Optical Fibre Sensors.pdf

7/29/2019 Optical Fibre Sensors.pdf

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Optical Fibre Sensors

Structure monitoring in Infrastructure field

(Monitoring of bridges` structure)


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Outlines

1.0 .Introduction 2

2. 2.Monitoring ways 32.1 .Fiber optic sensor systems. 4

3.0. Fiber Optic Sensor Classifications 5

3.1. fibre optical sensors based on sensing location.. 5

3.1.1.Extrinsic fiber optic sensor 5

3.1.2. Intrinsic fibre sensor... 6

3.2. Fibre optical sensors based on operating principle.. 7

3.3fibre optical sensors based on the application.. 7

4.0. Fibre optic sensors types... 7

4.1. Intensity Based Fiber Optic Sensors.. 7

4.2. Phase Modulated Fiber Optic Sensors 8

4.3. Polarization Modulated Fiber Optic Sensors.. 8

4.4. Wavelength Modulated Fiber Optic Sensors 8

5.0. Fiber Bragg Grating (FBG)... 9

5.1. THEORY OF FBG SENSORS 11

5.1.1. SENSOR DESIGN.. 11

6.0. FBG and structures of bridges.... 12

6.1. Advantages of FBG Sensors in Bridges monitoring.... 15

6.2. an example of FBG sensor in bridge health monitoring....15

7.0. Conclusion..17

8.0. References..17


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1.0.Introduction

The use of fibre optic sensors is becoming a valuable practice in sensory systems for the

health monitoring of structures. The paper presents a few significant case studies in which

fibre optic sensors have proven their effectiveness. Implementation of the sensory system as

well as data analysis and interpretation are discussed.

In the last decade, an increasing shift from investments in the construction of new

infrastructures to the maintenance and lifetime extension of the existing ones has taken place.

Generally, most of the transportation network, such as highways and railways, is completed

and in service. A similar situation is encountered in ports and maritime infrastructure, where

most of the facilities have been built 30 to 80 years ago. By the other hand, the establishment

of a common economical and political world spacelarger than ever is pushing the demand forfree circulation of people and freights, but the concern for the impact created by the

construction of new facilities in delicate natural and architectural environments is also

increasing. Therefore, the authorities managing civil infrastructure face the challenge of

maintaining the transportation network in a satisfactory state, using a limited budget and with

little perturbation to its normal use. This task is far more complex than that of building new

structures and requires new management instruments.

Due to the increasing loads, ageing of materials and environmental action, the

performance of many in-service structures has decayed and the inherent level of safety can be

shown inadequate relative to current design standards. Structural health monitoring is

certainly one of the most powerful tools for infrastructure management. In what we call the

information age, structural health monitoring seems to close the gap between the traditional

world of structural engineering and the frenetic one of information technology. Monitoring

includes the observation of deformations as well as environmentally induced processes.


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Climatic variables like temperature, humidity and wind loads shall be considered as well. A

central point consists in the observation of the chemical parameters in the form of

electrochemical potentials, resistivity, and penetration processes. However, an almost

complete instrumentation of all imaginable physical phenomena would exceed the reasonable

amount of financial efforts. Additionally, a larger number of collected data might not

necessarily improve the quality of the drawn conclusions. Therefore, the identification and

observation of the decisive parameters is fundamental for the development and calibration of

consistent engineering models describing the deterioration mechanisms threatening ultimate

limit state, serviceability, and durability .The definition of the objective of the

instrumentation program usually follows the realisation that something about the structure is

not known well enough and that measurements of a number of quantities at a certain location

would be desirable for the sake of economy or safety. The first step is to reflect on all

possible ways the construction might behave and to choose which quantities to measure,

where to measure them, and to select adequate instruments to do so. This requires an

estimation of the magnitudes of changes in the quantities to be measured, which allows the

definition of the range, resolution, accuracy, and sensitivity of the instruments selected to

measure them. In much the same way, the temporal behaviour of the observed phenomena

might be a criterion for the dynamic requirements for both the instruments and the readout

units. As next the instrument positions and the number of instrumented sections have to be

determined. After testing, the taking of the readings and their processing and analysis must be

carried out in a systematic, organised way.

2.0. Monitoring ways

The monitoring means the measurement of the deformation of a structure caused by loads

e.g, traffic. There are many monitoring systems that have been used for bridges structures


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health such as RTK-GPS differentiation system and sensing methods. Here, I will focus on

sensing methods and especially optical fibre sensors. Optical fibre sensors considered as new

generation of monitoring techniques for giving better and continuous acknowledge about the

bridge (major bridges) health. The traditional methods were limited and showed good results

only for vibration measurements and mostly for the external structure.

2.1. Fiber optic sensor systems

Why fibre optic sensors?

According to what mentioned above, optical fibre sensors take the advantage against other

techniques due to the wide range of the sensing such as harsh environment capability

(intensive EMI, high temperature, chemical corrosion, high pressure, high voltage Light

weight and small size), excellent performance (high sensitivity and large bandwidth) Long

range operation and Multiplexed or distributed measurements.

In many cases and especially for wireless methods, there were many flaws in getting good

results from long and curved bridges.

Comparing with the traditional electro-mechanics sensors, the optic fiber sensors has the

advantages of higher precision of testing, tiny size, smaller weight, essential explosion-proof,

immunity from the effect of electro-magnetic and corrosion resistance.

Here I tried to list the most benefits of using optical fibre sensors:

1. More accurate rating of bridge capacity2. to maximize the safe utilization of the transportation infrastructure and improve

the flow of commerce.

3. Measured data can be compared against computed values.4. More efficient utilization of bridge maintenance and inspection resources.


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5. Increased safety through early detection of structural abnormalities.6. Increased safety through monitoring and communication of icing and other conditions

on bridge decks.

7. Findings can be transferred to future bridge construction projects.Furthermore, continuous monitoring of bridge infrastructure can help in early fault

detection and protection against man-made threats. The remote location of many bridges

makes them especially vulnerable to security issues, which could lead to dangerous

consequences in terms of the economy and public safety.

The basic component of optical fibre sensing system(Springer Handbook of Experimental Solid Mechanics By William N. Sharpe, Jr., William N)

3.0. Fibre Optic Sensor Classifications

Optical fibre sensors can be classified under three classes, according to the sensing location,

the operating principle and the application.

3.1. Fibre optical sensors based on sensing location

In this class, optical fibre sensors are categorized to two groups, intrinsic and extrinsic.


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3.1.1.Extrinsic fiber optic sensor

In this type, the fiber is simply used to carry light to and from an external optical device

where the sensing takes place. The fibre just acts as means of getting the light to the sensing

location.Extrinsic fiber optic sensors provide excellent protection of measurement signals

against noise corruption.

An extrinsic fiber optic sensor(Springer Handbook of Experimental Solid Mechanics By William N. Sharpe, Jr., William N)

3.1.2 Intrinsic fibre sensor

In an intrinsic fibre optic sensor one or more of the physical properties of the fibre undergo a

change figure below Perturbations act on the fibre and the fibre in turn changes some

characteristic of the light inside fibre.

A particularly useful feature of intrinsic fiber optic sensors is that they can, if required,

provide distributed sensing over very large distances.

An extrinsic fiber optic sensor(Springer Handbook of Experimental Solid Mechanics By William N. Sharpe, Jr., William N)


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3.2. Fibre optical sensors based on operating principle

Based on the operating principle or modulation and demodulation process, a fiber optic

sensor can be classified as intensity, a phase, a frequency, or a polarization sensor. All these

parameters may be subject to change due to external perturbations. Thus, by detecting

these parameters and their changes, the external perturbations can be sensed

3.3. fibre optical sensors based on the application

Based on the application, a fiber optic sensor can be classified as follows:

1- Physical sensors: Used to measure physical properties like temperature, stress, etc.2- Chemical sensors: Used for pH measurement, gas analysis, spectroscopic studies, etc.3- Bio-medical sensors: Used in bio-medical applications like measurement of blood

flow, glucose content etc.

4.0. fibre optic sensors types

Fibre optic sensors can be categorized into four types as following:

4.1. Intensity Based fibre Optic SensorsIntensity-based finer optic sensors rely on signal undergoing some loss. They are made

by using an apparatus to convert what is being measured into a force that bends the fiber and

causes attenuation of the signal. Other ways to attenuate the signal is through absorption or

scattering of a target. The intensity-based sensor requires more light and therefore usually

uses multimode large core fibers .There are a variety of mechanisms such as microbending

loss, attenuation, and evanescent fields that can produce a measurand-induced change in the

optical intensity propagated by an optical fiber. The advantages of these sensors are:

Simplicity of implementation, low cost, possibility of being multiplexed, and ability to

perform as real distributed sensors.


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4.2. Phase Modulated Fiber Optic Sensors

Phase modulated sensors use changes in the phase of light for detection. The optical

phase of the light passing through the fiber is modulated by the field to be detected. This

phase modulation is then detected interferometerically, by comparing the phase of the light in

the signal fiber to that in a reference fiber. In an interferometer, the light is split into two

beams, where one beam is exposed to the sensing environment and undergoes a phase shift

and the other is isolated from the sensing environment and is used for as a reference. Once

the beams are recombined, they interfere with each other

4.3. Polarization Modulated Fiber Optic Sensors

The direction of the electric field portion of the light field is defined as the polarization

state of the light field. Different types of polarization states of the light field are linear,

elliptical, and circular polarization states. For the linear polarization state, the direction of the

electric field always keeps in the same line during the light propagation. For the elliptical

polarization state, the direction of the electric field changes during the light propagation. The

end of the electric field vector forms an elliptical shape; hence, it is called elliptical

polarized light.

4.4. Wavelength Modulated fibre Optic Sensors

Wavelength modulated sensors use changes in the wavelength of light for detection.

Fluorescence sensors, black body sensors, and the Bragg grating sensor are examples of

wavelength-modulated sensors. Fluorescent based fibre sensors are being widely used for

medical applications, chemical sensing and physical parameter measurements such as

temperature, viscosity and humidity. Different configurations are used for these sensors

where two of the most common ones are shown in. In the case of the end tip sensor, light


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propagates down the fiber to a probe of fluorescent material. The resultant fluorescent signal

is captured by the same fiber and directed back to an output demodulator.

There are many types of fibre optic sensors listed under wavelength Modulated but the most

widely used wavelength based sensor is the Bragg grating sensor. Fiber Bragg gratings

(FBGs) are formed by constructing periodic changes in index of refraction in the core of a

single mode optical fiber. This periodic change in index of refraction is normally created

by exposing the fiber core to an intense interference pattern of UV energy. The variation in

refractive index so produced, forms an interference pattern which acts as a grating. The light

propagates through the grating, and part of the signal is reflected at the Bragg wavelength.

The complimentary part of the process shows a small sliver of signal removed from the total

transmitted signal. This obviously shows the Bragg grating to be an effective optical filter.

5.0. fibre Bragg Grating (FBG)

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


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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.

Structure of fibre bragg gratings


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(spectral response of FBG)5.1 Theory Of FBG Sensors

5.1.1. Sensor Design

The fibre Bragg grating is a device commonly used in telecommunications and sensor

technology. Fibre gratings are formed by a periodic change of the fibre cored refractive index

in direction of propagation of optical radiation. In principle, the fibre 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:

where Bragg is the Bragg resonant wavelength, neff is the effective refraction index, and

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


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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.

6.0. FBG and monitoring structures of bridges

One of the most successful applications of fibre Bragg grating is in the field of infrastructure

and civil engineering. As it known, the spreading of bridges building around the world

increases the need of having continues observation of these bridges to avoid any sudden

accidents. Fiber optic sensors based on wavelength modulated (FBG) give good range of

monitoring bridges structure and providing an alert for any problems.

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 sensors,

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.Technically, the absorbent material is


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the sensing element, converting the amount of gas to a strain. The Bragg grating then

transduces the strain to the change in wavelength.

FBG in bridge monitoring (Fiber Optic Sensors,Second Edition)

The FBG purposes are to observe strain, crack and predict the bridges fatigue that may lead

to the damage. So, FBGs sensors give good solutions in that case and installing them to the

concrete structure will offer continues observation and this will directly archives the safety

and reduces the cost as well.

Due to the fragility of bare FBG, it is hard to directly apply in infrastructures (bridges) , so

it must be encapsulated. There are two kindes of encapsulated FBG sensors are used for this

purpose, capillary encapsulated FBG strain sensors and slice base encapsulated FBG strain

sensors. As in the fig (*^&%$) below.


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The way how FBG is encapsulated

Encapsulated FBG installed to a bridge

a mental tube of encapsulated FBG

FBG encapsulation strain sensor is FBG encapsulated in mental capillary by glue (especially

epoxy resin) under the air press difference between the inside and the outside of the capillary.

Capillary FBG can be conveniently used then in concrete structure of the bridge. The mental

holder ring is used to keep the FBGs deformation consistent with the concrete structures and

the stretched optical fiber is ready for temperature compensation connector. In slice base


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encapsulation FBG sensors the FBG encapsulated in slice base encapsulated by the glue,

which can be conveniently used on the surface.

Mental slice encapsulated FBG sensor

6.1. Advantages of FBG Sensors in Bridges monitoring

There are many advantages that achieved by using FBG sensors such as remote Sensing

Ability(Ideal for applications with long distances between sensors or sensors and instrument), Easy to

Install which can embed or mount directly to most materials via epoxy, screws or spot welding,

simple to Multiplex( Easily facilitates multiple sensors on a single optical fiber connection), non-

Electrical, Environmentally Stable Sensor Design(Immune to EMI and Lightning) And Ideal for

Harsh Environments: Small in size, uses light waves not visible to the naked eye, no electro-magnetic

field generated by the sensor. Also there are additional improvements via other sensors such as

electrically insulating materials (no electric cables are required high voltage environments, chemically

passive, not subject e.g. to corrosion, Immune to electromagnetic interference (EMI) and wide

operation temperature range.

6.2. An example of FBG sensor in bridge health monitoring

The example here is for installing the FBG sensors to a known long bridge in Hong Kong it

called Tsing Ma bridge (TMB), which is the world longest (1377 m) suspension bridge that

carried both railway and regular road traffic. Forty FBG sensors divided into three arrays

were installed on the deck, hanger cable, rocker bearing and truss girders of the TMB. The

aim of this installing is to prove that FBG sensors for structural health monitoring are very


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challengeable technology to other old once via monitoring the health of different parts of the

TMB under both the railway and highway loads as well as comparing the FBG sensors

performance with the conventional structural health monitoring system. Wind and Structural

Health Monitoring System (WASHMS) that has been operating at TMB since the bridges

commissioning in May 1997. The experimental observations in this case show that the results

using FBG sensors were in excellent agreement with those acquired by WASHMS.

Tsing Ma bridge (TMB) with forty FBG sensors

The process of FBG in Tsing Ma bridge (TMB)


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Applying this number of FBG sensors to the bridge gave excellent results. The information of the

bridge health was a continuous and accurate enough comparing to the old technologies.

7.0. Conclusion

In conclusion, we have demonstrated the application of FBG sensors and interrogation system to

monitor the dynamic strain and temperature on the Hong Kongs landmark Tsing Ming Bridge.

The FBG package technique was proposed to apply for structural monitoring application. The

measurement result of the interrogation system was in excellent agreement with the one obtained by

electrical strain gauge measurement. The FBG sensor system offers many advantages over electrical

strain gauge. These include remote sensing, ease of installation, non-corrosive and lower

maintenance cost. This shows that FBG sensor technology is a good alternative for civil and

structural dynamic strain monitoring.

8.0. References

Structural Monitoring: Making Bridges Safer Across the United States

Structural Health Monitoring and Intelligent Infrastructure: volume 1, J.P. Ou, H. Li,

Zhongdong Duan2006

Structural Health Monitoring 2005: Advancements And Challenges edited by Fu-Kuo Chang-

2003

Fiber Optic Sensors: Principles and Applications By B.D.Gupta, Gupta, Banshi Das

Structural Health Monitoring 2003: From Diagnostics & Prognostics to ... edited by Fu-Kuo

Chang

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


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Fiber Optic Sensors and Systems: Fos2, By Hui Pan, Editor, Paul Polishuk,2002

Structural Monitoring With Fiber Optic Technology, By Raymond M. 2001

Smart Fibres, Fabrics and Clothing, By Xiaoming Tao.2005

Optical sensing, B. Culshaw, Anna Grazia Mignani, Rainer Riesenberg, Society of Photo-optical

Instrumentation Engineers. April 2004

Modelling of Corroding Concrete Structures, By Carmen Mancini,2010

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