are RI and temperature coefficients of FBG, respectively.kMzh and kMzIT are RI and temperature coefficients of MZI,respectively.AA FBG and△A Mz1 are the wavelength shifts of FBG and MZI,respective— ly.The△A FBG and the△A MzI are get by lineal fitting 0f the measured data as shown in Fig.4 and Fig.7.re— spectively.When the range of the RI is 1.3426 to 1.3692,the Eq.(7)can be described as。 【△几l 一. 1 1 0985 L_0.Jr-118.一.005666  3759. 。0.00928 I l△A… I(『、8” ) When the range of the RI is 1.3692 to 1.4191, and the Eq.(7)can be described as。 I△ 1 . 1 I[一.48 29591. 1△A ̄FBG.04482 0 05666 0 00928L_.. _] A 1c (、 9) 3 CoNCLUSION In conclusion,a high sensitivity fiber RI sensor based on PMF MZ1 was proposed.In the interference patterns,the interference resonant dip wavelength shows shifts with the increasing of the surrounding RI and temperature,Benefited from using PMF MZI struc— ture with an FBG.The variations of surrounding RI and temperature can be measured simultaneously.The proposed sensor shows the sensitivities of一1 1 8.38 nm/ RIU and 48.3 nm/RIU.The high sensitivity and the simple fabrication process shows that proposed sensor has a great potential for many sensing applications. 参考文献: Jiang L,Yang J,Wang S,et a1.Fiber Mach—Zehnder Interferome。 ter Based on Mieroeavities for High—Temperature Sensing with High Sensitivity[J].OptLett,2011,26(19):3753—3755. 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[1 1]Iadiciceo A,Cusano A,Cutolo A,et a1.Thinned Fiber Bragg Grat. ings as High Sensitivity Refractive Index Sensor[J].IEEE Photo- nics Technol Lett,2004,16:1149-1151. 112j Shu X,Gwandu B A L,IJiu Y,et a1.Sampled Fiber Bragg Grating for Simuhaneous Refraetiveindex and Temperature Measurement [J].Opt Lett,2001,26:774—776. 113j Ding J F,ZhangA P,SbaoLY,et a1.Fiber—Taper SeededLong— Period Grating Pair as a Highly Sensitive Refractive—Index Sensor lJ J.IEEEPhotonTechnolLett,2005,17(6):1247—1249. [14]付华,谢森,徐耀松,等.光纤布拉格光栅传感技术在隧道火灾 监测中的应用研究[J].传感技术学报,2013,26(1):133—137.A 曾红燕(1991一),女,中国计量学院光电 子研究所的学生,导师为沈常宇,主要从 事光纤传感方面的研究,952253414@qq. eonl; 沈常宇(1977一),男,博士,教授,入选浙 江省新世纪151人才工程第二层次,现 为光电学院光电信息科学与工程专业 负责人。2002年在陕西师范大学获得 光学硕士学位,2009年在浙江大学获得 光学工博士学位;主要从事光纤传感、 LED固体照明、非线性光学等方面研究; 李展霞(1978一),女,博士,副教授,2010 年获得上海理工大学博士,主要从事光 学材料、光纤传感研究。 袅

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