光纤传感系统

410工程与技术科学基础学科

Data fusion in multi-parameter measurement of

optical fiber sensors system

Jiang Junfeng, Liu Tiegen*, Zhang Yimo, Sun Jie, Li Chuan, Ding Sheng

(College of Precision Instrument and Opto-Electronics Engineering, Tianjin University, Key Laboratory of Opto-electronics Information and Technical Science(Tianjin University),

Ministry of Education, Tianjin, 300072, China

ABSTRACT

Multi-parameter measurement is important for building health monitoring. In this paper, we use fiber Bragg grating(FBG) and extrinsic Fabry-Perot interferometer(EFBI) sensor to realize the measurement of strain, temperature and vibration. A universal demodulate method, which combines the low coherence interference demodulation of EFBI and fiber Fourier transform demodulation of FBG, is proposed. Thus it is possible to extract the signals from FBG and EFBI parallelly with only a Michelson interferometer. A data fusion model is established to process the origin data to get more precise results and distinguish the different parameters. The system is tested on a equi-intensity cantilever, and the results of experiment show that strain and temperature can be obtained efficiently.

Keywords: Data fusion； fiber Bragg grating,；extrinsic Fabry-Perot interferometer； multi-parameter measurement,；

strain,；temperature

光纤传感系统

江俊峰，刘铁根，张以谟，孙杰，李川，丁胜

（天津大学精密仪器与光电子工程学院，教育部光电信息与技术重点实验室）

摘要：多参数测量对建筑健康监测至关重要。本文使用光纤布拉格光栅（FBG）和非本征型法布里－珀罗干涉(EFBI)传感器实现了对应变，温度和振动的测量。提出了一种新的解调方法，该方法综合了非本征型法布里－珀罗干涉仪（EFBI）的低相干信号解调和光纤布拉格光栅（FBG ）的傅立叶传输信号解调方法各自优点。因此，仅用一个迈克尔逊干涉仪就能同时提取这两种信号。为了得到更精确的结果和分辨不同的参数，建立了一个数据融合模型来对原始信号进行处理。通过在等强度梁上的试验，结果显示能够有效完成对温度和应变的同时测量。 关键词：数据融合；光纤光栅；非本征型法布里－珀罗干涉仪；多参数测量；应变；温度

1. INTRODUCTION

contact. tgliu@tju.edu.cn; phone 86-22-27409621; fax 86-22-27409621; College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin, 30072 ,P.R.China.

*

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In the field of building and pipeline monitoring, which require a sensor network to cover a large area, fiber optic sensors have attracted much interest in recent years. Fiber optic sensor has advantages in constructing of distributed sensor network or quasi-distributed sensor network, it can be linked by one optical fiber, while conventional sensors, such as electric resistance strain gage, need a lot of wires to connect. Furthermore, fiber optic sensors are immune to electromagnetic interference (EMI) and radio interference (RI), and are compact, which make it suitable for embedding in composite material or concrete. Among a great variety of optical fiber sensors, extrinsic Fabry-Perot interferometer(EFBI) and fiber Bragg grating(FBG) are regarded as the most promising sensors in the build health monitoring. These two kind of optical fiber sensors both can be used for multi-parameters measurement, particularly, for the simultaneous measurement of strain and temperature. The two kind of optical fiber sensors are generally used independently in different sensor systems, because the demodulation methods of them are different. A typical example is the method proposed by Xu.M. G［1］, who use dual-wavelength fibre grating sensors to discriminate the temperature and strain. The matrix equation is easy to solved, but the condition number of matrix equation is too big and sensitive to the measure error. A group of similar equations can be obtained for EFBI when it used for multi-parameter measure.

Here we will combine the character of EFBI and FBG to construct the fiber sensor system. The great difference between the two kinds of optical fiber sensors ensure the matrix equation have a reasonable condition number. A universal demodulation method is proposed to simply the whole sensor system.

2. PRINCIPLE OF OPERATION

2.1 Universal Demodulation of EFBI and FBG There are many kinds of demodulation methods for wavelength detecting of FBG［2］, such as using a

Coupler source scanning filter to track wavelength of FBG or a edge Light λC=1550 nmCoupler EFBI 1 EFBI n Coupler FBG1 FBG n Reflector optical filter to convert the information of wavelength to intensity. As far as EFBI is concerned,

Grin len the most typical demodulations are direct spectroscopy and low coherence interference, and the latter is more accurate and attracts much concern in recent year for its property of absolute measure［3］. Generally, FBG and EFBI sensor have been demodulated independently with different demodulation methods. But the Fourier transform demodulation for FBG introduced by M.A.Davis and A.D.Kersey［4］give the chance to build up a universal demodulation both for FBG and EFBI. The optical signal returned from FBG array, which consist of

Data acquisition and process system Scanning mirrorGrin len Reflector Beam splitter Photodetector Fig.1. Illustration of universal demodulation setup

FBGs with different Bragg wavelengths, comprises a series of weak narrow band spectrum components. The situation is similar to the application of Fourier transform spectroscopy. The interferogram obtained from a Michelson

Fixed mirror

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interferometer and reflective spectrum of FBG array make a Fourier transform pairs, which expressed as the following when the constant bias value of interferogram function is set to zero:

S(x)=I(σ)cos(2πxσ)dσ (1)

0∞

∫

I(σ)=S(x)cos(2πxσ)dx (2)

0

∫

∞

where x is the optical path difference, σ is the wave number, S(x) is the interferogram, I(σ) is the reflective spectrum of FBG array. Thus we can get the wavelength of FBG through Fourier transform. This demodulation has a common thing when compared to low coherence interferometer of EFBI, that is, both of them use a scanning interferometer to get signal.

The universal demodulation that we proposed based on this fact. Fig.1 is the illustration of setup for universal demodulation. The key component is the Michelson interferometer. One mirror of Michelson interferometer is fixed, another, which is termed as scanning mirror, is installed on a translate stage having a high precision step resolution. A computer is used to control the scanning mirror to scan. The Michelson interferometer will work as low coherence receiving interferometer for EFBI. When the origin cavity lengths of EFBI are different, a serial of EFBI sensors can be multiplexed. At the same time, the interferogram of light reflected by FBG can be obtained when scanning mirror moves. The signals from EFBI and FBG are obtained parallelly and then they are sent to computer for further data processing. If the scanning mirror of Michelson interferometer is fixed at a position, which makes interferometer unbalanced, and a tunable optical filter is connected to the optical fiber line, this demodulation setup can also be used to measure the dynamic strain applied on the FBG. Thus this demodulation will simplify the structure of sensor system.

2.1 Data fusion model

The first step of data processing is to process the origin data from the output of Michelson interferometer. The Fourier transform or Hilber transform is used to get the change of wavelength of FBG and low coherence interference is used to obtain the cavity length of EFBI.

When EFBI and FBG are used to the simultaneous measurement of strain and temperature, a matrix equation relating change of measurand to the variation of cavity length of EFBI and the wavelength shift of FBG can be expressed

?λB

=kε?ε+[kε?(αH?αF)+kT]??T+kεT?ε??T (3)

λB

?LcavityLGauge

=ε+(αH?αF)??T (4)

where λB,?λB are the Bragg wavelength of FBG and its variation. kε and kT are the strain-optic coefficient and thermo-optic coefficient of FBG, kεT is the cross-sensitivity coefficient, αH and αF are the coefficient of thermal expansion for host material and optical fiber. ?Lcavity is the change of cavity length of EFBI and LGauge is gauge length of EFBI. Thus the variation of measurand can be solved through Eq.(3) and (4) .

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3. EXPERIMENT

The experiment system is set up according to the Fig.1. A broadband light source with output power of 10 dBm was used. A translate stage with step precision of 50 nm is adopted to move the scanning mirror. A equi-intensity cantilever is used to provide the strain. In order to simplify experiment, only one EFBI and one

Strain (με) Load (Kg)

Fig.2. Strain of cantilever when load is applied

FBG are glued to the cantilever. The Bragg wavelength of FBG is 1551.36 nm. The length LGauge of EFBI is 1mm. Resistance strain gage is used to scale the strain of cantilever at room temperature, and then cantilever is put into a heater box. The strain-optic coefficient kε of FBG measured at 25oC is0.67×10?6/με, and

thermo-optic coefficient kT measured without strain is

6.3×10?6/°C. The coefficient of thermal expansion of cantilever is

11.5με/°C, and αF=0.5με/°C. Those parameters value was used to get the strain and temperature with the

measured wavelength shift and change of cavity length, which decided by Fourier transform and low coherence interference. The experiment is carried out at temperature of 65 oC and different strain. And the results of strain are showed in the Fig.2. The linear of sensor is good. The mean measured temperature is 65.4 oC. It is satisfactory to some extent.

4. CONCLUSIONS

A universal demodulation system is proposed, which combined the low coherence interference of EFBI and Fourier transform of FBG. With this method, the demodulation system can be simplified. The difference between sensitivity of EFBI and FBG helps to create a well conditioned matrix equations, which is important for simultaneous measurement of strain and temperature. The primary experiment showed that the sensor system is feasible and a further work should be concentrated on the improvement of data processing.

ACKNOWLEDGMENTS

This work is supported by the National Nature Science Foundation of China(60077023)

REFERENCE

［1］ Xu.M.G,

Archambault. J.L,

Reekie. L, et al, “Discrimination between strain and temperature effects using

dual-wavelength fibre grating sensors,” Electro. Lett.30, pp. 1085 – 1087,1994.

［2］ A.D. Kersey, “A review of recent developments in fiber optic sensor technology,” Optical Fiber Technology,2,

pp.291-317,1996

［3］Rao.Yunjiang, David A Jackson, “Recent progress in fiber optic low coherence interferometry,”

Meas.Sci.Technol,7,pp.981-999,1996

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［4］ M.A.Davis, A.D.Kersey, “Application of a fiber Fourier transform spectrometer to the detection of wavelength

encoded signals from Bragg grating sensors,” Journal of Ligthwave Technology,13,pp.1289-1295, 1995

［5］ Li Chuan, Zhang Yimo, Liu Tiegen, Ding Yongkui, “Filtering researches of strain-tuned fiber Bragg gratings”,

Journal of Optoelectronics·Laser,13,pp.1237-1240, 2002 资助类别：国家级

资助来源：国家自然科学基金

课题名称及编号：多光纤传感数据融合的理论与方法 60077023