Study of Optical Fiber Grating Sensor Systems and its Applications in Civil Engi

【前言】Study of Optical Fiber Grating Sensor Systems and its Applications in Civil Engi由文秘帮小编整理而成，但愿对你的学习工作带来帮助。Grating Sensor System is mainly build up of light source, optical fiber grating sensors and the signal demodulation system. A broadband light source provides light energy for System. Optical Fiber Grating Sensor deals with the measurement informati...

Abstract.Fiber Bragg grating is one of the most important inventions of modern measurement field at the end of the twentieth Century and become the research focus of current sensor. The Optical Fiber Grating Sensor System is mainly build up of light source, optical fiber grating sensors and the signal demodulation system. In this paper, how can achieve proper relations between parts are attained at the premise of improve measurement precision and satisfy the needs of real time measurement is studied.

Key words: Fiber Bragg grating, Sensor system, Application

1. Introduction

Fiber Bragg grating (FBG) sensor is one of the current heat research fields in the optical fiber grating sensor technique fields. Optical fiber grating sensors have advantages of corrosion preventing and resisting electromagnetic to interfere. All that makes them work normally under abominable environment such as rain, snow and fog terms. It is widely applied in many fields such as the structure monitoring of the large compound material and the concrete, the performance monitoring of the intelligent materials, electric power industry, medicine, chemical engineering etc.

2. Optical Fiber Grating Sensor Systems

Grating Sensor System is mainly build up of light source, optical fiber grating sensors and the signal demodulation system. A broadband light source provides light energy for System.

Optical Fiber Grating Sensor deals with the measurement information through source of light. The outside information to be measured reflected in real time. The characteristic of Fiber Bragg grating Sensor Technology is a core having refractive index periodic changes in the structure of the fiber. The basic principle of Optical Fiber Grating Sensor is that Fiber Bragg grating can converts the light into a central peak plus side peaks, when incidence angle or azimuth is varied, the light of different wave band will be reflected. The incident light is a beam of laser with wavelength λB= 2neffΛ. The relation between Bragg Grating drift, strain and temperature is as follows: Δλ λ=(1-Pe)ε+(ξ+α)ΔT, ε is axial strain, ΔT is temperature difference, Pe is effective stress optical coefficient, αis thermal expansion coefficient, ξ is thermo-optic coefficient. The optical fiber grating for installation in the structure to be measured can be detected via reflection grating wavelength changes in Prague, deduced from the measured strain and temperature value structure. The essence of Optical Fiber Grating Sensor is optical fiber, its diameter is about 125μm, its main components is SiO2, fragile, poor anti-shearing capability, so the Optical Fiber Grating Sensor and optical fiber transmission line is relatively weak.

3. Optical Fiber Grating Sensor can gauge the physical quantity such as temperature and strain directly.

Fiber Bragg grating wavelength for simultaneous strain and temperature sensitive, simultaneous temperature and strain caused by fiber grating wavelength shift, so that by measuring the fiber grating wavelength mobile unable to distinguish between strain and temperature. So solving the cross sensitivity, temperature and stress sensor distinguishing measurement is practical. Through certain technologies to measure the stress and temperature change to realize temperature and stress discriminating measurement. These technical basic principle is the use of two or two segments with different temperature and strain response sensitivity of fiber Bragg grating double grating temperature and strain sensor, by identifying 2 of fiber Bragg grating temperature and strain sensitivity coefficient, using 2 two yuan of linear equation solution of temperature and strain. Measurement technology can be divided into two categories: Multiple fiber Bragg grating measurement and single fiber Bragg grating measurement. Multiple fiber Bragg grating measurement mainly consist of mixed FBG/ long period gratings method, method of double period fiber gratings, method of Fiber Bragg grating／F-P integration and reuse, method of double FBG overlapping writes, Each method has its advantages and disadvantages. Method of FBG／LPG simple demodulation, but it is difficult to ensure the measurement is the same point. Its accuracy is 9×10-6，1.5℃. Method of double period fiber gratings can ensure the measuring position, improve the measurement accuracy, but the grating strength is low, difficult signal demodulation. The feature of Fiber Bragg grating／F-P integration and reuse Method is sensor temperature stability, the mini volume, convenience to take and high precision, Its accuracy is 20×10-6，1℃, but cavity length of F-P adjustment difficulties, signal demodulation complex. Method of double FBG overlapping writes high precision, but grating writing difficulties, signal demodulation complex.

Single Fiber Bragg grating sensor measurement mainly consist of package with a different material single Fiber Bragg grating sensor measurement method, utilize different FBG combination method, and method of prefabricated strain. Package with a different material single Fiber Bragg grating sensor measurement method is through use of certain organic compounds on temperature and stress of different response to increased fiber grating on the temperature or stress sensitivity, overcome the cross sensitivity effect. This kind of method is easy to manufacture, but its difficulties to choose polymer material. Utilize different FBG combination method is writing the grating in different refractive index and temperature sensitivity temperature sensitivity and or different depart material concentration of 2 kinds of optical fiber connection, using different refractive index and temperature sensitivity of different discriminating measurement. This kind of method is simple to extract, and demodulation is wavelength encoding which can avoid stress concentration, but it has the problem such as high wastage, narrow measuring range, the welding areas is easy broken. Method of prefabricated strain is optical fiber grating to exert a certain strain, the strain under the condition of fiber Bragg grating part fixedly pasted on a cantilever beam. After releasing stress, stick the portion of the optical fiber grating deformation recovery, the reflected wavelength constant; and paste on the cantilever beam section deformation can not be restored, which led to the portion of the optical fiber grating reflection wavelength change center. Therefore, these Fiber Bragg gratings have two reflection peaks, one is sensitive to the strain and temperature both, and the other is only sensitive to the temperature, Wavelength drift of these two reflection peaks can measure temperature and strain at the same time.

4. Signal demodulation

In Optical Fiber Grating Sensor Systems, a part of signal demodulation is optical signal processing to realize the transformation the wavelength of the optical signal to electric parameter complete the wavelength of the optical signal to electric parameter conversion, the other part of signal demodulation is signal processing realize the electric parameter processing, withdraw the outside information and show by familiar style. Signal demodulation is the key of demodulation. The most direct detection apparatus of Optical Fiber Grating Sensor Center reflection wavelength is spectrometer. The advantage of this method is simple structure and easy for operation, its drawback including large size, high in price, low precision,and can not be directly output corresponding to the wavelength change of signal. It’s far from enough to meet the practical automatic control demand. So people study and put forward a variety of demodulation method to realize fast and accurate acquisition for the signal. There are filter method, interferometry, tunable narrowband light source method, and dispersion method. Filter method includes bulk filtering method, grating matching filter method, tunable F-P filter method. Element of bulk filtering method is Waveguide Multiplexer. The working principle of the light emitted from the coupler is divided into two beams, intensity, a beam of and in relation to the wavelength of the filter; the other beam is used as the reference beam, two emergent light beams through photoelectric detector into electrical signals, processed to eliminate optical power change, finally, get with the center wavelength of a fiber grating on output value. The method can realize the dynamic and static parameter measurement, resolution375x10-6. The measurement of dynamic strain response speed of not more than 100Hz matched grating filtering method is the use of other FBG or band-pass filtered device, in the drive element under the action of FBG wavelength change tracking, and then, by measuring the driving element drive signal to obtain the measured stress or temperature. This method has the advantages of simple structure, good linearity, resolution0.4×10-6, can achieve the static measurement. But this method has some shortcomings, as 2grating should matching strictly, and the measurement range was relatively narrow. Tunable F-P filter is a method of sensing array FBG reflected signal into the tunable optical fiber F-P filter ( FFP ), adjusting the FFP of the transmission wavelength and FBG reflection peak wavelengths, filtering the transmission intensity reaches the maximum value, is driven by FFP voltage transmission wavelength available FBG reflection peak wavelengths. Scan plus disturbance signal component wavelength locking loop, its stress resolution up to 0.3 x 10-6. The demodulation method can realize the dynamic and static measurement. Since FFP filter cavity tuning range is very wide, it can realize multi sensor demodulation. But the high precision FFP higher cost. Filter demodulation method has the advantages of simple structure, but it is difficult to further improve the sensing accuracy. Interference method has higher precision, can greatly improve the sensor resolution. Tunable narrowband light source demodulation method can obtain a high signal-to-noise ratio and resolution, experiment obtained minimum wavelength resolution of about 2.3pm, corresponding to the temperature resolution of approximately 0.2 degrees C, but because of the current fiber laser tunable range of stability and is not ideal, to a certain extent restricts the number of fiber Bragg grating sensor and scope of use.

5. applications of optical fiber grating sensor in civil engineering.

Optical fiber sensors can be embedded in the concrete, carbon fiber reinforced plastic and composite materials, used to test the stress relaxation, the construction stress and dynamic load stress to evaluate bridge short, construction stage and the long-term operating state of the structure performance.

5.1 Fiber optical raster intelligent vehicle load monitoring is the bridge, road, tunnel structure health monitoring and general load is an important testing tool, is used to effectively evaluate the bridge safety state highway, serving the basic data source, and based on the traditional electrical vehicle weighing ( weighbridge ) in durability indicators to meet international engineering need. Optical fiber grating smart dynamic vehicle scale, make full use of the high durability of the fiber Bragg grating intelligent sensing characteristic, according to the structure type sensing device. The structure thought, subtly changing load process, and uses no glue packaging technology, making the entire truck without durability restrict department, and has very high precision, repetitive, fatigue properties.

5.2 High durability welding type optical fiber grating strain sensor. It adopt FRP no plastic package techniques, overcome the traditional problems of durability. It can change its gauge length in response to request, the least of gauge length is 1～2cm, have the advantages of simple engineering layout , detachable, large measuring range, good durability, convenient layout, high accuracy and so on. FBG sensor has the merits of using conveniently and non-vitiating, can be used for surface of steel structure strain monitoring, using embedded parts or rivets can also be used for concrete surface should be used for testing, bridge, building, hydraulic and other civil engineering construction, the test completion and operation monitoring of surface strain sensor.

5.3 Typical sensor performance curve GFRP packaged fiber Bragg grating strain sensor. This product also can change its gauge length in response to request, especially for large strain range monitoring, and can be used to monitor the mixed concrete structure of internal crack information. The main performance index of this sensor as follow: measure range is greater than 5000 ε, testing precision is 2～3 ε, repeatability error is less than 0.6％, linearity error is less than 0.9％, sensitivity coefficient is 7.7~8.0E-7 （1.18~1.2ε /pm, hysteresis is less than 0.5%, in 1000 micro strain cycle 100000 performance does not change. There is no performance changes test 4 months working the environment of corrosion environment, temperature of 35 degrees C, salt fog for 3.5%NaCl solution of the salt fog box.

5.4 Influence of the prototype packaged fiber grating temperature sensor using external shielding method developed by the embedded, a heat conductor, with fast heat, without outside influence, keep the distributed sensing capability, good durability, good reliability. This product is especially suitable for not using the electrical temperature sensor temperature field，can be used to power stations, transmission lines, buried pipeline, civil construction monitoring, the tests on completion and operation monitoring temperature monitoring. At the same time, it can also be used as optical fiber grating strain sensor temperature compensation device.

5.5 Fiber grating ice pressure sensor ice pressure is high latitudes sea crossing, cross river long-span bridge in winter one of the important loads, the monitoring information is used to analyze this kind of structure fatigue, integrity and safety evaluation of the direct information. The sensing device has a temperature self compensation compensation, measurement and load point irrelevant, linearity and repeatability, high precision, suitable for ocean platform ice pressure test.

6. A Summary

According to these properties, the Fiber Bragg Grating will have a good prospect in application in the field of high buildings, girders, dams, nuclear reactors and etc. The study on FBG (Fiber Bragg Grating) promotes the development of FBG sensors. Fiber Bragg grating sensor and network technology maturity can meet most bridges, large construction and pipeline health monitoring level, The series products have already spread to all over various trades, it is still necessary to try and improve Optical Fiber Grating Sensor skills.

7. References

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[3] Zhao Yong,Liao Yan-biao .Discrimination methods and demodulation techniques for fiber Bragg grating sensars[J].Optics and Lasers in Engineering,2004,41(1):1-18.

[4] Zhi Zhou, OU Jinping. Techniques of temperature compensation for FBG strain sensors. Proceeding of Asian Pascifc Fundamental Problems of Opto- and Microelectronics (APCOM 2004), Khabarovsk, Russia，September 13 - 16, 2004:465-471

[5] L C S Nunes, L C G Valente, A M B Braga. Analysis of a demodulation system for fiber Bragg grating sensors using two fixed filters[J] .Optics and Lasers in Engineering, 2004,42, 42 :529-542 .