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Low Cost Fiber Bragg Grating( FBG) Sensors for Structural Health Monitoring

INITIAL PROJECT REPORT

CONTENTS:

1. ABSTRACT

2. DOCUMENTATION OF FBG SENSORS

a. WHAT IS FIBER BRAGG GRATING? b. CURRENT APPLICATIONS OF FBG’S c. TIMELINE FOR COMMERCIALIZATION OF VARIOUS FBG APPLICATIONS d. INDUSTRIAL APPLICATIONS WHERE FBG’S HAVE SPECIFIC

ADVANTAGES e. FIBER BRAGG GRATINGS AS SENSORS f. ADVANTAGES AND DISADVANTAGES OF FBG’S OVER TRADITIONAL

SENSORS: g. SENSORS BASED ON FBG FOR INFRATRUCTURES h. THEORY OF FBG SENSORS: SENSOR DESIGN

3. PROJECT SUMMARY

4. SPECIFIC PROJECT TASKS a. INTRODUCTION: b. DYNAMIC STRAIN MEASUREMENT USING FBG SENSORS c. CRACK DETECTION USING FBG SENSORS

5. CONCLUSION 6. REFERENCES

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ABSTRACT

In the fifth five year plan, India is expected to invest $ 1 Trillion in its various infrastructure projects. Today's civil structures depend tremendously on use of sensing technology for real time structural health monitoring to measure a multitude of parameters like strain, temperature, pressure and chemical and biological effect of environment. Currently, however, India's infrastructure lacks a rigorous mechanism for structural health monitoring. It is mostly done through visual inspections of the structure. As we build future structures which have to work safely under extreme weather conditions, it becomes imperative that these structures are inherently smart and sense their own structural health so their safety is never compromised. The structural health monitoring (SHM) is very important and definitely demanded for safely working of engineering structure such as concrete structures. This project aims at modelling Fiber Bragg Grating(FBG) based low cost structural health monitoring sensors.

WHAT IS FIBER BRAGG GRATING?

A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber core, which generates a wavelength specific dielectric mirror. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.

Fiber Bragg Gratings are made by laterally exposing the core of a single-mode fiber to a periodic pattern of intense ultraviolet light. The exposure produces a permanent increase in the refractive index of the fiber's core, creating a fixed index modulation according to the exposure pattern. This fixed index modulation is called a grating. At each periodic refraction change a small amount of light is reflected. All the reflected light signals combine coherently to one large reflection at a particular wavelength when the grating period is approximately half the input light's wavelength. This is referred to as the Bragg condition, and the wavelength at which this reflection occurs is called the Bragg wavelength. Light signals at wavelengths other than the Bragg wavelength, which are not phase matched, are essentially transparent. This principle is shown in Figure.

Working of FBG

Ref: Fibre Bragg Grating Sensors Technologies,

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The following image shows the structure of a positiverefractive index profile of the fiber core shows the change of the refractivespectral response on the Fiber Bragg Grating shows how the incident broadband signal is split into the transmitted and reflected components, about the Bragg wavelength.

Ref: http://en.wikipedia.

CURRENT APPLICATIONS OF FBG’S

Communications:

The primary application of fiber Bragg gratings is in optical communications systems. They are specifically used as notch filters. They are also used in optical multiplexers and optical circulator, or Optical Add-Drop Multiplexer (OADM).

Sensors:

Fiber Bragg gratings can then be used as direct sensing elements for strain and temperature. They can also be used as transduction elements, converting the output otemperature change from the measurand, for example fiber Bragg grating gas sensors use an absorbent coating, which in the presence of a gas expands generating a strain, which is measurable by the grating. Technically, the absorbent material is the sensing element, converting the amount of gas to a strain. The Bragg grating then transduces the strain to the change in wavelength.

Germany

image shows the structure of a positive-only index change Fiber Bragg Grating. The refractive index profile of the fiber core shows the change of the refractive index along the core. The spectral response on the Fiber Bragg Grating shows how the incident broadband signal is split into the transmitted and reflected components, about the Bragg wavelength.

Figure: Spectral Respose of FBG

http://en.wikipedia.org/wiki/Fiber_Bragg_grating

CURRENT APPLICATIONS OF FBG’S

The primary application of fiber Bragg gratings is in optical communications systems. They are specifically used as notch filters. They are also used in optical multiplexers and demultiplexers with an

Drop Multiplexer (OADM).

Fiber Bragg gratings can then be used as direct sensing elements for strain and temperature. They can also be used as transduction elements, converting the output of another sensor, which generates a strain or temperature change from the measurand, for example fiber Bragg grating gas sensors use an absorbent coating, which in the presence of a gas expands generating a strain, which is measurable by the grating.

ically, the absorbent material is the sensing element, converting the amount of gas to a strain. The Bragg grating then transduces the strain to the change in wavelength.

only index change Fiber Bragg Grating. The index along the core. The

spectral response on the Fiber Bragg Grating shows how the incident broadband signal is split into the

The primary application of fiber Bragg gratings is in optical communications systems. They are demultiplexers with an

Fiber Bragg gratings can then be used as direct sensing elements for strain and temperature. They can also f another sensor, which generates a strain or

temperature change from the measurand, for example fiber Bragg grating gas sensors use an absorbent coating, which in the presence of a gas expands generating a strain, which is measurable by the grating.

ically, the absorbent material is the sensing element, converting the amount of gas to a strain. The

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TIMELINE FOR COMMERCIALIZATION OF VARIOUS FBG APPLICATIONS

HIGH PERFORMANCE REQUIADVANTAGES OVER OTHER MATERIALS

Aerospace

• Aircraft structures, wing and hull (embedment in composites, light weight)• Spacecraft (nosafety objections,EMC)• Airship (electrical insulation,lightning safe)

Energy industry (electrical insulation, immunity to electromagnetic interference)

• Power generators, transformers, switches (multi• Wind power stations (embedment, lightning safe, multiplexing)• Superconductors, nuclear fusion

Transportation

• Railway overhead contact lines, Railway pantographs (electrical insulation)

Geo-technical &civil engineering (longtransmission lines, multiplexing)

• coal mining, petrol & gas exploration (explosion• Rock-bolts / anchors

Opto-chemical monitoring

INE FOR COMMERCIALIZATION OF VARIOUS FBG APPLICATIONS

HIGH PERFORMANCE REQUIREMENT INDUSTRIES WHERE FBG’S HAVE SPECIFIC ADVANTAGES OVER OTHER MATERIALS:

Aircraft structures, wing and hull (embedment in composites, light weight) Spacecraft (nosafety objections,EMC) Airship (electrical insulation,lightning safe)

ustry (electrical insulation, immunity to electromagnetic interference)

Power generators, transformers, switches (multi-sensor: T, ε, vibr., flow, H2, ..) Wind power stations (embedment, lightning safe, multiplexing) Superconductors, nuclear fusion experiment (electrical insulation,EMC)

Railway overhead contact lines, Railway pantographs (electrical insulation)

technical &civil engineering (longtransmission lines, multiplexing)

coal mining, petrol & gas exploration (explosion-proof, remote sensor)

INE FOR COMMERCIALIZATION OF VARIOUS FBG APPLICATIONS

S HAVE SPECIFIC

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Figure: Current Applications of FBG

Ref: Wolfgang EckeInstitute of Photonic Technology

FIBER BRAGG GRATINGS AS SENSORS

Figure: Current Applications of FBG

Wolfgang EckeInstitute of Photonic Technology -IPHT JenaAlbert-Einstein-Str. 9, 07745 Jena, Germany

FIBER BRAGG GRATINGS AS SENSORS

. 9, 07745 Jena,

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Since their fortuitous discovery by Ken Hill back in 1978 and subsequent development by researchers at the Canadian Research Center, United technologies, 3M and several others, intra-core fiber gratings have been used

Sensing extensively in the telecommunication industry for dense wavelength division multiplexing, dispersion compensation,laser stabilization, and erbium amplifier gain flattening, mostly at the 1550 nm, C-band wavelength range.

However, given their intrinsic capability to measure a multitude of parameters such as strain, temperature, pressure, chemical and biological agents, combined with their flexibility of design to be used as single point or multi-point sensing arrays and their relative low cost, also make of FBGs ideal devices to be adopted for a multitude of different sensing applications and implemented in different fields and industries.

FBG-based sensors have been developed for a wide variety of mechanical sensing applications including monitoring of civil structures (highways, bridges, buildings, dams, etc.), smart manufacturing and non-destructive testing (composites,laminates, etc.), remote sensing (oil wells, power cables, pipelines, space stations, etc.), smart structures (airplane wings,ship hulls, buildings, sports equipment, etc.), as well as traditional strain, pressure and temperature sensing.

ADVANTAGES AND DISADVANTAGES OF FBG’S OVER TRADITIONAL SENSORS:

§ Immunity against, i.e., applicable in-

• Electro-magnetic fields, high voltage, lightning-Explosive

• chemically aggressive + corrosive media-High

• low temperatures-Nuclear / ionising radiation environment

§ Light-weight, miniaturised, flexible; low thermal conductivity

§ Non-interfering, low-loss, long-range signal transmission ("Remote Sensing")

§ Multiplexing capability ("Sensor Networks")

§ Embedding in composite materials ("Smart Structures")

§ Wavelength encoded –transferable measurement, neutral to intensity drifts

§ Mass producible at reasonable cost

§ Durable to high strain 5..6% ("Draw Tower Gratings", with any kind of coating)

§ High and low temperatures (4 K .. 900 °C)

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SENSORS BASED ON FBG FOR INFRATRUCTURES

Bare FBG shows good compatibility with infrastructures, but due to its fragility, bare FBG without encapsulation (package) is not proper to be directly applied in practical infrastructures. Otherwise, we have to develop special in-situ installation and protection techniques for bare FBGs.

Unfortunately, the “perfect” installation techniques often conflict with the in-situ construction, or it can not meet the demand of critical schedule of construction. Besides, in order to make full use of FBG, we should develop indirect sensors device based on FBG and combine the FBG and construction materials. Many sensors based on FBG have been developed. Under the support of several projects related to heath monitoring, Harbin Institute of Technology, China, has developed some practical packaged FBG sensors, indirect sensor devices and FRP-combined sensors, which are commercially available.

Encapsulated FBG strain sensor for embedment

In order to overcome the embedment of FBG strain sensor in concrete structures, HIT has developed the technique of mental tube encapsulated FBG strain sensor, depicted as figure.

Figure 1 Picture of mental tube encapsulated FBG strain sensor

Ref: Zhi Zhou and Jinping OU. Development of FBG sensors for Structural Health Monitoring in

civil infrastructures. Proceeding of North American Euro-Pacific Workshop “Sensing Issues

in Civil Structural Health Monitoring” , 2004, Waikiki Beach, Oahu ,Hawaii ,USA

THEORY OF FBG SENSORS

SENSOR DESIGN

The fiber Bragg grating is a device commonly used in telecommunications and sensor technology. Fiber gratings are formed by a periodic change of the fiber cored refractive index in direction of propagation of optical radiation. In principle, the fiber Bragg grating acts as a spectral filter that reflects particular wavelengths of light near Bragg resonance wavelength and the rest of the optical signal spectrum is being released. The Bragg resonant wavelength is given by:

������������������������=2����������������Λ (1)

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where �Bragg is the Bragg resonant wavelength, neff is the effective refraction index, and Λ is the periodic variation of the FBG. FBGs used in sensors mostly rely on the spectral analysis of reflected light wavelengths. The Bragg resonant wavelength is determined by various factors applied on the FBG, which affect effectively refractive index or grating periodic variation; therefore, it is an indirect measurement resulting from modifying physical or geometrical properties of the FBG. Among the affected factors we have temperature, mechanical deformation (e.g., stretching, pushing, bending, and applying shear stress) to the fiber Bragg grating. In real applications, it is difficult to separate the effects of measured and parasitic variables that affect the same parameter (e.g., when the fiber Bragg grating deformation is measured, temperature also affects reflected light wavelengths). Pressure measurement is always based on the deformation of some sensing part (typically the membrane), which is afterwards measured by principles described in the first section. Applied stress on the fiber Bragg grating in the direction of the fiber axis results in the extension of its physical dimensions and in the change of the periodic variation; however, the influence of temperature also affects physical dimensions due to thermal expansion.

Figure: FBG Sensor Design

Ref: Design of a Pressure Sensor Based on Optical Fiber Bragg Grating Lateral Deformation

Frantisek Urban 1, Jaroslav Kadlec 1,*, Radek Vlach 2 and Radek Kuchta 1

APPLICATIONS OF FBG SENORS

FBG sensors have been applied in many infrastructures. HIT have developed intelligent monitoring system based FBG and applied it in several projects as follows. And more than 2000 FBGs has been used

Intelligent monitoring systems based on FBG:

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Practical devices for monitoring system based on FBGsensors Software of intelligent monitoring

system based on FBG sensors

Figure Intelligent monitoring system based on FBG sensors

Ref: Zhi Zhou and Jinping OU. Development of FBG sensors for Structural Health Monitoring in

civil infrastructures. Proceeding of North American Euro-Pacific Workshop “Sensing Issues

in Civil Structural Health Monitoring” , 2004, Waikiki Beach, Oahu ,Hawaii ,USA

FBG sensors applied in Bridges

Due to that the bridges is the key part of transportation, lots of large-span bridges are under construction in developing countries as well as China.

With the development of structural health monitoring, the bridge owners realize the importance of adding structural health monitoring system to the bridges under construction in order to avoid disastrous tragedy to happen. OU has developed several structural health monitoring systems based on FBG sensors to be applied in the several large-span bridges, shown as figure9-12, to monitor the performance of the bridges under construction and in service.

Figure: Songhua River Bridge in Figure: Dongying Yellow River Bridge

Heilongjiang (2003) in Shandong (2003, over1800 FBGs used)

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PROJECT SUMMARY

SPECIFIC PROJECT TASKS

INTRODUCTION:

The optical fiber sensors for the measurement of strain have been under development for a number of years and a lot of sensing techniques have been established. Moreover, the optical fiber sensors have been extensively employed as real-time damage detectiofiber sensors, with flexibility and small size, areshapes to embed within the structural material for the purpose offeature of optical fiber sensors is their inherent ability to serve as both the sensingtransmission medium. This allows the conventional instrumentation to be located remotely from themeasurement site and it is especially useful for remote monitoring of the condition of the bridges.

Many researches have been conducted on the implementation of FBG sensors for monitoring of composite materials and concrete structures. The technology has been experimented and used in aconstruction sites to monitor aircrafts, bridges,results. However, more studies are needed to investigate theterm stability and durability of the FBG sensor

1. DETAILED DOCUMENTATION

3. HARDWARE AND EMBEDDED DESIGN

SPECIFIC PROJECT TASKS

The optical fiber sensors for the measurement of strain have been under development for a number of sensing techniques have been established. Moreover, the optical fiber sensors have been

damage detection tools in advanced aircraft and space vehicles. Optical fiber sensors, with flexibility and small size, are geometrically versatile and can be configured to arbitrary shapes to embed within the structural material for the purpose of damage assessment. Thefeature of optical fiber sensors is their inherent ability to serve as both the sensing element and the signal transmission medium. This allows the conventional instrumentation to be located remotely from the

ecially useful for remote monitoring of the condition of the bridges.

Many researches have been conducted on the implementation of FBG sensors for monitoring of concrete structures. The technology has been experimented and used in a

construction sites to monitor aircrafts, bridges, buildings, and civil structures with some successful results. However, more studies are needed to investigate the performance of the FBG sensors for longterm stability and durability of the FBG sensors.

1. DETAILED DOCUMENTATION

2. CONSTRUCTION AND DESIGN SCHEMATIC

3. HARDWARE AND EMBEDDED DESIGN

4. SOFTWARE AND SYSTEM OPERATION

PROJECT SUMMARY

The optical fiber sensors for the measurement of strain have been under development for a number of sensing techniques have been established. Moreover, the optical fiber sensors have been

n tools in advanced aircraft and space vehicles. Optical geometrically versatile and can be configured to arbitrary

damage assessment. The most attractive element and the signal

transmission medium. This allows the conventional instrumentation to be located remotely from the ecially useful for remote monitoring of the condition of the bridges.

Many researches have been conducted on the implementation of FBG sensors for monitoring of concrete structures. The technology has been experimented and used in a few

buildings, and civil structures with some successful performance of the FBG sensors for long-

2. CONSTRUCTION AND DESIGN SCHEMATIC

4. SOFTWARE AND SYSTEM OPERATION

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At the same time, studies have been carried out in Taiwan in the last few years concerning the manufacturing of FBG sensors, tests of the FBG properties and the index change mechanism, the temperature measurement using FBG sensors and the development of the packaging method. Results of these studies have offered a background to use the FBG sensors in the static and dynamic strain measurement, and a full scale implementation to a prestressed concrete bridge is currently underway. In this paper,, two experiments are reported to demonstrate the feasibility of using the FBG sensors in structural monitoring applications.

1. DYNAMIC STRAIN MEASUREMENT USING FBG SENSORS

Recently, many researches have demonstrated that the FBG sensors can be used to measure the strain of a structure under static loadings. The resolution of FBG sensors also gives a reliable data while comparing with the traditional RSG sensors.

Under this situation, applying the FBG sensors to the dynamic measurement becomes an interesting topic in civil engineering. Since the “smart structure” concept gradually rises up in civil engineering, there is a need of large amount of sensors in the structure. The main reason to use the FBG sensors is that they can solve the complex wire problems if there are hundreds of sensors on the structure. On the other hand, the FBG sensors have another advantage: sequential measurement. This characteristic makes it possible to measure huge data from only one fiber and make the real time measuring of large amount of data become feasible.

In the first part of this paper, a small-scaled, three-story structure was used to study the dynamic measurement using the FBG sensors. Figure shows the small experimental model, and the locations of the FBG sensors are indicated in the figure.

There are two kinds of FBG sensors attached to the structure. The initial wavelengths are 1545.721 nm and 1541.808nm.

The conventional RSG sensors (resistance strain gauges) and the accelerometers are also installed on the structure to offer a benchmark value for comparison.

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Figure The three-story model

After setting up all the sensors, the structure will be excited by an impact hammer to make a free vibration test. The measurement sampling rate of the monitoring system is 100HZ and the resolution is set at 1pm (1µε ), respectively.

2. CRACK DETECTION USING FBG SENSORS

Since the FBG sensors have another advantage of multi-points measuring, the FBG sensors are also good candidates in crack detection. In order to prove the feasibility of using FBG sensors in crack detection, an Aluminum plate experiment is done after the dynamic measurement using FBG sensors.

Figure shows the size of the Aluminum plate. There are four RSG sensors and four FBG sensors on the plate. The locations of each sensor are described clearly in the figure. Two dial gauges are also used to measure the deflection of specified locations of the plate. The distance between the RSG sensors and FBG sensors is set as close as possible and in this case it is 1cm. This can make sure that the value measured by the RSG sensors and the FBG sensors to be almost the same. The loading is added in the mid-span of the plate and each step is 0.3 kg. The loading is controlled strictly to prevent the stress of the plate excess its elastic range.

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Figure: The specification of the plate

Two initial conditions are used to test the plate. They are 1.plate without any crack and 2.plate with two cracks in specified locations. Both of these two conditions are tested under the same loading history. There are ten steps to add the loading to the middle of the plate and each steps is 0.3 kg.

In order to prove how to use FBG sensors in detecting the cracks in the structure, the results will focus on the reading of the FBG sensors. Two FBG sensors are picked up and the strain readings history is compared.

It should be emphasized that the RSG sensors readings may also be abnormal if there is a crack near it. But this needs to know the exact location of the crack in advance and in real situation there may be huge difficulties to put lots of RSG sensors on the structure. Since it is possible to put large amount of FBG sensors on structure and the FBG sensors could solve the traditional measurement time and equipment noise problem, the FBG sensors will be more effective in detecting the structure crack and the experiment result also prove this viewpoint.

In other words, if the distribution of the FBG sensors could be as uniform as possible, the technology of FBG sensors measurement system could find out every random crack on the real civil structure and offer a complete monitoring on the structure to prevent any serious damage in advance. It is known that every big damage such as fallen bridge starts from an initial crack. If the FBG sensors could be applied on real structure in the future, there is a belief that the monitoring system can help us to prevent some serious damage.

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CONCLUSION

The use of fiber optic technology for the monitoring of smart structures is very promising and the future is sure to bring further advancements and improvements. Because this is a rather new technology and few people have experience with Bragg grating sensors, is of utmost importance to detail the practical problems encountered during the implementation and installation of the sensors. The overall results of the research have provided great encouragement for the continued study of the development and use of fiber optic sensors for monitoring of smart structures.

The Bragg grating sensors themselves adapt well to use on, “smart structure”. The tests discussed in this research show a reasonable correlation between the strain values obtained from the fiber optic sensors and those obtained from conventional foil gauges; from a maximum difference of seven percent for a properly applied gauge to as close as 0.9 percent for a Bragg grating sensor. The factor that most limits the use of foil gauge is the instruments used with them. The Data Logger that was used in these tests is one of the earlier models. It is anticipated that improvements will be achieved with other models and the development of a demodulator chip.

The development of the optoelectronic demodulator chip will be a significant advancement in the use of fiber optic sensors in monitoring networks resulting in cost savings and a simplification of the overall system.

As a new kind of sensor for structural health monitoring system, popularization of FBG is still a challenge for researchers. Following problems should be the directions of future studies on FBG:

1) Develop lowcost standard bare FBG with stable sensing coefficient;

2) Effective temperature compensation techniques for FBG strain sensors used in longterm monitoring systems;

3) How to develop standardized FBG encapsulated sensors;

4) How to develop smart devices based on FBG to detect special parameters of infrastructures, such as corrosions, slip, debonding, and so on;

5) How to avoid the damage of the FBG sensors and cables during constructions;

6) How to develop low-cost multi-channel interrogators when we have to face the embarrassment that we have no choice but to cut down the number of FBG sensors along one cable, even sometimes one sensor on one cable, which makes us not able to make full use of the advantages of

FBG;

7) How to develop large integrated FBG monitoring system and make it become the important part of the Structural Health Monitoring System and give useful information for damage identification.

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REFERENCES

1. Forrest, S., L.A. Coldren, S. Esener, D. Keck, F.Leonberger, G.R. Saxonhouse and P.W. Shumate, 1996. Optoelectronics in Japan and the United States. Japanes Technology Evaluation Center, Panel report. 2. Zdunek, A.D., D.W. Prine, Z. LI, e. Landis and S. Shah, 1995. Early detection of steel rebar corrosion by Acoustic Emission monitoring. CORROSION95, The NACE Intl. Ann. Conf. and Corrosion Show. Paper No. 547, pp: 1. 3. ISIS Canada Homepage. What is ISIS Canada? 4. Prine, D.W., 1994. Application of acoustic emission and strain gauge monitoring to steel highway bridges. Northwestern University, Evanston, Illinois, pp: 1. 5. Huston, D.R. and F.L. Peter, 1993. Intelligent materials for intelligent structures. IEEE Communications Magazine, Oct., pp: 40-45. 6. Wu ZS. Proceeding of the first international conference on structural health monitoring and intelligent Infrastructures, Tokyo, Japan, 2003. 7. Hill KO. “Photosensitivity in optical fiber waveguides application to reflection filter fabrication”. App. Phys. Lett., 32(10), pp. 647-653, 1978. 8. Meltz G., Morey W.W., Glenn W.H. “Formation of Bragg gratings in optical fibers by a transverse holographic method”. Optical letter, 14 (15), pp.823-825, 1989. 9. Ou JP. “Some recent advance of intelligent monitoring system for civil infrastructures in mainland China”. Proceeding of the first international conference on structural health monitoring and intelligent Infrastructures, Tokyo, Japan, 2003, pp. 131-144. 10 . Ou JP. Zhou Z. “Encapsulation Techniques for FBG and Smart Monitoring for Bridges with FBG Sensors”, Proceeding of the 4th International Workshop on Structural Health Monitoring at Stanford University, Stanford University, 2003, pp. 180-187. 11. Ou JP. Zhou Z., Wu ZJ. “The sensing properties and practical application in civil infrastructures of optical FBGs”. SPIE, 5129, 2003, pp. 10-17. 12. Zhou Z. Thomas W. G., Luke H. and Ou JP. “Techniques of Advanced FBG sensors: fabrication, demodulation, encapsulation and their application in the structural health monitoring of bridges”. Pacific Science Review, 5 (1), pp.116-121, 2003.