chapter 1 - Bentham Science

FIBER BRAGG GRATING SENSORS: RECENT ADVANCEMENTS, 

INDUSTRIAL APPLICATIONS AND MARKET EXPLOITATION 

Editors: 

Andrea Cusano, Antonello Cutolo and Jacques Albert

eBooks End User License Agreement 

Please read this license agreement carefully before using this eBook. Your use of this eBook/chapter constitutes your agreement 

to the terms and conditions set forth in this License Agreement. Bentham Science Publishers agrees to grant the user of this 

eBook/chapter, a non-exclusive, nontransferable license to download and use this eBook/chapter under the following terms and 

conditions: 

1. This eBook/chapter may be downloaded and used by one user on one computer. The user may make one back-up copy of this 

publication to avoid losing it. The user may not give copies of this publication to others, or make it available for others to copy or 

download. For a multi-user license contact permission@bentham.org 

2. All rights reserved: All content in this publication is copyrighted and Bentham Science Publishers own the copyright. You may 

not copy, reproduce, modify, remove, delete, augment, add to, publish, transmit, sell, resell, create derivative works from, or in 

any way exploit any of this publication’s content, in any form by any means, in whole or in part, without the prior written 

permission from Bentham Science Publishers. 

3. The user may print one or more copies/pages of this eBook/chapter for their personal use. The user may not print pages from 

this eBook/chapter or the entire printed eBook/chapter for general distribution, for promotion, for creating new works, or for 

resale. Specific permission must be obtained from the publisher for such requirements. Requests must be sent to the permissions 

department at E-mail: permission@bentham.org 

4. The unauthorized use or distribution of copyrighted or other proprietary content is illegal and could subject the purchaser to 

substantial money damages. The purchaser will be liable for any damage resulting from misuse of this publication or any 

violation of this License Agreement, including any infringement of copyrights or proprietary rights. 

Warranty Disclaimer: The publisher does not guarantee that the information in this publication is error-free, or warrants that it 

will meet the users’ requirements or that the operation of the publication will be uninterrupted or error-free. This publication is 

provided "as is" without warranty of any kind, either express or implied or statutory, including, without limitation, implied 

warranties of merchantability and fitness for a particular purpose. The entire risk as to the results and performance of this 

publication is assumed by the user. In no event will the publisher be liable for any damages, including, without limitation, 

incidental and consequential damages and damages for lost data or profits arising out of the use or inability to use the publication. 

The entire liability of the publisher shall be limited to the amount actually paid by the user for the eBook or eBook license 

agreement. 

Limitation of Liability: Under no circumstances shall Bentham Science Publishers, its staff, editors and authors, be liable for 

any special or consequential damages that result from the use of, or the inability to use, the materials in this site. 

eBook Product Disclaimer: No responsibility is assumed by Bentham Science Publishers, its staff or members of the editorial 

board for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any 

use or operation of any methods, products instruction, advertisements or ideas contained in the publication purchased or read by 

the user(s). Any dispute will be governed exclusively by the laws of the U.A.E. and will be settled exclusively by the competent 

Court at the city of Dubai, U.A.E. 

You (the user) acknowledge that you have read this Agreement, and agree to be bound by its terms and conditions. 

Permission for Use of Material and Reproduction 

Photocopying Information for Users Outside the USA: Bentham Science Publishers Ltd. grants authorization for individuals 

to photocopy copyright material for private research use, on the sole basis that requests for such use are referred directly to the 

requestor's local Reproduction Rights Organization (RRO). The copyright fee is US $25.00 per copy per article exclusive of any 

charge or fee levied. In order to contact your local RRO, please contact the International Federation of Reproduction Rights 

Organisations (IFRRO), Rue du Prince Royal 87, B-I050 Brussels, Belgium; Tel: +32 2 551 08 99; Fax: +32 2 551 08 95; E-mail: 

secretariat@ifrro.org; url: www.ifrro.org This authorization does not extend to any other kind of copying by any means, in any 

form, and for any purpose other than private research use. 

Photocopying Information for Users in the USA: Authorization to photocopy items for internal or personal use, or the internal 

or personal use of specific clients, is granted by Bentham Science Publishers Ltd. for libraries and other users registered with the 

Copyright Clearance Center (CCC) Transactional Reporting Services, provided that the appropriate fee of US $25.00 per copy 

per chapter is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers MA 01923, USA. Refer also to 

www.copyright.com

CONTENTS 

Foreword i 

Preface ii 

Contributors iv 

CHAPTERS 

1. Fiber Bragg Grating Sensors: A Look Back 1 

J. Albert 

2. Fiber Bragg Gratings: Advances in Fabrication Process and Tools 9 

K. Sugden and V. Mezentsev 

3. Fiber Bragg Gratings: Analysis and Synthesis Techniques 35 

J. Skaar 

4. Photonic Bandgap Engineering in FBGs by Post Processing Fabrication Techniques 53 

A. Cusano, D. Paladino, A. Cutolo, A. Iadicicco and S. Campopiano 

5. Fiber Bragg Grating Interrogation Systems 78 

J.L. Santos, L.A. Ferreira and F.M. Araújo 

6. Multiplexing Techniques for FBG Sensors 99 

M. López-Amo and J.M. López-Higuera 

7. Polarization Properties of Fiber Bragg Gratings and Their Application for Transverse Force 

Sensing Purposes 116 

C. Caucheteur, S. Bette, M.Wuilpart and P. Mégret 

8. Fiber Bragg Grating Sensors in Civil Engineering Applications 143 

J.P. Ou, Z. Zhou and G. Chen 

9. Fiber Bragg Grating Sensors in Aeronautics and Astronautics 171 

N. Takeda and Y. Okabe 

10. Fiber Bragg Grating Sensors in Energy Applications 185 

C.B. Staveley 

11. Fiber Bragg Grating Sensors for Railway Systems 197 

H.Y. Tam, S.Y. Liu, S.L. Ho and T.K. Ho 

12. Fiber Bragg Grating Sensors in Nuclear Environments 218 

F. Berghmans and A. Gusarov 

13. Fiber Bragg Grating Evanescent Wave Sensors for Chemical and Biological Applications 238 

A. Cusano, D. Paladino, A. Cutolo, A. Iadicicco and S. Campopiano 

14. Fiber Bragg Grating Sensors in Microstructured Optical Fibers 270 

T. Nasilowski 

15. Polymer Fiber Bragg Gratings 292 

D.J. Webb and K. Kalli 

16. Fiber Bragg Grating Sensors: Market Overview and New Perspectives 313 

J.W. Miller and A. Méndez 

Index 321

FOREWORD 

Optical fiber based sensor systems have fascinated the researcher and tantalized the application engineer for over 

forty years. The fascination lies within the countless ways through which the technically inclined optical scientist 

can cause light guided within an optical fiber to interact with the world outside and the myriad principles through 

which these modulation phenomena could be detected. The applications engineer finds the benefits which this 

approach offers to be many and intriguing. These include predominantly that the area or point which is to be 

measured needs no electrical contact and yet remains in intimate proximity to the parameter of interest. Add this to 

ideas like highly multiplexed all electrically passive networks, distributed sensing systems and interrogation ranges 

of 10 or more kilometers and a whole new spectrum of applications becomes possible. 

Of the multiple techniques which have been explored a few have emerged into reality. The fiber optic gyroscope 

now navigates spacecraft and the hydrophone array helps in mapping the seabed. Other techniques are exciting 

considerable interest and among these one of the principal contenders for extensive exploration is undoubtedly the 

Fiber Bragg grating – the subject of this book. 

The Bragg grating is intrinsically a simple device. A periodic structure is written into the core of an optical fiber and 

this periodic structure will reflect specific optical wave length dependent on the periodicity. Vary the periodicity, 

vary the wavelength. Since this period depends upon environmental temperature and externally applied strains and 

pressures we have the basis for a simple sensor which is easily and unambiguously interrogated. Furthermore long 

strings of these sensors each operating at a different wave length can be written into a single fiber giving an array 

technology linked through just a single fiber giving an array technology which can extend over kilometers. 

The basic concepts for the fiber grating go back more than twenty years from early exploratory work undertaken by 

Ken Hill and his colleagues in Canada. Since then the grating has attracted significant attention from scientists and 

application engineers alike and has demonstrated its applicability in areas ranging from monitoring large and 

complex highway bridges to measuring stresses in teeth. 

This book explores every nuance of this important sensing concept. The contributors are, without exception, 

internationally recognized experts in their field and present the diversity of techniques, applications and perceptions 

through which the potential offered by the Bragg grating can be appreciated. This ranges from basic fabrication 

processes through the design of the details of the grating structure itself into multiplexing techniques and 

technologies, the interrogating electro-optic system and the numerous interfaces into application. Market prospects 

and technology roadmaps also feature to complete the overall picture. 

The book promises to be an invaluable addition to the sensing engineer’s library and will provide essential 

background to stimulate the future prospects for this important and intriguing technology. 

i 

Brian Culshaw 

University of Strathclyde 

United Kingdom

ii 

PREFACE 

The field of fiber optics has undergone tremendous growth over the past 40 years. Initially conceived as a medium 

to carry light and images for medical endoscopic applications, optical fibers were later proposed in the mid 1960’s 

as an information-carrying medium for telecommunication applications. The outstanding success of this concept is 

embodied in millions of miles of telecommunications fiber that have spanned the earth, the seas, and utterly 

transformed our capacity to communicate and the means by which we do it. The award of the 2009 Nobel Prize in 

physics to C. K. Kao, who first proposed the use of optical fibers for data communication, is the crowning jewel on 

this fantastic story. Among the reasons why optical fibers are such an attractive communication medium are their 

low loss, high bandwidth, electromagnetic interference immunity, small size, light weight, safety, relatively low 

cost, low maintenance, etc. 

As optical fibers consolidated their dominant position in the telecommunications industry, the technology and 

commercial markets both matured, especially from the late 1980s onward. In parallel over the same period, several 

groups around the world have been carrying out research to exploit some of the key benefits of optical fibers for 

another very important application: sensing. Initially, fiber sensors were laboratory curiosities and simple proof-ofconcept 

demonstrations. However, more and more, optical fiber technologies are making an impact and have 

achieved commercial respectability and viability in industrial sensing, bio-medical laser delivery systems, aerospace 

and military gyroscopes, as well as automotive lighting & control – to name just a few – and span applications as 

diverse as oil-well downhole pressure/temperature sensors and intra-aortic catheters. This transition has taken the 

better part of the last 20 years and reached the point where fiber sensors enjoy widespread use for structural sensing 

and monitoring applications in civil engineering, aerospace, marine, oil & gas, composites, smart structures, biomedical 

devices, electric power industry and many others. Of course all these advances have resulted from the 

development of a complete understanding of the physical and chemical transducing mechanisms and of the 

appropriate sensor interrogation systems and technologies. For instance, there is a variety of commercial discrete 

sensors based on Fabry-Perot cavities and Fiber Bragg Gratings (FBGs), as well as distributed sensors based on 

Raman and Brillouin scattering methods, all readily available along with relevant interrogation instruments. 

Amongst all of these, FBG based sensors – more than any other particular sensor type – have become widely known, 

researched, and popular within and outside of the photonics community, leading to a significant increase in their 

utilization and commercial impact. Given the capability of FBGs to measure a multitude of parameters such as 

strain, temperature, pressure, chemical and biological agents and many others, coupled with their design flexibility 

for single point or multi-point sensing arrays and their relative low cost, FBGs are ideal for a multitude of sensing 

applications in many fields and industries. 

Since 2000, more than twenty companies have been active in the FBGs sensor market. In 2000, Micron Optics was 

able to launch in the market the first line of advanced FBG interrogators, while in 2003 LxSix (now LxDATA) and 

Sabeus launched the first reel-to-reel production of high reliability FBGs arrays. Finally, in May of 2007 HBM – the 

world’s largest supplier of strain sensing systems – began offering optical strain gages and interrogators based on 

FBG technology. This marked the first time that a conventional foil strain gage manufacturer adopted and embraced 

FBGs as an essential part of their product portfolio. A broad and intense commercial pull is expected to follow from 

this breakthrough. 

On the research side, many groups around the world are hard at work on new fabrication methods, FBG reliability, 

new device designs, and new data processing techniques. For instance, FBGs can now be fabricated in many types 

of glasses using ultrafast IR writing and new post processing techniques have been developed to customize device 

spectra for enhanced sensitivity in specific applications. New industrial applications have opened up in many 

strategic sectors, especially in the case of chemical or biological sensing. The prospects of using polymer optical 

fibers (POF) in sensing applications is expected to lead to the development of POF FBGs for inexpensive, simple

and low-cost disposable platforms (in the automotive industry for instance). Similarly, microstructured optical fibers 

are expected to have a major impact in the development of new chemical and biological systems based on 

optofluidics as well as active and passive microfluidics. 

All of these advances have created a need for a comprehensive overview of the FBG Sensor Technology. The book 

that we have put together covers every aspect of this technology, starting all the way back from the fortuitous 

discovery of Hill in 1978. The remainder of the book details the enormous efforts carried out by both the scientific 

and industrial communities over the last two decades to bring the field to its current level of success and its promise 

of further advances. The different chapters will illustrate and describe: 

- new fabrication methods and advancements in photosensitivity; 

- new post processing techniques for spectral tailoring; 

- new applications in strategic industrial sectors starting from civil, aeronautic energy up to nuclear and 

extreme environments; 

- exciting new developments in the field of chemical and biological sensing; 

- new perspectives involving plastic and micro-structured fibers; 

- a clear identification of the market situation and a forecast for the next decade. 

In summary, our goal was to provide a complete, very exciting source of information on FBG sensors for the huge 

community of people now involved in the field, including both the scientific and the industrial sectors. Of special 

interest is the fact that the book also contains a very wide range of topics and innovative concepts that have not yet 

been presented as comprehensive reviews elsewhere. Finally, it is important to remark that each of the chapters has 

been written by recognized experts in their fields, either from academia or the private sector according to the topic 

of the chapter. It has been a pleasure for the editors to work with all these authors as they have worked very hard to 

provide us with authoritative, clear and elaborate reviews of their specialties. We hope that the readers of the book 

will share our pleasure in reading these reviews, and maybe learn a thing or two along the way to further their 

research or business. That, surely, would be our most satisfying reward. 

iii 

Andrea Cusano & Antonello Cutolo 

University of Sannio, Italy 

Jacques Albert 

University of Carleton, Canada

iv 

CONTRIBUTORS 

Jacques Albert 

Department of Electronics, Carleton University, 1125 Colonel By Drive, Ottawa, ON, Canada K1S 5B6 

Francisco M. Araújo 

INESC Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal 

FiberSensing S.A., Rua Vasconcelos Costa 277, 4470-640 Maia, Portugal 

Francis Berghmans 

Department of Applied Physics and Photonics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium 

SCK·CEN Belgian Nuclear Research Center, Boeretang 200, 2400 Mol, Belgium 

Sébastien Bette 

Electromagnetism and Telecommunications Department, Faculté Polytechnique, Université de Mons, Boulevard 

Dolez 31, 7000 Mons, Belgium 

Stefania Campopiano 

Department of Technology, University of Naples “Parthenope”, Centro Direzionale di Napoli Isola C4, 80143 

Napoli, Italy 

Christophe Caucheteur 



Genda Chen 

Center for Infrastructure Engineering Studies, Missouri University of Science and Technology, Rolla, MO 65409- 

0710, USA 

Andrea Cusano 

Optoelectronic Division – Engineering Department, University of Sannio, Corso Garibaldi 107, 82100 Benevento, 

Italy 

Antonello Cutolo 


Italy 

Luís A. Ferreira 


FiberSensing S.A., Rua Vasconcelos Costa 277, 4470-640 Maia, Portugal 

Andrei Gusarov 

SCK·CEN Belgian Nuclear Research Center, Boeretang 200, 2400 Mol, Belgium 

Siu-lau Ho 

Photonics Research Centre, Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung 

Hom, Kowloon, Hong Kong SAR, China 

Tin-kin Ho 



Agostino Iadicicco 

Department of Technology, University of Naples “Parthenope”, Centro Direzionale di Napoli Isola C4, 80143 

Napoli, Italy

Kyriacos Kalli 

Nanophotonics Research Laboratory, Cyprus University of Technology, Cyprus 

Shun-yee Liu 



Manuel López-Amo 

Grupo de Comunicaciones Ópticas, Universidad Pública de Navarra, Campus de Arrosdía, E-31006-Pamplona, 

Spain 

Jóse M. López-Higuera 

Grupo de Ingeniería Fotónica, Universidad de Cantabria, ETSII y Telecomunicación, Avda. de los Castros s/n E- 

39005 Santander, Spain 

Patrice Mégret 



Alexis Méndez 

MCH Engineering LLC, 1728 Clinton Avenue, Alameda, CA 94501, USA 

Vladimir Mezentsev 

Photonics Research Group, Aston University, Aston Triangle, Birmingham B4 7ET, United Kingdom 

Jeff W. Miller 

Micron Optics Inc., 1852 Century Place, Atlanta, GA 30345, USA 

Tomasz Nasilowski 

Department of Applied Physics and Photonics, Faculty of Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 

building F, B-1050 Brussels, Belgium 

Yoji Okabe 

Department of Mechanical and Biofunctional Systems, Institute of Industrial Science, The University of Tokyo, 4-6- 

1 Komaba, Meguro-ku, Tokyo 153-8505, Japan 

Jinping Ou 

School of Civil Engineering, Harbin Institute of Technology, Harbin, Heilongjiang, 150090, P.R. China 

School of Civil and Hydraulic Engineering, Dalian University of Technology, Dalian, 116024, P.R. China 

Domenico Paladino 


Italy 

José L. Santos 


Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal 

Johannes Skaar 

Department of Electronics and Telecommunications, Norwegian University of Science and Technology, Trondheim, 

Norway; University Graduate Center, Kjeller, Norway 

Christopher B. Staveley 

Smart Fibres, Limited, 12 The Courtyard, Eastern Road, Bracknell, RG12 2XB, United Kingdom 

v

vi 

Kate Sugden 


Hwa-yaw Tam 



Nobuo Takeda 

Department of Advanced Energy, Graduate School of Frontier Sciences, The University of Tokyo, Mail Box 302, 5- 

1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan 

David J. Webb 

Photonics Research Group, Aston University, United Kingdom 

Marc Wuilpart 



Zhi Zhou 

School of Civil Engineering, Harbin Institute of Technology, Harbin, Heilongjiang, 150090, P.R. China 

Center for Infrastructure Engineering Studies, Missouri University of Science and Technology, Rolla, MO 65409- 

0710, USA

Fiber Bragg Grating Sensors: Recent Advancements, Industrial Applications and Market Exploitation, 2011, 1-8 1 

Fiber Bragg Grating Sensors: A Look Back 

Jacques Albert* 

Department of Electronics, Carleton University, 1125 Colonel by Drive, Ottawa, ON, Canada K1S 5B6 

Andrea Cusano, Antonello Cutolo and Jacques Albert (Eds) 

All rights reserved - © 2011 Bentham Science Publishers Ltd. 

CHAPTER 1 

Abstract: The development of Fiber Bragg Gratings (FBGs) is closely associated with the field optical fiber 

sensors. This chapter presents a personal overview of the history of FBGs, with particular emphasis on the 

interrelation and impact of FBGs with sensing. The major milestones for FBG-based sensing are identified, from 

the very discovery of fiber gratings to current developments that are described in full detail in the following 

chapters of this book. 

THE DISCOVERY YEARS 

Fiber Bragg gratings (FBGs) were discovered in 1978 by Ken Hill at the Communications Research Center in 

Ottawa [1]. It is one of those rare discoveries that are undoubtedly associated with an individual, to the point that 

FBGs were called “Hill” gratings in the early days by several researchers (a term that was later reserved to gratings 

made using the same technique as the original discovery, i.e. internal writing) [2]. I do not intend to write a history 

of FBGs in this chapter, but rather to relate how this history in closely associated with optical fiber sensors in 

general. And this association starts from the very beginning. One of the ways that were used to measure the strength 

and line widths of these gratings was to measure the transmission of the grating with monochromatic light and a 

detector while stretching or heating the grating. The grating spectrum was then recorded as a function of strain or 

temperature, from which the grating period could be calculated. Therefore the first FBGs were also the first FBG 

sensors! This was out of necessity. In 1977 it was very difficult to couple high brightness broadband light into the 

core of an optical fiber and to gather enough light in transmission or reflection to measure spectra with 

monochromators (even single mode couplers did not exist, only multimode ones had just been invented, also in 

Hill’s group [3]). Furthering the problem was that writing process of the first Hill gratings was internal to the fiber 

core, arising from a standing wave pattern set up by the Fresnel reflection at the end of the fiber. The grating period 

was thus fixed to reflect only light at the writing wavelength. The fibers used were also very lossy (gratings were 

written directly with blue-green light at relatively low intensity because these early fibers were full of defects and 

color centers). These issues limited severely the applications of these gratings but it was widely recognized that if 

gratings of arbitrary periods could be written in “good” fibers the potential for new devices for processing and 

measuring guided light was enormous. In the years that followed the mechanism of the formation of FBGs was more 

or less elucidated as a two-photon process due to the interaction of ultraviolet (UV) light with defects states just 

below the bandgap of the doped silica forming the core [4]. 

I am not quite sure what led Gerry Meltz and colleagues from the United Technologies Research Center to achieve 

the next breakthrough in FBG technology, i.e. a practical way to use ultraviolet light directly to write FBGs. 

However, I must side track for a moment to mention a related activity in those days: the generation of second 

harmonic light from “prepared” fibers, a discovery by Walter Margulis and Ulf Osterberg [5]. In this process, intense 

pulses of light propagating down a single mode fiber for a certain amount of time eventually cause a material 

modification that leads to the formation of a second-order nonlinearity in the glass (a normally impossible feature in a 

material with inversion symmetry, such as glass). Moreover, the second order nonlinearity thus formed is 

automatically phase matched to the light that caused it because the induced effect is periodic, i.e. a χ 2 grating is 

formed inside the fiber [6]. Basically, they saw green light coming out of fibers in which only infrared light was 

launched. This process becomes significantly more efficient if the fiber is “prepared” by sending a weak probe signal 

at the second harmonic alongside the pump beam. The reason I am mentioning this here is that the FBG process and 

the self-organized second harmonic generation (SHG) effects were long thought to arise from the same physical 

mechanisms and that there was a strong overlap between the research communities. It is likely that advances in the 

*Address correspondence to this author Professor Jacques Albert: Department of Electronics, Carleton University, 1125 Colonel By Drive, 

Ottawa, ON, Canada K1S 5B6; Tel: +613 520-2600; Fax: +613 520-5708; Email: Jacques_albert@carleton.ca

2 Fiber Bragg Grating Sensors Jacques Albert 

SHG field helped several advances in FBG science and technology. In fact, the first scientific workshops and 

meetings on these two topics were held jointly (more on this later). 

FIBER BRAGG GRATINGS BECOME A REALITY 

It was the 1989 breakthrough publication by Meltz, Morey and Glenn actually made FBGs practical [7-8]. Since 

FBGs were basically permanent holograms written in the core of an optical fiber, they decided to try holographic 

techniques to write FBGs from the outside of the fiber. Since it had been determined by then that UV light was the 

source of the FBG writing process, two questions had to be addressed: would the cladding of the fiber be 

transparent enough to the UV light to reach the core and was there a light source with enough power and spatial 

coherence to form interference fringes of sufficient contrast and intensity to write FBGs? Luckily, the cladding of 

most optical fibers (even those at the time) is made of very pure undoped synthetic silica, whose absorption edge 

lies beyond 180 nm in the UV. Since the most prominent defect band thought responsible for the formation of 

FBGs is a germanium oxygen deficient color center with absorption near 242 nm, the cladding would not pose a 

problem. For the UV source there was not much to choose from in the late 1980s. They had to use a rather intricate 

system made up using a tunable excimer-pumped dye laser, operated at a wavelength in the range of 486-500 nm, 

along with a frequency-doubling crystal to provide UV light near 242 nm adequate coherence. The advantage of that 

particular system was that they could tune the laser wavelength and thus prove that the FBG writing process was more 

efficient near 242 nm. Of course, forming the interference fringe pattern from the outside also allowed the writing of 

gratings with almost arbitrary periods, and hence to make wavelength filters and sensors for practical wavelength 

bands (near 800, 1300, and 1500 nm for instance). The same group also demonstrated very early on the formation of 

tilted gratings that were used then to tap some of the light from the core of the fiber to make it radiate outside [9]. 

But back to sensors! The FBG is essentially a wavelength filter, bandpass in reflection and band reject in 

transmission. Since the period of the grating necessary to reflect core guided light in glass fibers is of the order of 

half a micrometer, even millimeter long gratings have Q-factors (number of grating periods / grating length) of at 

least 2000, resulting in pass bands that are of the order of 1 nm and less for light at visible and near IR wavelengths. 

Furthermore the combined thermo-optic coefficient of the reflection wavelength (hereafter called the “Bragg” 

wavelength) was soon calculated and measured to be near 10 pm/°C, arising from a combination of the (weak) 

coefficient of thermal expansion of silica glass and the relatively stronger thermo-optic coefficient of the refractive 

index (+10 -5 /°C). Similarly for strain, whereas fibers can be stretched without breaking by as much as 1%, with a 

corresponding change in the period of the FBG [7, 10]. Therefore with suitable instrumentation to launch light with 

at least a few nanometers of bandwidth and to measure the reflected optical spectrum (fused fiber couplers for single 

mode fibers and optical spectrum analyzers had been invented and mass produced by the late 1980s!), a fiber with 

an embedded FBG could be used to measure temperature or strain in very small spaces, remotely from relatively 

large distances, and in areas where electrical devices were not welcomed. Interestingly, the original patent on side 

writing, filed in October of 1986, explicitly mentions temperature and strain sensing as claimed applications and 

provides the equations to calculate the wavelength shifts of FBGs as a function of these two parameters. Knowing 

that Meltz’s group at UTC was concerned with the development of fiber optic (strain and temperature) sensors from 

at least the mid 1970s (US patents from 1981), it is fair to say that the need for better fiber sensors was the main 

reason FBGs were developed in the first place. The attractive sensing features of FBGs were immediately 

recognized by the photonics community and led to an incredible flurry of activity worldwide from the early 1990s 

onward to this day. Another aspect of optical sensing where FBGs were recognized to have great potential was in 

the development of narrowband, high coherence lasers through the use of FBGs as mirrors for fiber laser cavities 

and also as pigtailed output couplers for semiconductor lasers [11-13]. These efforts were made in parallel with 

similar research activities to find applications for FBGs in other fields, especially telecommunications. The 1990s 

were the golden age of optical telecommunications research as the growth of the demand for long distance 

bandwidth was fueled by the birth of the internet and fed by several breakthroughs (low loss single mode fibers and 

erbium-doped fiber optical amplifiers). Just to give a flavor of the enthusiasm associated with FBGs in that field, in 

2000, near the peak of what has been called the “telecom bubble” the market forecast for FBGs in telecom alone was 

predicted by some analysts to reach 1 B$/year within the first half of the decade. At grating prices between 10 $ and 

1000 $ depending on the application, that would have been a lot of gratings. 

Back to sensors again! It is not so well known as I write this that a large fraction of the early research on FBGs was 

for sensing applications. The Optical Fiber Sensors conference had had its first occurrence in 1983, and the field was

Fiber Bragg Grating Sensors: A Look Back Fiber Bragg Grating Sensors 3 

certainly as exciting during the ensuing years as the optical communications field. In parallel, the FBG community 

organized itself and pioneers of the field, Francois Ouellette and Raman Kashyap organized the first two 

international conferences or workshops on the topic in 1991 and 1993 in Quebec City [14-15]. It is actually 

fascinating to browse through the proceedings of these conferences to see who was involved then and what they 

were working on. Surprisingly, some of the research topics discussed then are still very relevant today, such as the 

photosensitivity of rare earth doped glasses and the UV-induced birefringence of FBGs. As indicated earlier, these 

two conferences, and also the following one that was organized as an Optical Society of America Topical meeting in 

Portland, Oregon in 1995 [16], also included what was known then as “self-organization” and quadratic 

nonlinearities in optical fibers and waveguides. For the most part these were concerned with light-induced 

modifications of glassy materials to induce a second order nonlinearity and hence fabricate devices for second 

harmonic generation and for electro-optic modulation in glass fibers and waveguides. Achieving these goals with 

enough efficiency for practical devices would have had a tremendous impact in photonics, possibly as important as 

the invention of the EDFA. 

A MOST NOTABLE YEAR: 1993 

The year 1993 is possibly the single most important year in the history of FBGs, with the discovery of two more 

enabling technologies on the path to wide acceptability and opening up real life applications: the phase mask and 

hydrogen-loading. 

First, looking back to 1991, and following the publication of the side-writing paper by Meltz et al., Ken Hill’s group 

at the Communications Research Center resumed an intense research effort in FBGs. They had both significant 

hurdles to overcome and also some distinct advantages over other groups that jumped in this field. The hurdle was 

that the laser source that was used for side writing at the time was by no means easy to put together or to operate. 

However, Hill’s group had an old excimer laser available and they were actively working on SHG and selforganization 

at the time, using intense pulsed light to fabricate point-by-point mode converter gratings in fibers (the 

ancestors of Long Period Gratings), using side exposure [17]. Of course they had intimate knowledge of every 

aspect of FBG modeling and applications. It is said that necessity is the mother of invention. The lack of coherence 

of the excimer laser turned out to be the main reason Ken Hill was driven to look for a way to generate an 

interference pattern of UV light without an interferometer. This is how the phase mask came about [18]: using a 

diffractive optical element to generate two light beams from a single one, at such an angle between the beams that 

the resulting interference pattern has the necessary periodicity to print a FBG in a fiber. Because the phase mask can 

be placed so close to the fiber (within microns of the fiber surface) the two interfering beams generated by the phase 

mask need only to have spatial coherence of the order of a few tens of microns for their interference pattern to have 

sufficient visibility to write the gratings. Relatively simple calculations and tests confirmed all the necessary features 

of the phase mask and a supplier was found to fabricate them. It is worth mentioning that the invention of the phase 

mask was contested in court by another group who had a similar idea for writing FBGs, but with using an amplitude 

mask instead. Because it is possible to null the zeroth-order transmission with a phase mask, the FBG writing 

efficiency is greatly optimized and amplitude masks are no longer used for FBGs. The second crucial development 

in 1993 was the discovery that the diffusion of molecular hydrogen gas in optical fibers at relatively high pressure 

and for a duration long enough to saturate the fiber led to an extremely large enhancement of the photosensitivity of 

the fibers to UV light [19] (it is interesting to note that a similar enhancement had been observed a few years earlier 

for SHG experiments in hydrogen-loaded fiber by Francois Ouellette, a member of Ken Hill’s group at the time 

[20]). Instead of FBGs with grating modulation amplitudes barely reaching 10 -4 without hydrogen, modulation 

amplitudes approaching 10 -2 were achieved. This improvement by two orders of magnitude not only increases the 

reflectivity of gratings, it is actually crucial in making some devices possible at all, such as FBG-based gain 

equalizing filters for which the whole dynamic range of the photosensitive process is necessary to achieve the 

product specifications. 

It was also at that time that “photosensitive” fibers were invented, i.e. fibers with dopant materials and profiles 

customized to enhance the formation of permanent refractive index changes under UV irradiation [21]. One of the 

driving forces for such developments was the perceived need to avoid hydrogen-loading, since that technique 

provided as much photosensitivity as needed in low cost, high quality standard telecommunications fiber. The 

problem with hydrogen-loading is that there is a safety issue related to handling this gas at high pressure and that the


Fiber Bragg Gratings: Advances in Fabrication Process and Tools 

Kate Sugden* and Vladimir Mezentsev 




CHAPTER 2 

Abstract: Successful commercialization of a technology such as iber Bragg ratings requires the ability to 

manufacture devices repeatably, quickly and at low cost. Although the first report of photorefractive gratings was 

in 1978 it was not until 1993, when phase mask fabrication was demonstrated, that this became feasible. More 

recently, draw tower fabrication on a production level and grating writing through the polymer jacket have been 

realized; both important developments since they preserve the intrinsic strength of the fiber. Potentially the most 

significant recent development has been femtosecond laser inscription of gratings. Although not yet a commercial 

technology, it provides the means of writing multiple gratings in the optical core providing directional sensing 

capability in a single fiber. Femtosecond processing can also be used to machine the fiber to produce micronscale 

slots and holes enhancing the interaction between the light in the core and the surrounding medium. 

INTRODUCTION 

When Hill discovered the formation of photorefractive gratings in 1978 few would have imagined the commercial 

promise this technology would eventually hold [1-2]. The Hill gratings were fabricated using the interference 

between counter propagating laser beams from an Argon ion laser operating at 488 nm. The gratings were long, 

which meant they were of extremely narrow bandwidth, and they reflected the same wavelength of light that was 

used to fabricate them. These two factors limited their practical applications in sensing. 

An approach yielding somewhat more useful devices was demonstrated in 1986 [3], where periodic filters were 

written using a photoresist layer and surface relief etching. The cladding layer of the fiber was polished down to 

give a flat surface which was overlaid with the photoresist and then exposed and etched. Strong gratings with greater 

than 90% reflectivity were made. However, although similar to conventional fiber Bragg gratings in reflection this 

process induced large transmission losses due to modal out-coupling which ultimately limited the number of 

gratings that could be interrogated on a single piece of fiber. In addition, because of the multistage process, this 

approach was never going to be one of low cost volume fabrication. 

What was arguably the most significant development came in 1989 when a bulk optic interferometer was used to 

directly write gratings into the fiber using side illumination with a UV laser [4]. Here the writing method itself was 

not that novel but the realization that the photorefractive change with ultraviolet light was enough to allow direct 

side writing of devices was a key development. This significant advance heralded a period of rapid development in 

the field of fiber Bragg gratings and, combined with the demonstration of phase mask writing technique in 1993 [5- 

6], provided the foundation of most of the work in this area today. 

This chapter concentrates on the areas of the fabrication of fiber Bragg gratings that are most relevant to the sensing 

field today. It first summarizes the background, including photosensitivity, then looks at the different grating types that 

can be fabricated, grating stability and choice of fiber and laser sources. It then reviews the more practical schemes 

proposed for fabricating fiber Bragg gratings, including the use of interferometers, phase masks, phase mask 

interferometers, direct writing, and higher order masks. Draw tower fabrication, writing through the coating and 

packaging are also discussed in this section. The final section of this chapter is given over to the fabrication of fiber 

Bragg gratings with femtosecond lasers which is one of the most exciting developments in the field in recent years. 

*Address correspondence to this author Dr. Kate Sugden: Photonics Research Group, Aston University, Aston Triangle, Birmingham B4 

7ET, United Kingdom; Tel: +44 121 204 3498; Fax: +44 121 204; Email: k.sugden@aston.ac.uk

10 Fiber Bragg Grating Sensors Sugden and Mezentsev 

BACKGROUND 

Photosensitivity 

Before discussing the techniques for fabricating fiber Bragg gratings it is important to understand the underlying 

mechanisms. The term ‘photosensitivity’ is used to describe the permanent change in the refractive index of a 

material by exposure to light. It is this photosensitivity effect that is exploited in the fabrication of fiber Bragg 

gratings since it enables a variation in laser light intensity to be recorded directly into the fiber as a refractive index 

variation. The result of this is seen in (Fig. 1) which shows a microscope image of a number of grating fringes 

inscribed in the core of a single mode optical fiber. 

Figure 1: Microscope image of fiber Bragg grating fringes written in the core of a fiber [7]. 

The majority of commercial gratings to date have been written using ultraviolet (UV) lasers in a single photon 

excitation of germanosilicate based defects. Hill’s early demonstration of photosensitivity using a 488 nm laser to 

produce Hill gratings relied on a two-photon process [8] and was related to an absorption band around 240 nm. 

In the single photon regime the ultraviolet radiation interacts with various defect centers in the fiber [9-10]. The 

exposure initiates a bleaching in the absorption band at 240 nm and the creation of other absorption bands that lead 

to a refractive index change that can be described using the Kramers-Kronig relationship [11]. The photosensitivity 

in germanosilicate fibers is generally proportional to the concentration of the Ge dopant. 

It is possible to produce significantly larger refractive index changes by ‘hydrogen loading’ the fiber [12]. Hydrogen 

loading involves soaking the fiber in a high pressure hydrogen environment either at room temperature for 1-2 

weeks or at an elevated temperature for a shorter time. For example, at 80°C the fiber only requires around 24 hours 

to absorb sufficient hydrogen. 

By studying the growth dynamics of gratings in a variety of different fibers, laser powers and wavelengths a number 

of distinct gratings types have been identified. The distinctions between them depend on the exposure conditions, 

the type of induced changes within the glass, and the final grating characteristics. 

The technology developed largely due to the push of the telecommunications market which had requirements for 

long lifetimes and high stability but where devices operate in relatively benign environments below 100°C. Since 

sensing gratings are often used in harsh environments such as in oil and gas applications, careful consideration needs 

to be given to the temperatures the gratings will encounter over their lifetime. Tests must carried out on the thermal 

decay rate of gratings made under the same fabrication conditions as the gratings to be deployed (power, exposure 

time, hydrogenation conditions, fiber type) before setting up a successful anneal regime since all of these things 

affect the decay rate and long-term stability. 

Grating Types 

A number of different grating types have been identified to date, however initially the names given to these different 

types were somewhat misleading. A new classification system was recently proposed [13] which groups grating 

types according to the similarity in growth mechanisms and this system is followed here.

Fiber Bragg Gratings: Advances in Fabrication Process and Tools Fiber Bragg Grating Sensors 11 

There are three basic types of grating associated with UV inscription and these are known as Type I, Type II and 

Regenerated. Type I gratings are associated with refractive index changes that occur below the damage threshold of 

glass whereas Type II gratings are associated with changes in refractive index above the damage threshold of glass. 

Each type is summarized below. There are several Type I sub-classes that are identified according to the writing 

conditions and properties of the gratings. 

(Fig. 2) shows three different types of gratings written with the same period phase mask. The three gratings reflect 

light at different wavelengths. This difference in the reflected wavelength occurs because the refractive index 

change produced by each process is associated with a different mechanism and has a different value. 

Figure 2: The spectra of Type I (blue), Type IHp (red), and Type In (green) gratings written in B/Ge fiber [14]. 

Type I 

Type I – These are currently the most common gratings used and often referred to as standard gratings. These 

gratings can be fabricated in most types of germanosilicate fiber. In the reflected spectrum there is negligible light 

lost to the cladding modes or by absorption. The refractive index change associated with this process is usually 

(although not in the case of Type In) positive i.e. the average refractive index in the exposed area is higher than the 

unexposed core. 

The underlying mechanism, at least for small refractive index changes, is a one-photon absorption effect [5]. The 

process works by exciting defect centers where the energy levels correspond to oxygen deficiency centre (ODC) 

absorption bands at wavelengths around 244 nm and 320 nm. These same bands can also be excited by multi-photon 

processes using femtosecond lasers, this will be discussed later in the chapter. For large refractive index changes it is 

likely that some type of densification change is also involved [15]. 

Erdogan showed that Type I Bragg gratings exhibited a temperature dependent decay [16]. It was shown that the 

decay in the reflectivity could be characterized using a power law. Immediately after the inscription the decay rate 

was at its maximum and over time this decay rate decreased. The starting rate of decay was dependent on the 

surrounding temperature. This decay is due to the thermal depopulation of the trapped excited states created during 

the UV irradiation. Elevating the temperature gives the carriers in the shallowest traps enough energy to escape and 

return to the ground state. The carriers remaining are associated with more stable traps, however if the temperature 

is steadily increased progressively more of these will decay resulting in a further decrease in the grating strength. 

Since it is the most unstable carriers that decay at low temperature Bragg gratings can be annealed at an elevated 

temperature prior to use and this removes the least stable part of the induced refractive index change. Post-anneal the 

grating exhibits a much greater level of stability over the longer term. This annealing process can also be used to 

stabilize other Type I grating sub-types although for each type the related decay properties must be individually 

characterized.


Fiber Bragg Gratings: Analysis and Synthesis Techniques 

Johannes Skaar* 



CHAPTER 3 

Department of Electronics and Telecommunications, Norwegian University of Science and Technology, Trondheim, 

Norway; University Graduate Center, Kjeller, Norway 

Abstract: Common methods for modeling, analysis, and synthesis of Fiber Bragg Gratings are reviewed in detail, 

including coupled-mode theory, transfer matrix methods, and layer-peeling algorithms. 

INTRODUCTION 

In order to design, fabricate, and use fiber Bragg gratings in various applications, tools for analysis, synthesis and 

characterization are crucial. For example, when designing gratings, a suitable mathematical model is central to 

understand the relation between the spatial grating structure and the reflection and transmission spectra. Moreover, 

fabrication of gratings is facilitated by synthesis methods, to extract the actual spatial structure from spectral 

measurements. Similarly, when using fiber gratings as sensor elements, synthesis methods can be used to extract any 

distributed sensing parameter (e.g., temperature, strain, etc.) along the fiber. 

A simple model of a fiber Bragg grating is the following: the grating reflects a narrow band of the incident optical 

field by successive, coherent scattering from the index variations. When the reflection from a crest in the index 

modulation is in phase with the next one, we have maximum mode coupling or reflection. Then the Bragg condition 

is fulfilled, i.e., 

λB eff 

2n 

Λ , (1) 

where λB is the Bragg wavelength, neff is the effective modal index, and Λ is the perturbation period. Although this 

model captures one of the most important properties of fiber Bragg gratings, it cannot tell us how much light is 

actually reflected, how large bandwidth, detailed spectral dependence including reflectivity and group delay, impact 

of perturbations or imperfections, and so forth. We thus need a model that can describe the detailed wave 

propagation in fiber Bragg gratings, and therefore how the optical filter characteristics relate to the amplitude and 

phase modulated quasi-periodic refractive index profile. 

The most common mathematical model governing wave propagation in fiber Bragg gratings is coupled-mode 

theory. Other techniques are available; however, we will only consider coupled-mode theory since it is by far the 

most popular method. Coupled-mode theory is relatively straightforward and intuitive, and for most practical fiber 

gratings of interest it is accurate. In Section 2 we will derive the coupled-mode equations from the scalar wave 

equation. Once the governing equations are established we use them in Section 3 for analyzing two important 

special cases for which closed-form expressions for the reflectivity exist; uniform gratings and weak gratings. In 

Section 4 we develop general numerical techniques for computing the spectrum associated with any grating profile. 

Section 5 contains a description of the inverse problem, namely to obtain the grating profile from the reflection 

spectrum. Finally, we sum up some important spectral properties and constraints in Section 6. 

COUPLED-MODE THEORY 

The relation between the spectral dependence of a fiber grating and the corresponding grating structure is usually 

described by coupled-mode theory. Coupled-mode theory is described in a number of texts; detailed analysis can 

be found in Refs. [1-5]. The notation in this section follows most closely that of Refs. [1, 4]. We assume that the 

fiber is lossless and single mode in the wavelength range of interest. In other words, only one forward and one backward 

*Address correspondence to this author Professor Johannes Skaar: UNIK – University Graduate Center, Postboks 70, NO-2027 Kjeller, 

Norway; Tel: +47 64844748; Fax: +47 63818146; Email: johannes.skaar@iet.ntnu.no

36 Fiber Bragg Grating Sensors Johannes Skaar 

propagating modes are considered. Moreover, we assume that the fiber is weakly guiding, i.e., the difference 

between the refractive indices in the core and the cladding is very small. Then the electric and magnetic fields are 

approximately transverse to the fiber axis, and we can ignore all polarization effects due to the fiber structure and 

consider solely the scalar wave equation [1]. For analysis and synthesis of birefringent gratings or gratings in 

multimode fibers the reader is referred to Refs. [6-7]. The fiber axis is oriented in the z direction and we assume that 

the electric field is x-polarized. The implicit time dependence is exp(–iωt); thus a forward propagating wave with 

propagation constant β > 0 and frequency ω > 0 has the form exp[i(βz–ωt)]. 

The grating is treated as a perturbation of the fiber. The unperturbed fiber has refractive index profile n(x,y), and the 

perturbed fiber has the z-dependent index n(x,y,z). Both fibers are assumed to be weakly guiding, so in the core and 

the cladding we have n(x,y,z) n(x,y) neff ncl nco. Here ncl and nco are the indices in the cladding and the 

core, respectively, and neff is the effective index of the supported mode in the absence of the grating. We write the 

total electric field as a superposition of the forward and backward propagating modes, 

Ex 

 

(x,y,z) b (z)Ψ(x, 

y) b (z)Ψ(x, 

y) , (2) 

where the coefficients b± contain all z-dependence of the modes. It is clear that b± are dependent on frequency since 

they include the harmonic propagation factor exp(±iβz), with β = β(ω) = neff ω/c the scalar propagation constant. The 

transverse dependence is described by the function Ψ, which satisfies the scalar wave equation for the unperturbed 

fiber, 

2 2 2 2 

k n (x,y) β Ψ 0 

t (3) 

where t 2 = ∂ 2 /∂x 2 + ∂ 2 /∂y 2 , and k = ω/c is the vacuum wavenumber. The total electric field Ex must satisfy the 

scalar wave equation for the perturbed fiber, i.e., 

2 2 2 

2 2 

n (x,y,z) / z 

E 0 

t x 

k (4) 

By substituting Eq. (2) into Eq. (4), and using Eq. (3), we obtain 

2 

 

z 

2 

(b 

 

b )Ψ [β k (n 

 

2 

2 

2 

n 

2 

)](b 

 

b )Ψ 0 

Multiplication by Ψ and integration over the xy-plane leads to 

2 2 

b 

d b 

2 

(β 

2 2 

 

d 

dz 

dz 

where we have defined the coefficient D as 

D(z) 

k 

2n 

co 

(n 

2 

n 

2 

Ψ 

dA 

 

2knco 

D(z))(b 

b ) 0 

(6) 

2 

)Ψ 

2 

dA 

The integrations extend over the entire xy-plane. Eq. (6) can be decomposed into the following set of first order 

differential equations [1] 

db 

i(β D)b 

iDb 

, (8a) 

dz 

db 

i(β D)b 

iDb 

, (8b) 

dz 

(5) 

(7)

Fiber Bragg Gratings: Analysis and Synthesis Techniques Fiber Bragg Grating Sensors 37 

as is readily realized by differentiation and summation of the two Eqs. (8). This decomposition amounts to 

separating the total field in Eq. (2) into its forward and backward propagating components. Indeed, when n = n, we 

observe that the solution of Eqs. (8) reduces to b±(z) = B± exp(±iβz) with constant B±, i.e., b± correspond to the 

forward and backward propagating modes. In the absence of the grating, n = n, the modes propagate without 

affecting each other; otherwise the modes will couple to each other through the quantity D(z). 

For a fiber grating, the z-dependence of the index perturbation is approximately quasi-sinusoidal in the sense that it 

can be written 

2 2 2π 

 

n n Δεr,ac(z) 

cos 

z θ(z) Δεr,dc(z) 

, (9) 

Λ 

 

 

where Λ is a chosen design period so that θ(z) becomes a slowly varying function of z compared to a period Λ. The 

functions Δεr,ac(z) and Δεr,dc(z) are real and slowly varying, and satisfy 

r,ac 

2 

co 

2 

co 

|Δ (z)| n , |Δ (z)| n . (10) 

r,dc 

It is therefore convenient to express D as a quasi-sinusoidal function 

D(z) 

2π 

* 2π 

 

κ(z) exp i 

z κ (z) exp 

i z σ(z) , (11) 

Λ 

Λ 

 

 

where κ(z) is a complex, slowly varying function of z and σ(z) is a real, slowly varying function that accounts for the 

dc index variation from εr,dc(z). In order to simplify Eqs. (8), we define new field amplitudes u and v by setting 

b (z) 

 

 

 

π 

z 

u(z) exp i z exp( 

i 

σ(z')dz' ) 

(12a) 

Λ 

0 

π 

z 

b(z) 

v(z) exp i z exp( i0 

σ(z')dz' ) . (12b) 

Λ 

By substituting Eqs. (11) and (12) into Eqs. (8), ignoring the terms that are rapidly oscillating since they contribute 

little to the growth and decay of the amplitudes, we arrive at the coupled-mode equations 

du 

dz 

iδu 

q(z)v , (13a) 

dv * 

iδv 

q (z)u . (13b) 

dz 

In Eqs. (13) we have defined the wavenumber detuning δ = β – π/Λ, and the coupling coefficient q of the grating 

q(z) 

z 

0 

iκ(z) 

exp ( 2i 

σ(z')dz' ) . (14) 

Note that all phase factors in Eqs. (12) are independent of the propagation constant β and therefore the frequency. 

Hence, we can simply treat the new variables u and v as the fields themselves once the reference planes have been 

fixed, since they differ only from b± by constant phase factors. For example, the reflection coefficient b–(z0)/b+(z0) 

can as well be computed by the expression v(z0)/u(z0) once the position z0 has been fixed because the two 

expressions only differ by a constant phase factor 1 . Also note that all functions u, v, and q are slowly varying with z 

compared to the period Λ because β π/Λ when the wavelength is close to the Bragg wavelength λB = 2neff Λ. 

1 Strictly speaking, σ(z) is dependent on frequency (see Eq. (16)). However, since fiber gratings usually have narrow bandwidths, this dependence 

can be ignored to a very good approximation.




CHAPTER 4 

Photonic Bandgap Engineering in FBGs by Post Processing Fabrication 

Techniques 

Andrea Cusano 1,* , Domenico Paladino 1 , Antonello Cutolo 1 , Agostino Iadicicco 2 and 

Stefania Campopiano 2 

1 Optoelectronic Division – Engineering Department, University of Sannio, Corso Garibaldi 107, 82100 Benevento, 

Italy and 2 Department of Technology, University of Naples “Parthenope”, Centro Direzionale di Napoli Isola C4, 

80143 Napoli, Italy 

Abstract: The incredible growth of Fiber Bragg Gratings (FBGs) from both the research and the commercial 

points of view, has forced the development of several methods able to tailor their spectral characteristics for 

specific applications. Basically, two different approaches can be adopted to specialize the grating spectra: one is 

based on complex grating profiles induced directly at the fabrication stage. Another, more interesting, approach 

relies on post processing methodologies that introduce localized defects along the grating, breaking its periodic 

structure. The presence of the defects leads to the formation of allowed bands or defect states within the grating 

bandgap. The introduction of finer scale spectral features results in new interesting perspectives in both 

telecommunications and sensing fields. In addition, the possibility to accurately control the defect states spectral 

features – depth, bandwidth, and spectral position – could allow the complete engineering of the FBG bandgap, 

opening the way to the realization of several new advanced and attractive photonic devices. This chapter is 

focused to review the main advancements in FBGs post processing to easily control the spectral features of the 

final device for specific applications. 

INTRODUCTION 

Thanks to their peculiar spectral characteristics, Fiber Bragg Gratings (FBGs) are widely popular devices within and 

out the photonic community [1-2]. Basically – by means of a UV induced periodic modulation of the core refractive 

index along the hosting optical fiber – FBGs are able to selectively reflect only a narrow wavelength range (typically 

0.1-1 nm), giving rise to an effective Photonic BandGap (PBG). Consequently, FBGs – as they stand – represent an 

useful tool to manipulate the propagating features of the light traveling along an optical fiber [3], and can be usefully 

exploited to develop wavelength encoded sensing elements in numerous application fields [4-5]. As a consequence 

of the technological assessment in FBGs fabrication, in the mid 1990’s many research groups have been engaged in 

the study and realization of new fiber grating devices through more complex refractive index modulation profiles [6- 

12]. The common idea was to complicate the core refractive index modulation allowing further manipulation of the 

light propagating within the fiber grating structures. In this way, it is possible to realize fiber grating with peculiar 

spectral characteristics either in reflection and transmission. 

Since FBGs can be considered one-dimensional PBG structures realized within optical fibers by UV writing, among 

the different complex grating structures, a particular and interesting approach relies on the inclusion of local defects 

breaking the FBG periodicity along the grating axis. One of the most important modern research field, in fact, is 

related to the novel and unique way to control many features of the electromagnetic radiation by opportune PBG 

structures. These artificial structures are characterized by one-, two-, or three-dimensional periodic arrangements of 

dielectric materials and are commonly called Photonic Crystals (PhCs) [13]. 

PhCs are the optical analogue to electronic semiconductors, i.e., PhCs can be considered semiconductors for photons: 

in the electronic case, the wave functions of electrons interact with the periodic potential of the atomic lattice and, for 

a certain range of energies (similar to the frequencies of photons), electronic states cease to exist, thus giving rise to 

an electronic bandgap. For PhCs, the analogous of the electronic potential in an atomic crystal is the spatial 

distribution of the dielectric constant of the PhC structure. Moreover, due to a periodic interaction, certain PBGs 

appear, forbidding the propagation of certain frequency ranges of light. In such a structure, a line or point defect, which 

*Address correspondence to this author Professor Andrea Cusano: Optoelectronic Division – Engineering Department, University of Sannio, 

Corso Garibaldi 107, 82100 Benevento, Italy; Tel: +39 0824305835; Fax: +39 0824305846; Email: a.cusano@unisannio.it

54 Fiber Bragg Grating Sensors Cusano et al. 

breaks the lattice periodicity, can be introduced wherein a mode is localized in virtue of being suppressed by the 

lattice. In practice, defects break the PhC structure symmetry and may give rise to a defect state or an allowed 

frequency within the PBG region. Several research groups proposed and experimentally demonstrated optical 

trapping and dropping functions by exploiting resonant tunneling [14], cavity wave-guiding coupling phenomena in 

one-dimensional [15-16], two-dimensional [17], and three-dimensional [18] structures. 

In the case of FBGs, the introduction of local defects introducing a phase shift along the grating can be substantially 

obtained by adopting two different methods: 

- by acting at the fabrication stage through the modification of the photo-induced core refractive index 

modulation generating punctual or distributed phase shift; 

- by post processing approaches aimed to locally modify the FBG physical and/or geometrical features 

generating distributed phase shift. 

The common spectral effect is to induce narrow transmitted windows or defect states within the standard FBG stopband. 

Clearly, narrower spectral features would further increase the FBG potentialities in both telecommunications 

and sensing. 

Even if this chapter is focused on the second class of FBG based devices, here it is important to briefly summarize 

also the main milestones characterizing the first category. 

In 1986, it was originally demonstrated by Alferness et al. [19] that the insertion of a single quarter-wave phase shift 

at the center of a Bragg grating waveguide opens a band-pass peak within the stop-band. 

The same principle of operation was adopted in 1990 by Reid et al. to realized phase-shifted Moiré grating 

resonators [20]. The Moiré approach to fiber grating transmission filter fabrication was reported for application to 

side-etched, surface-relief structures. Later, the same technique was adopted using the UV direct-write grating 

fabrication method and uniform FBGs [21]. The Moiré grating resonators were fabricated using the same technique 

as for standard FBGs, but adopting the holographic exposure. Moirè gratings were formed by a double exposure to 

two interference patterns of slightly different periods. 

In 1994, Agrawal and Radic modeled the response of phase-shifted FBGs and showed that the transmitted peak 

shifts with the amount of the phase shift in the middle of the grating [22]. FBGs fabricated with single or multiple 

phase shifts at various precise locations along the grating have been proposed for creating narrow spectral 

resonances for both filtering and laser applications [23-24]. Such structures can be produced at the fabrication stage 

by using specially designed phase masks incorporating the required phase shifts with restrictions on the magnitude, 

position, and number [24-26]. However, the cost of this procedure, the dependence on the operating wavelength and 

the need of a large variety of phase masks to produce structures with different and optimized spectral properties for 

specific applications are the major limitations of this approach. 

Phase shifts were also realized by detuning the relative position between the phase mask and the optical fiber 

hosting the grating at properly selected longitudinal positions [27-28]. In this case a piezoelectric ceramic translation 

stage was used to change the relative position during the fabrication stage, but, in principle, it would be extremely 

useful to separate the FBG fabrication stage from the successive introduction of defects along their structure. In this 

sense, during the last decades several mechanisms have been adopted to introduce phase shifts along FBGs, 

allowing also a more accurate and adjustable control of the induced phase shift amount. 

In particular, many research studies focused their attention on the development of wavelength independent post 

processing techniques able to induce distributed phase shifts along the grating structure enabling the tailoring of the 

bandgap features. In this chapter, the main technological steps are carefully reviewed. 

POST PROCESSING APPROACHES: A VIEW BACK 

The first attempt to introduce a defect along a FBG by post processing methods was dated 1994 by Canning and Sceats 

[29]. Starting from the demonstration of grating writing by point-by-point technique [30] – having the significant 

advantage of allowing arbitrary phase gratings to be written – here the basic idea relied on the demonstration that a

Photonic Bandgap Engineering in FBGs by Post Processing Fabrication Techniques Fiber Bragg Grating Sensors 55 

local UV post processing treatment of a periodic grating can be used to generate a complex grating. The local 

irradiation by UV light raises the general refractive index at a certain region along the fiber grating as shown in (Fig. 1). 

Such post processing produces two gratings out of phase with each other which act as a wavelength selective Fabry- 

Pérot resonator allowing light at the resonance to penetrate the stop-band of the original grating. The resonance 

wavelength depends on the size of the phase change. For the experimental realization an homemade strong uniform 4 cm 

long FBG (reflectivity of more than 99% and Full Width at Half Maximum (FWHM) of about 0.15 nm, kL ~ 3) was 

selected. 240 nm light was focused directly onto the centre of the grating over a length of about 1 mm. The FBG 

transmission spectrum was monitored with a resolution of 1 pm. A defect state was observed to grow into the reflection 

bandwidth and after 12000 shots the transmission spectrum of (Fig. 2) was obtained. Continued processing resulted in 

no further changes, indicating that the processed region had reached a saturated level of index change. 

Figure 1: Introduction of a phase shift by raising the refractive index at a point along the FBG. (J. Canning et al. (1994), 

“π-phase-shifted periodic distributed structures in optical fibres by UV post-processing,” Electron. Lett. 30, 1344-1345. © IEEE. 

Reproduced with permission) 

transmission 

1.2 

1.0 

0.8 

0.6 

0.4 

0.2 

0.0 

q1 

L 

1 

UV beam 

q2 

1519.8 1519.0 1520.0 

wavelength,nm 

Figure 2: Normalized transmission spectrum of the grating after UV post-processing. (J. Canning et al. (1994), “π-phase-shifted 

periodic distributed structures in optical fibres by UV post-processing,” Electron. Lett. 30, 1344-1345. © IEEE. Reproduced with 

permission) 

The experimental results revealed also that, once the phase shift was introduced, an overall broadening of the whole 

bandwidth of the spectrum results. Theoretical calculations confirmed the experimentally measured broadening of 

the bandwidth. The obtained defect state demonstrated that a phase shift close to π was introduced. The narrowness 

of this defect state – close to the utilized resolution, in this case – is influenced by several factors, but, above all, by 

the strength of the grating. The most obvious applications include production of very narrowband transmission 

filters. In addition, the length of the grating resulted in high sensitivity to external perturbations such as temperature 

and strain where potential applications in sensor devices involving detuning of the resonator to detect small changes 

in the measurand can be envisaged. It is worth noting that such a technique could be used to introduce multiple 

phase shifts along the same grating to produce other devices such as comb filters. 

This technique was used in 2005 by Canning and associates to finely tune the phase shift notch spectral position in a 

Distributed Feed-Back (DFB) Photonic Crystal Fiber (PCF) laser configuration [31]. The DFB laser structure relied 

L 2

78 Fiber Bragg Grating Sensors: Recent Advancements, Industrial Applications and Market Exploitation, 2011, 78-98 

Fiber Bragg Grating Interrogation Systems 

José Luís Santos 1,2,* , Luís Alberto Ferreira 1,3 and Francisco Manuel Araújo 1,3 



CHAPTER 5 

1 INESC Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal; 2 Faculdade de Ciências da Universidade do 

Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal and 3 FiberSensing S.A., Rua Vasconcelos Costa 277, 

4470-640 Maia, Portugal 

Abstract: Fiber Bragg Gratings are structures with remarkable characteristics that have induced new qualitative 

developments in the broad field of optical fiber technology, most notably in optical communications and in 

optical sensing. When these devices are applied for sensing, the underlying concept is the modulation of the 

grating Bragg wavelength by the measured and, therefore, a central issue is the sensitive and accurate conversion 

of the resonant wavelength into a proportional electrical signal with the adequate format for further processing. 

This topic is broadly known as Fiber Bragg Grating interrogation and is the subject of the present chapter. It is 

organized in two parts: in the first one, the techniques developed by the scientific community looking for this 

functionality are reviewed, with emphasis on the identification of general conceptual classes where they fit; in the 

second part, illustrative and state-of-the-art commercial Fiber Bragg Grating interrogation systems are described. 

INTRODUCTION 

Since the beginning the optical fiber sensing concept promised novel developments in the vast area of 

instrumentation and measurement, conviction anchored in the unique feature of this technology which is the fact that 

the optical fiber is simultaneously sensing element and communication channel. Therefore, there is no need to 

consider a specific telemetry system that is always required in other sensing technologies and, probably more 

important, this dual characteristic of the optical fiber opens the natural consideration of fiber optic sensing networks 

for multi-point or distributed measurement. Up to the outcome of the fiber Bragg grating technology, a vast number 

of sensing solutions based on optical fibers were demonstrated, but only a few could overcome the demanding 

obstacles that are present in the industrialization and commercialization paths. In that sense, and certainly by other 

reasons, fiber Bragg gratings (FBG) were a technological breakthrough. In fact, these structures can be considered as 

almost ideal sensing elements. One central feature is the circumstance that the measurand information is, in the vast 

majority of situations, encoded in the resonance wavelength, which is an absolute parameter and provides unique 

self-referencing capacity. There are also other favorable characteristics, common to the optical fiber sensing 

technology, such as small size and weight, as well as immunity to electromagnetic interference. Additionally, the 

wavelength encoded characteristic of these devices confer them excellent multiplexing capability, which permits to 

address a large number of sensors with a single optical fiber cable [1]. 

In principle, the action of a particular measurand on a fiber Bragg grating can affect one or more of the 

characteristics of the device spectral signature, i.e., its resonance wavelength, the spectral width or the reflectivity. 

However, in most of the cases reported up to now the focus has been on the resonance wavelength direct modulation 

and, therefore, the subject known as FBG interrogation addresses the problem of transforming this wavelength 

modulation into an optical intensity modulation, and later into a corresponding electrical signal compatible with the 

common standards of instrumentation. Desirably, this transformation is accurate in the determination of the absolute 

Bragg wavelength and sensitive in the detection of small Bragg wavelength shifts, a perspective that in practice 

needs to be balanced by the factors complexity and cost in a particular FBG interrogation configuration. 

This chapter addresses the problem of interrogation of fiber optic sensors based on Bragg gratings. The first part 

reviews the interrogation concepts developed so far, emphasizing their core characteristics, performance and 

requirements, while the second part describes some state-of-the-art equipments commercially available and 

illustrative of the main FBG interrogation approaches. 

*Address correspondence to this author Professor José Luís Santos: INESC Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal; Tel: 

+351 220 402 302; Fax: +351 220 402 437; Email: josantos@fc.up.pt

Fiber Bragg Grating Interrogation Systems Fiber Bragg Grating Sensors 79 

FIBER BRAGG GRATING INTERROGATION 

There are several types of fiber Bragg gratings with specific optical transfer functions, but the standard structure has 

a reflectivity function with an approximately Gaussian shape centered at the resonance wavelength of the device and 

a spectral width around 0.2 nm. When a particular measurand direct or indirectly affects the effective refractive 

index of the core mode, or the period of the grating refractive index modulation, or both, then the resonance 

condition changes and there is a spectral shift of the grating signature. FBG interrogation means to convert this 

wavelength shift into a variation of an electrical signal with adequate characteristics to obtain the information about 

the measurand. The general principle of FBG interrogation is shown in (Fig. 1). The optical source can be a 

broadband source (LED, SLD, ASE, Supercontinuum), in which case it operates passively and normally there is no 

control from the processing unit. This is not the case when spectrally narrow illumination is used, most of the cases 

from laser sources, in which the wavelength modulation of the emitted light can be a component of the FBG 

interrogation technique. 

 

Vout f B 

 

Figure 1: General layout for interrogation of fiber Bragg grating sensors. 

B 

Mensurand( ) 

Table 1 groups and classifies the FBG interrogation techniques that have been proposed along the years. As will be 

described next sections, the underlying physical principles are diverse, which is also reflected in the performances 

achievable with each particular configuration. Independently of the specificities of each FBG interrogation 

approach, some general goals can be indicated. First of all, an appropriated interrogation technique must provide a 

reproducible transduction of the Bragg wavelength. Additional merit factors will be high sensitivity, large 

measurement range, immunity to optical power fluctuations, low environmental susceptibility, amenability to sensor 

multiplexing, simplicity and low cost. Certainly, some of these characteristics do not coexist and, therefore, for any 

particular demodulation solution a compromise is required. 

Bulk Optics 

The first group of interrogation systems indicated in Table 1 is based on utilization of diffraction gratings and 

volume holograms. The designation “bulk” is normally utilized to mention conventional optical devices, i.e., not 

supported in optical fibers. Of historical importance is the utilization of a monochromator, by Morey et al. [2], to 

perform in 1989 some of the first studies involving fiber Bragg gratings. Normally, the diffraction gratings are 

integrated in equipment such as monochromators and optical spectrum analyzers, but can also be used externally in 

association with CCD cameras (Blair et al. [3], Xu et al. [4]). These systems are versatile and can interrogate a large 

number of sensors. However, to obtain good sensitivity in the determination of the gratings resonance wavelengths it 

is required high density diffraction gratings associated with high precision rotation stages or, alternatively, large 

distances between the diffraction gratings and the detectors. This increases the size of the systems and their cost, 

eventually making them an attractive solution only in the cases where it is needed to read a large number of Bragg 

FBG

80 Fiber Bragg Grating Sensors Santos et al. 

sensors. However, for applications in which the requested sensitivity is not demanding, the utilization of 

spectrometers based on low resolution diffraction gratings and small CCD arrays can be a favorable solution, 

particularly when sub-pixel algorithms are explored (Askins et al. [5], Liu et al. [6]). Besides these situations, the 

utilization of monochromators and optical spectrum analyzers is essentially confined to the laboratory environment. 

Table 1: Interrogation techniques of FBG based sensors. 

Group Principle Configuration References 

Monochromator [2] 

Bulk Optics Diffraction 

Optical Spectrum Analyzer 

Diffraction Grating + CCD 

[3-4] 

[5-6] 

Volume Hologram + CCD [7] 

Bulk Filter Transmissive [8-9] 

Integrated Optic Filter Arrayed Waveguide Grating [10-11] 

Biconical Filter [12] 

Long-Period Grating [13-15] 

Passive Edge Filtering Optical Fiber Filters 

Fused Coupler [16-17] 

Chirped Bragg Grating [18-19] 

Sagnac Loop [20] 

Sources/Detectors 

Source Spectrum 

Detector Spectral Responsivity 

[21-22] 

[23] 

Fabry-Pérot [24-27] 

Bulk Optical Filters 

Acousto-Optic [28-30] 

Hybrid Optical MEMS [32] 

Receiving Bragg Grating [34-36] 

Active Bandpass Optical Fiber Filters 

WDM Coupler [37] 

Filtering 

Dynamic Long-Period Grating [38] 

Optical Sources 

Singlemode Laser Diode 

Multimode Laser Diode 

[39-43, 46] 

[44] 

Carrier Generation 

Receiving Bragg Grating 

Multimode Laser Diode 

[50-51] 

[48-49] 

Passive Chirped Grating + Sagnac loop [53] 

Homodyne Mach-Zenhder [52, 54-56] 

Interferometric Carrier Generation Mach-Zenhder [57, 61] 

Fourier Domain Variable OPD Scan [62, 65] 

Optical Coherence Function [66] 

Laser Sensing FBG Laser Cavity FBG as Cavity Mirror [67-74] 

Angular Dispersion Receiving Blazed FBG + CCD [75-76] 

Miscellaneous 

OTDR [77-80] 

Wavelet Processing [81] 

A different approach consists in using volume holograms written in photo-refractive materials as diffractive 

elements (James et al. [7]). In such cases some benefits can result, considering it is then practical to store in a single 

passive device a large number of filters with different spectral responses, which can later be tuned in order to fulfill 

the needs of a specific application. This flexibility, as well as the ability to tune the filters central wavelengths by 

application of electric fields to the material where the hologram is written, indicate that the utilization of these 

elements is a conceptually elegant solution for interrogation of fiber Bragg sensors. However, their bulk nature and 

high insertion losses are factors that probably will limit their dissemination in this field.


Multiplexing Techniques for FBG Sensors 

Manuel López-Amo 1,* and Jóse Miguel López-Higuera 2 



CHAPTER 6 

1 Grupo de Comunicaciones Ópticas, Universidad Pública de Navarra, Campus de Arrosdía, E-31006-Pamplona, 

Spain and 2 Grupo de Ingeniería Fotónica, Universidad de Cantabria, ETSII y Telecomunicación, Avda. de los 

Castros s/n E-39005 Santander, Spain 

Abstract: One of the main goals of fiber optic sensor technology is to multiplex together a high number of 

sensors in the same fiber in order to share expensive terminal equipment and reduce the size and weight of the 

optical cable. Both passive and active multiplexing techniques are used for telemetry in sensor networks. The 

different multiplexing techniques for Fiber Bragg Grating sensors will be described and compared here, including 

networks using optical amplification and lasing multiplexing systems. 

INTRODUCTION 

Multiplexing is the simultaneous transmission of two or more information channels along a common path. A fiber sensor 

system includes three main parts or subsystems: the sensing elements or transducers, the optical fiber channel and the 

optoelectronic unit [1]. Because of the high cost of the last subsystem, when it is possible to multiplex a high number of 

sensing points in the same network using a common optoelectronic unit, the cost per sensing element decreases. 

125 Systems employing multiplexed arrays of fiber Bragg gratings (FBGs) (surface mounted or embedded in 

structures and materials) have performed successfully in numerous field trials and applications involving a wide 

variety of structures, as will be shown in the next chapters. There are numerous companies that commercialize FBG 

sensors, arrays of FBGs and optoelectronic units for FBGs multiplexed networks. 

The development of the technology and components used in the optoelectronic units and multiplexing networks has 

been helped by the fast growing of the fiberoptic telecommunications technology. High performance tunable lasers, 

couplers, optical switches, optical amplifiers, filters and detectors are available for sensors multiplexing due to the 

major market that supposes telecommunications. Furthermore, certain multiplexing techniques, like Wavelength 

Division Multiplexing, come from the telecommunications side. However, other multiplexing techniques have been 

developed or adapted specifically for fiber optic sensors multiplexing [2]. 

In this chapter we are going to focus on the most utilized techniques for FBGs multiplexing, omitting other 

techniques, like coherence division multiplexing or polarization division multiplexing, that are little tied to FBGs. 

The sensor networks can be designed using only passive fibers (without utilizing optical gain) or introducing optical 

amplification in some key parts of the networks and hence passive or active networks can be developed [3-4]. 

Especially interesting for long distance applications are the systems that transform the multiplexing structure in a 

multiwavelength laser by the utilization of optical amplifiers. 

The aim of the chapter is to give a comprehensive overview of multiplexing techniques for FBGs, including a 

review of the most important achievements in modern multiplexing systems. 

GENERAL CONCEPTS 

FBG based sensors are wavelength selective devices that may be interrogated using different modulation formats. It 

is possible to interrogate them by using their reflected signal or the transmitted one. However, the reflective 

response is the most usually employed. 

A variety of multiplexing techniques based on different modulation formats have been developed, each one with 

their specific advantages for a particular application. As it is shown in (Fig. 1), the modulation formats generally fall 

*Address correspondence to this author Professor Mauel López-Amo: Grupo de Comunicaciones Ópticas, Universidad Pública de Navarra, 

Campus de Arrosadía, E-31006-Pamplona, Spain; Tel: +34 948169055; Fax: +34 948169720; Email: mla@unavarra.es

100 Fiber Bragg Grating Sensors López-Amo and López-Higuera 

into one of the following categories: Wavelength Division Multiplexing, Time Division Multiplexing (TDM), 

Frequency Division Multiplexing, Coherence Multiplexing and Polarization Division Multiplexing [5]. The last two 

techniques are not usually employed for FBGs multiplexing, but we must consider also hybrid approaches 

(simultaneous utilization of two modulation formats inside the same network). 

Figure 1: Multiplexing modulation formats: (a) Wavelength Division Multiplexing (WDM); (b) Time Division Multiplexing 

(TDM); (c) Frequency Division Multiplexing (FDM); (d) Coherence Multiplexing (CM) and (e) Polarization Division 

Multiplexing (PDM). 

When a single fiber is devoted to each sensor, we also use the term Spatial Division Multiplexing as another 

multiplexing technique. As depicted in (Fig. 2), to build up a sensor network, four basic configurations or topologies 

can be used. In all of them, the optimized received signal power is a key factor. The optoelectronic unit of the 

system must receive from the network an optical power level that ranges from the saturation power of the detector to 

the minimum accepted power (in which the system works with an acceptable signal to noise ratio). This difference is 

called the dynamic range of the system, which is another important design factor. Couplers and optical circulators 

are basic devices for networking, being their losses a fundamental parameter in the network behavior. In the star 

topology, the chosen splitting ratio uses to be 1/M, where M is the number of sensors to be multiplexed. However, in 

branched buses (buses which includes couplers for power distribution) or dual buses, the selection of the coupling 

factor is not an easy task [6]. The usual case, in which it is desirable that all couplers have the same splitting ratio, 

the optimum value should be 1/M in a branched single bus configuration. On the other hand, the received signal 

level from the M sensors may be equalized by tailoring the coupling ratio of each coupler. This last strategy is not 

usually selected, because of economical and operative reasons. 

a) 

b) 

c) 

O.S 

O.D 

O.S 

T 

T 

O.D 

T 

T 

O.S 

T 

O.D 

T 

1 

3 

2 

C 

Star 

coupler 

1xM 

Serial Bus 

S1 S2 SM-1 S M 

Dual Bus 

S1 S2 SM-1 S M 

Figure 2: Typical fiber optics sensor network topologies: (a) serial bus; (b) dual bus; (c) star and (d) tree topologies. 

S1 

S2 

SM-1 

S M 

d) 

O.S 

T 

O.D 

T 

1 2 

3 

Tree 

O.D 

T 

S1 

S2 

S M-1 

S M

Multiplexing Techniques for FBG Sensors Fiber Bragg Grating Sensors 101 

Multiplexing optical fiber sensors includes four basic tasks [3]: i) Launching an optical signal into the network having a 

suitable power, spectral distribution, polarization and modulation; ii) The detection of the signal which has been coded or 

modulated by the sensor element, and sent back to the optoelectronic unit by transmission or reflection; iii) Uniquely 

identifying the information corresponding to each sensor in the network, by means of proper addressing, polling and 

decoding; iv) The evaluation of the generated optical signals by modulating each sensor with a calibrated electrical signal. 

To efficiently design the most appropriate sensor network, it is necessary to take into account different aspects: the 

modulation and coding format of the optical signal, the network topology, the inclusion or not of optical 

amplification technologies, the decoding method for the received signal, and the type or types of sensors 

multiplexed in the same network. Finally, it must be also considered the economic conditions, which would 

eventually determine the most appropriate network (see Fig. 3). 

Figure 3: Illustration of the interdependence of the main concepts on a typical fiber optics sensor network. 

It must be noted that there is not a single ‘ideal’ approach for all the applications. However, there is indeed an 

optimum approach for each application. The choice of a different multiplexing technique depends on the 

requirements of the sensor network. The relative importance of parameters such as cost, noise, Bandwidth (BW), 

and flexibility constitutes the basis for making the right selection. 

SPATIAL DIVISION MULTIPLEXING 

This technique is also known as fiber multiplexing, and it is the first natural approach to build up fiber optic sensor 

networks. As illustrated in (Fig. 4), a single light source feeds M fiber channels and interrogates M sensing points, 

sharing in this way the cost of the laser or the broadband incoherent source over the entire network. In the typical 

reflective configuration, M fibers (one per path) are used to interrogate the M FBG sensors. By using optical 

circulators located at the M outputs of the optical coupler (OC), each sensor response is detected by an optical 

detector (OD). In the unusual transmissive response (dashed lines), 2·M fibers are needed. In the reflective case, the 

optical power at each detector drops by a factor of M, which corresponds to a signal power drop of 1/M 2 . Assuming 

shot noise limited operation, the signal to noise ratio (SNR) in fiber multiplexed systems falls to (1/M 2 )/(1/M) = 1/M, 

i.e. the SNR degradation in decibels is given by 10·logM. 

Figure 4: Illustration of a spatial or fiber multiplexed network, using one optical source, M detectors on the Optoelectronic Unit. 

Continuous line: reflective configuration. Dashed line: transmissive configuration.




CHAPTER 7 

Polarization Properties of Fiber Bragg Gratings and Their Application for 

Transverse Force Sensing Purposes 

Christophe Caucheteur*, Sébastien Bette, Marc Wuilpart and Patrice Mégret 



Abstract: Due to the lateral inscription process, Fiber Bragg Gratings (FBGs) written into standard single-mode 

fiber can exhibit birefringence. The birefringence value is however too small to be perceived in the FBG spectral 

response but it can lead to significant polarization-dependent properties such as polarization dependent loss 

(PDL) and differential group delay (DGD). A transverse force applied on the FBG can enhance the birefringence, 

which increases both the PDL and DGD values. Hence, although these properties are not desired in 

telecommunication applications, they can be advantageously used for transverse force sensing measurements, 

allowing the use of FBGs written into standard single-mode optical fiber, which fail to work when they are 

interrogated through amplitude spectral measurements. This chapter first analyzes the normalized Stokes 

parameters, PDL and DGD evolutions with wavelength when the FBG parameters and the birefringence value are 

modified. It then focuses on the realization of a transverse force sensor based on the monitoring of the PDL and 

DGD evolutions. 

INTRODUCTION 

Fiber Bragg grating (FBG) sensors are widely used to measure temperature and axial strain. During the last decade, 

there has been increasing attention on the application of FBG sensors in smart structures, where sensors are 

deployed to measure not only the axial strain but also the transverse force. Transversal strain sensors are of high 

importance in the civil engineering domain. Moreover, FBGs can be used in embedded applications for structural 

health monitoring. As an example, in composite materials, sensors are used for cure monitoring during the 

manufacturing process and also for health monitoring of the final product. 

It is known that the transverse strain or force induces birefringence in FBGs written into standard fiber and thus cause 

their reflection band to split into two bands characterized by orthogonal polarizations [1]. The amount of 

birefringence is directly linked to the transverse force value [2]. However, for force values below a few hundreds of 

Newtons, the mechanically-induced birefringence is not sufficient to reach the complete separation between the two 

bands corresponding to the two orthogonal polarization modes. They overlap and the resulting amplitude spectrum is 

completely deformed. Accurate measurements are therefore not possible with a single FBG fabricated into a standard 

single mode fiber (SMF), unless a polarizer is optimized in front of the sensor to discriminate between the amplitude 

spectra corresponding to the two polarization modes [3]. Such discrimination is essentially visual and thus fails in 

being automated and precise. It is the reason why FBGs written into polarization maintaining fiber (PMF or highbirefringence 

fiber) have been privileged for transverse force sensing purposes [4]. 

PMFs generally contain stress applying regions in the cladding that break the circular symmetry of the fiber cross 

section and lead to an important intrinsic fiber birefringence of the order of 10 -4 . The pristine birefringence in PMF is 

hundreds or thousands times higher than in standard SMF. This stress-induced birefringence leads to different 

propagation constants for the two orthogonal polarization modes (also called slow and fast modes – the slow mode 

corresponds to the axis passing through the stress applying regions) so that the FBG fabrication in PMF results in the 

formation of dual well-separated resonance bands [5]: the wavelength separation between these two bands, which is 

linked to the birefringence value, is generally of the order of a few hundreds of picometers [6-8]. 

When a transverse force is applied on an FBG written into a PMF, the resonance bands amplitudes and central 

*Address correspondence to this author Dr. Christophe Caucheteur: Electromagnetism and Telecommunications Department, Faculté 

Polytechnique, Université de Mons, Boulevard Dolez 31, 7000 Mons, Belgium; Tel: +32 65374149; Fax: +32 65374199; E-mail: 

christophe.caucheteur@umons.ac.be

Polarization Properties of Fiber Bragg Gratings Fiber Bragg Grating Sensors 117 

wavelengths are simultaneously modified. The rate of change depends on the direction of the applied force with respect 

to the PMF slow and fast axes [6]. Moreover, the maximum wavelength shift of a given resonance band is obtained 

when the loading direction is oriented along the corresponding fiber axis. It corresponds to the minimum sensitivity 

for the orthogonal axis. Hence, the resonance band wavelength shift exhibits a periodic dependence on the load 

orientation, with a period of about 180° [6, 8-10]. The sensitivity of the slow axis has been reported higher than that 

of the fast axis [8]. Depending on the fiber birefringence value, it can reach a maximum of about 0.2 nm/(N/mm) for 

the slow axis. However, as the temperature and axial strain effects also shift the resonance bands [5], monitoring 

separately the resonance bands shifts does not provide temperature-insensitive and axial strain-insensitive transverse 

force measurements. Therefore the operating principle of transverse force sensing with FBGs written in PMF results 

in the monitoring of the wavelength spacing between the two resonance bands [6-10]. This provides measurements 

of both the amplitude and direction of the transverse force thanks to the fiber-orientation dependence of the 

amplitude spectral response. Accurate measurements require a careful control of the state of polarization of the input 

signal at 45° between the fiber axes so that the two orthogonal modes exhibit the same optical power. Because the 

state of polarization is varied over the distance and the transversal force modifies the birefringence value, this 

requirement is difficult to ensure in practice so that important measurement errors can readily occur. To avoid this 

important complexity as well as the high cost linked to the use of PMF, an interesting alternative remains the use of 

FBGs written in standard SMF. For the reasons outlined above, amplitude spectral measurements of standard FBGs 

are not suited for transverse force sensing purposes. Recent researches have demonstrated that the polarization 

dependent properties related to standard FBGs can be used for this purpose [11]. Due to the lateral inscription 

process, photo-induced birefringence is present in FBGs written into standard SMF [12]. The birefringence value, 

typically of the order of 10 -6 , is too small to be perceived in the FBG amplitude spectral response. However, it leads 

to significant polarization-dependent properties such as polarization dependent loss (PDL) and differential group 

delay (DGD). The birefringence value can be mechanically enhanced by a transverse force [2], which increases both 

the PDL and DGD values. As a result, the PDL and DGD evolutions with wavelength can be monitored for 

transverse force sensing purposes, allowing to use FBGs written into standard SMF, which fail to work when they 

are interrogated by means of amplitude spectral measurements. 

The aim of this chapter is double: it first defines and analyzes the evolutions with wavelength of the normalized 

Stokes parameters, PDL and DGD associated to the transmitted signal of uniform FBGs, which completes the study 

of the amplitude and phase spectral characteristics presented in Ref. [13] when birefringence is taken into account. It 

then details the exploitation of the polarization related phenomena in FBGs for transverse force sensing purposes. 

In the following, Section 2 introduces some concepts of light polarization in optical fiber while Section 3 details and 

analyzes the polarization dependent phenomena in FBGs. Their dependence with respect to the FBG parameters and 

the birefringence value is identified in Section 4. Finally, the feasibility of using the FBGs polarization dependent 

properties for transverse strain sensing is demonstrated in Section 5. 

REVIEW OF SOME CONCEPTS OF LIGHT POLARIZATION IN OPTICAL FIBER 

This section reviews some important concepts of light polarization in optical fiber. These concepts are essentially 

the Jones and Stokes formalisms and will be useful for the definition of the polarization dependent properties 

associated to FBGs, which is the subject of the next section. 

State of Polarization 

A polarized lightwave signal propagating in an optical fiber or in free space is represented by electric and magnetic 

field vectors perpendicular to each other in a transverse plane itself perpendicular to the direction of propagation. 

Commonly, the attention is focused on the electric field since it has the most direct effect during the interaction 

between wave and matter [14]. 

The state of polarization is defined as the pattern drawn in the transverse plane by the extremity of the electric field 

vector as a function of time at a fixed position in space. A monochromatic light source emits a single frequency and is 

always totally polarized. In some cases, the electric field vector can occupy random orientations in the transverse 

plane as a function of time. If the orientation changes are fast enough to be beyond observation in the physical context 

of a particular measurement or application, the light is said to be unpolarized [15]. It is the case of naturally produced

118 Fiber Bragg Grating Sensors Caucheteur et al. 

light, such as sunlight and firelight, which is characterized by a very large spectral width. In that case, the polarization 

behavior of each frequency is different and it is therefore impossible to clearly define a fixed polarization state. 

Between the cases of totally polarized and unpolarized light, we find partially polarized light for which the electric 

field vector is still characterized by random orientations but stays around a polarization state of maximum probability. 

It is typically the case when an optical source emits a quasi-monochromatic wave with a small but not null spectral 

width. Hence, each spectral component of the wave produces a different polarization state at a fixed point of space but 

all these states remain distributed around the same state. 

Partially polarized light can be represented as a superposition of both fully polarized and completely unpolarized 

lightwaves. The degree of polarization (DOP) describes partially polarized light and is defined as the ratio of the 

intensity of the totally polarized component to the total intensity of the wave. The degree of polarization varies from 

0 for unpolarized light to 1 for totally polarized light and takes intermediate values for partially polarized light. In 

free space propagation, the degree of polarization of a wave is maintained. In a transmission medium, it can evolve 

depending on the spectral width of the source and the dispersive properties of the transmission path. 

The Polarization Ellipse 

For a plane and monochromatic lightwave (DOP = 1) propagating along the z direction of a Cartesian coordinate 

system (x, y, z), the transverse components of the electric field can be expressed as [14]: 

Ex 0 x 

x 

(1) 

(z,t) E cos(ω 

t δ k z) 

Ey 0 y 

y 

(2) 

(z,t) E cos(ω 

t δ k z) 

where E0x and E0y are the amplitudes of the x and y components, x and y are the corresponding phases. The 

elimination of (·t – k·z) from Eqs. (1) and (2) yields the following relationship: 

E 

E 

2 

x 

2 

0x 

2 

Ey 

Ex 

Ey 

2 

2 cos δ sin δ 

(3) 

2 

E E E 

0 y 

0x 

0 y 

where = y – x is the phase difference between the two components. 

Eq. (3) is the equation of an ellipse drawn in the transverse plane by the extremity of the electric field vector at a 

fixed point in space as a function of time. The state of polarization of a fully polarized lightwave is therefore in 

general elliptical. The ellipse defined by Eq. (3) is illustrated in (Fig. 1) where one can see that parameters other than 

E0x, E0y and can be used to define a state of polarization. These parameters are the angle , called the azimuth, 

between the major axis and the x axis, the sense of rotation of the electric field vector materialized by an arrow on 

the ellipse and the ellipticity degree de defined as: 

Figure 1: Polarization ellipse.


Fiber Bragg Grating Sensors in Civil Engineering Applications 

Jinping Ou 1,2,* , Zhi Zhou 1,3 and Genda Chen 3 



CHAPTER 8 

1 School of Civil Engineering, Harbin Institute of Technology, Harbin, Heilongjiang, 150090, P.R. China; 2 School of 

Civil and Hydraulic Engineering, Dalian University of Technology, Dalian, 116024, P.R. China and 3 Center for 

Infrastructure Engineering Studies, Missouri University of Science and Technology, Rolla, MO 65409-0710, USA 

Abstract: Fiber Bragg Gratings (FBG) have been regarded as one of the most promising local monitoring sensors 

and are widely deployed in civil infrastructures. In this chapter, those FBG-based sensors aiming at civil 

structures have been briefly presented, including direct packaged strain and temperature sensors, indirect sensors 

constructed using FBG as a sensing element, and FBG based smart civil structures. Specific issues concerning 

methods of effectively and correctly applying the FBG sensors to civil infrastructures have been discussed. Those 

issues involve sensor installation technique, strain transfer based error modification, and temperature 

compensation. Finally, more than ten case studies of critical infrastructures outfitted with FBG sensors have been 

reported. Both research and practical applications show that FBG sensors are now competitive with conventional 

electrical sensors in long-term structural health monitoring. 

INTRODUCTION 

Civil infrastructure can be defined as a network of large scale structures with long service lives, existing in harsh 

operating environments with limited maintenance budget. For example, long-span bridges are often constructed to 

link ground transportation networks across rivers for 75 to 100 years. During a typical lifespan, civil infrastructure is 

inevitably subjected to environmental effects, long-term loading effects, material deterioration and/or extreme 

loading effects such as corrosion, fatigue, aging, and/or earthquakes and hurricanes. Predictably, the accumulated 

damage on a structure will result in the degradation of structural performance; the structural loading resistance can 

be greatly lowered, possibly leading to disasters in cases of extreme loading. Therefore, Structural Health 

Monitoring (SHM) has recently become an important innovative technology for both the detection and 

characterization of a damage process. Understanding this process will ensure the ability to maintain healthy 

conditions for civil infrastructures [1]. 

China has constructed a number of long-span bridges annually since the early 1990s. For example, the Sutong cablestayed 

bridge has a main span of 1088 m, and the Hanzhou Bay Bridge is 36 km in length. Similarly, other 

structures, such as the Olympics Swimming Center, Bird Nest, Three Gorges Dam, Guangzhou New TV Tower, and 

Tibet Railway have also been rapidly developed. In the near future, more skyscrapers and long-span structures 

expects to be constructed. Maintaining structural health throughout their lifespan and mitigating potential 

catastrophes will be a major challenge. 

Traditional sensors such as electrical strain gauges and vibration wires are unable to meet the ever-increasing demand 

for long-term monitoring of large-scale infrastructure in terms of durability. In recent years, Optical Fiber (OF) 

sensors have been used for long-term SHM due to their distinct advantages, such as electromagnetic immunity, 

compact size, corrosion resistance, and the ability for a multiplexed sensor array to be distributed along a single fiber. 

To date, Fiber Bragg Grating (FBG) has attracted the most attention in various SHM applications in composite 

structures [2-5]. The technology has been validated and used for case studies on more than 100 real-world structures. 

In 2003, approximately 1,700 FBG sensors were installed on the Dongying Yellow River Bridge for long-term SHM, 

making it the largest FBG case study. In 2006, 179 FBG strain sensors and 24 temperature sensors were used to monitor 

stress and temperature levels during the construction of a national swimming center in China. FBG sensors were 

also attached to rails along a railway to monitor dynamic strain based on the Wavelength Division 

Multiplexer/demultiplexer (WDM) [6]. FBG temperature sensors were also integrated into a fire alarm system in 

*Address correspondence to this author at Professor Jinping Ou: School of Civil Engineering, Harbin Institute of Technology, Harbin, 

150090, P.R. China; Tel: +8645186282209; Fax: +8645186282209; E-mail: oujinping@hit.edu.cn

144 Fiber Bragg Grating Sensors Ou et al. 

highway tunnels at Luliang Mountain, Baiyan Brook, and the Zhang Jiachao Tunnel in west Shanghai-Chengdu 

Highway [7]. Furthermore, FBG strain and temperature sensors were applied to monitor offshore platform Numbers 

CB371 and CB32A [8-9]. 

In this chapter, some of the latest advances in FBG sensing technologies concerning long-term monitoring of largescale 

infrastructure are reported in two parts. The first part will introduce the latest progress in the development of 

direct FBG-based sensors, indirect FBG based sensors, and FBG-based smart structures. The second part will 

demonstrate various applications of the FBG sensors and smart structures in more than 10 case studies on critical 

infrastructures. 

FBG-BASED SENSORS FOR CIVIL INFRASTRUCTURES 

Direct OF Sensors 

Packaged Strain Sensors 

Due to their fragile behavior in a harsh environment, bare FBG sensors cannot be directly placed in civil 

infrastructure without proper packaging. Considering the creep and aging effect of glues and the corrosion of metals, 

Fiber Reinforced Polymer (FRP) has been extensively studied and successfully applied for FBG and OF packaging. 

FRP is an elastic and durable composite material with superior fatigue behavior. FRP is naturally compatible with 

optical fiber material because both have a key element of silica. (Fig. 1) shows a Scanning Electron Microscope 

(SEM) view of the bonding surface between FRP and bare OF. Through the cross section analysis under axial 

strains, the strain transfer error and the error correction coefficient are approximately 3.3% and 1.03, respectively, 

for Glass FRP (GFRP) rods of 4~8 mm in diameter, and those for Carbon FRP (CFRP) bars of 4~8 mm in diameter 

are approximately 2% and 1.1, respectively, indicating that the FRP is well bonded to the OF. Therefore, FRP is 

well qualified for both FBG and OF packaging. Additionally, because FBG or OF sensors with a diameter of 

125 μm or less will not affect the mechanical properties of FRP elements, the FRP packaging can be integrated with 

bars and plates to promote load capacity. (Fig. 2) validates this theory. 

(a) (b) 

Figure 1: Interface SEM of bare OF and GFRP/CFRP: (a) GFRP and (b) CFRP. 

In order to make full use of the superior durability and elasticity of the FRP material, various FRP-packaged FBG 

strain sensors has been developed as represented by those in (Fig. 3). These sensors show excellent characteristics, 

including: 1) a strain measurement range of up to 8000~15000 , 2) a strain measurement resolution of 1~2 , 

depending on the FBG interrogator, 3) exceptional precision, with deviations of less than 0.5%, 4) a normalized 

sensitivity coefficient of 7.8×10 -7 , 5) a hysteresis error of less than 0.5%, and 6) a fatigue life of over 200 million 

cycles at 30001000 . After 12 months of corrosion tests, the FRP-packaged sensors have the same sensing 

capabilities as the original ones. They can be designed for various needs and specifically are appropriate in longterm 

SHM for large-scale infrastructure. In addition, for a strain measurement of over 15,000 with a resolution of 

less than 0.5 , a sensitivity-increasing or decreasing technology based on strain transfer mechanisms has been 

developed for various applications [10-13].

Fiber Bragg Grating Sensors in Civil Engineering Applications Fiber Bragg Grating Sensors 145 

Stress(MPa) 

700 

600 

500 

400 

300 

200 

100 

CFRP1 

CFRP2 

CFRP-FBG1 

CFRP-FBG2 

0 

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 

Strain(X10 -6 ) 

(a) (b) 

Figure 2: Strength comparison of FRP-FBG bar and FRP bar under loading: (a) CFRP and (b) GFRP. 

(a) 

(c) 

Figure 3: Representative FRP-packaged FBG strain sensors: (a) overall embeddable, (b) end-enlarged, (c) weldable, and 

(d) long-gauge. 

To address the large-scale nature of infrastructure, an integrated global and local SHM technology with a single OF 

was developed by combining the Brillouin Optical Time Domain Analysis/Reflectometry (BOTDA/R) with FBG 

sensors. The BOTDA/R and FBG are for distributed and local high-precision strain measurements, respectively. The 

combined sensing heads can be easily integrated using the FRP-package method and applied at a length of over 

5 km, as depicted in (Fig. 4). (Fig. 5) shows that the strain measured with BOTDA/R is in agreement with that from 

FBG (marked with dots) as long as the center wavelength of the FBG is far away from the BOTDA/R’s operation 

wavelength of 1550 nm. For practical applications, the BOTDA/R-FBG sensor is installed on a structure so that the 

Bragg gratings are located in potential damage areas based on a pre-analysis and assessment of the structure. The 

actual location of damage areas and their spatial distribution can be identified by the BOTDA/R measurements, 

while the detailed damage data can be acquired through the FBG sensors [14]. (Fig. 6) shows some other metalpackaged 

FBG sensors developed by Micron Optics, Inc. (MOI). They are examples of products with the advantages 

of fast response time, high accuracy, long-term stability and premium performance under harsh environmental 

conditions. 

Stress(MPa) 

500 

450 

400 

350 

300 

250 

200 

150 

100 

50 

0 

GFRP-FBG1 

GFRP-FBG2 

GFRP1 

GFRP2 

-50 

0 1000 2000 3000 4000 5000 6000 

(b) 

(d) 

Strain(X10 -6 )


Fiber Bragg Grating Sensors in Aeronautics and Astronautics 

Nobuo Takeda 1,* and Yoji Okabe 2 



CHAPTER 9 

1 Department of Advanced Energy, Graduate School of Frontier Sciences, The University of Tokyo, Mail Box 302, 5- 

1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan and 2 Department of Mechanical and Biofunctional Systems, 

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan 

Abstract: Fiber optic sensors, Fiber Bragg Grating (FBG) sensors in particular, are among most promising 

sensors for structural health monitoring of aerospace structures in order to assess the safety and durability during 

a long period of time in use. These sensors are expected to provide a low-cost maintenance methodology for 

carbon fiber reinforced composite structures which are now extensively being used for the primary structures. 

This chapter presents an overview of current use of FBG sensors in aeronautics and astronautics. 

INTRODUCTION 

In the aerospace filed, the weight saving of the structures is one of the important demands to increase the flight 

performance of airplanes and the mission operability of spacecrafts. Therefore lightweight composite materials, such 

as carbon fiber reinforced plastics (CFRP), have been applied to most main members of aerospace structures in 

recent years. Whereas the composite materials actualize lightweight high-performance structures, the fracture 

process of the materials is really complex due to the combination of different constituents of reinforcing fibers and 

matrix. Hence structural health monitoring (SHM) technologies are attracting attention to increase the reliability and 

safety in the lightweight design of the aerospace structures. 

In the SHM field, currently, the methods using piezoelectric actuators/sensors for generating and detecting 

ultrasonic waves and optical fiber sensors for static and dynamic strain sensing are mainly researched. Ultrasonic 

propagation techniques use the response of the waves to the geometric change caused by the damage occurrence. On 

the other hand, optical fiber sensors basically measure strain with high accuracy in order to detect damages in 

structural members. Among the many optical fiber sensors, fiber Bragg grating (FBG) sensor is one of the most 

promising sensors for SHM, because the FBGs can measure the strain with a high degree of accuracy and reliability 

and are also easy to handle. 

Excellent summary of fiber optic sensors including FBG sensors to be used in aerospace applications can be found 

in Refs. [1-3]. In this chapter, some recent detailed researches using FBG sensors for SHM of aerospace structures 

are reviewed based on the authors’ previous review article [4]. 

SHM OF AIRCRAFT WITH FBG SENSORS 

Most researches on SHM with FBG sensors in the aerospace field targets aircrafts in order to decrease the 

maintenance cost and increase the safety of the structures. FBG sensors can be easily integrated into structural 

materials of aircrafts because optical fibers are thin, lightweight, and flexible. 

As an example of early fundamental researches, Wood et al. embedded FBG sensors into CFRP laminates for a 

future morphing aircraft and measured the Bragg wavelength using their demodulation system that determined the 

locations of multiplexed FBGs by Fourier transformation. The strain of the laminate was successfully measured with 

the embedded FBG [5]. They also attempted to combine the single mode optical fiber for the FBG sensor with near 

infrared spectroscopy to measure a variety of chemical and physical changes in the airframe structure using a single 

embedded fiber [6]. 

*Address correspondence to this author Professor Nobuo Takeda: Department of Advanced Energy, Graduate School of Frontier Sciences, 

The University of Tokyo, Mail Box 302, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan; Tel: +81-3-5841-6642; Fax: +81-3-5841- 

6642; Email: takeda@smart.k.u-tokyo.ac.jp

172 Fiber Bragg Grating Sensors Takeda and Okabe 

ANALYSIS/EVALUATION 

(by Kawasaki Heavy Industries, Ltd.) 

EMBEDDED SMALL-DIAMETER 

OPTICAL FIBER SENSORS 

VISUALIZATION 

(by Univ. Tokyo) 

Figure 1: Arrangement of embedded small-diameter FBG sensors in upper panel of composite fuselage demonstrator and the 

impact detection/localization system. 

The precise strain measurements with FBG sensors have been applied to detection of damages in aircraft structures. 

Guemes et al. bonded or embedded some FBGs into three blade-stiffened CFRP panels with co-cured stiffener webs 

[7]. The sensors successfully measured the strain during the buckling behavior of the stiffened panels under the 

compression loading. In the stiffened CFRP panels, debonding between the skin panel and the stiffener is a typical 

damage. They also detected the debonding damage based on the response of the FBG sensor to the non-uniform 

strain distribution in the grating length [8]. 

Other typical structural panel used in the aerospace structures is a sandwich panel that consists of a lightweight thick 

core and two thin high-stiffness facesheets to increase the bending stiffness of the lightweight panel. However, since 

the sandwich panels are vulnerable to out-of-plane impact loadings, Bocherens et al. embedded FBG sensors and 

multimode optical fibers for the optical time domain reflectometry (OTDR) in a radome sandwich structure for 

detection of impact damages [9]. These SHM methods were indicated to be efficient in detecting permanent damage 

induced by the impacts. 

The FBG strain measurement is also applied to monitoring of the deformation behavior of wing structures. When the 

flight speed of aircrafts exceeds a limit, the oscillation of the wing panel diverges and leads to destruction of the 

wing. This hazardous phenomenon is called flutter. If the flutter can be detected and suppressed by smart material 

systems, the limitation of the flight speed can be increased. Lee et al. embedded FBG sensors in the skin panel of a 

1/25th scale wing model of a commercial airliner and conducted a wind tunnel testing [10]. Using their 

demodulation system with a wavelength swept fiber laser, the flutter was able to be detected. The deformation 

monitoring is also needed for development of “morphing wing” that is a new concept of wing structures. The 

morphing wing changes the wing area, sweepback angle, and camber smoothly to maximize the flight performance. 

In order to control the wing shape precisely, the shape sensing of the morphed wing is important. Hence Tai 

investigated theoretically the applicability of multiple FBG sensors to estimate the shape of the morphed wing [11]. 

Moreover, modal parameters in vibration are often sensitive to damages in structures. Some attempts have been 

conducted using high-speed FBG interrogation systems [12-13]. Also, vibration observations with FBGs are

Fiber Bragg Grating Sensors in Aeronautics and Astronautics Fiber Bragg Grating Sensors 173 

combined with smart actuator systems to control the vibrations. For example, the strain measurement with an FBG 

was used to suppression of the vortex-induced vibration of a cylinder by piezo-ceramic THUNDER actuators [14]. 

In a MESEMA project in the aeronautics field, a smart magnetostrictive auxiliary mass damper equipped with an 

FBG sensor was developed [15]. This smart device was mounted on a real stiffened aeronautical panel and 

succeeded in the reduction of the vibration level. 

When the FBG sensors are embedded into composite materials, connection method to the embedded optical fibers is 

also an important issue. In order to overcome this problem, various optical fiber connecters integrated in the 

composite laminates have been developed [16-17]. The method using polytetra-fluorethylene dummies enabled 

trimming of the composite parts with embedded optical fiber connection hardware [17]. 

At the same time as the above fundamental researches, investigations on practical application to actual aircrafts have 

been performed. Daimler Chrysler glued FBG sensors to the surface of a newly developed CFRP wing structure and 

tested the sensing performance at the DASA Airbus test center Hamburg for over one year from September 1998 to 

December 1999 [18]. They also attempted to embed the sensors into the varnish of structures for protection of the 

optical fibers. Then Airbus Espana had evaluated the feasibility of FBGs embedded in CFRP structures for 

measurement of manufacturing induced residual stresses and SHM of the large structure [19]. EADS Deutschland 

GmbH bonded FBG sensors on the surface of a CFRP aircraft fuselage test barrel and measured the strain change 

during the impact and the permanent deformation caused by the impact damage [20]. Alenia Aeronautica in Italy 

also has investigated the main requirements for FBG sensors and the application possibility to SHM of aerospace 

structures [21]. A micro-size interrogation system is under development for aircraft use [22]. 

Damage identification has been conducted in some Japanese national projects on fiber optic sensors in aircraft 

applications [23]. Development of real-time detection of impact damage with embedded small-diameter optical fiber 

sensors was conducted as a collaborative work between the University of Tokyo and Kawasaki Heavy Industries and 

demonstrated in a CFRP fuselage structure of 1.5 m in diameter and 3 m long. In the upper panel, the small-diameter 

sensors were embedded in order to detect impact-induced damages (Fig. 1) [24-25]. The small-diameter FBG 

sensors were used to obtain the impact location through the dynamic strain measurement, and multi-mode smalldiameter 

optical fibers were embedded to judge the occurrence of the impact-induced damages using the optical loss 

due to bending. An impact detection and localization system was also developed and could successfully detect the 

impact locations and impact-induced damages. A special embedded optical connector was developed for a smalldiameter 

optical fiber so that it could be trimmed after the autoclave composite fabrication and then connected to a 

normal-diameter optical fiber. More details on small-diameter FBG sensors can be found in reference [26]. 

Another example is a grid structure (Fig. 2) made of CFRP unidirectional composites, named an advanced grid 

structure (AGS), has specific characteristics such as simplicity of stress path/damage feature and fail-safe structural 

redundancy [27]. The HRAGS system equipped with an SHM system utilizing FBG sensors embedded in every rib 

of AGS has been proposed, so that the size and intensity of operational or accidental damages can be evaluated 

through the strain measurement of every rib. A recent advanced six-axis-controlled tape placement machine can be 

utilized to place CFRP unidirectional tapes and optical fibers with FBG sensors. Recent high-speed optical switch 

can also be used to scan all the strain data from a number of FBG sensors. A statistical damage recognition system 

for SHM was established to distinguish the damage location from strain data [28]. 

When new demodulation systems of FBG sensor were developed, their practicality was sometimes demonstrated by 

mounting the system in airplanes and monitoring the strain by FBG during the flight. When BAE Systems 

developed their FBG sensor system, the system was equipped in the cabin of a test aircraft and FBG strain rosettes 

were bonded to the external lower surface of the wing [29]. The flight tests over a two-week period successfully 

demonstrated that the FBG sensor system was capable of measuring strains during the actual operation of airplanes. 

Recently, Technobis Fiber Technologies developed a high-speed multi sensor FBG interrogator, and they conducted 

a flight test using a test aircraft [30]. The system succeeded in measurement of the stress and the temperature of the 

aircraft structure during the flight. 

As other examples of SHM researches relating to the aircrafts, the dynamic behavior of the main rotor blade in a 

helicopter [31] and that of a parachute during inflation [32] were evaluated using FBG sensors.


Fiber Bragg Grating Sensors in Energy Applications 

Christopher Barry Staveley* 

Smart Fibres, Limited, 12 The Courtyard, Eastern Road, Bracknell, RG12 2XB, United Kingdom 



CHAPTER 10 

Abstract: Fiber Bragg Grating sensing systems have found application in numerous market sectors, wherein the 

unique capabilities of the technology are either displacing existing, limited sensing technologies, or providing 

solutions where sensing was hitherto impractical or impossible. Here, we review the energy market for the 

technology, a market where some of the most challenging measurement conditions are presented, yet some of the 

largest value propositions can be seen. 

INTRODUCTION 

Fiber Bragg grating (FBG) technology came to light toward the end of the 1970s. Through the remainder of the 20 th 

century, the development of the technology almost exclusively focused on telecoms applications. In the 1990s a few 

pioneering Companies recognized the potential sensing opportunities of the technology and began to develop 

products towards the markets that might emerge to utilize the same. 

After just over a decade of this shift in focus towards sensing, FBG measurement systems are finding increasing 

application in a number of industry sectors for design optimization, smart control of active systems, and structural 

health monitoring. Of these industry sectors, which include aerospace, civil infrastructure, transportation and 

medical, the single largest one is energy. 

ENERGY APPLICATIONS 

The generation, harvesting, conversion, transportation and storage of energy utilizes an enormous range of industries 

and scientific disciplines, and represents one of the single largest global market sectors. As shown in (Fig. 1), the 

market is projected to continue growing at a significant rate for the foreseeable future, 

Figure 1: Projected growth in global energy demand (source: IEA WEO 2007). 

The industry is very diverse, reflecting the diversity of the forms of energy that humankind relies upon in everyday 

life, from ancient hydrocarbons that are mined or pumped from subterranean reserves, though nuclear reactions 

carried out within safety critical containment systems, to naturally renewable forms of energy such as hydroelectric, 

solar, wind and wave. 

*Address correspondence to this author Christopher Barry Staveley: Smart Fibres, Limited, 12 The Courtyard, Eastern Road, Bracknell, 

RG12 2XB, United Kingdom; Tel: +44 1344 484111; Fax: +44 1344 423241; Email: info@smartfibres.com

186 Fiber Bragg Grating Sensors Christopher Barry Staveley 

The forms of distribution of energy are equally diverse, from high power electric cables feeding transformer 

stations, to pipelines and ocean vessels carrying hydrocarbons, either in their stable state or super-cooled to become 

easier to handle. 

It is no wonder then, that the applications found by FBG technologies in this huge, diverse market are themselves 

diverse also. This brief summary lists some of these applications, and cites some noteworthy examples of them 

being developed. It is, however, far from exhaustive, and seeks only to capture a snapshot in time of this quickly 

growing and dynamically changing field of sensing. 

Table 1 lists a number of applications that can be found within the energy industry where FBG technology is either 

already established or showing promise for future adoption. 

Table 1: Example FBG applications in the energy industry. 

Upstream Oil and Gas 

Multi-drop downhole pressure and temperature sensing for intelligent well optimization 

Structural health monitoring of offshore structures 

Condition monitoring of subsea pumps 

4D seismic monitoring 

Midstream Oil and Gas 

Pipeline integrity monitoring 

Structural health monitoring of LNG carrier cargo containment tanks 

Hull stress monitoring of product carriers 

Fossil Fuel Power generation 

Temperature sensing of power plant boilers and generators 

Wind Energy 

Structural health monitoring of rotorblades 

Independent pitch control of rotorblades 

Condition monitoring of rotating turbine machinery 

Structural health monitoring of turbine tower and foundations 

Tidal Energy 

Blade, drivetrain and marine life impact monitoring 

Nuclear 

In-core temperature measurement of nuclear reactors 

Structural health monitoring of containment and cooling infrastructure for nuclear power plants 

Electricity Transmission 

Power transformer hot spot monitoring 

Health and usage monitoring of power transmission and distribution systems 

UPSTREAM OIL AND GAS 

Upstream is a term commonly used to refer to the searching for and the recovery and production of crude oil and 

natural gas. The upstream oil sector is also known as the exploration and production (E&P) sector. There is one fiber 

optic sensing technique that is already well established and recognized as a valuable tool in the upstream sector. 

Distributed fiber optic sensors (based on Raman or Brillouin light scattering), measure a change in light wave 

scattering at different frequencies due to displacements within an optical fiber. The fiber itself is used for sensing

Fiber Bragg Grating Sensors in Energy Applications Fiber Bragg Grating Sensors 187 

and, in the case of distributed temperature sensors, also tends to be used for data transmission. Such distributed 

sensing systems employ only a single fiber, and can interrogate the fiber over distances as great as around 30 km. 

The energy markets for distributed temperature sensing (DTS) are well served by a number of global service 

providers including Sensa, Sensornet, Lios, AP Sensing, Omnisens and Sensortran. The experiences gained by the 

upstream industry over the past decade with these DTS technologies, particularly for in-well measurements, has 

been very useful in paving the way for FBG technology to follow. As a result, two particular applications for FBG 

technology in upstream oil and gas have emerged. 

Multi-Drop Downhole Pressure and Temperature Sensing for Intelligent Well Optimization 

DTS is primarily used in-well for early detection of problems with injectors, producers and downhole pumps, flow 

profiling, and well integrity monitoring. One particular DTS campaign is described by Simonits and Franzen [1], 

whilst new applications continue to arise such as fracturing optimization, as reported by Shell [2]. 

In recent years, the benefits offered by adding multi-point pressure and temperature (P/T) gauges in providing higher 

accuracy measurements at certain points have become apparent. 

Electronic pressure sensors (for example, quartz resonator gauges and piezoresistive sensors) have key limitations in 

downhole monitoring applications, such as low MTBF, inability to readily and cost-effectively take multi-point 

measurements, and limited temperature capability. FBG based pressure sensors have no downhole electronics and so 

do not suffer from these limitations. Instead they offer the advantages of longer operating lifetimes at elevated 

temperatures, intrinsic safety, the ability to multiplex and run in hole many tens of sensors on a single optical fiber 

cable, and the ability to deploy sensors many tens of kilometers away from a surface electronic readout unit without 

the need for signal amplification. 

This has been recognized by, for instance, Weatherford, who added several optical pressure and temperature gauges 

to a DTS installation in offshore Brunei, as reported by Pallanich [3]. 

Bani et al. [4] reported on an intelligent completions project carried out by Saudi Aramo, Baker Oil Tools and 

Weatherford International in which numerous FBG-based P/T gauges deployed demonstrated excellent long-term 

field performance, providing real-time zonal P/T for production monitoring and well diagnostics. 

Another example is a joint project between Smart Fibres and Shell, who have collaborated since 2003 to develop a 

low-cost, multi-point FBG P/T gauge technology known as SmartCell®. The objective of the project was to realize 

an FBG-based P/T sensing system with a price/performance ratio that represented a value proposition for the 

majority of operating wells, even in brownfield reserves with declining production. 

The development culminated in the downhole deployment pictured in (Fig. 2) of a string of 9 SmartCell P/T gauges 

in the Natih field of Shell operating unit Petroleum Development Oman [5-6]. 

Figure 2: Installation of SmartCell Multi-Point FBG P/T System (Shell / Smart Fibres) [5-6].


Fiber Bragg Grating Sensors for Railway Systems 

Hwa-yaw Tam*, Shun-yee Liu, Siu-lau Ho and Tin-kin Ho 



CHAPTER 11 



Abstract: Fiber Bragg Grating (FBG) sensor technology has been attracting substantial industrial interests for the 

last decade. FBG sensors have seen increasing acceptance and widespread use for structural sensing and health 

monitoring applications in composites, civil engineering, aerospace, marine, oil & gas, and smart structures. One 

transportation system that has been benefited tremendously from this technology is railways, where it is of the 

utmost importance to understand the structural and operating conditions of rails as well as that of freight and 

passenger service cars to ensure safe and reliable operation. Fiber-optic sensors, mostly in the form of FBGs, 

offer various important characteristics, such as EMI/RFI immunity, multiplexing capability, and very long-range 

interrogation (up to 230 km between FBGs and measurement unit), over the conventional electrical sensors for 

the distinctive operational conditions in railways. FBG sensors are unique from other types of fiber-optic sensors 

as the measured information is wavelength-encoded, which provides self-referencing and renders their signals 

less susceptible to intensity fluctuations. In addition, FBGs are reflective sensors that can be interrogated from 

either end, providing redundancy to FBG sensing networks. These two unique features are particularly important 

for the railway industry where safe and reliable operations are the major concerns. Furthermore, FBGs are very 

versatile and transducers based on FBGs can be designed to measure a wide range of parameters such as 

acceleration and inclination. Consequently, a single interrogator can deal with a large number of FBG sensors to 

measure a multitude of parameters at different locations that spans over a large area. 

FBG is the most promising, cost-effective and distributed sensor technology that provides an ideal platform to 

monitor the condition and structural health of tracks, carriages and underframe equipment in railway systems. In 

the last few years, a number of field trial railway projects using FBG sensors for axle counting, track and train 

vibration measurements, monitoring of bogie conditions, structural health monitoring of train bodies, and 

interaction between overhead contact lines and current collectors (pantograph) were successfully conducted by a 

few research institutions. These studies demonstrated the superiority of FBG sensors over conventional sensors in 

many crucial aspects. However, major barriers, such as lack of proprietary and custom specifications, packaging 

and reliability standards, insufficient field experience, have yet to be resolved before major railway operators are 

to embrace the FBG sensor technology. 

INTRODUCTION 

The rail industry is enjoying its biggest development boom worldwide in recent years. Fuelled by growing trade and 

rising environmental concerns on road transportation, the United States invested nearly US$ 10 billion in 2008 and 

allocated US$ 8 billion for high-speed trains in 2009. Indian Railways will invest about US$ 50 billion under the 

11 th Five Year Plan to modernize its railway system. As part and parcel of China’s rapid economic rise to become a 

modern nation, the scale of railway investment in China is gargantuan. In 2009, China spent US$ 50 billion on its 

high-speed rail system with a top speed of up to 350 km/hr. By 2020, China will have added more than 25,000 km of 

high-speed tracks and spend up to US$ 300 billion. Undoubtedly, the improvement of safety, reliability and 

productivity will continue to be the most important directive for the railway industry. This can be achieved through 

advances in on-board computers, on-board train condition monitoring systems, and wireless data transmission from 

wayside monitoring systems. There is also an increasing demand for better system reliability, availability, 

maintainability and safety from the communities. A smart condition monitoring system allows real-time and 

continuous monitoring of the structural and operational conditions of trains [1], overhead contact lines [2-3], as well 

as monitoring of the structural health of rail tracks and the location, speed and weight of passing trains of the entire 

rail systems. Ultimately, the inclusion of train location, speed restrictions, and train, track and overhead contact line 

conditions to the ‘intelligent systems’ will herald a safer railway industry with reduced maintenance cost, optimized 

*Address correspondence to this author Professor Hwayaw Tam: Photonics Research Centre, The Hong Kong Polytechnic University, Hung 

Hom, Kowloon, Hong Kong SAR, China; Tel: +852 27666175; Fax: +852 23301544; Email: eehytam@polyu.edu.hk

198 Fiber Bragg Grating Sensors Tam et al. 

performance and capacity. Therefore, the need of a smart condition monitoring system is imminent as indicated by 

the increase in railway/underground accidents/incidences around the world. Smart condition monitoring systems for 

the railway industry would require extensive sensor networks with large number (1,000s’) of multifunctional sensors 

for the measurements of temperature, strain/stress, vibration, acceleration, etc. Fiber Bragg grating sensors, in 

comparison to electrical sensors and other types of fiber-optic sensors, offer many advantages that are particularly 

well suited to railway transportation systems. These include immunity to EMI/RFI, long life-time (>20 years), and 

massive multiplexing capability – hundreds of sensing points along a single strand of optical fiber with length up to 

230 km [4], fast measurement speed, self-referencing and inherent redundancy feature. 

Quasi-distributed fiber-optic sensor based on fiber Bragg gratings (FBGs) is an excellent candidate for the 

realization of smart condition monitoring systems for the railway industry. There are more established distributed 

fiber-optic sensors based on Raman or Brillouin optical time-domain reflectometry but they are less suitable for 

condition and structural monitoring of railways which demand fast measurement time and high spatial resolution. 

In this chapter, the important characteristics of distributed photonic sensors, potential applications for the railway 

industry and some field trials will be described. Some of the FBG sensor-based monitoring systems are fully 

operational and in present service use – are providing valuable information about stresses experienced during 

service, both static and dynamic, under different operational conditions. The sensors also provide information on the 

loading and traffic status of the passenger cars; temperature-induced stresses and deformations on rails and 

carriages; temperatures in and around axles and wheel brakes; dynamic axle vibrations due to corrosion and bearing 

wear; and many other parameters relevant to railroad health monitoring and integrity. 

DISTRIBUTED FIBER-OPTIC SENSOR TECHNOLOGIES 

In distributed photonic sensor systems, a single fiber is used as the sensing element to substitute for thousands of 

conventional point sensors. The concept of distributed sensing was initially promoted in the late 70’s by optical 

fibers based on Rayleigh back scattering mechanism and through the technique of optical time-domain reflectometry 

(OTDR) [5] for locating faults in optical fibers, up to 250 km with a resolution of ~3 meters. 

Figure 1: Principle of optical time-domain reflectometer for distributed measurement along an optical fiber. 

(Fig. 1) shows the basic configuration of an OTDR in which a short pulse (~10 ns) of light is launched into a test 

fiber. The back-scattered spectrum due to Rayleigh, Brillouin and Raman is also shown. The position of the 

measured quantity is computed via the time of flight of the backscattered light pulse propagating in the fiber. In the 

late 80’s, a number of sensor configurations were proposed, mostly originated from the Raman and Brillouin 

scatterings in optical fibers [6-11]. Raman distributed sensing is based on the spontaneous scattering process 

generated by thermally-activated acoustic waves. Information about temperature is retrieved from the comparison 

between the intensities backscattered into the Stokes and the Anti-Stokes waves. Raman distributed sensor is now 

very mature and commercial units have typical performance of 1 K temperature accuracy with 1 m spatial resolution 

for fiber lengths up to 10 km. The measurement time varies from 1 minute to 10 minutes, depending on the required 

accuracy and spatial resolution. On the other hand, Brillouin OTDR does not base on intensity measurement, but the

Fiber Bragg Grating Sensors for Railway Systems Fiber Bragg Grating Sensors 199 

frequency shift of the Brillouin scattered wave. The Brillouin frequency shift depends on both temperature and 

strain, and the backscattered power depends solely on temperature. Brillouin OTDR can thus be used to measure 

temperature and strain simultaneously. 

Commercial Brillouin OTDR has the capability of measuring distributed strain and temperature with a resolution of 

20 µε and 1 K respectively, and with 1 m spatial resolution over a distance of 30 km in 1 minute. Distributed 

photonic sensors based on Raman and Brillouin OTDRs offer many advantages over conventional sensors in 

applications where a large number of sensing points is required and the environment is hazardous. In addition, 

optical fibers are non-conductive, non-corrosive, unaffected by EMI and RFI, low loss and small size. A common 

application of Raman OTDR is in the measurement and identification of hot spots along power transmission lines. 

Brillouin OTDRs are being employed in locating leaks in oil-pipe lines. However, the Raman and Brillouin 

distributed sensing systems require long measurement time and generally exhibit spatial resolution of the order of 

meter. Consequently, they are not suitable for applications where fast response time is needed or the required 

sensing regions are small. 

On the contrary, fiber Bragg grating sensors [12-13] can be interrogated at very high-speed of up to 2.5 Msamples/s 

[14]. FBGs are very small – short length of single-mode fiber (down to 0.1 mm) with periodic refractive-index 

variation in its 9-μm core, as shown in (Fig. 2a). FBGs can be created to reflect narrow-bands of spectrum (typically 


Fiber Bragg Grating Sensors in Nuclear Environments 

Francis Berghmans 1,2,* and Andrei Gusarov 2 



CHAPTER 12 

1 Department of Applied Physics and Photonics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium and 

2 SCK·CEN Belgian Nuclear Research Center, Boeretang 200, 2400 Mol, Belgium 

Abstract: Fiber Bragg Grating sensors are evaluated for applications in environments where the presence of 

highly energetic radiation is a concern, such as space and nuclear industry. To assess the feasibility of using this 

sensor technology in these particular application environments it is essential to evaluate the behavior of Fiber 

Bragg Gratings when exposed to different types of ionizing radiation. We, therefore, review the effects of 

ionizing radiation on several types of Fiber Bragg Gratings. 

INTRODUCTION 

Nuclear radiation effects on optical materials and photonic devices have been studied since several decades. In the 

early days one of the main concerns was to determine whether advanced technologies could withstand military and 

tactical environments such as surveillance satellite missions and nuclear weapon explosions. Today many other 

applications associated with presence of highly energetic radiation can benefit from the enhanced functionalities 

offered by photonic technologies for communication and sensing. Examples of these application fields include 

space, healthcare, civil nuclear industry and high energy physics experiments. Future thermonuclear fusion plasma 

reactors will also rely on optical communication and sensing techniques. 

Electronic and photonic components are well known to suffer from exposure to nuclear radiation [1]. The radiation 

interacts with the materials and alters their characteristics. This most often modifies the performance and affects the 

reliability of the device. Resulting device failures and system malfunctions may have dramatic consequences on 

safety and carry significant financial repercussions. One has to bear in mind that human intervention to replace 

components or to repair systems in radiation environments is mostly impossible due to the radioactivity levels 

involved or for reasons of accessibility. It is indeed almost impossible to repair onboard equipment once a satellite 

has been put into orbit. One can hence understand the importance of carefully investigating how radiation affects the 

operation and the reliability of photonic components intended for use in these harsh environments and of ensuring 

that devices and systems will survive the entire mission. 

Several years ago optical fibers carried a relatively bad reputation in terms of resistance to ionizing radiation. The 

fibers were indeed known to darken rapidly during exposure with substantial levels of so called radiation induced 

attenuation (RIA) as a result. Modern fiber fabrication technologies now allow obtaining fibers with low to moderate 

levels of RIA, depending on the wavelength range. This together with the undeniable and well-known advantages of 

fiber optic technology and the increased availability of compact, efficient and lower cost devices promoted renewed 

interest in the applications of optical fiber based systems in radiation environments. 

A fiber Bragg grating (FBG) is a typical example of a versatile photonic component that can be applied in both 

optical communication and sensing systems. FBGs are for example being considered as sensor elements for 

structural health monitoring of spacecrafts and as dosimeter in nuclear facilities. Both types of applications put 

entirely different demands on the FBGs: for the first application the gratings should be insensitive to radiation 

whereas for the second they should essentially be as sensitive as possible. 

The response of different types FBGs to various kinds and intensities of highly energetic radiation has therefore been 

the subject of many studies in the last decade. This chapter intends to review the main results obtained so far. Since 

radiation environments bring along considerably different environmental constraints from those conventionally 

*Address correspondence to this author Professor Francis Berghmans: Department of Applied Physics and Photonics, Vrije Universiteit 

Brussels, Pleinlaan 2, 1050 Brussels, Belgium; Tel: +32 2 629 34 53; Fax: +32 2 629 34 50; Email: fberghma@vub.ac.be

Fiber Bragg Grating Sensors in Nuclear Environments Fiber Bragg Grating Sensors 219 

encountered we chose to introduce the subject with an overview of different related application fields. Each of these 

fields is characterized by the presence of particular radiation types and intensities. These together with the basic 

radiation-matter interactions and the typical quantities and units involved are recalled as well. This should help the 

reader to understand how radiation induced ionization and displacement processes can significantly alter material 

and device characteristics. We then deal with radiation effects on fiber Bragg gratings by elaborating on a 

substantial amount of studies reported in open literature. 

For descriptions of the basic features of FBGs, such as spectral characteristics, fabrication methods and sensor 

properties we refer to other chapters in this book. 

AN OVERVIEW OF RADIATION ENVIRONMENTS 

This overview deals with three types of radiation environments: space, civil nuclear industry and future 

thermonuclear fusion reactors. The choice for these environments stems from the different radiation effect studies on 

FBGs that will be reported later in the text. These studies were indeed conducted with the intention to assess the 

feasibility of applications in one of these three environments. 

In this section and further down the chapter we will use different units to quantify and compare the intensity and the 

amount of radiation that can be encountered by a FBG in the different radiation environments. For particle radiation 

the flux is given in number of particles per unit area and per unit time. The particle fluence is then given in number 

of particles per unit area. For purely ionizing radiation such as gamma rays, the total ionizing dose is expressed in 

“Gray” (Gy) which corresponds to 1 J of energy absorbed in 1 kg of material and is the S.I.-unit of dose. 

Alternatively we can use “rads” with 1 Gy ≡ 100 rad. The gamma dose-rate is given in Gy per hour or rad per hour. 

The meaning of these quantities will also be described in the section dealing with the basics on radiation-matter 

interactions. 

Space 

The main sources of energetic particles that affect space borne devices can be classified in (1) protons and electrons 

trapped in the Van Allen belts, (2) heavy ions trapped in the magnetosphere, (3) cosmic ray protons and heavy ions, 

and (4) protons and heavy ions from solar flares. The levels of all of these sources are affected by the activity of the 

sun. Particle energies range from keV to GeV and beyond [2]. 

The Van Allen belts correspond to two torus shaped zones of charged particles which are trapped by our planet’s 

magnetic field (Fig. 1). The inner belt reaches about 1.5 earth radii far and is essentially fed by protons with energies 

in the 10-100 MeV range and electrons with energies from say 1 to 5 MeV. The protons can penetrate spacecrafts 

and damage instruments. They are also very dangerous for astronauts. The outer belt has maximum radiation flux 

values between 4 and 5 earth radii and contains essentially 0.1-20 MeV electrons. This belt can be hazardous to 

communication satellites [4]. 

Figure 1: Simplified drawing of the Van Allen radiation belts [3].

220 Fiber Bragg Grating Sensors Berghmans and Gusarov 

Cosmic rays include all particles traveling through the Cosmos with very high energies. The cosmic rays can have a 

solar, galactic and extragalactic origin. Almost 90% of all the incoming cosmic ray particles are protons, about 9% 

are helium nuclei (alpha particles) and about 1% are electrons [5]. 

Solar flares correspond to releases of magnetic energy built up in the sun’s atmosphere. With this energy release 

charged particles and nuclei are accelerated in the solar atmosphere. Solar flare protons, together with electrons and 

alphas in smaller quantities, are emitted by the sun in bursts during solar storms. The flux varies with the solar cycle. 

The energy spectra of solar protons are likely to be softer than those associated with trapped protons, but a 

spacecraft may nevertheless be exposed to considerable total fluence levels. The degree of exposure to such particles 

is highly dependent on orbital parameters. The Earth’s magnetic field exhibits a shielding effect in the equatorial 

regions, but allows the proton flux to be tunneled in towards the magnetic poles. 

Space agencies such as the National Aeronautics and Space Administration (NASA) and the European Space 

Agency (ESA) have defined guidelines for testing the radiation hardness of onboard equipment in terms of exposure 

levels and radiation intensities. ESA has for example specified the space environment in standard ECSS-E-10-04A 

[6]. This standard is intended to assist in the consistent application of space environment engineering to space 

products through specification of required or recommended methods, data and models to the problem of ensuring 

best performance, problem-avoidance or survivability of a product in the space environment. 

To assess the radiation dose experienced by a FBG in a spacecraft one has to consider the spacecraft orbit as well as the 

shielding. Typical orders of magnitude used in space radiation testing are a total ionizing dose of 10 kGy (≡1 Mrad) 

and a proton fluence of 10 12 cm -2 s -1 for a 10 year mission. Proton energy also has to be considered and is typically a few 

tens of MeV. 

In terms of applications FBGs have been considered for structural integrity monitoring of spacecrafts, leak detection 

on fuel tanks, sensor systems in nuclear propulsion systems, wavelength division multiplexing (WDM) element in 

intra- and inter-satellite communication systems [7-9]. 

Nuclear Fission and Nuclear Power Plants 

Civil nuclear industry essentially refers to the entire nuclear fuel cycle. The two main types of radiation which needs 

to be taken into account in this application field are gamma-rays emitted by fuel elements and by the surrounding 

structures which have become radioactive following the exposure to neutrons, as well as the neutrons themselves. 

The latter are however only to be considered for applications inside a reactor core during reactor operation. 

The actual flux/dose-rate and fluence/dose levels will strongly depend on the particular application. One can expect 

large differences in radiation fluxes whether one considers operation in a hot-cell or in the core of an operating 

reactor. Table 1 lists typical orders of magnitude for various applications. 

Table 1: Examples of radiation conditions that can be encountered for various applications in civil nuclear environments. 

Φ n = neutron flux; Φ γ = gamma dose-rate; D γ = gamma total dose [1, 10-11]. 

Application Environment 

In reactor core n ≤ 10 14 cm -2 s -1 , ≤ 10 7 Gy/h 

Containment building D ≤ 5·10 5 Gy (normal operation) / D ≤ 1.5·10 6 Gy (accident) 

Reactor maintenance ≈ 10 Gy/h, D ≈ 10 4 Gy 

Hot-cell ≈ 0-10 4 Gy/h, D ≈ 0-10 6 Gy 

Decontamination ≈ 10 -2 -10 Gy/h, D ≈ 10-10 3 Gy 

Spent fuel manipulation ≈ 10 2 -10 3 Gy/h, D ≈ 10 6 -10 7 Gy 

FBGs have been considered for various sensing tasks in nuclear industry. Examples include structural health 

monitoring of containment buildings, in reactor core temperature and mechanical stress measurements, and longterm 

monitoring of underground nuclear waste storage facilities [12-13].




CHAPTER 13 

Fiber Bragg Grating Evanescent Wave Sensors for Chemical and Biological 

Applications 

Andrea Cusano 1,* , Domenico Paladino 1 , Antonello Cutolo 1 , Agostino Iadicicco 2 and 

Stefania Campopiano 2 

1 Optoelectronic Division – Engineering Department, University of Sannio, Corso Garibaldi 107, 82100 Benevento, 

Italy and 2 Department of Technology, University of Naples “Parthenope”, Centro Direzionale di Napoli Isola C4, 

80143 Napoli, Italy 

Abstract: While Fiber Bragg Grating (FBG) sensors continue to act as valuable sensing platforms specially when 

physical parameters have to be measured in single or multipoint configuration, great efforts have been focused in 

the last decade to convey the great advantages of FBG technology towards the development of new devices and 

components employable in modern chemical and biological applications. Since the first attempt carried out in 

1996 by Meltz et al., many sensing schemes have been proposed based on the evanescent wave effect arising 

from the surrounding refractive index sensitization of the final device by tailoring either the grating structure or 

the host fiber. In this chapter, we review the main advances in the area of FBG evanescent wave sensors, with 

particular emphasis on principles of operation, technological developments, overall performances and discussing 

in details perspectives and challenges that lie ahead. 

INTRODUCTION 

Among the large number of fiber optic sensors configurations, Fiber Bragg Grating (FBG) based sensors, more than 

any other particular sensor type, have become widely known and popular within and out the photonics community 

and seen a rise in their utilization and commercial growth. Given the capability of FBGs to measure a multitude of 

parameters such as strain, temperature, pressure, and many others coupled with their flexibility of design to be used 

as single or multipoint sensing arrays and their relative low cost, make them ideal devices to be adopted for a 

multitude of sensing applications and implemented in different industrial fields [1-5]. 

With numerous life sciences applications in mind, the photonic science and technology have recently been evolved 

into an important interdisciplinary field – bio-photonics [6]. One highly attractive area in this large field, the optical 

biosensor, is experiencing a new surge in activity, seeking to exploit novel optical structures and bio-coating 

materials and techniques, driven by the demand for high performance analytical tools capable of detecting and 

discriminating among large classes of bio-molecules. Through rapid advances in this area, the highly biosensitive/selective 

optical sensors appear set to become viable and preferred alternatives to traditional “solution 

based” assay biosensors for applications in genomics, proteomics and drug discovery research and development, as 

well as in food industry, homeland security and environmental monitoring applications. 

Over the last decade, FBGs have provided the basis for families of optical sensors for applications in aerospace, 

transportation, energy and civil engineering, just to name a few. Nevertheless, there has also been an increasing 

flurry of activity aimed at implementing optical biochemical sensors by exploring the grating’s response to the 

Surrounding-medium Refractive Index (SRI) or of thin specific and functionalized overlays. In fact, besides the 

direct influence of temperature and strain onto the optical length of the fiber grating, there is the possibility to 

modify the effective refractive index of the guided mode via the evanescent wave interaction [7]. This feature 

combined with the intrinsic multi-measurand capability in single and multipoint sensor configuration open the way 

to develop high performances FBGs evanescent wave sensors as valuable technological platforms for chemical and 

biological applications. 

Among fiber gratings written perpendicularly to the fiber axis, only Long-Period Fiber Gratings (LPFGs) are readily, 

intrinsically sensitive to the SRI via the coupled light from core to cladding penetrating the surrounding medium. As a 

core-to-core mode coupling device, the light in a FBG is well screened by the cladding, effectively precluding strong 

*Address correspondence to this author Professor Andrea Cusano: Optoelectronic Division – Engineering Department, University of Sannio, 

Corso Garibaldi 107, 82100 Benevento, Italy; Tel: +39 0824305835; Fax: +39 0824305846; Email: a.cusano@unisannio.it

Fiber Bragg Grating Evanescent Wave Sensors for Chemical and Biological Applications Fiber Bragg Grating Sensors 239 

interaction with the surrounding medium. However, FBGs can be SRI-sensitized by tailoring either the grating 

structure or the host fiber, creating the basis for the development of possible technological platforms for chemical and 

biological sensing applications. Furthermore, despite high SRI sensitivity in comparison with FBGs, LPFGs exhibit 

several disadvantages as deployable devices. LPFGs possess much higher temperature and bending cross-sensitivities 

and, thus, can be severely influenced by their environmental conditions. Another disadvantage of the LPFG is that its 

spectral response can be measured only in transmission, and often with poor resolution due to the broad (typically 

tens of nanometers) transmission loss-type resonances. To reduce the bandwidth of the LPFG significantly, the device 

length has to be increased substantially, compromising its use as a localized or point sensor device. 

Here, we review the main advances in the area of fiber grating evanescent wave sensors focusing the attention on 

short period FBGs. To this aim, we can envision most of the works proposed in literature as divided in four main 

classes employing the evanescent wave interaction as transducing principle: 

- FBGs written in unconventional optical fibers (D-shaped optical fibers and Micro-structured Optical 

Fibers (MOFs)) ; 

- Uniform Thinned FBGs (ThFBGs); 

- Tilted FBGs (TFBGs); 

- Micro-structuring of conventional FBGs by post processing techniques. 

In the following these classes are separately illustrated with particular emphasis on their principle of operation, 

technological development, overall performances and discussing in details perspectives and challenges that lie ahead. 

FBGS WITHIN UNCONVENTIONAL OPTICAL FIBERS 

From the historical point of view, the first attempt to realize a FBG based evanescent wave sensor relies on the use 

of D-shaped optical fibers [8]. Following this first experimental evidence, many efforts have been mainly devoted to 

optimize the structure through the use of side polished optical fibers or surface relief gratings within D-fibers. More 

recently, a new approach has been envisaged via grating writing within MOFs, providing the basis for modern optofluidics 

components and devices. 

FBGs within D-Shaped Optical Fibers 

The first attempt to use FBGs as chemical transducers via evanescent wave interaction was demonstrated in 1996 by 

Meltz et al. [8]. Unlike LPFGs, FBGs are intrinsically insensitive to the SRI, since the light coupling takes place 

only between well-bound core modes that are well screened from the influence of the SRI by the cladding. To 

address this issue, the basic idea proposed by Meltz et al. was to use fiber gratings written in D-fibers sensitized to 

the SRI through post processing cladding stripping. 

Figure 1: Schematic of a D-shaped fiber as supplied by KVH Industries Inc. (Source: Brigham Young University). 

As reference, (Fig. 1) illustrates the cross section of a D-shaped fiber supplied by the KVH Industries, Inc. The two 

primary features of the D-fiber are the elliptical core, which maintains the polarization, and the proximity of the 

fiber core to the flat surface above the core. This proximity allows access to the light in the core by a weak cladding


stripping above the core area. Because of the D-shape of the fiber, the mechanical integrity and approximately the 

same dimensions of the fiber are preserved. 

As few microns of cladding are removed from the flat side of the host fiber, the core mode is influenced by the 

external medium through evanescent wave interaction. As main consequence, the shift of the Bragg wavelengths 

related to the two polarization states occurs. The Bragg lines of both the fast and slow eigenmodes, indeed, are blueshifted 

when the silica cladding layer is removed and replaced with water or methanol films (lower than the core 

refractive index). Changes in the fiber birefringence were observed because the perpendicular and parallel modes 

decay into the cladding at different rates. By using a tunable laser with a narrowband Bragg grating filter or threegrating 

Fabry-Pérot interferometer, refractive index variations of 5 by 10 -6 could be detected. Temperature 

compensation methods were also discussed including the use of an isolated reference grating and the simultaneous 

combination of birefringence and Bragg line wavelength shift measurements. 

Successively, planar side polishing [9] was demonstrated as effective technique to develop reliable chemical 

transducers based on FBGs written in D-shaped fibers with normal birefringence and depressed cladding [10]. 

In this experiment, standard single mode optical fiber was embedded in a glass block and successively polished 

down to residual cladding thickness less than 2 m (see Fig. 2). Finally, Bragg gratings were written in the central 

part of the polished fiber allowing multipoint sensor array being addressed by wavelength multiplexing of sensors 

heads in series along a single optical fiber. In follow up works, the sensing performances of the proposed 

configuration have been in details numerically and experimentally investigated revealing that [11]: 

- due to the asymmetric removal of the cladding, the dependence of the effective index of the 

fundamental mode on the surroundings is affected by light polarization, in particular, TM polarization 

exhibits a higher efficiency when compared to the TE polarization state; 

- the effective refractive index of the fundamental mode increases non-linearly approaching its 

maximum for SRI values close to the core refractive index (cut-off of the fundamental mode); 

- maximum sensitivity is obtained in case of cladding layer completely removed on the flat side of the 

structure leading to the maximum evanescent wave interaction; 

- in the case of coated structures with low refractive index overlays, sensing performances decrease as 

the coating is thinned compared with the penetration depth of the evanescent wave; 

- due to the dependence of the penetration depth on the optical wavelengths, sensing performances can 

be improved if longer wavelengths are used. 

Analyte n A 

l = 2n eff [n ] 

A 

B 

Figure 2: Schematic of a side polished FBG chemical sensor. (Source: IPHT Jena) 

Bragg Wavelength [nm] 

839.0 

838.8 

838.6 

838.4 

838.2 

838.0 

Figure 3: SRI sensor characteristic at 838 nm. (Source: IPHT Jena) 

Fiber Bragg grating in core 

of side-polished fiber 

837.8 

1.30 1.35 1.40 

1.45 

DAO 

n Ethanol 

A H O 

OZ91 

2 

LMO 

26%NaCI/H O OZ98 

2 

SPO 

L 

Embedded side-polished 

single mode fiber 

OZ91, OZ98 - 

Octane number of Petrol 

products 

SPO - Spindle oil 

LMO - Light machine oil 

DAO - De-asphalted oil of 

Furfural extraction product


Fiber Bragg Grating Sensors in Microstructured Optical Fibers 

Tomasz Nasilowski* 



CHAPTER 14 

Department of Applied Physics and Photonics, Faculty of Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 

building F, B-1050 Brussels, Belgium 

Abstract: The unusual properties of Fiber Bragg Gratings in microstructured fibers deliver significantly 

improved performance in some respect to traditional fibers. They are also superior in one or few characteristics 

and eventually unveil the novel properties of fiber Bragg grating sensors, leading to the superiority of 

implementations and original applications, which are briefly discussed in this chapter. Moreover, a very 

promising functionality of doped core microstructured fibers in perspective of material and geometrical guiding 

mechanism competition is clarified and encouraged. Furthermore, such fibers are greatly advantageous for Fiber 

Bragg Grating inscription and instead of scarifying important properties they pave the way to novel applications 

fulfilling the industrial requirements. Additionally, very interesting and hopeful research and development of pure 

silica microstructured Fiber Bragg Gratings are presented as well. 

INTRODUCTION 

One of a very important and popular type of fiber sensors rely on a fiber Bragg grating (FBG) [1-2]. FBG sensors 

are commonly used in civil engineering (bridges, dams, railways, mines etc.). They consist 70% to 80% of the 

optical sensors in structural health monitoring. 

Some of the reasons for this common use of FBG sensors [3-5] is based in the ability to measure physical changes 

such as multi-axis and transverse strain, vibration, displacement, and also moisture, ice, and in general, refractive 

index changes of the sensor neighborhood. Additionally, because the temperature and stress directly affect the 

reflectivity spectrum of FBGs, they can provide unique measurements availability, such as detecting changes in 

stress in buildings, bridges, airplane wings or other bodies; depth measurements in rivers, reservoirs for flood 

control; temperature and pressure measurements in deep oil wells, dislocations of railways, non circularity of train 

wheels, cracks in mines etc. They also offer exceptional resolution, large dynamics, low weight, absolute 

measurements and modest cost per channel. Furthermore, due to their nature FBG sensors can be either time- or 

wavelength-multiplexed, which allows for quasi-distributed sensing – an essential benefit for structural health 

monitoring [6]. 

The majority of FBG sensors are inscribed in conventional glass optical fibers. A new class of microstructured fibers 

(MOFs) or photonic crystal fibers (PCFs), also called holey fibers (HF) [7-8], became in recent years a subject of 

extensive research. Generation of supercontinuum, improvement in efficiency of fiber lasers, very low bending 

losses and compensation of chromatic dispersion are some of the numerous applications of MOFs already 

demonstrated in literature [7-8]. Endlessly single mode light propagation for very broad wavelength range (from UV 

up to IR) was the first paradigm shift unveiled by new generation fibers. Moreover, polarization properties of MOFs 

are also very interesting. For example, it has been shown that modal birefringence in MOFs may exceed 10 -3 , which 

is one order of magnitude higher than birefringence in classical highly birefringent (HB) fibers [7-9]. Furthermore 

single-polarization operation can be achieved as well in photonic crystal fibers [10-12]. 

MOFs for sensing purposes cover various types of sensors including interferometric and polarimetric sensors of 

different physical parameters (temperature, hydrostatic pressure, elongation, force, bending, etc.) as well as 

evanescent field sensors for monitoring specific chemical compounds in gas and liquids and their refractive index 

changes. Many of these interesting sensing applications are presented in Refs. [13-14]. 

The combination of a microstructured fiber with FBG is an architecture which joins two interesting ideas. On one hand 

*Address correspondence to this author Professor Tomasz Nasilowski: Department of Applied Physics and Photonics, Faculty of 

Engineering, Vrije Universiteit Brussel, Pleinlaan 2, building F, B-1050 Brussels, Belgium; Tel: +32 2 629 35 67; Fax: +32 2 629 34 50; Email: 

tnasilowski@tona.vub.ac.be

Fiber Bragg Grating Sensors in Microstructured Optical Fibers Fiber Bragg Grating Sensors 271 

we have the Bragg grating, a device which has been thoroughly studied [15-17] and successfully employed in 

numerous applications. On the other hand the photonic crystal fiber with its many, recently discovered possibilities. 

To rather big extend we can consider such components as three dimensional photonic crystal structures. 

Fiber Bragg gratings in conventional fibers have played a major role in many applications, and therefore in recent 

years there have been several attempts to incorporate a Bragg grating in MOF through different techniques. 

One of the most attractive fiber sensing applications steams from the use of highly birefringent MOF with a 

dedicated design that allows inscribing FBG in the fiber core. Such 3D microstructure can serve as pressure or 

transverse force transducers with a large sensitivity, while exhibiting a very low sensitivity to temperature drifts 

and/or longitudinal strain if adequately designed. Therefore, Bragg gratings in PCF may offer a viable alternative to 

traditional optical fiber sensors that require temperature compensation mechanisms and that are not intrinsically 

capable of distinguishing stress and temperature [18-20]. 

The unusual properties of MOFs deliver significantly improved performance in respect to traditional fibers. They are 

superior in several characteristics and also unveil the novel properties of fibers. All this leads to the superiority of 

implementations and original applications of FBGs in MOFs. 

The intention of this chapter is to show that the most realistic applications of microstructured fiber Bragg 

gratings for the next years steam from the doped core MOFs, which are enough UV photosensitive to use 

standard FBG inscription setups [21]. Moreover, such fibers give additional degrees of freedom (shape and level 

of doped core) over pure silica MOFs in designing sensor properties, especially in case of birefringent MOFs. The 

most important advantage seams to be the selectivity of the measured parameters, like temperature insensitivity. 

In longer time scale, the point-by-point inscription technology [22] should prove a great potential in writing 

FBGs in low UV sensitive MOFs. However, presently this technology is still far from being mature for 

industrial implementation. 

FBG in MOFs are becoming the new basic component which leads to the novel and large scale development 

of fiber sensor industrial applications. As it is shown in this chapter, already now, in many cases such sensors 

can be fabricated on industrial scale with necessary reliability and repeatability for sensing applications. 

The following paragraphs give a short description of guiding mechanisms in microstructured fibers followed by the 

explanation of propagation mechanism competition in case of doped MOFs. Next, there is a brief presentation of 

different technologies for FBGs inscription in MOFs. After that the main results and achievements of FBG in solid 

bandgap fibers (SBF) is mentioned. Other sections demonstrate the breakthroughs of FBGs in MOFs for sensing 

applications of gases and liquids refractive index measurements, as well as monitoring the mechanical stresses with 

use of FBGs in low and high birefringent fibers, which are often temperature insensitive. 

There are still many unanswered questions and non established limitations of FBGs in MOFs that have to be studied 

within next years. 

GUIDING MECHANISM IN MICROSTRUCTURED FIBER 

The spite of high maturity of classical fibers, they have limits in designing their properties. This might sometimes 

limit their applications or make them less competitive e.g. with electronic devices. This situation was often taking 

place in case of sensing purposes and for that reason fiber sensors were mostly employed in niche applications. 

Microstructured fibers consist of a periodic lattice (or another type of structure) of air holes that run along the length 

of the fiber and that allow confining and guiding light along the fiber (Fig. 1). A defect in the centre of the lattice 

serves as fiber core. Adapting the particular features of the microstructure, such as the air filling fraction and the 

lattice period, allows obtaining optical fibers with very particular properties in terms of dispersion, mode-field 

confinement, endlessly single mode operation, unusual polarization properties etc. [7-8]. The optical and mechanical 

properties of MOFs can also be controlled by changing the size, shape and location of the air holes.

272 Fiber Bragg Grating Sensors Tomasz Nasilowski 

The unusual properties of MOF deliver significantly improved performance in some respect to traditional fibers. 

They are also superior in one or few characteristics and eventually unveil the novel properties of fibers. All this 

leads to the superiority of implementations and original applications of MOFs. 

Classical optical fibers are today finding wide use in areas covering telecommunications, sensor technology, 

spectroscopy, medicine etc. [23]. Their operation usually relies on light being guided by the difference in material 

properties sometime called index guiding, and often mistaken with total internal reflection phenomena due to its 

intuitive property taken from geometrical optics. Precisely, light in classical fibers is guided in the core region filled 

by the material with higher refractive index and surrounded by the cladding region filled by the material with lower 

refractive index. Some of the modes allowed by Maxwell equations or wave equation [24] in the core are not 

allowed in the cladding region. In other words, some stationary solutions of Maxwell equations in the higher 

refractive index material do not exist in the lower refractive index material. For this reason, if such modes are 

excited in the core region they have to propagate along the core down the length of the fiber since their existence is 

forbidden in the cladding. Because the propagation in classical fibers is steamed by the difference in the core and the 

cladding materials such guiding mechanism can be called material guiding. 

In case of microstructured fibers the difference between core and cladding is not in different material properties, but 

in different geometries of the same material. Due to this difference in geometry some of the modes are allowed in 

the core and are not allowed in the microstructured cladding. If these modes are excited in the core they are trapped 

over there and they can propagate only along the core. This is intuitively illustrated on the (Fig. 1a). Whenever the 

modes excited in the core are similar to the cladding modes they are not propagating because of coupling to the 

cladding modes, as shown on (Fig. 1b). Since the propagation is founded by the difference in geometries such 

guiding mechanism can be called geometrical guiding. 

The geometrical guiding explains the propagation in both types of microstructured fibers, namely index guiding and 

photonic bandgap (PBG) PCFs. 

Typical index guiding PCF [25-26] consist of solid glass core and microstructured hollow cladding very often 

surrounded again by larger solid glass region (Fig. 1). In this case the effective refractive index of the mode guided 

in the solid core is higher from the index of the microstructured cladding modes. For that reason this type of fibers 

are compared with classical, material guided, fibers. However, the great distinction of very high dispersion 

properties, due to the scaled with wavelength geometry and, additionally, the selectivity of cladding modes in 

contrast with almost continuum of modes in large and solid cladding of classical fiber are remarkable differences, 

which can not be neglected and are basis for most of the extraordinary properties of PCFs. 

n 1 eff 

n cl eff 

neff 

(a) (b) 

Figure 1: Schematic drawing of a typical microstructured fiber presents guided (a) and not guided (b) modes. If mode excited in 

the core has effective refractive index (n 1 eff) different from effective index of cladding modes (n cl eff) it is trapped by the 

surrounding cladding structure and has to propagate along the core. In case the excited mode in the core has similar effective 

refractive index (neff) to the one of the cladding modes it becomes the leaky mode and is coupled out from the core. 

n cl eff


Polymer Fiber Bragg Gratings 

David John Webb 1,* and Kyriacos Kalli 2 



CHAPTER 15 

1 Photonics Research Group, Aston University, United Kingdom and 2 Nanophotonics Research Laboratory, Cyprus 

University of Technology, Cyprus 

Abstract: This chapter deals with gratings recorded in polymeric optical fibers (POFs); predominantly those 

based on poly (methyl methacrylate) (PMMA). We summarize the different mechanical and optical properties of 

POFs which are relevant to the application of POF Bragg gratings and discuss the existing literature on the 

subject of the UV photosensitivity of PMMA. The current state of the art in POF grating inscription is presented 

and we survey some of the emerging applications for these devices. 

INTRODUCTION 

Work on the UV photosensitivity of poly (methyl methacrylate) (PMMA) dates back to the early 1970s, well before 

the discovery of photosensitivity in silica fibers. However, single mode polymer optical fiber (POF) only became 

available in the 1990s and it was not until the end of that decade that the first POF Bragg grating (POFBG) was 

reported. Compared to the current state of the art with silica FBGs, the field of POFBGs is still very immature. 

Considerable research challenges exist in photosensitive fiber manufacture, developing new fiber materials, 

understanding the mechanisms behind the photosensitivity and improving the reproducibility of grating inscription. 

Nevertheless, applications research is now starting to emerge, seeking to exploit the rather different properties of 

POF compared to silica fiber. 

This chapter summarizes the current state of the art in the field. We have attempted to provide useful information to 

help those starting to work with POF, with regard to handling the fiber and the practicalities of inscribing gratings, 

and have also highlighted some of the areas where further research is particularly needed, for example in 

understanding and improving photosensitivity. Finally, we summarize the properties of gratings produced to date 

and survey some of the applications that are starting to arise. 

TECHNOLOGICAL DRIVERS 

In this section we discuss the motivation for work on POF. Within the telecommunications industry, POF has a 

reputation for being low cost so this might seem like the first area we should explore but unfortunately, the features 

of POF that provide this attraction to network installers do not carry over to grating based sensing. In short distance 

moderate bandwidth applications, the typical POF is 1mm in diameter, most of which is taken up by the core [1]. 

Even at this diameter, POF is highly flexible and tolerant of sharp bends, thereby being easy to install. Furthermore, 

the fiber can be illuminated using low cost, broad emission area sources, such as light emitting diodes. Finally, fiber 

connection is straightforward as the positional tolerance required is very relaxed and the fiber can be easily 

“cleaved” using a sharp knife. The low cost of POF in network systems is therefore not so much related to the actual 

fiber cost but to the ease of handling and installation coupled with the low cost of ancillary components. 

Where grating based sensors are concerned, the fiber will usually need to be single mode, which imposes very tight 

positional tolerances on connections between fibers. In addition, single transverse mode sources are usually 

required. These factors are all likely to preclude any cost advantage over silica fiber and consequently we have to 

look elsewhere for the motivation to develop this technology. 

Fortunately, the physical and chemical properties of polymeric materials are rather different to silica and it is in this 

area that there appear to be advantages. We will explore the main differences below; to do so we will often make use 

of the properties of PMMA as a representative polymer. It should be borne in mind though that whilst PMMA may 

be the most widely used polymer for optical fibers, there are many other polymers available that may well be better 

for certain applications. 

*Address correspondence to this author Dr. David J. Webb: Photonics Research Group, Aston University, Aston Triangle, Birmingham, B4 

7ET, United Kingdom; Tel: +44 121 204 3541; Email: d.j.webb@aston.ac.uk; kkalli@cytanet.com.cy

Polymer Fiber Bragg Gratings Fiber Bragg Grating Sensors 293 

Mechanical Properties 

Let’s start with Young’s modulus: for silica fiber it is 73 GPa [2], while a typical value for bulk PMMA is 3.3 GPa 

[3]. The Young’s modulus for various PMMA based fibers has been measured yielding values of: 1.6-2.1 GPa [4], 

2.75 GPa [5], 2.8 GPa [6] and 2.8-3.4 GPa [7]. The variations in Young’s modulus for POF observed here are 

unsurprising given the various co-polymers used in the fiber cores, the different processing conditions likely to have 

been used, the various molecular weight distributions that could exist and the addition of plasticizers that may have 

taken place. Note that when we talk about Young’s modulus for POF we are implicitly discussing stress and strain 

along the fiber axis. Following the drawing process, there is thought to be some alignment of the polymer molecules 

along the axis, which will probably lead to anisotropic mechanical properties. 

In many FBG sensing applications, the difference in the values of Young’s modulus for silica and POF will be 

unimportant; where it does matter is when the FBG sensor is being used to monitor a material that itself has a low 

Young’s modulus. The potential problem here is that a stiff silica fiber can act to locally stiffen the material, 

reducing the strain experienced in the region of the sensor in response to a given stress, whereas POF can have much 

less of an effect [8]. 

We next consider other tensile properties of the materials. Silica fiber has a typical failure strain of 5-10% [9] and 

below this value exhibits good elastic behavior. Of course, extreme care must be taken during grating fabrication not 

to introduce any scratches onto the fiber surface, which have been shown to considerably reduce the strength [10]. 

For PMMA and polymers in general, the situation is more complex. The good news is that extremely high failure 

strains are achievable. Despite the failure strain of bulk PMMA being as low as 4-5.5% (greater for higher molecular 

weights) [11], for fibers a value has been reported in excess of 100% [12], though it should be noted that to achieve 

these values it is important that the fiber is drawn under low tension. High tension drawing appears to lead to 

significant molecular alignment along the fiber axis which considerably reduces the failure strain, as shown in (Fig. 

1). Fiber drawn under high tension can be annealed to obtain the properties associated with low-tension drawing. 

Annealing at 95°C for a few days has the effect of: reducing Young’s modulus, yield point and tensile strength and 

increasing ductility (failure strain) [4]. The yield strain (limit of quasi-elastic behavior) is reported to be around 6% 

for PMMA [4-5]. 

True stress (MPa) 

300 

250 

200 

150 

100 

50 

0 

00:00:00:00 00:00:00:00 00:00:00 00:00:00 

Increasing drawing 

temperature 

0 10 20 30 40 50 60 70 80 

True strain (%) 

Figure 1: Tensile properties of PMMA fiber drawn under different tensions. The photographs show the morphology of the fiber 

break [12]. 

The bad news is that as a visco-elastic material, the tensile properties of polymers are complicated, displaying both 

hysteresis and a dependence on the timescales involved. For example, in one study, the yield strain of POF was 

00:00:00

294 Fiber Bragg Grating Sensors Webb and Kalli 

studied as a function of the rate of application of strain [4]. As strain rate was varied from 0.1 to 0.5 per minute, the 

yield strength increased from 76 to 85 MPa, with a concomitant increase in the tensile strength. The authors 

conclude that at low strain rates PMMA is able to conform to the applied load better than at high rates, when the 

internal viscosity leads to a brittle failure. 

PMMA based POF can display hysteresis when the applied strain is cycled. (Fig. 2) shows the effect of repeatedly 

stretching a POF by the same strain increment, starting each cycle from the point where the stress has dropped to 

zero in the previous cycle [5]. In this work, the strain was applied at a rate of 0.5%/minute. Given time in the 

unloaded state, the POF would relax back towards its initial value. One study reports that after strains as high as 3% 

the POF will return to its original length when the strain applied over a short period of time, of the order of a minute 

[13]. On the other hand, when the strained fiber was held for 10 hours prior to release, the relaxation was much 

slower. After a strain of 1%, the fiber relaxed back to essentially its original length after 10 hours, while from a 

strain of 4%, the length had only relaxed half way to its original value in that time [14]. This study also noted a 

relaxation of the fiber when under constant strain, revealed as a decrease in the stress needed to cause that strain. 

16 

12 

8 

4 

0 

Stress (MPa) 

Strain (%) 

0 0.2 0.4 0.6 0.8 1.0 

Figure 2: First, second, third and tenth cycles from repetitive tensile testing of PMMA fiber (after Ref. [5]). 

Finally, even in its quasi-elastic region, the stress-strain curve for PMMA fiber is not linear [7]. In fact the nonlinearity 

is approximately an order of magnitude larger than for silica fiber, and as a consequence can be important 

for strains above 1%, as opposed to 3% for silica fiber. This same study looked at the behavior of PMMA fiber 

under strain rates from 0.01 to 3/minute and noted – as mentioned earlier – that higher rates increased yield strain 

and yield stress but not the value of Young’s modulus. 

Chemical Properties 

The drawing of PMMA based fiber takes place at around 200°C, as opposed around 2000°C for silica. Many organic 

substances can survive the former temperature but certainly not the latter. Furthermore, as an organic material itself, 

PMMA is readily susceptible to processing using a range of organic chemistry techniques. The ability to incorporate 

specific organic molecules, either directly as part of the host polymer or added as copolymers or dopants, offers 

many possibilities for modifying the fiber, for example to enhance non-linear properties [15], to enable optical 

amplification [16], to provide sensitivity to specific chemical – or biochemical – species [17] or to increase 

photosensitivity [18]. 

Biocompatibility 

Both silica and PMMA are considered to be biocompatible in many situations. An advantage for polymer fibers 

arises due to their mechanical properties discussed above: POF can have a much greater failure strain than silica, 

leading to less chance of fiber breakage in the body. Perhaps more importantly, when silica fiber breaks there is a 

high chance that a sharp edge or point will result that could cause damage to surrounding tissue; the much softer 

polymer will not pose this kind of threat.




CHAPTER 16 

Fiber Bragg Grating Sensors: Market Overview and New Perspectives 

Jeff Wayne Miller 1 and Alexis Méndez 2,* 

1 Micron Optics Inc., 1852 Century Place, Atlanta, GA 30345, USA and 2 MCH Engineering LLC, 1728 Clinton 

Avenue, Alameda, CA 94501, USA 

Abstract: Over the last few years, optical fiber sensors have seen increased acceptance and widespread use. 

Among the multitude of sensor types, Fiber Bragg Grating (FBG) based sensors—more than any other particular 

sensor type—have become widely known and popular. FBGs have an intrinsic capability to measure a variety of 

parameters along a single fiber, such as: strain, temperature, pressure, chemical and biological agents, and many 

others. These multi-point sensing arrays of many relative low cost FBGs, provide great flexibility of design and 

make them ideal devices to be adopted for a multitude of different sensing applications and implemented in 

different fields and industries. However, some technical hurdles and market barriers need to be overcome in order 

for this technology—and fiber sensors in general—to gain more commercial momentum and achieve faster and 

more pronounced market growth. Other relevant factors are the need for industry standards on FBGs and FBGbased 

sensors, adequate and reliable packaging designs, as well as training and education of prospective 

customers and end-users. 

INTRODUCTION 

The fiber optics field has undergone a tremendous growth and advancement over the past 40 years. Initially 

conceived as a medium to carry light and images for medical endoscopic applications, optical fibers were later 

proposed in the mid 1960’s as an information-carrying medium for telecommunication applications. The outstanding 

success of this concept is embodied in the millions of miles of telecommunications fiber that have spanned the earth 

and the seas, utterly transforming the means by which we communicate. This has all been documented with awe 

over the past several decades. Among the reasons why optical fibers are such an attractive communication medium 

are their low loss, high bandwidth, EMI immunity, small size, lightweight, safety, relatively low cost, low 

maintenance, etc. 

As optical fibers cemented their position in the telecommunications industry—and its associated technology and 

commercial markets matured—parallel efforts were carried out by a number of different groups around the world to 

exploit some of the key fiber features and utilize them in sensing applications. 

Initially, fiber sensors were lab curiosities and simple proof-of-concept demonstrations. However, nowadays, optical 

fibers are making an impact and serious commercial inroads in other fields besides communications such as in 

industrial sensing, bio-medical laser delivery systems, military gyro sensors, as well as automotive lighting & 

control—to name just a few—and spanned applications as diverse as oil well downhole pressure sensors to intraaortic 

catheters. This transition has taken the better part of 20 years and reached the point where fiber sensors enjoy 

increased acceptance as well as a widespread use for structural sensing and monitoring applications in civil 

engineering, aerospace, marine, oil & gas, composites, smart structures, bio-medical devices, electric power industry 

and many others [1-2]. Optical fiber sensor operation and instrumentation have become well understood and 

developed, and a variety of commercial discrete sensors based on Fabry-Perot (FP) cavities and Fiber Bragg Gratings 

(FBGs), as well as distributed sensors based on Raman and Brillouin scattering methods, are readily available along 

with pertinent interrogation instruments. Among all of these, FBG based sensors—more than any other particular 

sensor type—have become widely known, and seen a rise in their utilization and commercial growth. 

FIBER BRAGG GRATINGS AS SENSORS 

Since their fortuitous discovery by Ken Hill back in 1978 [3] and subsequent development by researchers at the 

Communications Research Centre, United technologies, 3M and several others [4], intra-core fiber gratings have been 

*Address correspondence to this author Dr. Alexis Méndez: MCH Engineering LLC, 1728 Clinton Avenue, Alameda, CA 94501, USA; Tel: 

+1 (510) 521-1069; Fax: +1 (510) 521-5079; Email: alexis.mendez@mchengineering.com

314 Fiber Bragg Grating Sensors Miller and Méndez 

used extensively in the telecommunication industry for dense wavelength division multiplexing, dispersion 

compensation, laser stabilization, and erbium amplifier gain flattening, mostly at the 1550 nm within the C-band 

wavelength range. However, one of the primary FBG benefits is the ability to be multiplexed, allowing multiple 

sensors and multiple parameters to be measured along a single fiber. These, multi-point, FBG sensing arrays provide 

great measurement flexibility and make them 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 sensing applications (see Fig. 1) 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. To date, there are a diverse number of commercial FBG-based sensors designed and packaged 

to measure a variety of different mechanical, electrical and chemical parameters, as shown in (Fig. 2). 

Figure 1: FBG sensors are used in a variety of industrial applications. 

Figure 2: Diverse types of commercial FBG-based sensors. 

Accelerometer Displacement meter 

Strain meter Pressure meter 

Thermometer 

The main advantage of fiber gratings for mechanical sensing is that these devices perform a direct transformation of 

the sensed parameter into optical wavelength—independent of light levels, connector or fiber losses, or other FBGs 

operating at distinct wavelengths. For instance, when compared to one of the most common and popular basic 

electronic sensors—the foil strain gage—the relevant advantages of FBG-based sensors become evident: 

• Totally passive no resistive heating or local power needed 

• Small size can be embedded or laminated 

Incline meter 

• Narrowband with wide wavelength operating range can be multiplexed

Fiber Bragg Grating Sensors: Market Overview and New Perspectives Fiber Bragg Grating Sensors 315 

• Non-conductive immune to electromagnetic interference 

• Environmentally more stable glass compared to copper 

• Low fiber loss at 1550 nm remote sensing 

Table 1: Pros and cons of FBG sensors. 

Table 1 summarizes several of the many benefits to intra-core FBGs used as sensing elements. However, there are 

also some limitations and cons associated with gratings. Most fundamentally, it is the fact that they are 

simultaneously sensitive to strain, temperature, pressure and radiation effects. Hence, adequate temperature 

compensation is always essential in the design and commercial offering of reliable and repeatable physical sensors. 

Although the theory and use of FBGs dates back to the late 1980s, the actual commercial transition did not happen 

until the mid-1990s, when it was strongly driven by the vast communications needs at the time coupled with the 

rampant demand for components brought on by the so-called telecommunications bubble, which saw a tremendous 

explosion on the number of companies and research groups engaged with the design, fabrication, packaging and use 

of gratings. The significant milestones and timeline evolution of the FBG industry over the past 30 years is 

illustrated in (Fig. 3). 

Figure 3: Historical evolution of fiber Bragg gratings. 

Soon after the telecommunications bubble collapse (circa 2002), there was a significant shift by many players in the 

industry from communications to sensing applications. At the time, this was a prudent and strategic move on the part 

of FBG manufacturers to keep exploiting the technical and manufacturing infrastructure they had available and ride 

the telecomm crisis until a possible comeback. At the present time, with the dust from the telecomm “bubble” 

already settled, it is clear that the original expectations and market potential originally envisioned for FBG 

components in the communications sector has not materialized and that the industry is operating at more realistic 

levels that match the trends at pre-bubble years. 

Nevertheless, the sensing sector benefited tremendously from this shift and resulted in an increase in activity and 

demand of FBG-based sensors. However, the impact of the frenzy of mergers and acquisition at the peak of the

A 


Accelerometer 134-159 

B 

Birefringence 110-133 

Bulk and integrated optics 73-91 

C 

Composite structures 160-173 

Coupled mode theory 33-47 

E 

Electricity transmission 174-185 

F 

Femtosecond laser 9-32 

Fossil fuel power generation 174-185 

G 

Grating inscription within polymeric fibers 276-295 

Guiding mechanisms in optical fibers 255-275 

I 

Interferometric interrogation 73-91 

Interferometric technique 9-32 

Inverse scattering methods 33-47 

L 

Laser sensing 73-91 

Light polarization 110-133 

Liquid pressure sensing 134-159 

Load sensing 134-159 

Local heat treatment 48-72 

M 

Market barriers 296-303 

Microstructured fiber Bragg grating 48-72 

Multi-functionality 186-206 

Multiplexing 186-206 

N 

Nuclear 174-185 

INDEX 


All rights reserved - © 2011 Bentham Science Publishers Ltd.


O 

Oil and gas applications 174-185 

P 

Passive sensing 186-206 

Peculiar properties of polymers 276-295 

Phase mask 9-32 

Photonic bandgap 48-72 

Photosensitivity 1-8 

Physical sensing 255-275 

Polymeric materials 276-295 

R 

Radiation effects on fiber Bragg gratings 207-224 

Radiation environments 207-224 

Radiation-matter interaction 207-224 

Refractive index sensing 255-275 

S 

Sensing 1-8 

Sensors market 296-303 

Space structures 160-173 

Spatial division multiplexing 92-109 

Standardization 296-303 

Strain sensing 134-159 

Structural health monitoring 160-173 

T 

Telecommunications 1-8 

Temperature sensing 134-159 

Tidal energy 174-185 

Tilt sensing 134-159 

Tilted fiber Bragg gratings 225-254 

Time division multiplexing 92-109 

Transfer matrix method 33-47 

Transverse strain sensing 110-133 

U 

Ultra-long reach sensing 186-206 

Unconventional optical fibers 225-254 

W 

Wavelength division multiplexing 92-109 

Wavelength encoding 186-206 

Wet chemical etching 225-254 

Wind energy 174-185

chapter 1 - Bentham Science

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?