Research on Lower Coherence Interferometer andcorresponding optic fiber sensorCheng-yu Hong Jian-hua YinDepartment of Urban and Civil Engineering Department of Civil and Structural Engineering Shenzhen Graduate School, Harbin Institute of Technology Hong Kong Polytechnic UniversityShenzhen, P. R. China, 518055 Hong Kong. P. R. ChinaE-mail：joeyhcy@ E-mail：cejhyin@.hkYou-hua Fan Chao WangDepartment of Urban and Civil EngineeringShenzhen Graduate School, Harbin Institute of Technology Bookham companyShenzhen, P. R. China, 518055 Shenzhen, P. R. China, 518055E-mail：yhfan@ E-mail：wangchao_prc@AbstractApplying lower coherence interferometer executes scan of optic fiber sensor under different environmental state. Factors influence accuracy of results can be obtained through comparison between above tests, which are shown in details in this paper. Repeating test of optic fibers is completed as a part of calibration, an accuracy of 4um can be achieved as well. Data acquisition and data analysis are accomplished by the software of Labview. Installing reference point on signal arm to eliminate the deviation of initial scan time and reduce the error of lower coherence system.Keywords：optic fiber sensor，repeating test，Labview，reference point1. IntroductionLower coherence interferometer is widely used in civil engineering area[1], and its principal is similar to Mach–Zehnder interferometer [2], meanwhile, the strain sensing array has been produced [2]. New apparatus is produced according to Michelson interference principal as is shown in Figure 1. Wide spectrum light split into signal arm from laser source, generate signal R1 and R2 by the sensors with different reflectivity along signal arm, as is shown in Figure 1. An optical path difference b generated. Signal R1 and R2 reflected passes through the 2×2 coupler with power ratio 50: 50, divided into two branches, one is to the reference arm, and the other is to the mirror. R1 and R2 will be reflected both by reference arm and mirror on the electromotor, split into coupler again to achieve PIN[3].Interference willFigure 1: Working principal of optic fiber sensor based on Michelson white light interferometerbe generated as soon as light path differences satisfy equation 2-1:T c 0102n L n L L +−≤[4] (2-1)0n is the refractive index, c L is the lowest length of interference. Position of the highest peak, whichlocates in the center of the interference peak, corresponds to the exact optical path matching of these two arms. That is T 0102n L n L +=[5]. Light path difference between two arms will change along with scan continuing.Light power output satisfies the following equation:{}I R P +P (t)-(t)]ref dut ref dut φφ=[6](2-2)As is shown in equation 2-2, I is power of interference signal finally. R is responsibility of diode; P ref is power of reference light reflected, P dut is the power of the light reflected by mirror, (t)ref φ is phaseof reference light reflected, (t)dut φis phase of light reflected by mirror.Speed of electromotor is invariable; meanwhile interval of peaks generated can be recorded by computer, thus changing in displacement of the section can be detected through the product of interval and electromotor speed.2. Repeating TestScanning is carried out on fibers under different environmental conditions: first one is encapsulated in aluminum slot with epoxy[2] as is shown in figure 2, material for encapsulation is epoxy; the other one sticks to desk directly with adhesive tape, no force applied on both of these two conditions.Figure 2: Fiber encapsulating into aluminum slotBased on the working principal of lower coherence Michelson interferometer, calibration test is carried out in lab. Continue scanning on the same fiber under no force; and execute data acquisition by Labview software. Comparison work can be carried out in this way. Keep repeating scan for five times on a fiber with five sensors which have no encapsulation, therefore, three differences of distance between twoFiber encapsulated with epoxy in aluminum slotOptic Fiberadjacent sensors can be obtained, as is shown in figure 3 to 5 , fiber isn’t encapsulated in aluminum slot: According to three scan results in figure3-5, maximum value can be obtained separately from above results. They are 7.3709mm , 6.0436mm , and 8.7050mm , which tell the difference of adjacent section on fiber. Meanwhile minimum value can be obtained through figure1-3: 7.3600mm , 6.0277mm , and 8.6815mm . Therefore, differences can be reached through calculation.Figure 3: First section of scan result Figure 4: Second section of scan resultFigure 5: Third section of scan result Figure 6: First section of encapsulate fiberFigure 7: Second section of encapsulate fiber Figure 8: Third section of encapsulate fiberMaximum differences are 9.1um , 15.9um and 13.5um separately. Scan result should be stable as neither force nor displacement applied. Scan test is continuing as soon as fiber is encapsulated in aluminum slot, corresponding results are shown in figure 4-6. Scan test is carried out for eight times on the same fibersensor, maximum values of repeating tests are 17.0654mm, 9.4818mm, and 17.5488mm. The corresponding minimum values are 17.0610mm, 9.4769mm, and 17.5419mm. It is evidently that the differences reduced a lot as environmental influence reduced to minimum. Effect of epoxy increases accuracy to higher level. The maximum gap between maximum and minimum is 6.9um, the average of these data is right the average of maximum and minimum. Accuracy is less than 4um if average value is taken as the final result.It is apparently that environmental influence is the main reason that higher accuracy can’t be reached through scanning on fibers without encapsulation. Therefore, more attention should be paid on environmental influence when carrying out fiber scanning.3. Improvement on interferometer systemElectromotor as a part of lower coherence system is significant to the result accuracy, the initial time of electromotor may vary from time to time, and following scan results can be obtained if difference of initial time exists according to figure 9: the update way to improve accuracy is installing sensor “a” on signal arm as is shown in figure.1, following results can be obtained in figure 10, scan result should be shown in figure 10 theoretically if the reference point “a” installed on signal arm. After improvement, the result is irrelevant to the start time deviation, which could be as accurate as the value from peak-to-peak calculation.Fig.9 Start time variety of scan Fig.10 Final result after improving4. Discussion on electromotor of low coherence interferometerApplying electromotor in lower coherence interferometer system will be one of the error sources as speed of electromotor may not be as stable as we believe. Labview software is the data acquisition and analysis software. Corresponding relationship between length of stage T and sensor length b in figure 1 is adopted to investigate length difference of adjacent sensors [6]. The product of scan speed and interval that adjacent peaks appear is b in figure 1. Unstable speed of electromotor maybe caused by electromotor and changing in friction of scan stage, therefore it should be noticed that keeping the scan stage T clean in figure 1 and maintain good quality of electromotor is the crucial section to get high accuracy result.5. ConclusionsComparison between fibers with and without encapsulation shows following conclusions:1 Environmental factor is the main reason that higher accuracy result of scan can’t be reached from thetwice repeating scan tests. Error decreases from more than 10 micro strains without encapsulation to less than 4 micro strains with encapsulation with epoxy.2 Adoption of reference point can reduce the deviation in initial time of electromotor, which will decreasethe fluctuation of interferential peaks as is shown in figure 9 and figure 10.3 Speed of the electromotor is another factor that influences accuracy of scan result, which should be stableall the time, meanwhile maintains cleanness of scan stage is also important.References[1] D.Inaudi, A.Elamari, L.Pflug, N. Gisin, J. Breguet, S. Vrupillot. (1994) Low-coherence deformation sensors for themonitoring of civil-engineering structures, Sensors and Actuators A 44 pp. 125-130[2] Qingbin Li, Libo Yuan, Chongjie Liu, Yuxin Jie, Guangxin Li, (2007) Twin multiplexing strain sensing array basedon a low-coherence fiber optic Mach–Zehnder interferometer, Sensors and Actuators. A 135 pp. 152–155[3] Yang Zhao, Farhad Ansari. 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