opto-mechanical fiber optic sensors€¦ · opto-mechanical fiber optic sensors research,...

OPTO-MECHANICALFIBER OPTIC SENSORS

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OPTO-MECHANICALFIBER OPTIC SENSORSResearch, Technology,and Applicationsin Mechanical Sensing

Edited by

HAMID ALEMOHAMMAD

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CONTENTSList of Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiBiography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiiiPreface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv

Chapter 1 Opto-Mechanical Modeling of Fiber BraggGrating Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Hamid Alemohammad

1.1 Fiber Bragg Gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Opto-Mechanical Properties of Optical Fibers . . . . . . . . . . . . . . . . . . . . . . . 21.3 Fiber Bragg Gratings With Structurally and Thermally

Induced Index Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.4 Light Propagation in Optical Fibers With Induced

Optical Anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.5 Coupled-Mode Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.6 Derivation of Coupled-Mode Theory for Fiber

Bragg Gratings With Uniform Grating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.7 Coupled-Mode Theory for Superstructure

Fiber Bragg Gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Appendices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Chapter 2 Superstructure Fiber Bragg Grating Sensorsfor Multiparameter Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Hamid Alemohammad

2.1 Superstructure Fiber Bragg Gratings With PeriodicOn-Fiber Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.2 Opto-Mechanical Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.3 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.4 Geometrical Features of Fabricated SuperstructureFiber Bragg Gratings With On-Fiber Films . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.5 Measurement Test Rig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382.6 Optical Response Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Chapter 3 Flat-Cladding Fiber Bragg Grating Sensors for LargeStrain Amplitude Fatigue Tests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49Xija Gu

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.2 Experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.3 Sensor Validation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523.4 Application in the Fatigue Test of a Friction

StireWelded Aluminum Alloy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.5 Application in Asymmetric Fatigue Deformation

of a Magnesium Alloy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Chapter 4 Fiber Bragg Grating Strain Sensor for Microstructurein Situ Strain Measurement and Real-TimeFailure Detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75Hua Lu, Xija Gu

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 754.2 Fiber Bragg Grating Basics and Sensor Fabrication . . . . . . . . . . . . . . . . 774.3 Comparison of Cantilever Strain Measured by

a Fiber Bragg Grating Sensor and a Strain Gauge . . . . . . . . . . . . . . . . . 804.4 Printed Circuit Board Assembly Test Sample Preparation

for Bend Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.5 Strain Gauge A and Fiber Bragg Grating Sensor Installation

on Assembly Packages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

vi CONTENTS

4.6 Comparison of Ball Grid Array Substrate Strain Results byFiber Bragg Grating Sensor Array and Finite ElementAnalysis Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.7 Four-Point Bending System and Test Setup. . . . . . . . . . . . . . . . . . . . . . . . 844.8 Dye-and-Pry Failure Visual Inspection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874.9 Test Results and Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

4.10 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

Chapter 5 Distributed Brillouin Sensing Using PolymerOptical Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Yosuke Mizuno, Neisei Hayashi, Kentaro Nakamura

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 975.2 Characterization of Brillouin Scattering in Polymer

Optical Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 985.3 Distributed Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155.4 Polymer Optical Fiber Fuse. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

Chapter 6 Femtosecond Laser-Inscribed Fiber Bragg Gratingsfor Sensing Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137Stephen J. Mihailov

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1376.2 The Fiber Bragg Grating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1386.3 The Fiber Bragg Grating Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1416.4 Femtosecond Laser-Induced Bragg Gratings. . . . . . . . . . . . . . . . . . . . . 1436.5 Applications of Femtosecond Laser-Induced Fiber Bragg

Gratings for Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1526.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

CONTENTS vii

Chapter 7 Innovative Fiber Bragg Grating Sensors for HighlyDemanding Applications: Considerations, Concepts,and Designs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175Lun-Kai Cheng, Peter Martijn Toet

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1757.2 Fiber Bragg Grating Sensor System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1767.3 High-Demand Fiber Bragg Grating Sensor System

Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1787.4 Fiber Bragg GratingeBased Sensors for Dedicated

Operational Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1817.5 Fiber Bragg GratingeBased Sensors for Special

Physical Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

Chapter 8 Fiber Optic Sensors in the Oil and Gas Industry:Current and Future Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211Christopher Baldwin

8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2118.2 Breakdown of the Oil and Gas Industry . . . . . . . . . . . . . . . . . . . . . . . . . . 2128.3 Thermal Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2158.4 Pressure Monitoring in the Downhole Environment . . . . . . . . . . . . . 2228.5 Flow Monitoring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2268.6 Seismic Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2298.7 Acoustic Monitoring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2318.8 Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236

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Chapter 9 Aerospace Applications of Optical FiberMechanical Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237Craig Lopatin

9.1 Introduction and Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2379.2 Measurements for Flight Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2439.3 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2449.4 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260Further Reading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

Chapter 10 Fiber Optical Sensors in Biomechanics . . . . . . . . . . . . . . . . . . . . 263Antonio B. Lobo Ribeiro, Paulo Roriz

10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26310.2 Why Fiber Optical Sensors in Biomechanics?. . . . . . . . . . . . . . . . . . . 26510.3 Applications in Biomechanics of Rigid Bodies . . . . . . . . . . . . . . . . . . 26810.4 Applications in Biomechanics of Deformable Bodies . . . . . . . . . . . 27510.5 Applications in Biomechanics of Fluids . . . . . . . . . . . . . . . . . . . . . . . . . 28210.6 Final Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

Chapter 11 Fiber Optic Sensors for Biomedical Applications . . . . . . . . . . 301Daniele Tosi, Sven Poeggel, Iulian Iordachita,

Emiliano Schena

11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30111.2 Biomedical Fiber Optic Sensor Systems . . . . . . . . . . . . . . . . . . . . . . . . 30411.3 Optical Fiber Sensors for Diagnostics. . . . . . . . . . . . . . . . . . . . . . . . . . . 31211.4 Optical Fiber Sensors for Robotic Microsurgery . . . . . . . . . . . . . . . . 31811.5 Smart Textiles and Wearable Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 327

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

CONTENTS ix

LIST OF CONTRIBUTORS

Hamid AlemohammadAOMS Technologies Inc., Toronto, ON, Canada

Christopher BaldwinWeatherford, Laurel, MD, United States

Lun-Kai ChengTNO, Delft, The Netherlands

Xija GuRyerson University, Toronto, ON, Canada

Neisei HayashiTokyo Institute of Technology, Yokohama, Japan

Iulian IordachitaJohns Hopkins University, Baltimore, MD, United States

Antonio B. Lobo RibeiroUniversity Fernando Pessoa, Porto, Portugal

Craig LopatinTechnion-Israel Institute of Technology, Haifa, Israel

Hua LuRyerson University, Toronto, ON, Canada

Stephen J. MihailovNational Research Council of Canada, Ottawa, ON, Canada

Yosuke MizunoTokyo Institute of Technology, Yokohama, Japan

Kentaro NakamuraTokyo Institute of Technology, Yokohama, Japan

Sven PoeggelUniversity of Limerick, Limerick, Ireland

Paulo RorizUniversity Institute of Maia (ISMAI), Maia, Portugal; INESC TEC, Porto,

Portugal; LABIOMEP, Porto Biomechanics Laboratory, Porto, Portugal;

CIDESD-ISMAI, CIDESD, Maia, Portugal

Emiliano SchenaUniversita Campus Bio-Medico di Roma, Rome, Italy

Peter Martijn ToetTNO, Delft, The Netherlands

Daniele TosiNazarbayev University, Astana, Kazakhstan

xii LIST OF CONTRIBUTORS

BIOGRAPHY

Hamid Alemohammad, PhD, PEng, is the cofounder andCEO of AOMS Technologies Inc. Dr. Alemohammad has PhD inMechanical Engineering from the University of Waterloo inOntario, Canada. He is specialized in industrial and academicresearch on fiber optic sensors along with the commercializationof fiber optic sensor technologies for harsh environment andindustrial sensing applications.

PREFACE

The 1960s was a turning point for the fiber optic industryfollowing the breakthrough discovery by Charles Kao (2009Nobel Prize Laureate in Physics) and George Hockham fromStandard Telecommunication Laboratories in England onreducing the loss in glass fibers by removing impurities. In thefollowing decade, the work conducted by scientists at Corningand Bell Laboratories resulted in the development of a scalablemass production process for the manufacturing of low-lossoptical fibers, which are now widely deployed to transmitvoice and data over long distances.

Optical fibers, which are a commodity for the telecommuni-cations industry, have found their way into the sensing industry.In the early days, fiber optic sensor technology was adopted onlyby the oil and gas and defense sectors, because of the relativelyhigh cost of the technology. However, thanks to new advance-ments in the development of low-cost optoelectronic systems, thetechnology is finding niche markets in other industry sectorsincluding biomedical, environmental, transportation, structuralhealth monitoring, and process industries. The industrial adop-tion of fiber optic sensors stems from unique features and tech-nical capabilities unmatched by electronic sensors; these featuresinclude low-loss remote sensing, the ability to work in harshenvironments, immunity to electromagnetic interference, smallsize, and capability of integrated and distributed sensing. Thenumbers of patents and scholarly articles published in the area offiber optic sensing, new companies commercializing state-of-artfiber optic sensor technologies, and research and development(R&D) investments by renowned research centers indicate theglobal growth of this technology. According to the marketresearch report Fiber Optic Sensors: Global Markets published byBCC Research in 2017, the global market size for fiber optic sen-sors is projected to reach $3.2 billion by 2021 from $2.0 billion in2016 with a 5-year compound annual growth rate of around 10%.The world-class research on specialty optical fiber sensors (i.e.,polymer fibers, photonic crystal fibers, femtosecond written fiber

Bragg gratings, etc.) and the development of low-cost andaffordable optical fiber sensor interrogators are the primarydrivers for the adoption of the technology and emergence of newuse cases for fiber optic sensing.

This book relays state-of-the-art research results andprospective advances in the field of fiber optic sensing withemphasis on opto-mechanical sensing applications. It is aconsolidated collection of contributions by researchers inacademia, research centers, and industrial R&D departments.The book aims at agglomerating recent research into onesingle source that is accessible to a wide range of audience.It provides a reference source for R&D engineers, scientists,application engineers, and technical managers in industriesrelevant to test and measurement and for university facultymembers, postdoctoral fellows, and graduate students prac-ticing research in various engineering and applied sciencedisciplines.

Hamid AlemohammadAOMS Technologies, Inc.

Toronto, Canada

xvi PREFACE

5DISTRIBUTED BRILLOUINSENSING USING POLYMEROPTICAL FIBERSYosuke Mizuno, Neisei Hayashi, Kentaro NakamuraTokyo Institute of Technology, Yokohama, Japan

5.1 IntroductionPolymer (or plastic) optical fibers (POFs) [1,2], which provideextremely easy and cost-effective connections compared to otherstandard glass fibers, are sufficiently flexible to withstand a largestrain of several tens of percent [3,4]. Therefore, despite theirhigher loss compared to that of silica glass fibers, POFs havebeen utilized in medium-range communication applicationssuch as home networks and automobiles [5] as well as in high-strain monitoring applications [3,6]. On the other hand, Brillouinscattering in optical fibers [7,8], which is one of the most signifi-cant nonlinear effects, has been extensively studied. Its applica-tions include a variety of useful devices and systems, such asoptical amplifiers [7], lasers [7,9], optical comb generators [9],microwave signal processors [10], slow light generators [11], phaseconjugators [12], fiber-core aligners [13], tunable delay lines [14],optical memories [15], and distributed strain and temperaturesensors [16e20]. As of this writing, Brillouin scattering has beenstudied not only for silica fibers but also for some specialty glassfibers including tellurite fibers [21,22], As2Se3 chalcogenide fibers[23,24], bismuth oxide fibers [25,26], photonic crystal fibers[27,28], and multicore fibers [29]. However, until our first observa-tion [30], no experimental reports had been provided on Brillouinscattering in POFs, which adds a variety of advantages of POFsover the conventional application field of Brillouin scattering.

Here we focus on the sensing applications of Brillouin scat-tering. In addition to their high flexibility, one attractive featureof POF-based sensors is a unique function called a “memory”effect [31], with which the information on the applied large straincan be stored owing to their plastic deformation. Based on this

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http://dx.doi.org/10.1016/B978-0-12-803131-5.00005-2

effect, we have created a novel concept: “we need not always putexpensive analyzers at the ends of the sensing fibers; after earth-quakes, an officer has only to go round with a single analyzer.”With this concept, the application range of fiber optic sensingtechnology, which has been limited to only large-scale civil struc-tures owing to its high cost, can be extended to smaller-scalemultifamily residences and individual houses. A memory effectregarding temperature has also been reported [32,33].

This chapter reviews current knowledge of Brillouin scatteringin POFs and its application to distributed measurement. InSection 5.2, we first present the fundamental properties of Bril-louin scattering in POFs at 1.55 mm, such as the Brillouin fre-quency shift (BFS), Brillouin linewidth, Brillouin gain coefficient,and Brillouin threshold power [30]. For sensing applications, wealso describe the BFS dependence on strain and temperature inPOFs, including a BFS hopping phenomenon [34e36]. Further-more, some methods for enhancing the Brillouin signal aredetailed [37,38]. Then in Section 5.3, we present the first demon-stration of truly distributed strain and temperature sensing with ahigh spatial resolution in POFs using a correlation-domain tech-nique. The performance limitation of POF-based sensingsystems is fully discussed [39]. Section 5.4 deals with a so-calledPOF fuse phenomenon, the fundamental properties of whichneed to be well investigated to perform distributed Brillouinmeasurement with a signal-to-noise ratio (SNR) that is as highas possible [40e42]. Finally, Section 5.5 summarizes this chapter,and an outlook on future work is given.

5.2 Characterization of Brillouin Scatteringin Polymer Optical Fibers

Here we present the unique characteristics of Brillouin scatteringin POFs. In addition to the fundamental properties, the BFSdependence on strain (from small strain of <1.0% to large strainof 60%) and temperature is clarified. Induction of stimulatedscattering and employment of POFs with smaller cores are alsodescribed as promising methods for enhancing the Brillouinsignal in POFs.

5.2.1 Fundamental PropertiesIn this section, we describe the first observation of Brillouin scat-tering in POFs and present its fundamental properties, such asBFS, Brillouin linewidth, Brillouin gain coefficient, and Brillouinthreshold power [30].

98 Chapter 5 DISTRIBUTED BRILLOUIN SENSING USING POLYMER OPTICAL FIBERS

5.2.1.1 Brillouin Scattering in Optical FibersWhen a light beam is injected into an optical fiber, it interactswith acoustic phonons and generates backscattered light calledStokes light. This phenomenon is called spontaneous Brillouinscattering. Because the phonons decay exponentially, the back-scattered Brillouin light spectrum, also known as the Brillouingain spectrum (BGS), takes the shape of a Lorentzian functionwith a bandwidth of several tens of megahertz. The frequency atwhich the peak power is obtained in the BGS is downshifted byseveral gigahertz from the incident light frequency, and theamount of this frequency shift is known as the BFS. In opticalfibers, the BFS nB is given as [7]

nB ¼ 2nvAlp

¼ 2nlp

ffiffiffiffiEr

s; (5.1)

where n is the refractive index, vA the acoustic velocity in the fiber, lpthewavelength of the incident pump light, E Young’smodulus, and r

the density. If tensile strain is applied or the temperature is changedin a standard silica single-mode optical fiber (SMF), the BFS movesto a higher frequency in proportion to the applied strain(þ580 MHz/%) [43] and the temperature change (þ1.18 MHz/K)[44]. In some specialty fibers, such as tellurite glass fibers, it is knownthat the BFS moves to a lower frequency with increasing appliedstrain (�230 MHz/%) [22] and temperature (�1.14 MHz/K) [26].In both cases, we can derive the strain amplitude and temperaturechange by measuring the BFS in the fiber.

5.2.1.2 Experimental SetupA standard POF composed of polymethyl methacrylate (PMMA)[2] is optimally designed for visible light transmission at 650 nm,with a propagation loss ofw150 dB/km. However, its loss at tele-communication wavelength is so high (>>1 � 105 dB/km) thatthe Brillouin signal cannot be detected. In the meantime, toobserve Brillouin scattering in a PMMA-based POF at 650 nm,we need to prepare all the necessary optical devices at this wave-length, which is not easy. Therefore, we use a perfluorinatedgraded-index (PFGI) POF [1,45] instead of a PMMA-based POF.It consists of a core (120 mm diameter), cladding, and overclad-ding (750 mm diameter) encased in polyvinyl chloride. Thecore and cladding layers are composed of doped and undopedpoly(perfluoro-butenylvinyl ether), respectively. The refractiveindex at the center of the core is 1.356, whereas that of thecladding layer is 1.342; these values do not depend strongly on

Chapter 5 DISTRIBUTED BRILLOUIN SENSING USING POLYMER OPTICAL FIBERS 99

the optical wavelength [45]. The numerical aperture (NA) is0.185. The polycarbonate reinforcement overcladding reducesmicrobending losses and increases the load-bearing capability.The propagation loss is relatively low (w250 dB/km; comparableto that of a PMMA POF at 650 nm) even at 1.55 mm, and inexpen-sive optical amplifiers can be used to boost the optical power.

Fig. 5.1A depicts the experimental setup for investigating theBrillouin scattering properties in the POF. For BGS measurementwith a high resolution, we employed so-called self-heterodynedetection [20]. All the optical paths except the POF itself werecomposed of silica SMFs. A distributed-feedback laser diode(DFB-LD) at 1.552 mm was employed as a light source, and itsoutput was divided into two light beams with an optical coupler.

Figure 5.1 (A) Experimental setup for investigating the Brillouin scattering properties in perfluorinated graded-index(PFGI) polymer optical fibers (POFs). BFS, Brillouin frequency shift; DAQ, data acquisition; DC, direct current; DFB-LD,distributed feedback laser diode; EDFA, erbium-doped fiber amplifier; ESA, electrical spectrum analyzer; PC, polariza-tion controller; PD, photodiode. (B) Brillouin gain spectrum in the 100-m-long POF at 20-dBm pump power. The insetshows its magnified view around the peak. BGS, Brillouin gain spectrum. (C) Relative power of the Stokes light back-scattered from the 100-m-long POF plotted as a function of pump power.


One of the beams was, after passing a polarization controller(PC), directly used as the reference light of the heterodyne detec-tion. The other beam was amplified with an erbium-doped fiberamplifier (EDFA) and injected into the POF as the pump light.Then, the optical beat signal between the backscattered Stokeslight and the reference light was converted to an electrical signalwith a photodiode (PD). Finally, the signal was amplified by 23 dBwith an electrical preamplifier and monitored with an electricalspectrum analyzer (ESA).

We optically coupled the silica SMF and the POF using a buttcoupling technique [46]. Because the core diameters are largelydifferent (8 mm for SMF versus 120 mm for POF), a large opticalloss is expected when light travels from the POF into the SMF.However, this loss contributes only to the attenuation of theStokes light once generated in the POF, and it was measured tobe approximately 12 dB. On the other hand, when light travelsfrom the SMF into the POF, the loss was less than 0.2 dB, whichis sufficiently low to investigate the Brillouin scattering propertiesin the POF.

5.2.1.3 Brillouin Gain SpectrumThe BGS was observed when the 100-m-long PFGI POF waspumped at 20 dBm (Fig. 5.1B). The peak corresponding to theBFS was observed at 2.83 GHz, which is about four times lowerthan that of standard silica fibers. This allows the use of a PDand an ESA that are less expensive with a lower bandwidth. Theacoustic velocity vA can be calculated using the BFS nB as in Eq.(5.1). With n of 1.35 and lp of 1.552 mm, vA in this POF was calcu-lated to be 1627 m/s, which is much lower than that of standardbulk PMMA, w2700 m/s [47]. By Lorentzian fitting, the 3-dB Bril-louin linewidth DnB was measured to be 105 MHz, which is threeto five times broader than that of silica fibers [48], resulting indeterioration of the sensitivity of time-domain sensors [17].

Fig. 5.1C shows the dependence of the relative Stokes poweron pump power. The Stokes power is generally known to growexponentially at the Brillouin threshold power Pth and thenreaches saturation, which indicates the transition from sponta-neous to stimulated Brillouin scattering (SBS). Although a roughestimation of Pth is often performed using this kind of figure[7,21,23,48,49], saturation of the Stokes power is not observedin Fig. 5.1C. Therefore, Pth of this POF appears to be much higherthan 30 dBm (¼1 W). The detailed estimation of Pth is providedlater in this section.


We subsequently evaluated the Brillouin gain coefficient gB.Using the acoustic velocity vA and the Brillouin linewidth DnB, gBis given by [49]

gB ¼ 2pn7p212

cl2prvADnB; (5.2)

where p12 is the longitudinal elastooptic coefficient and c the lightvelocity. Because the accurate values of p12 and r are not knownfor perfluorinated PMMA, we used the values of standardPMMA [50] in this calculation. Using the measured values ofvA ¼ 1627 m/s and DnB ¼ 105 MHz, along with n ¼ 1.35,p12 ¼ 0.297, lp ¼ 1.552 mm, and r ¼ 1187.5 kg/m3, gB was calcu-lated to be 3.09 � 10�11 m/W, which is close to that of silica fibers(3e5 � 10�11 m/W) [7]. Owing to the multimode nature of thePOF, the actual gB value may be larger than this value.

Finally, we estimated the Brillouin threshold power Pth. Analternative way to calculate gB is to use the following equation [51]:

gB ¼ 21bAeff

KPthLeff; (5.3)

where Aeff is the effective cross-sectional area, and Leff is the effec-tive length defined as

Leff ¼ ½1� expð�aLÞ�=a. (5.4)

Here, a is the propagation loss and L is the fiber length. Formultimode fibers, a correction factor b is needed [52], whichcan be treated as 2 when the NA is approximately 0.2. K is a con-stant that depends on the polarization properties of the fiber[49,53] and is 1 if the polarization is maintained and 0.667 other-wise. Then, using the values of gB ¼ 3.09 � 10�11 m/W, b ¼ 2 [52],Aeff ¼ 209 mm2 [54], K ¼ 0.667, a ¼ 0.056/m, and L ¼ 100 m, Pthcan be calculated to be 24 W. This value is valid compared tothe theoretical values of w10 W (L ¼ 300 m) [54] or w100 W(L ¼ 100 m) [55]. Because Pth is in proportion to b ‧Aeff inEq. (5.3), Pth can be reduced to a moderate power level by usingPOFs with smaller core diameters (refer to Section 5.2.4).

5.2.2 Strain and Temperature DependenceIn this section, we present the BFS dependence on relativelysmall strain (<1.0%) and low temperature (<80�C) in the POF,and clarify that Brillouin scattering in POFs can be utilized to


develop highly sensitive temperature sensors with reduced strainsensitivity [36]. We also show that this BFS dependence is prob-ably caused by the dependence of Young’s modulus on strainand temperature in the POF.

5.2.2.1 ExperimentsWe used a 5-m-long PFGI POF with the same physical propertiesas those used in the previous experiments. The experimentalsetup for investigating the BFS dependence on strain and temper-ature in the POF is basically the same as that described in Section5.2.1.2. The Brillouin signal generated in the 1-m-long SMFbetween the circulator and the POF is included in the Stokes light,but it has no influence on the BGS measurement, because the BFSin the SMF is typically 11 GHz, about four times higher than that inthe POF. The whole length of the POF was fixed using epoxy glueonto a translation stage, to which different strains were applied.Temperature was adjusted with a heater along the whole lengthof the POF.

Fig. 5.2A shows the strain dependence of the BGS in the POF.The pump power was 19 dBm, and strains of 0.2%, 0.4%, 0.6%,0.8%, and 1.0% were applied. As the applied strain increased, theBGS shifted toward lower frequency. Fig. 5.2B shows the straindependence of the BFS. The slope was almost linear, and its coef-ficient was �121.8 MHz/%. While the negative sign is the same asfor tellurite glass fibers [22], the absolute value was approximatelyone-fifth of that of a standard silica SMF (þ580 MHz/%) [43]. Next,Fig. 5.2C shows the temperature dependence of the BGS in thePOF. The pump power was 23 dBm, and the temperature wascontrolled from 30�C up to 80�C with a step of 10�C. As tempera-ture increased, the BGS also shifted toward lower frequency. TheStokes power at high temperature over 40�C was lower than thatat 30�C by about 0.7 dB, probably because of the nonuniformtemperature distribution of the heater. Fig. 5.2D shows the temper-ature dependence of the BFS, and its coefficient was�4.09 MHz/K.Although the negative sign is also the same as for tellurite glassfibers [26], the absolute value was about 3.5 times as large as thatof an SMF (þ1.18 MHz/K) [44]. The larger temperature coefficientleads to sensitivity enhancement of the temperaturemeasurement,whereas the smaller strain coefficient means that PFGI POF-basedBrillouin sensors are less susceptible to the applied strain. There-fore, the Brillouin scattering in the PFGI POF can be potentiallyutilized to implement highly sensitive temperature sensors withlow strain sensitivity.


Figure 5.2 (A) Brillouin gain spectrum (BGS) dependence on strain in the polymer optical fiber (POF). (B) Brillouinfrequency shift (BFS) plotted as a function of strain. (C) BGS dependence on temperature in the POF. (D) BFSversus temperature. (E) Youngs modulus of polymethyl methacrylate (PMMA) bulk versus temperature. (F)Density of PMMA bulk versus temperature. Plotted using the data reported in the literature: Saneyoshi J, Kikuchi Y,Nomoto O. Handbook of ultrasonic technology. Nikkan Kogyo; 1978 [chapter 5].


5.2.2.2 DiscussionThe origins of the BFS dependence on strain and temperature inthe PFGI POF are discussed. The strain coefficient of the normal-ized BFS is given, by differentiating Eq. (5.1) with respect tostrain, as

1nB

vnB

vε¼ 1

nvnvε

þ 12E

vEvε

þ�� 12r

vr

vε

�: (5.5)

Here the following two equations hold true [43]:

1nvnvε

¼ �n2p12 � kðp11 þ p12Þ2

; (5.6)

�1r

vr

vε¼ 1� 2k

2; (5.7)

where p11 and p12 are the elastooptic coefficients, and k is thePoisson ratio. As their values in PFGI POFs are unknown, weused the values for bulk PMMA: p11 ¼ 0.3 [56], p12 ¼ 0.297 [56],and k ¼ 0.34 [57]. Then the first and the third terms in Eq. (5.5)were calculated to be �0.0857 and þ0.16, respectively. Althoughthe second term is reported to drastically vary depending bothon the method for applying strain and on the fabrication qualityof the fiber, we used �5.75 as the second term, which is the valuereported for a standard PMMA-based POF [58]. Compared to itsabsolute value, the first and the third terms are negligibly small.Then the theoretical strain coefficient was calculated tobe �160.6 MHz/%. Considering that each term in Eq. (5.5) wasestimated using the values of PMMA, this value is in moderateagreement with the experimental value of �121.8 MHz/%. Thus,the strain dependence of the BFS appears to originate from thedependence of Young’s modulus on strain in the PFGI POF.

Next, we discuss the BFS dependence on temperature in thesame manner. The temperature coefficient of the normalizedBFS is expressed as

1nB

vnB

vT¼ 1

nvnvT

þ 12E

vEvT

þ�� 12r

vr

vT

�. (5.8)

The first term can be assumed to be �0.0000889, which is the valuereported for a standard PMMA-based POF [50]. To estimate the sec-ond and the third terms, we plotted Young’s modulus and the den-sity of bulk PMMA at various temperatures using the data in theliterature [59] (Fig. 5.2E and F). Using their slopes (�0.0295 GPa/Kand �0.409 kg/m3/K), along with Ew 6 GPa and r¼ 1187.5 kg/m3,


the second and the third terms were calculated to be �0.00246and þ0.000172, respectively. Then the theoretical temperature coef-ficient was calculated to be �6.72 MHz/K, which is in rough agree-ment with the experimental value of �4.09 MHz/K. Thus, thetemperature dependence of the BFS also seems to originate fromthe large negative dependence of Young’s modulus on temperaturein the PFGI POF. Note that the BFS dependence on temperature in aPOF with a 50-mm core diameter has also been investigated in awider temperature range [60].

5.2.3 Induction of Stimulated Brillouin ScatteringIn this section, we describe the observation of SBS in a POF withpumpeprobe technique [38]. The BGS is detected with anextremely high SNR, even with a 1-m-long POF, scrambled polar-ization state, and no averaging. We also investigate the BGSdependence on probe power and temperature, and confirm thatSBS in a POF measured with this technique can be utilized todevelop high-accuracy temperature sensors as well.

5.2.3.1 Motivation and PrincipleThe Brillouin scattering in POFs observed in the previous experi-mental setup (Fig. 5.1A) was not stimulated but spontaneous,because the Brillouin threshold of POFs was estimated to be ashigh as tens of watts (refer to Section 5.2.1.3). Consequently, thepower of the reflected Stokes light was so low that we had toface the following four problems: (1) a POF longer than severalmeters was required, (2) the polarization state had to be opti-mized, (3) averaging of the spectral data had to be conductedseveral tens of times, and (4) the SNR of the BGS was extremelylow, even when (1), (2), and (3) were cleared. To implement prac-tical Brillouin sensors and other systems using POFs, these prob-lems need to be resolved.

One solution is to employ the so-called pumpeprobe tech-nique. As described in Section 5.2.1.3, when the pump power ishigher than the Brillouin threshold, a transition to SBS occurs,leading to drastic enhancement of the Stokes light. On the otherhand, when probe light at the same frequency as the Stokes lightis also injected into the other end of the fiber, SBS is inducedeven when the power of the pump light is much lower than theBrillouin threshold, because the probe light itself acts as a seedof stimulated scattering [61]. This technique, called the pumpeprobe technique, has been used to develop Brillouin systemswith a high SNR [62].


5.2.3.2 Experimental SetupThe fiber under test was a 1-m-long PFGI POF with a core diam-eter of 120 mm (see Section 5.2.1.1 for details). The experimentalsetup is schematically shown in Fig. 5.3A, and it is similar to thatof Brillouin optical correlation-domain analysis (BOCDA)[18,62e64]. The light beam from a 1.55-mm three-electrode LDwas divided into two. One was used as the pump light, after beingchopped with an intensity modulator for lock-in detection andbeing amplified with an EDFA. The other was used as the probelight, after passing two EDFAs, a single-sideband modulator(SSBM), and a polarization scrambler (PSCR). The SSBM wasemployed with a microwave generator and a proper DC biascontrol to suppress the carrier (pump) and the anti-Stokes compo-nent of the two first-order sidebands and to maintain a stablefrequency downshift from the pump light. This frequency down-shift was swept from 2.5 to 3.5 GHz with a period of 300 ms toobtain the BGS of the POF, which is observed approximately at2.8 GHz. The suppression ratio of the other frequency componentswas kept higher than 25 dB (Fig. 5.3B). The PSCR, which canmodu-late the polarization state at 1 MHz, was inserted to suppress thepolarization-dependent fluctuations of the signal. The POF andthe silica SMFs were butt-coupled with the gaps filled with index

Figure 5.3 (A) Experimental setup for observing stimulated Brillouin scattering in the polymer optical fiber (POF)with a pumpeprobe technique. BFS, Brillouin frequency shift; DAQ, data acquisition; DC, direct current; EDFA,erbium-doped fiber amplifier; FG, function generator; FUT, fiber under test; IM, intensity modulator; LD, laserdiode; LI-A, lock-in amplifier; MG, microwave generator; OSC, oscilloscope; PD, photodiode; PSCR, polarizationscrambler; SSBM, single-sideband modulator; VOA, variable optical attenuator. (B) Optical spectrum of the SSBMoutput measured when the frequency of the MG was set to 2.83 GHz.


matching oil (n ¼ 1.46) to minimize the Fresnel reflection. TheStokes light was adjusted in power with a variable optical attenu-ator, and converted to an electrical signal with a 125-MHz PD. Afterpassing a lock-in amplifier (LI-A) with a chopping frequency of5.018 MHz and a time constant of 10 ms, the electrical signal wasobserved as a BGS with an oscilloscope synchronized with thefrequency sweep of the SSBM.

5.2.3.3 Experimental ResultsFig. 5.4A shows the BGS measured without averaging when thepump power and the probe power were 23 and 22 dBm, respec-tively. The power was normalized so that the peak power was1.0. Although the POF length was only 1 m and the polarizationstate was scrambled, the BGS was observed with a much higherSNR than that in Section 5.2.2.1. The BFS was 2.86 GHz, which isslightly higher than the value of 2.83 GHz obtained in Section5.2.1.3. This discrepancy seems to originate from the difference

Figure 5.4 (A) Brillouin gainspectrum (BGS) in thepolymer optical fiberobserved without averaging.(B) BGS dependence onprobe power. (C) BGSdependence on temperature.(D) Brillouin frequency shiftdependence on temperature.


in temperature and the time constant of the LI-A, which is notsufficiently short. The 3-dB bandwidth of the BGS measured inthis experiment was about 160 MHz, but further research isneeded on the bandwidth because it is also dependent on thetime constant of the LI-A (when the time constant was shorterthan 10 ms, the BGS was distorted).

Fig. 5.4B shows the dependence of the BGS on probe powerwhen the pump power was fixed at 23 dBm. The probe powerwas reduced from 22 down to 6 dBm, and averaging was con-ducted 30 times for the observable readout when the Stokespower was very low. As the probe power decreased, the Stokespower also decreased, which proves that this BGS is caused bythe interaction between the pump light and the probe light,i.e., SBS.

The dependence of the BGS on temperature was alsomeasured (Fig. 5.4C). The pump power and the probe powerwere 23 and 22 dBm, respectively, and averaging was conducted30 times. The temperature was set to 20�C, 40�C, and 60�C.With increasing temperature, the BGS shifted toward lowerfrequency. Fig. 5.4D shows the temperature dependence of theBFS. The slope of �4.05 MHz/K is in good agreement withthe value obtained in Section 5.2.2.1, which confirms that theBGS in a POF observed with the pumpeprobe technique can beapplied to high-sensitivity temperature sensing. Note thatthe SBS in a POF has also been detected with a lock-in-freepumpeprobe technique [65].

5.2.4 Influence of Core Diameter and Fiber LengthAs described in Section 5.2.1.3, the power of the spontaneousBrillouin Stokes light generated in the PFGI POFs with a 120-mmcore diameter was low, and it needs to be enhanced for detailedinvestigations of the BGS including the linewidth narrowing effect.Because the BGS observed with the pumpeprobe techniquedescribed in Section 5.2.3 is easily influenced by the time constantof lock-in detection, detailed evaluation of its linewidth was notfeasible. Another approach to enhance the Stokes signal is tomake use of POFs with core diameters smaller than 120 mm. Inthis section, by characterizing the BGS in POFs with a 62.5-mmcore diameter, we investigate the influence of the core diameterand the fiber length on the Brillouin properties, and observe theBrillouin linewidth narrowing effect [37].


5.2.4.1 Effects of Small Core DiametersFig. 5.5A shows the measured BGS of a 5-m-long PFGI POF with62.5-mm core at pump power Pin of 5, 10, 15, and 20 dBm. Thepolarization state optimized for Pin of 20 dBm was employed forall the measurements. The center frequency of the BGS, i.e., BFS,was approximately 2.81 GHz, which is slightly lower than the valueof 2.83 GHz obtained in Section 5.2.1.3 because of the difference inroom temperature. Even when Pin was as low as 5 dBm, small butclear BGS was observed. Fig. 5.5B shows the Pin dependence of therelative Stokes power, when 5-m-long POFs with core diameters of62.5 and 120 mm were employed. The reference power was set toabout �63 dBm, which is the Stokes power when Pin is sufficientlylow. The dependence curve of the POF with the 62.5-mm core wasabout 10 dB lower in pump power than that with the 120-mm core,which indicates that, even at the same pump power, we can largelyenhance the Stokes signal by using a POF with a smaller corediameter.

One of the reasons for the 10-dB curve shift is the difference inBrillouin threshold power Pth. By substituting into Eqs. (5.3) and(5.4) the values of b ¼ 2 [52], K ¼ 0.667 [7], gB ¼ 3.09 � 10�11 m/W [30], a ¼ 0.056/m (¼250 dB/km), and L ¼ 5 m, Pth of the POFwith the 120-mm core (Aeff ¼ 209 mm2 [54]) was calculated to be97.7 W. On the other hand, Pth of the PFGI POF with the 62.5-mmcore (Aeff ¼ 108.9 mm2) was calculated to be 53.3 W, which is lowerthan 97.7 W by 2.6 dB. Thus, the curve shift observed in Fig. 5.5Bcan be partially explained by the difference in Pth, but its amountof 10 dB is much larger than the calculated value.

Figure 5.5 (A) Brillouin gain spectrum dependence on pump power Pin. PFGI-POF, perfluorinated graded-indexpolymer optical fiber. (B) Relative Stokes power plotted as a function of pump power; comparison between POFswith 62.5- and 120-mm core diameters.


Another reason for the curve shift is the improvement of opti-cal coupling efficiency at the butt coupling when the Stokes lightgenerated in the POF propagates back and is injected into theSMF. Although, owing to the unstable core alignment and therough surface of the POF, it is difficult to measure the couplingloss accurately, we confirmed that the loss with the POF withthe 62.5-mm core was several decibels lower than that with the120-mm core. This fact, along with the difference in internal struc-ture designed by different manufacturers, moderately explainsthe 10-dB curve shift.

5.2.4.2 Effects of Long Fiber LengthAccording to Eqs. (5.3) and (5.4), to employ long POFs is anotherway to reduce Pth and to enhance the Stokes signal. Fig. 5.6Ashows the measured BGS of 80- and 200-m-long POFs with a62.5-mm core at Pin of 5, 10, 15, and 20 dBm. Much larger Stokessignals (w7 dB higher at Pin of 20 dBm) compared to those ofthe 5-m-long POF (Fig. 5.5A) were observed.

There was, however, almost no difference between the BGSof the 80-m-long POF and that of the 200-m-long POF, only aslight discrepancy of the BFS caused by the room-temperaturedifference. This means that the incident light is considerablyattenuated after propagation for 80 m in the POF. To estimatethis effect quantitatively, the effective POF length Leff was plottedas a function of actual length L (Fig. 5.6B), where Leff graduallyapproaches 18 m (Pth w 13 W) with increasing L. Thus, weproved that employing a POF longer than w50 m is not an effec-tive way to enhance the Stokes signal at 1.55 mm.

According to Eqs. (5.3) and (5.4), as the core diameterdecreases, the Brillouin threshold Pth also becomes lower. WhenPin is higher than Pth, SBS is induced and consequently the Stokessignal is exponentially enhanced [7]. Here, under the roughassumption that the multimode nature, NA, refractive index,and loss do not change with core diameters, we calculated Pthof a 50-m-long POF with a 10-mm core diameter (hypothetical;Aeff ¼ 17.4 mm2) to be 2.22 W (¼33.5 dBm). This value is morethan 1 order of magnitude higher than the pump power of severaltens to hundreds of milliwatts commonly used in BGS character-ization in silica SMFs [48]. Even when the POF is treated as an SMF[i.e., b ¼ 1 [7] but Aeff becomes larger [66] in Eq. (5.3)], thisdifference cannot be compensated for. Thus, it seems to be diffi-cult to reduce Pth of POFs down to the same level as that of longsilica SMFs by decreasing the core diameter, which is due to thelimited effective length of 18 m associated with the high propaga-tion loss at 1.55 mm.


5.2.4.3 Brillouin Linewidth Narrowing EffectAlthough the Brillouin linewidth of a POF is reported to be105 MHz at Pin of 20 dBm (refer to Section 5.2.1.3), detailed inves-tigations were difficult because the Stokes power was extremelylow. Here, by making use of the enhanced Stokes power, we inves-tigated the Brillouin linewidth dependence on Pin.

Fig. 5.6C shows the measured BGS of the 200-m-long PFGI POFwith a 62.5-mm core at Pin of 14.5, 17.5, and 23.5 dBm, where theStokes power is normalized so that the maximum power is 0 dB.Fig. 5.6D shows the Brillouin linewidth dependence on Pin. Fromthese figures, we can see that, with the increasing Pin, the 3-dBlinewidth of the BGS decreases, but that its slope gradually

Figure 5.6 (A) Brillouin gain spectrum (BGS) dependence on pump power Pin; comparison between the 80-m-longperfluorinated graded-index polymer optical fiber (PFGI-POF) (solid line) and the 200-m-long POF (dotted line). (B)Calculated effective fiber length versus fiber length. (C) Normalized BGS at pump powers of 14.5, 17.5, and23.5 dBm. (D) Brillouin linewidth versus pump power.


becomes small. This behavior agrees well with the experiment andtheory of the linewidth narrowing effect in silica-based SMFs [48].

5.2.5 Brillouin Frequency Shift Dependenceon Large Strain

In Section 5.2.2, the BGS (and BFS) dependence on relativelysmall strain (<1.0%) in a POF was presented. In this section, weinvestigate the BGS dependence on larger strain of up to 60% ina POF, and observe a nonmonotonic behavior [35] caused by aBFS hopping phenomenon [34].

5.2.5.1 Experimental SetupWe employed a 0.6-m-long PFGI POF with a 50-mm core diam-eter. Instead of the standard experimental setup based on self-heterodyne detection described in Section 5.2.1.2, we used anewly developed Fresnel-assisted setup [67], which can detectthe BGS in POFs with a higher SNR. Large strain was applied tothe POF with two computer-controlled motorized stages atroom temperature of 20�C.

5.2.5.2 Experimental ResultsFirst, we measured the BGS dependence on large strain of up to60% in the POF (Fig. 5.7A). The pump power was 26 dBm, andthe strain rate was 200 mm/s. The Brillouin peak observed atw2.8 GHz in the absence of strain shifted to lower frequencyat <2.3% strain, and then shifted to a higher frequency; its peakpower gradually reduced with increasing strain (>10%), whereasan additional peak appeared at w3.2 GHz when the strain was>7.3%. At 31% strain, the power of the two peaks was almost thesame, and at 60% strain, the initial peak almost disappeared(note that the peak at w2.85 GHz originated from the w3-cm-long unstrained POF section near the connector). The BFS of thetwo peaks were then plotted as a function of strain (Fig. 5.7B).The dependence of the initial peak showed a nonmonotonicbehavior (>20%, the BFS cannot be accurately measured). TheBFS of the newly appeared peak was almost constant, independentof the applied strain, in this range.

Next, Fig. 5.7C shows side views of the POF in the presence ofw7.3% strain. Several sections were slimmed down in a stepwisemanner, and with increasing strain, the slimmed sections grewlonger (i.e., spread along the POF), while their outer diameterwas maintained. This explains the independence of the BFS from


large strain (>7.3%). The core diameter of the slimmed-down POFwas estimated to be 0.84 times that of the unstrained POF from across-sectional view (Fig. 5.7D). This phenomenon is probablycaused by the yielding of the overcladding layer made up of poly-carbonate, and not by the core or cladding layers. The upper yieldpoint of w8.0% [68] agrees with the strain at which the POF wasslimmed down (according to the specification sheet, the upperyield point of the core and cladding materials is approximately20%). This abrupt change in the core diameter seems to haveinduced the change in the acoustic velocity, therefore resultingin the BFS hopping. Further, the unstable stressestrain curve ofthe POF in the range from w10% to 60% (see Fig. 5.7E) can alsobe explained by this phenomenon.

Finally, after the whole length of the POF was slimmed down at60% strain, its BGS and BFS dependence on strain and temperature

Figure 5.7 (A) Brillouin gain spectrum dependence on large strain. (B) Brillouin frequency shift (BFS) dependenceon large strain. The BFS of the initial peak was not accurately measured at strains of >20% (gray). (C) Sideviews of the slim-down process of the polymer optical fiber (POF); taken every 4 s. (D) Cross-sectional view ofthe slimmed-down POF. (E) Measured stressestrain curve of the POF.


was investigatedafter the strainwas released (Fig. 5.8AeD). The BFSdependence coefficients were �65.6 MHz/% and �4.04 MHz/K(Fig. 5.8B and D), which are 0.5 times [36] and 1.3 times [60] thevalues of an unstrained POF. This result indicates that even morehighly sensitive temperature sensing with lower strain sensitivityis feasible by exploiting the Brillouin signals in the slimmed-down POFs.

5.3 Distributed MeasurementIn this section, we present the first demonstration of truly distrib-uted strain/temperature sensing with a high spatial resolution inPOFs based on Brillouin optical correlation-domain reflectometry

Figure 5.8 (A) Brillouin gain spectrum (BGS) dependence on strain. (B) Brillouin frequency shift (BFS)dependence on strain. (C) BGS dependence on temperature. (D) BFS dependence on temperature.


(BOCDR) [39]. The performance limitation of this system is alsodiscussed.

5.3.1 MotivationLow-resolution distributed temperature sensing in a POF hasbeen demonstrated [69] based on Brillouin optical frequency-domain analysis (BOFDA) [16]. They detected a 4-m-long heatedsection located at one end of a 20-m-long POF, but the spatialresolution and SNR were not sufficiently high for practical use;the relatively high cost of the devices, such as a vector networkanalyzer and a microwave generator, is also a problem. Althougha 3-cm spatial resolution has been obtained by BOFDA [70] in asilica SMF, such a high resolution has not been achieved in aPOF, not only because of the high propagation loss, but alsobecause of the weak Brillouin signal resulting from its largecore diameter and multimode nature. Employing Brillouin opti-cal time-domain analysis (BOTDA) to acquire the BGS distribu-tion has also been experimentally shown to be extremelydifficult [71].

In this section, we report on the first demonstration of distrib-uted strain and temperature sensing with a centimeter-orderspatial resolution in a POF based on BOCDR [20], which is highlycost-effective. A 10-cm-long heated section located away fromboth ends of a 1.3-m-long POF is successfully detected with atheoretical spatial resolution of 7.4 cm and a sampling rate of3.3 Hz per measured point (corresponding to a measurementtime of w1 min, if the number of measured points is 200). Wealso discuss how the characteristics of POFs (BFS, Brillouin band-width, propagation loss, etc.) affect the sensing performance ofBOCDR.

5.3.2 PrincipleSeveral distributed measurement techniques based on Brillouinscattering in optical fibers have been proposed so far, which areclassified into two categories: “reflectometry” and “analysis.” Inreflectometry, based on spontaneous Brillouin scattering, a lightbeam is injected into only one end of the fiber, whereas in anal-ysis, based on SBS, two light beams are injected into both endsof the fiber. Analysis systems proposed so far include BOTDA[17,71e73], BOFDA [16,69,70], and BOCDA [18,63,74], in which arelatively large signal and thus a high SNR can be obtained.Two-end access is, however, less convenient, because the systemdoes not work completely when the fiber has even one breakage


point. Moreover, expensive devices are often required to prepareso-called probe light to induce SBS. In contrast, even though thesignal is weak, reflectometry such as Brillouin optical time-domain reflectometry [19,75,76] and BOCDR [20,77,78] can resolvethese problems. As the interface between a silica SMF and a POFis easily damaged by injecting short optical pulses with highpeak power [79], here we focus on BOCDR.

First proposed in 2008 [20], BOCDR has been used as a prom-ising distributed sensing technique with one-end accessibility, ahigh spatial resolution, a high sampling rate (i.e., fast measurementspeed), and cost efficiency. Its operating principle is based on thecorrelation control of continuous light waves [81]; namely, thepump light and the reference light in a standard self-heterodynescheme for analyzing Brillouin signals (refer to Section 5.2.1.2)are sinusoidally frequency modulated at fm, producing periodicalcorrelation peaks in the fiber to be measured. The measurementrange dm is determined by their interval, which is inversely propor-tional to fm as

dm ¼ c2nfm

; (5.9)

where c is the velocity of light in a vacuum and n is the refractiveindex of the fiber core. By sweeping fm, the correlation peak, i.e.,the sensing position, can be scanned along the fiber to acquirethe BGS or BFS distribution. According to theory [78], when fmis lower than the Brillouin bandwidth DnB, the spatial resolutionDz is given

Dz ¼ cDnB2pnfmDf

; (5.10)

where Df is the modulation amplitude of the optical frequency.Considering that fm higher than DnB does not contribute to theenhancement of Dz [78], and that Df is practically limited to halfof the BFS nB of the fiber because of the Rayleigh noise [20,78],the limitation of the spatial resolution Dzmin is given by

Dzmin ¼ cpnnB

. (5.11)

The number of effective sensing points NR, which can be regardedas an evaluation parameter of the system, is given by the ratio ofdm to Dz, as

NR ¼ dm

Dz¼ pDf

DnB. (5.12)


To obtain higher NR, Df needs to be raised but it should be lowerthan nB/2; NR is thus limited to

NRmax ¼ pnB

2DnB. (5.13)

5.3.3 Experimental SetupPFGI POFs with a 50-mm core diameter were employed as thefibers under test (see Section 5.2.1.1 for details). The schematicsetup of BOCDR for distributed measurement in a POF(Fig. 5.9A) is basically the same as the basic configuration [20].All the optical paths except the POF were silica SMFs. A DFB-LD at 1.55 mm with 1-MHz linewidth was used as a light source,and its output frequency was sinusoidally modulated by direct

Figure 5.9 (A) Schematic setup of Brillouin optical correlation-domain reflectometry. AC, alternating current; DC,direct current; EDFA, erbium-doped fiber amplifier; ESA, electrical spectrum analyzer; FG, function generator; PC,polarization controller; PD, photodiode; POF, polymer optical fiber. (B) Structure of the POF under test (Expt. 1).(C) Brillouin frequency shift (BFS) distribution obtained when the 50-cm-long section of the POF was strained. (D)BFS distribution obtained when the 50-cm-long section of the POF was heated.


modulation of the driving current. Its output was divided into twolight beams with a coupler. One was directly used as the referencelight of heterodyne detection, after passing through a 1-km-longdelay fiber, to adjust the correlation peak order, and an EDFA toenhance the beat signal. The other beam was amplified withanother EDFA, and injected into the POF as the pump light (inci-dent power: 27 dBm). The optical beat signal between the Stokeslight and the reference light was then converted to an electricalsignal with a PD, which was finally monitored with an ESA witha 300-kHz frequency resolution. Polarization state was optimizedwith a PC at the beginning of each distributed measurement sothat the Rayleigh noise was minimal [80].

5.3.4 Experimental ResultsFirst, we demonstrate distributed strain and temperature sensingwith a moderate spatial resolution but with a high SNR. The mod-ulation frequency fm was swept from 11.654 to 11.698 MHz, corre-sponding to the measurement range dm of 9.5 m according toEq. (5.9). The modulation amplitude Df was set to 0.9 GHz, result-ing in the theoretical spatial resolution Dz of 34 cm from Eq. (5.10)(the Brillouin bandwidth DnB is w100 MHz in a POF). Their ratioNR was 28. The 56th correlation peak was used. The overall sam-pling rate of single-location measurement was 3.3 Hz. Fig. 5.9Bshows the structure of a 2-m-long POF to be measured, in whichstrains of <1.2% [within the elastic region (refer to Section 5.2.5)]were applied to a 50-cm-long section fixed on a translation stage,or the same section was heated up to 65�C (sufficiently lower thanthe glass transition temperature [82]). One end of the POF wasbutt-coupled to a silica SMF (second port of the circulator) viaan SC connector, and the other end was cut at 8 degrees to sup-press the Fresnel reflection. The room temperature was 18�C.

The measured BFS distribution when strain was applied isshown in Fig. 5.9C. The measurement time was approximately1 min (200 points), which can be set shorter by reducing themeasured points. The 50-cm-long strain-applied section wassuccessfully detected. The BFS shifted to lower frequency withincreasing strain with a proportionality constant of �115.3 MHz/%, which was moderately consistent with that in Section 5.2.2.1(�121.8 MHz/%). The BFS changed even along the strain-freesections by about �10 MHz, which indicates that the strain mea-surement error was �0.09%. The measured BFS distributionwhen temperature was changed is also shown in Fig. 5.9D, wherethe 50-cm-long heated section was clearly detected. The measure-ment time was also about 1 min. The proportionality constant of


temperature dependence was �3.27 MHz/�C, which is in goodagreement with a previous result (�3.2 MHz/% [60]). The temper-ature measurement error was evaluated to be w�3.1�C.

Next, we demonstrate distributed temperature sensing with acentimeter-order spatial resolution. The modulation configura-tions of the light source were fm ¼ 53.321e53.451 MHz andDf ¼ 0.9 GHz, corresponding to dm of 2.1 m and Dz of 7.4 cm(NR ¼ 28). Fig. 5.10A shows the structure of a 1.3-m-long POFemployed, of which a 10-cm-long section was heated to 40�C.

Fig. 5.10B shows the measured distribution of normalized BGSalong the POF, and Fig. 5.10C shows the BGS examples atunheated and heated positions (relative positions of 67 and104 cm, respectively). Fig. 5.10D shows the BFS distribution corre-sponding to Fig. 5.10B. The measurement time was approximately40 s (130 points). The BFS clearly downshifted at the 10-cm-longheated section. The amount of the BFS shift was approximately26 MHz, which agrees well with the actual temperature (40�C).

Figure 5.10 (A) Structure of the polymer optical fiber (POF ) under test (Expt. 2). (B) Normalized Brillouin gainspectrum (BGS) distribution. (C) Examples of BGS [Z1 at 67 cm (room temperature); Z2 at 104 cm (heated)]. (D)Brillouin frequency shift distribution obtained with a centimeter-order resolution.


The gradual BFS changes at the relative positions of w90 andw115 cm were probably caused by the overlap of two broadBGSs (DnB w 100 MHz) from the sections with and without thetemperature changed.

5.3.5 DiscussionFinally, we compare the performances of POF-based BOCDRwith those of silica SMF-based BOCDR. First, according toEq. (5.11), the highest spatial resolution Dzmin theoreticallyachievable in POF-based BOCDR (nB w 2.8 GHz; n w 1.35) iscalculated to be 23 mm, which is approximately 1/4 of that inSMF-based BOCDR (nB w 10.8 GHz; n w 1.46). However, aweak Brillouin signal in a POF, leading to a low SNR, practicallylimits the spatial resolution, as shown in the aforementionedexperiment. Next, according to Eq. (5.13), the maximal numberof effective sensing points NRmax of POF-based BOCDR(DnB w 100 MHz) is calculated to be 44, which is w1/13 of thatof SMF-based BOCDR (DnB w 30 MHz). This problem can bemitigated by employing so-called temporal-gating [83] anddouble-modulation schemes [84]. Note that the measurementrange dm itself is limited not only by its trade-off relation to Dzbut also by the high propagation loss (250 dB/km at 1.55 mm)of the POF. Currently, the practical limitation of dm is severaltens of meters (depending on Dz, incident power, and manyother parameters); we believe it can be elongated to severalhundreds of meters by using shorter pump wavelengths at whichthe propagation loss is much lower (for instance, w10 dB/kmloss is reported at 0.98 mm [1]). As for the sampling rate ofsingle-location measurements, 3.3 Hz demonstrated in theexperiment is restricted by the speed of signal acquisition fromthe ESA via a general purpose interface bus, which might befurther enhanced by use of faster data acquisition methodsthat have been implemented in SMF-based BOCDR [77] andBOCDA [64]. As for cost efficiency, a simplified configuration ofBOCDR has been developed and demonstrated using POFs[85]. Highly accurate discriminative sensing of strain and tem-perature [86] using POFs is another important problem to betackled.

5.4 Polymer Optical Fiber FuseAs detailed in Section 5.2.1.3, the SNR of Brillouin measurementin POFs is not sufficiently high because of the weak Brillouin


signal that originates from their large core diameter and multi-mode nature. One method for mitigating this problem is toemploy higher-power incident light, but power that is too highwill induce damage or burning at the butt-coupled interfacesand a so-called optical fiber fuse phenomenon. In this section,we describe the fundamental properties of POF fuse [40e42].

5.4.1 Motivation and PrincipleFiber fuse is the continuous self-destruction of a fiber by propa-gating light [87e89]. High-power light propagating through thefiber results in local heating and the creation of an opticaldischarge that is then captured in the fiber core and travels backalong the fiber toward the light source, consuming the light energyand leaving a train of voids [90]. While fiber fuse propagation isstunningly beautiful [91], the fiber cannot be used after the pas-sage of the fuse. This effect is now regarded as one of the criticalfactors limiting the maximal optical power that can be delivered[92,93]. The fuse properties must be well characterized so that allpossible measures are taken to avoid the creation of a fiber fuse.

The fuse properties in various glass fibers, including standardsilica SMFs [87,88,90,91,94e96], microstructured fibers [97], fluo-ride fibers [98], chalcogenide fibers [98], erbium-doped fibers [99],photonic crystal fibers [100], and hole-assisted fibers [100], arewell documented. The fiber fuse is reported to be typicallyinduced at an input optical power of one to several watts (oneto several megawatts per square centimeter) and to have a prop-agation velocity of one to several meters per second. These prop-erties differ according to the type of glass fiber; the thresholdpower, for instance, is reported to be much higher in photoniccrystal and hole-assisted fibers than in silica SMFs [100], andnonlinear saturation of the fuse velocity has been observed inerbium-doped fibers [99]. However, no reports detailing similarproperties of POFs had been provided until our first observation,despite their pressing need.

In this section, we characterize the POF fuse and discuss itsunique properties. The propagation velocity of the bright spotis 1 to 2 orders of magnitude slower than that in standard silicaSMFs. The threshold power density is 1/180 of the reported valuefor silica SMFs. We find that, after the passage of the fuse, anoscillatory continuous curve is formed in the POF. We alsoshow that the POF fuse can be easily terminated by local elasticdeformation of the waveguide structure, and that, by spectralmeasurement, the bright spot is not a plasma but an opticaldischarge, the temperature of which is w3600 K.


5.4.2 Fundamental CharacterizationA PFGI POF with a 50-mm core diameter was used (see Section5.2.1.1 for other parameters). Fig. 5.11A depicts the experimentalsetup, in which 7-dBm (5-mW) output light from a 1.55-mmDFB-LD was amplified by an EDFA to up to 23 dBm (200 mW) andinjected into a 15-m-long POF. Two optical isolators were

Figure 5.11 (A) Schematic of the experimental setup. The silica single-mode fibers (SMFs) are indicated by bluelines (gray lines in print versions). EDFA, erbium-doped fiber amplifier; POF, polymer optical fiber. (B) Compositephotograph of the fiber fuse propagating along the POF; photographs were taken at 1-s intervals. The light wasinjected from the right-hand side, and the fuse propagated from the left-hand side. The fiber arrangement wasthat of the literature [90] to allow a direct comparison between the POF and the silica SMF. (C) Magnified view ofthe propagating fuse. The light was injected from the left-hand side. (D) Propagation velocity of the fiber fuse in aPOF measured at 1.55 mm as a function of the maximum power density in the core. Measured data are shown asblue circles (dark gray circles in print versions), and the red line (light gray line in print versions) is a linear fit.The slope of the line is 1590 mm/s/MW$cm2, and the threshold intensity is 6.6 kW/cm2. (E) Propagation velocity ofthe fiber fuse as a function of the power density. The measured data for the silica SMF at 1.48 mm are shown asgreen circles (gray circles in print versions), and the green line (gray line in print versions) is a linear fit (slope of11.7 mm/s/MW$cm2); the theoretical threshold power density [95] is 1.16 MW/cm2. The blue line (light gray line inprint versions) is a theoretical prediction [95] for the silica SMF at 1.55 mm (slope of 9.41 mm/s/MW cm2). The datain (D) is also reproduced for comparison. (E) Data extracted from the literature: Atkins RM, Simpkins PG, Yablon AD. Trackof a fiber fuse: a Rayleigh instability in optical waveguides. Optics Letters 2003;28:974e76.


inserted to protect the laser and EDFA from reflected or back-scattered light. The end of the silica SMF fitted with an FCconnector was connected to one end of the POF fitted with anSC connector via an FC/SC adaptor. We confirmed that the fiberfuse can be initiated in the same way as in glass fibers[87,88,90,91,94e100] by external stimuli such as heating,bending, or bringing the fiber output end into contact with anabsorbent material. For the demonstration discussed here, weused a POF end that was surface polished roughly with 0.5-mmalumina powder.

From observations of the propagation of the fuse along thePOF (Fig. 5.11B), the propagation velocity was calculated to beapproximately 24 mm/s, which is extremely slow in comparisonto Todoroki’s [91] result for a silica SMF. The optical power ofthe propagating light was calculated, using the measured powerof the injected light, the coupling loss at the SMF/POF interface,and the propagation loss in the POF, to be approximately 75 mW,corresponding to a maximal power density of 7.6 kW/cm2 (referto the next paragraph for the calculation method). A magnifiedview of the fuse propagation along a straight portion of the POFis shown in Fig. 5.11C (70.5 mW, 22.8 mm/s).

Here, we derive an equation for the maximal power density Iin the core when light with a certain power P is injected into thegraded-index (GI) POF. We consider a refractive index in thecore that takes a parabolic profile [1]. Under the assumptionthat all modes propagate with equal attenuation withoutcoupling, the optical power profile is given, in the same way asthe refractive index profile, by [101]

pðrÞ ¼ pð0Þ�1�

�rR

�g�; (5.14)

where r is the radial distance from the core center, R is the coreradius, and g is the refractive index profile coefficient. Conse-quently, the maximal power density I can be calculated as

I ¼ limr/0

Ppr2

R 2p0 dq

R r0

�1�

�rR

�g�rdr

R 2p0 dq

R R0

�1�

�rR

�g�rdr

¼ PpR2

g þ 2g

z2PpR2

;

(5.15)

where we assumed gz 2 in the GI POF [1]. Eq. (5.15) indicates thatthemaximal power density in the GI POFwith an incident power Pis equal to the average power density in a step-index POF of the


same core diameter with twice the incident power. For instance,for P ¼ 75 mW and R ¼ 25 mm, I is calculated to be 7.6 kW/cm2.

We found that the fuse propagation velocity in the POF,measured at 1.55 mm, had an almost linear dependence on themaximal power density with a slope of 1590 mm/s/MW cm2

(Fig. 5.11D). The power density at which the fuse ceased, i.e., thethreshold power density, was 6.6 kW/cm2 at a propagation velocityof 21.9 mm/s. Comparing these results with those of silica SMFs(Fig. 5.11E; results [94] at 1.48 mm and the theoretical line [95] at1.55 mm) revealed that at 1.55 mm the slope in the POF data (corre-sponding to the efficiency of the velocity control) was 170 times assteep as that in the silica SMF (9.41 mm/s/MW$cm2), and thethreshold power density of the POF was 180 times lower thanthat of the silica SMF (w1.2 MW/cm2). The minimal propagationvelocity achieved at 1.55 mm was 11 times as low as that experi-mentally obtained in a silica SMF at 1.48 mm (250 mm/s) [96].

5.4.3 Microscopic ObservationDigital micrographs taken after the passage of the fuse disclosethe extent of the damage to the fiber. The fuse was initially trig-gered by exploiting the rough surface at the end of the POF(Fig. 5.12A) and was verified to be induced at the center of thecore, which supports the assumption in our calculation that themaximal power density in the fiber cross section affects the fuseinduction and can be used to determine the threshold powerdensity. The passage of the fuse (Fig. 5.12B) appeared as a contin-uous black carbonized curve that oscillated periodically along thelength of the POF, which is considerably different from the bullet-shaped voids observed in glass SMFs. The oscillation period wasapproximately 1300 mm, which is in general agreement with thetheoretical oscillation period of the ray [102]. Fig. 5.12C showsthe position where the fuse ceased after the incident opticalpower was reduced to below the threshold; because the fuseremained at this point for several seconds, it melted a relativelylarge area of the POF, which resulted in the observed bending.

Optical propagation loss in the POF after the passage of thefuse was measured for incremental cutbacks from 30 to 20 cm(Fig. 5.12D) and a fixed input power of 10 dBm (10 mW) at1.55 mm. A loss of 1.4 dB/cm indicates that, unlike silica SMFs, lightcan propagate through the POF for several tens of centimeters afterthe passage of the fuse. We believe this is because undamagedregions remain in the core and cladding layers, as these diametersare relatively large, which is a unique characteristic of POFs. Yetthis propagation loss is somewhat significant for communication


Figure 5.12 (A) Digital micrograph of the polymer optical fiber (POF) end at which the fuse was initiated byexploiting the rough surface. (B) Path of the fuse in the POF. (C) Point at which the fuse was terminated bydecreasing the input optical power. (D) Output power dependence on the cut-back length. The open circles aremeasured points, and the solid line is a linear fit. (E) Image of the fuse termination in the POF at the position of anickel ring. (F) Emission spectra of the bright spot of the POF fuse, an incandescent light bulb, and background(sunlight). The red circles (dark gray in print versions) indicate some characteristic peaks of the POF fusespectrum.

applications, and so once the fuse is induced, it is crucial to stopthe propagation as soon as possible.

Several methods for terminating fiber fuses have been devel-oped for glass fibers [100,103e105], and these are in principlealso applicable to POFs. One method is to thin the outer diameterof the fiber at a certain position while maintaining the core diam-eter [104]; this can reduce the internal pressure and arrest thepropagating fuse via deformation. In silica SMFs, this structureis fabricated using hydrofluoric acid as an etchant [104], but in aPOF, chloroform could be used to etch the overcladding layer[82]. An even easier method, which we present here, is topressure-bond a small metal ring around the fiber; this methodis applicable only to POFs with an extremely high flexibility. Theoptical power of the particular propagating mode that providesthe bright spot with energy is decreased below the threshold bydeformation, and the propagating fuse is thus terminated. Theresulting induced optical loss is negligibly low, and an image ofthe fuse termination at the position of the ring (Fig. 5.12E) showsthat bending did not occur. Once the ring is detached, the poly-mer material will return to the original configuration (elasticdeformation).

5.4.4 Spectral AnalysisFig. 5.12F shows the measured emission spectrum of the brightspot propagating along the POF (w300 mW incident power). Itscomparison with the blackbody-like spectra of an incandescentlight bulb and background (i.e., sunlight) shows that, althoughthe POF fuse spectrum has some characteristic peaks, all threespectra are similar. This would indicate that the bright spot ofthe POF fuse originates not so much from plasma emission asfrom thermal radiation, because if the bright spot mainly consistedof plasma, the emission spectrum would generally contain someline-shaped components [106,107]. This result may raise doubtsas to whether the fast-propagating fuse in glass fibers should bereferred to as plasma, as this conclusion has been reached withoutconvincing evidence (the emission spectra that have so far beenreported [108e110] do not provide completely clear evidence ofthe fuse being composed mainly of plasma). Spectra theoreticallycalculated using Planck’s law indicate that the temperature of thebright spot is w3600K, which can also be verified using Wien’sdisplacement law.

5.4.5 DiscussionFiber fuse in glass multimode fibers (MMFs) has also beenreported; the shape of the molten area corresponds to a


summation of the optical paths of all the propagating modes[111,112]. By stark contrast, the entire cross section of the coreand cladding of a POF seems to melt (see Fig. 5.12B), partlybecause of their low glass transition temperature of <108�Cand partly because of the relatively slow fuse propagation veloc-ity; and the boundary between the molten and the solid areas ofthe fiber cannot be observed. In the molten area of the POF, thebright spot travels only along the optical path of a particularpropagating mode (that with the highest energy) that providesthe bright spot directly with energy. The GI profile may bedestroyed by high temperature, but the observed oscillatingdamage curve suggests that its time constant is longer than thefuse propagation velocity.

We point out here that the bright spot probably originatesfrom the carbide. Once carbide is generated at high temperature(>500�C), it absorbs the light and heats the neighboring polymerabove the decomposition temperature, resulting in its growthalong the optical path. This behavior is analogous to metal parti-cle manipulation by laser irradiation in glass [113], i.e., carbideserves as an equivalent to the metal particle that moves, emittingbright visible light and melting the surrounding glass by photo-thermal conversion. This interpretation supports the result thatPOF fuse is not a plasma but an optical discharge. The oscillatorycarbonized curve indicating the passage of the POF fuse is thusformed.

It is noteworthy that, unlike the case for a silica MMF, lightand electric current can simultaneously propagate through thePOF after the passage of the fuse, because the generated contin-uous carbonized curve is electrically conductive. The opticalpropagation loss of approximately 1.4 dB/cm (Fig. 5.12D) is toohigh for telecommunication but sufficiently low for not so long(centimeter-order) light propagation. This feature will provide apossible scheme for a long photoelectric interaction length, andthe optical absorption (or electric current/resistance) might becontrolled by adjusting the electric current (or optical power)propagating along the POF, which will be useful in developingvarious optical/electrical devices.

5.5 ConclusionWe have reviewed some unique characteristics of Brillouin scat-tering in PFGI POFs, and demonstrated POF-based distributed Bril-louin sensing of strain and temperature. In Section 5.2, the BFS inthe POFs was measured to be w2.8 GHz at 1.55 mm, which showed


negative dependence on strain and temperature. The measureddependence coefficients of �122 MHz/% and �4.1 MHz/K wererespectively �0.2 and �3.5 times the values in silica glass fibers,which suggests that the Brillouin scattering in POFs is applicableto highly sensitive temperature sensing with low strain sensitivity.We also presented a nonmonotonic BFS dependence on large straincaused by a BFS hopping phenomenon. Furthermore, enhancementof Brillouin signal was demonstrated by induction of SBS andemployment of a POF with a smaller core diameter. In Section 5.3,POF-based distributed Brillouin sensing was demonstrated usingBOCDR. Strain and temperature distributions were successfullydetected with a high SNR and a high spatial resolution. The perfor-mance limitation of the POF-based system was compared with thatof glass fiberebased systems. In Section 5.4, we described the funda-mental properties of a POF fuse phenomenon, which should beavoided to performPOF-based Brillouin sensing. The POF fuse prop-agation velocity was 21.9 mm/s, which was 1e2 orders of magnitudeslower than that in standard silica fibers. The threshold powerdensity was 1/180 of the value for silica fibers. We also found thata unique oscillatory continuous carbonized curve is formed afterthe passage of the fuse, which can be terminated easily. In addition,its engineering applications were discussed. Thus, prior efforts inBrillouin scattering in POFs have already achieved substantial prog-ress toward establishing a framework for practical distributed strainand temperature sensing. The key to practical applications is theimprovement of the sensing performance, such as the spatial resolu-tion, measurement range, sampling rate, SNR, measurement stabil-ity, and system cost, as well as the assignment of new functions, suchas the strain and thermal memory and discriminative sensing ofstrain and temperature.

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INDEX

Note: ‘Page numbers followed by “f” indicate figures, “t” indicate tables.’

Accelerometersbending beam concept, 199fiber optic hydrophone, 200massespring system,

199, 200fAcoustic monitoringhydraulic fracture monitoring,

232pipeline intrusion detection,

231e232slugging, 232

Aerospace applications, opticalfiber mechanical sensors

accelerometers, 255aircraft design, 244e245aircraft industry, 248e249antenna, 240BOCDA, 251Bulldog light aircraft, 250e251cantilever beam theory, 256CVM, 253damage, 245e246, 252e253data mining, 248design and test phases, 248D-fiber, 242embedding fibers, 258e259ESA, 241e242fail-safe philosophy, 244e245fatigue failures, 245e246fiber cavity etalons, 241flight control, measurements

formeasuring acceleration,243e244

MEMS, 243e244roles, 244SHM, 244ultrasonic system, 244

Fourier transform, 254g-loads, 250e251graphite epoxy composite, 240

grating sensors, 258interrogation methods, 254Ko theory, 247e248long-period gratings, 249e250matched grating approach, 257NASA, 246NDE, 238NDI, 237optical fiber Bragg grating

sensors, advantages of, 239optical spectrum analyzer, 257Pak notes, 252palladium-coated Bragg

grating fibers, 242photodetector, 257photogrammetry, 252piezoelectric sensors and

actuators, 246e247POF sensor, 249radial vibration, 250Rayleigh scattering, 254safe-life philosophy, 245shape reconstruction

methods, 253SHM, stages of, 239smart structures, 241e242SNR, 256solar sails, 243strain data, 246, 259structural transfer functions, 247surface-mounted sensors, 253tangential vibrations, 250threat factor, 257e258truss structure, 240e241vibrations, 256wing-tip displacement, 247

Bending plate hydrophonedesign, 199

BGS. See Brillouin gain spectrum(BGS)

Biomechanics, fiber opticalsensors

adhesion to biologicaltissues, 268

body fluids, 264Camino pressure sensors,282e283

human body liquids,282e283

intraarticular pressure,284e286

intramuscular/intracompartmentalpressure, 283e284

intravascularand intracardiac,282e283

deformable bodies, 264biomechanical materialstesting, 275e277

body structure,275e277

bone cements, 278force prediction, 280FORP system,281e282

macrobending technology,281e282

SGs, 277e278stainless steel bone plates,279, 279f

traumatic head and dentalinjuries, 278

electrical conductivity, 267geometrical versatility, 268immunity to electromagnetic

interference, 267inertness and

biocompatibility,265e267

light source, 265

Biomechanics, fiber opticalsensors (Continued)

pertinent physicalparameters, 268

quantitative discipline, 264remote operation

and sensing, 267rigid bodies, 263e264assessing body kinematics,269

concept of, 268e269data stations, 269e270electric goniometers andtorsiometers, 273e274

EMG systems, 271force platform, 271FOV, 269e270high natural frequencyplatforms, 271

MEMS-based IMU, 275MoCap optical systems,hardware components of,269e270

multiplexed FBG arrays, 273passive retroreflectivemarkers, 269e270

pedobarograph, 273pressure mapping devices,272e273

SHAPE TAPE, 274e275shear stress, 272e273three-dimensional MoCapsystems, 271

small dimensions and lightweight, 267

thermal expansion andthermal conductivity, 267

Biomedical fiber optic sensorsystems

biophysical parameters,302e303

cardiovascular diagnosticsFFR, 315e316, 315fgastroenterology,316e317

IAB counterpulsationtherapy, 314e315, 314f

MEMSs, 315e316urology, 317e318

diagnostic technologies,312e314

fiber bragg grating, 308e311FPIEFPI, 304e305, 307e308fabrication technique,305e306

FP cavity, 304e305nanothick silver diaphragm,306e307

standard multimode fiber,305e306

standard single-mode fiber,305e306

metrologic, 301physical, 301pressure-sensing

applications, 303robotic microsurgerycalibration, 327clinical use, 327optical fiber diameter,326e327

optical fiber materials, 327retinal microsurgery,319e325

tool-shaft force feedback,325e326

VRS, 318e319smart textiles and wearable

sensors, 327e330system, 302

Birefringence, 7Birefringent refractive index

change (Type II), 148Bladder outlet obstruction

(BOO), 317BOCDA. See Brillouin optical

correlation domainanalysis (BOCDA)

Body fluids, biomechanics of, 264Camino pressure sensors,

282e283human body liquids, 282e283intraarticular pressure,

284e286intramuscular/

intracompartmentalpressure, 283e284

intravascular and intracardiac,282e283

Bragg wavelength sensitivity,different film thicknesses

axial force, 34e36, 36ttemperature, 36e38, 38t

Bragg wavelength, 1, 77e78Brillouin frequency shift

(BFS), 98Brillouin gain spectrum (BGS),

99, 101e102Brillouin light spectrum, 99Brillouin optical correlation

domain analysis(BOCDA), 251

Brillouin optical correlation-domain reflectometry(BOCDR), 115e116

Brillouin optical frequency-domain analysis(BOFDA), 116

Brillouin scattering, 97, 128e129BFS dependence, large strainexperimental results,113e115

experimental setup, 113core diameter and fiber

length, influence ofBrillouin linewidthnarrowing effect, 112e113

long fiber length, 111small core diameters, effectsof, 110e111

fundamental propertiesBGS, 101e102experimental setup, 99e101optical fiber, 99

induction ofexperimental results,108e109

experimental setup, 107e108motivation and principle,106

strain and temperaturedependence

BFS, 105experiments, 103PMMA, 105strain coefficient, 105

336 INDEX

theoretical temperaturecoefficient, 105e106

Brittle fracture, 93

Cantilever test, 88Cardiovascular diagnosticsFFR, 315e316, 315fgastroenterology, 316e317IAB counterpulsation therapy,

314e315, 314fMEMSs, 315e316urology, 317e318

Coefficient of thermal expansion(CTE), 223

Comparative vacuummonitoring (CVM),252e253

Coupled-mode theory, 12first-order differential

equations, 16e17forward and backward modes,

12, 13foptical fiber gratings, 23e25phase matching, 15e16superstructure FBGs

exponential componentsfor, 18

Fourier series, 18Gaussian/cosine gratingprofile, 18

periodic variations, 17phase matching conditions,18e19

reflection spectrum, 19, 19fresonance wavelengths,18e19

CVM. See Comparative vacuummonitoring (CVM)

Cyclic steam stimulation (CSS),214e215

Data mining, 248DBR. See Distributed Bragg

reflector (DBR)Deformable bodies,

biomechanics of, 264biomechanical materials

testing, 275e277

body structure, 275e277bone cements, 278force prediction, 280FORP system, 281e282macrobending technology,

281e282SGs, 277e278stainless steel bone plates,

279, 279ftraumatic head and dental

injuries, 278Degree of freedom (DOF), 320DFB. See Distributed feedback

(DFB)Different pressure

sensitivity, 189Distributed acoustic sensing

(DAS), 211, 229Distributed Bragg reflector

(DBR), 164e165Distributed feedback (DFB),

164e165, 199Distributed pressure-sensing

(DPS), 191e192Distributed temperature sensing

(DTS), 181, 211Distributed-feedback laser diode

(DFB-LD), 100e101Drawdown, 224D-shape fiber, 162Ductile fracture, 92e93Dye-and-pry failure visual

inspection, 87e88,95e96

EDFA. See Erbium-doped fiberamplifier (EDFA)

Electric strain gauge,78e79

Electrical spectrum analyzer(ESA), 100e101

Electromyography (EMG), 271Enhanced oil recovery (EOR),

214e215Erbium-doped fiber amplifier

(EDFA), 100e101European Space Agency (ESA),

241e242

FabryePerot (FP), 284FabryePerot interferometry

(FPI), 303e304EFPI, 304e305, 307e308fabrication technique,

305e306FP cavity, 304e305nanothick silver diaphragm,

306e307standard multimode fiber,

305e306standard single-mode fiber,

305e306Failure map, 90fe91f, 92e93Failure-onset PCB strain, 94e95Fatigue test

aluminum alloy, 52e55friction stirewelded

aluminum alloycyclic hardening/softening,61e62

microhardness profile,60e61, 61f

motivation, 56e57NZ, 60plastic deformation, 62e63plastic strain amplitudes,59, 59f

sample preparations, 57e59TMAZ, 60

magnesium alloy, 55e56FBGs. See Fiber bragg gratings

(FBGs)Femtosecond laser-induced

Bragg gratingsactive sensing, 164e165bulk interferometers, 148e150chemical sensing, 162e164energy deposition, 143e144energy transfer, 143e144free electron plasma

formationavalanche ionizationprocess, 144e146

conduction band electrons,144e146

critical plasma density,144e146

INDEX 337

Femtosecond laser-inducedBragg gratings(Continued)

electron density, 144e146seed electrons, 144e146subpicosecond pulses, 146transparent dielectricmaterials, 144

fs-IR laser systems,143e144

harsh environments,multiparameter sensingin, 159e161

high pressure, 161e162high radiation, 158e159high-sensitivity strain

measurements, 164e167high temperatureFabryePerot structures,156e157

fs-IR laser/phase maskapproach, 155e156

gas turbine monitoring,157e158

inhomogeneouscombustion, 157e158

metallic coatings, 155SFBGs, 157silica single-mode fibers,154e155

silica-based optical fibers,156e157

stainless steel tubing/ceramic alumina tubing,155

type I and type II gratings,153e154

volume Bragg gratings, 157induced index change,

regimes ofbirefringent refractive indexchange (Type II), 148

type I/smooth refractiveindex change, 146e147

void formation, 148phase maskBragg resonance, 150e151Fourier components,150e151

nonsinusoidal modulatedgratings, 150e151

phase mask orderwalk-off, 151

traditional UV lasereinducedgratings, 150

point-by-point gratinginscription, 151e152

Fiber Bragg grating (FBG) strainsensors

basics and sensor fabricationBragg wavelength, 77e78electric strain gauge, 78e79Hooke’s law, 78e79laser beam, 78object deformation, 78e79phase mask method, 78polyimide coating, 78reflection spectra, 80, 80f

BGA, 83e84cantilever strain, 80e81capabilities, 95different mechanical

properties, 76e77dye-and-pry failure visual

inspection, 87e88, 95e96FEA, 84, 85tfour-point bending system

and test setupmechanical testparameters, 86

PCB deflection, 86e875-V trigger signal, 87

microstructures, 75e76pad crater, 76e77PCBA, 75e76, 81e82strain distribution pattern,

83e84, 84fstrain gauges, 77, 82test resultsbrittle fracture, 93cantilever test, 88crosshead dwelling, 89e90ductile fracture, 92e93general strain release, 93e95pad craters, 88e89strainand loadcurves, 89e90

Fiber Bragg gratings (FBGs),303e304

Bragg wavelength (lB), 1core refractive index, 139coupled-mode theory. See

Coupled-mode theorydamage-like process, 140grating structure, 138e139femtosecond laser-induced

Bragg gratingsapplications of, 152e167bulk interferometers,148e150

energy deposition, 143e144energy transfer, 143e144free electron plasmaformation, 144e146

fs-IR laser systems,143e144

induced index change,regimes of, 146e148

phase mask, 150e151point-by-point gratinginscription, 151e152

high-intensity portions, 139hydrogen gas, 141laser-induced damage, 140phase mask approach, 140photosensitivity, 141remnant indexmodulation, 141sensorBragg gratingebased sensorsystem, 141e142, 142f

smart skin sensor, 142e143telecommunicationsindustry, 141e142

thermooptic effect, 142e143spectral response, 1structurally and thermally

induced index changesbirefringence, 7transverse straincomponents, 5

temperature-dependentdecay, 140

UV photon absorptionprocess, 140

Fiber cavity etalons, 241Fiber optical respiratory

plethysmography (FORP)technique, 281e282

338 INDEX

Fiber optic sensors (FOSs)acoustic monitoring

hydraulic fracturemonitoring, 232

pipeline intrusiondetection, 231e232

slugging, 232advantages, 232e233biomechanics. See

Biomechanics, fiberoptical sensors

biomedical applications. SeeBiomedical fiber opticsensor systems

downhole environment,pressure monitoring in

Bragg gratingebasedsensors, 222e223

CTE, 223drawdown, 224FabryePerot-based sensors,222e223

interference testing, 226lift monitoring, 224e226pressure and temperature,225

pressure transient analysis,224

SAGD applications, 226zonal monitoring, 225e226

flow monitoringinjectionmonitoring, 228e229interferometric flowmeter,227e228

production monitoring, 229multiparameter sensing,

232e233oil and gas industry

categories, 212e213CO2, 215CSS, 214e215downstream sector, 212e213hydraulic fracturing, 215hydrocarbon productionprocesses, 212e213

SAGD, 214e215seismic monitoring

microseismic monitoring,230e231

seismic surface arrays, 231VSP, 230

thermal monitoringdownhole thermalmonitoring applications,217e219

pipeline monitoring,216e217

SAGD Optimization,221e222

Field of view (FOV), 268Finite element analysis (FEA),

20, 84, 278Flat-cladding fiber Bragg grating

sensorsexperiments, 51e52fatigue test of

aluminum alloy, 52e55friction stireweldedaluminum alloy, 56e64

magnesium alloy, 55e56fiber optic sensors, 49e50large strain amplitudes, 49e50magnesium alloy of,

asymmetric fatiguedeformation

AZ31 extruded,stressestrain hysteresisloops of, 66e68, 70e71

motivation, 64e65plastic strain amplitude,68e70

sample preparations, 65e66Flip-chip ball grid array

(FC-BGA), 75e76Flow monitoringinjection monitoring, 228e229interferometric flowmeter,

227e228production monitoring, 229

Flowmeterhot-wire anemometry-based

FBG flow sensor, 203e204vortex flowmeter, 201e203

FP. See FabryePerot (FP)Fractional flow reserve (FFR),

315e316Friction stir welding (FSW),

56e57

Ground reaction force(GRFz), 272f

Heat-affected zone (HAZ), 57High-pressure high-temperature

(HPHT), 191e192High-pressure sensors

commercial bending platetype, 191e192

enhanced side-hole fiberpressure sensor, 191

mechanical transducer (plate,tube), 189e190

second fiber Bragg gratingtemperature sensor,187e188

sensor design concepts, 187spliceless distributed pressure

sensing, 192e193using common-mode

configuration, 188e189Hooke’s law, 78e79Hydraulic fracture

monitoring, 232Hydrogen, 184e185Hydrogen gas, 141Hydrogen loading, 141Hydrophone

bending plate hydrophonedesign, 199

frequency response, 197mandrel type, 196, 196fpiston design, 197e199

Inertial measurement units(IMU), 275

Injection monitoring, 228e229Innovative fiber Bragg grating

sensorsBragg wavelength, 176coupled-wave theory, 176dedicated operational

conditionscryogenic temperature, 183DTS system, 181fiber optic sensors, 181high operational pressure,183e184

high temperature, 182e183

INDEX 339

Innovative fiber Bragg gratingsensors (Continued)

hydrogen, 184e185low stiffness fiber, 185radiation, 184vacuum, 184

high-end performance, criticalproperties/characteristicsof

dedicated interrogators,development of,180e181

high sensitivity, 178e179high-speedmeasurement, 179

large number of sensors,179e180

large-scale sensor networksystem, 175e176

nonstandardapplications, 177

physical parameters,177e178

accelerometer, 199e201flowmeter, 201e204high-pressure sensors,186e193

hydrophone, 196e199miniaturized pressuresensor, 193e196

primary sensingparameters, 177

reflection wavelength, 176revolutionary

developments, 175standard specifications, 177

Interference testing, 226Interferometric flowmeter,

227e228Intervertebral disc (IVD), 281Intraarticular pressure (IAP),

284e286

KarhuneneLoeve transform(KLT), 305e306

Large strain amplitude fatiguetests

aluminum alloy, 52e55

friction stireweldedaluminum alloy

cyclic hardening/softening,61e62

microhardness profile,60e61, 61f

motivation, 56e57NZ, 60plastic deformation, 62e63plastic strain amplitudes,59, 59f

sample preparations, 57e59TMAZ, 60

magnesium alloy, 55e56Lift monitoring, 224e226Low stiffness fiber, 185Low-cycle fatigu (LCF) tests, 56e57

Measurement test rig, 38e39Mechanical transducer (plate,

tube), 189e190Microelectromechanical

systems (MEMS),243e244, 275

Microseismic monitoring,229e231

Minimal detectable strain(MDS), 164

Multimode fibers (MMFs),127e128

Nondestructive evaluation(NDE), 238

Nondestructive inspection(NDI), 237

Nucleus pulposus (NP), 285Nugget zone (NZ), 57Numerical aperture (NA), 99e100

Optical fiber sensors, roles, 244Optical path difference (OPD),

197e198Optical spectrum analyzer

(OSA), 78e79Opto-mechanical modelingperiodic on-fiber films, 31e32stressestrainetemperature

relations, 30e31structural modeling, 29

Pad crater, 76e77Partial differential equations

(PDEs), 10e11PbP. See Point-by-point (PbP)PCBA. See Printed circuit board

assembly (PCBA)Perfluorinated graded-index

(PFGI), 99e100Periodic on-fiber films, 27e28Phase matching condition, 15e16Photodiode (PD), 100e101Photosensitivity, 141Physical parameters, 177e178accelerometersbending beam concept, 199fiber optic hydrophone, 200massespring system,199, 200f

flowmeterhot-wire anemometry-basedFBG flow sensor, 203e204

vortex flowmeter, 201e203miniaturized pressure sensor,

193e196high-pressure sensorscommercial bending platetype, 191e192

enhanced side-hole fiberpressure sensor, 191

mechanical transducer(plate, tube), 189e190

second fiber Bragg gratingtemperature sensor,187e188

sensor design concepts, 187spliceless distributedpressure sensing,192e193

using common-modeconfiguration, 188e189

hydrophonebending plate hydrophonedesign, 199

frequency response, 197mandrel type, 196, 196fpiston design, 197e199

Pipeline intrusion detection,231e232

Piston design, 197e199

340 INDEX

Plastic strain amplitudecyclic hardening, 68e69stress amplitude, 70

PMMA. See Polymethylmethacrylate (PMMA)

Pockels’ photoelastic constant,2e5

Point-by-point (PbP), 151e152Polymer/plastic optical fibers

(POFs), 185, 249Brillouinscattering,97,128e129

BFS dependence, largestrain, 113e115

core diameter and fiberlength, influence of,109e113

fundamental properties,98e102

induction of, 106e109strain and temperaturedependence, 102e106

concept, 97e98distributed measurement

double-modulationschemes, 121

experimental results,119e121

experimental setup,118e119

motivation, 116principle, 116e118SMF-based BOCDR, 121temporal-gating, 121

memory effect, 97e98POF fuse

carbide, 128electric current, 128fundamentalcharacterization, 123e125

GI profile, 127e128microscopic observation,125e127

MMFs, 127e128motivation and principle, 122spectral analysis, 127

Polymethyl methacrylate(PMMA), 99e100, 185

Pressure transient analysis, 224Pressure/temperature (P/T), 211

Printed circuit board assembly(PCBA), 75e76

Production monitoring, 229Pumpeprobe technique, 106

Radiation, 184Radiation-hard fibers, 184Refractive index

distribution, 189Riccati ordinary differential

equation (ODE), 31e32Rigid bodies, biomechanics of,

263e264assessing body kinematics, 269concept of, 268e269data stations, 269e270electric goniometers and

torsiometers, 273e274EMG systems, 271force platform, 271FOV, 269e270high natural frequency

platforms, 271MEMS-based IMU, 275MoCap optical systems,

hardware components of,269e270

multiplexed FBG arrays, 273passive retroreflective

markers, 269e270pedobarograph, 273pressure mapping devices,

272e273SHAPE TAPE, 274e275shear stress,

272e273three-dimensional MoCap

systems, 271Robotic microsurgerycalibration, 327clinical use, 327optical fiber diameter,

326e327optical fiber materials, 327retinal microsurgery

3-DOF force-sensing pickinstrument, 323e325

2-DOF force-sensingtool, 320

tool-tip force feedback,319e320

transverse force calculation,320e322

two degrees of freedommotorized microforceps,322e323

tool-shaft force feedback,325e326

VRS, 318e319

Sapphire fiber (SFBGs), 157SBS. See Stimulated Brillouin

scattering (SBS)Seismic monitoring

microseismic monitoring,230e231

seismic surface arrays, 231VSP, 230

Seismic surface arrays, 231Self-heterodyne detection,

100e101SGs. See Strain gauges (SGs)SHM. See Structural health

monitoring (SHM)Signal-to-noise ratio (SNR),

98, 256Single-mode fiber (SMF-28), 80Steam-assisted gravity drainage

(SAGD), 214e215Stimulated Brillouin scattering

(SBS), 101Stokes power, 110Strain gauges (SGs), 77,

82, 264Structural health monitoring

(SHM), 239Superstructure fiber Bragg

gratings (SFBGs), 17measurement test rig, 38e39geometrical features, 38optical response analysisstrain and temperature,simultaneousmeasurement of, 44e46

structural loading, 41e43temperature variations,39e41

opto-mechanical modeling

INDEX 341

Superstructure fiber Bragggratings (SFBGs)(Continued)

periodic on-fiber films,31e32

stressestrainetemperaturerelations, 30e31

structural modeling, 29periodic on-fiber films,

27e28simulation resultsdifferent film thicknesses,axial force for, 34e36,35f, 36t

on-fiber silver coatings,32e34, 34f

optical constants,32, 32t

reflection spectra, 34e38,34f

temperature for differentfilm thicknesses, 36e38,38t

Theory and opto-mechanicalmodeling of fiber Bragggratings (FBGs)

Bragg wavelength (lB), 1coupled-mode theory, 12first-order differentialequations, 16e17

forward and backwardmodes, 12, 13f

optical fiber gratings,23e25

phase matching, 15e16SFBGs, 17e19

FEACartesian coordinates, 20linear nonuniform axialstrain, 21e23, 21f

modeling parameters,21e23

PDEs, 20refraction, effective modeindex of, 21e23, 22f

triangular quadraticelement, 20, 21f

light propagation in opticalfibers

anisotropy, 8boundary condition, 11Cartesian coordinates, 8, 8fMaxwell’s equations, 8PDE, 10e11

optical fibers,opto-mechanicalproperties of

dielectric material, 2e5, 2fisotropic material, 2e5photoelastic andthermooptic effects, 2e5

spectral response, 1, 2fstructurally and thermally

induced index changesbirefringence, 7transverse straincomponents, 5

Thermal monitoringdownhole thermal monitoring

applicationsgas entry, 218gas lift optimization, 219

injection monitoring, 219liquid flow, 217e218wax buildup, 219

pipeline monitoringleak detection, 216e217temperature and strain,Brillouin monitoringof, 217

SAGD optimization, 221e222Thermomechanical-affected

zone (TMAZ), 57, 60Time domain multiplexing

(TDM), 180Type I/smooth refractive index

change, 146e147color center defects, 147Ge-doped silica, 146e147hydrogen loading process, 147micro-Raman spectroscopy,

146e147

Vacuum, 184Vertical seismic profile

(VSP), 230Vitreoretinal surgery (VRS),

318e319Void formation, 148Vortex flowmeter, 201e203

Wavelength divisionmultiplexing (WDM),211e212

Wavelength domainmultiplexing (WDM), 179

Zonal monitoring, 225e226

342 INDEX

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