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This section contains the title page, foreward, table of contents, and the glossary.

Library of Congress Preassigned Control Number Data

Spillman, William B., author.

Field Guide to Fiber Optic Sensors / William B. Spillman, Jr. and Eric Udd

pages cm. – (The field guide series; FG34)

Includes bibliographical references and index.

ISBN 978-1-62841-334-2

1. Fiber optics–Design and application. 2. Sensor design. I. Title.

TA1800 2014

681.2–dc23

2014954702

Published by

SPIE

P.O. Box 10

Bellingham, Washington 98227-0010 USA

Phone: 360.676.3290

Fax: 360.647.1445

Email: Books@spie.org

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The content of this book reflects the thought of the author(s). Every effort has been made to publish reliable and accurate information herein, but the publisher is not responsible for the validity of the information or for any outcomes resulting from reliance thereon.

Printed in the United States of America.

Last updated 02/10/2015

Introduction to the Series

Welcome to the SPIE Field Guides—a series of publications written directly for the practicing engineer or scientist. Many textbooks and professional reference books cover optical principles and techniques in depth. The aim of the SPIE Field Guides is to distill this information, providing readers with a handy desk or briefcase reference that provides basic, essential information about optical principles, techniques, or phenomena, including definitions and descriptions, key equations, illustrations, application examples, design considerations, and additional resources. A significant effort will be made to provide a consistent notation and style between volumes in the series.

Each SPIE Field Guide addresses a major field of optical science and technology. The concept of these Field Guides is a format-intensive presentation based on figures and equations supplemented by concise explanations. In most cases, this modular approach places a single topic on a page, and provides full coverage of that topic on that page. Highlights, insights, and rules of thumb are displayed in sidebars to the main text. The appendices at the end of each Field Guide provide additional information such as related material outside the main scope of the volume, key mathematical relationships, and alternative methods. While complete in their coverage, the concise presentation may not be appropriate for those new to the field.

The SPIE Field Guides are intended to be living documents. The modular page-based presentation format allows them to be updated and expanded. We are interested in your suggestions for new Field Guide topics as well as what material should be added to an individual volume to make these Field Guides more useful to you. Please contact us at fieldguides@SPIE.org.

John E. Greivenkamp, Series Editor

College of Optical Sciences

The University of Arizona

The Field Guide Series

Keep information at your fingertips with the SPIE Field Guides:
  • Adaptive Optics, Second Edition, Robert Tyson & Benjamin Frazier

  • Atmospheric Optics, Larry Andrews

  • Binoculars and Scopes, Paul Yoder, Jr. & Daniel Vukobratovich

  • Diffractive Optics, Yakov Soskind

  • Digital Micro-Optics, Bernard Kress

  • Displacement Measuring Interferometry, Jonathan Ellis

  • Fiber Optic Sensors, William Spillman, Jr. & Eric Udd

  • Geometrical Optics, John Greivenkamp

  • Holography, Pierre-Alexandre Blanche

  • Illumination, Angelo Arecchi, Tahar Messadi, & John Koshel

  • Image Processing, Khan M. Iftekharuddin & Abdul Awwal

  • Infrared Systems, Detectors, and FPAs, Second Edition, Arnold Daniels

  • Interferometric Optical Testing, Eric Goodwin & Jim Wyant

  • Laser Pulse Generation, Rüdiger Paschotta

  • Lasers, Rüdiger Paschotta

  • Lens Design, Julie Bentley & Craig Olson

  • Microscopy, Tomasz Tkaczyk

  • Nonlinear Optics, Peter Powers

  • Optical Fabrication, Ray Williamson

  • Optical Fiber Technology, Rüdiger Paschotta

  • Optical Lithography, Chris Mack

  • Optical Thin Films, Ronald Willey

  • Optomechanical Design and Analysis, Katie Schwertz & James Burge

  • Physical Optics, Daniel Smith

  • Polarization, Edward Collett

  • Probability, Random Processes, and Random Data Analysis, Larry Andrews

  • Radiometry, Barbara Grant

  • Special Functions for Engineers, Larry Andrews

  • Spectroscopy, David Ball

  • Terahertz Sources, Detectors, and Optics, Créidhe O'Sullivan & J. Anthony Murphy

  • Visual and Ophthalmic Optics, Jim Schwiegerling

Field Guide to Fiber Optic Sensors

Fiber optic sensor work began in the mid-1970s with a variety of multimode fiber optic sensors and early iterations of Sagnac and Mach–Zehnder interferometers. Concerted efforts developed fiber optic gyros and acoustics sensors in the form of hydrophones; fiber gyros are currently used for aircraft and spacecraft navigation, including the Spirit and Opportunity rovers on Mars. They are used for naval and land navigation, in robots that mow soccer fields and clean floors, and in remote-controlled helicopters that spray crops. Acoustic technology originally developed for undersea surveillance is now used to support oil and gas exploration and extraction operations.

In the mid-1980s the field of fiber optic smart structures emerged as a way to monitor the health and integrity of structures through the manufacturing process, during installation, and throughout the lifetime of the structure. Distributed sensors based on Raman and Brillouin scattering have been used to support cure monitoring on large concrete structures such as dams and to monitor oil holes and pipelines.

The continued improvement and reduction in costs associated with fiber optic technology associated with fiber sensors permit application areas that were previously inaccessible. These trends are expected to continue as new techniques become available and older ones are successfully adapted to new applications.

This Field Guide provides a broad introduction to a variety of fiber optic sensors that have been successfully developed from the 1970s to the present. We hope that the included examples inspire readers with ideas for new sensors and applications.

William B. Spillman, Jr.

Floyd, Virginia

Eric Udd

Fairview, Oregon

Table of Contents

Glossary xi

Introduction 1

What Is a Fiber Optic Sensor? 1

Why Use a Fiber Optic Sensor? 2

Light as an Electromagnetic Wave 3

Light Modulation 4

Optical Signal-to-Noise Ratio 5

Transduction 6

Some Optical Transducer Mechanisms 7

Components 8

Optical Fibers and Modes 8

Optical Fiber Types 9

Light-Emitting Diode 10

Semiconductor Laser Diode 11

Photodiode Detector 12

Avalanche Photodiode Detector 13

Color Sensors 14

Connectors and Splices 15

Lenses and GRIN Lenses 16

Waveplates 17

Polarizers 18

Diffraction Gratings 19

Fiber Bragg Gratings 20

Modulators 21

Sensor Design 22

Specification and Transducer Selection 22

Component Selection 23

Sensor Modeling 24

Intrinsic Multimode Sensors 25

FTIR Sensor Theory 25

FTIR Hydrophone 26

FTIR Fluid-Level Sensor 27

Microbend Sensor Theory 28

Microbend Pressure Sensor 29

Macrobend Sensor Theory 30

Macrobend Pressure Sensor 31

Modal Domain Sensor Theory 32

Modal Domain Vibration Sensor 33

Extrinsic Multimode Sensors 34

In-Line Fiber Coupling Theory 34

Fiber Coupling Pressure Sensor 35

Polarization Theory 36

Polarization Rotation Sensor 37

Photoelastic Sensor Theory 38

Photoelastic Pressure and Acoustic Sensing 39

Diffraction Grating Theory 40

Diffraction Grating Rotation Sensor 41

Schlieren Grating Theory 42

Schlieren Displacement and Acoustic Sensors 43

Chirped Grating Sensor Theory 44

Chirped Grating Displacement Sensor 45

Magneto-Optic Sensor Theory 46

Magneto-Optic Speed Sensor 47

Optrode Sensor Theory 48

Optrode pH Sensor 49

Fluorescence Theory 50

Fluorescence Temperature Sensor 51

Blackbody Theory 52

Blackbody Temperature Sensor 53

Optical Time Domain Reflectometry Theory 54

Optical Time Domain Wavefront Speed Detector 55

Evanescent-Wave Theory 56

Evanescent-Wave Chemical Sensor 57

Interferometers 58

Single-Mode Interferometers 58

Mach–Zehnder Interferometers 59

Mach–Zehnder Hydrophone 60

Mach–Zehnder Transducer Geometries 61

Michelson Interferometer 62

Sagnac Theory 63

Sagnac Rotation Sensor/Fiber Optic Gyro 64

Sagnac Acoustic Sensor 65

Sagnac Interferometer Strain Sensor 66

Bragg Gratings 67

Bragg Grating Fabrication 67

Bragg Grating Theory 68

Bragg Grating Simultaneous Strain and Temperature 69

Bragg Grating Transverse Strain 70

Bragg Grating Pressure Sensor 71

Fiber Etalon 72

Fiber Etalon Theory 72

Fiber Etalon Sensors 73

Multiplexing 74

Time Division Multiplexing 74

Wavelength Division Multiplexing 75

Optical Frequency Domain Reflectometry 76

Frequency Division Multiplexing 77

Coherence Multiplexing 78

Spatial Multiplexing 79

Distributed Sensors 80

Distributed Raman Sensor 80

Distributed Brillouin Sensor 81

Brillouin Distributed Sensor Capabilities 82

Distributed Sagnac Sensor 83

Distributed Sagnac–Mach–Zehnder Sensor 84

Applications 85

Sagnac Secure-Communication System 85

Low-Coherence Strain-Sensor System 86

Civil Structure Applications 87

Damage-Assessment Microbend Sensor 88

Position Sensors 89

Smart Bed 90

Catheter Shape Sensing 91

Radiation Dosage Sensing 92

Proximity Fiber Sensors 93

Oil and Gas Applications 94

Strain Imaging and Monitoring Composite Materials 95

Plastic Optical Fibers 96

Integrated Optics 97

Fiber Light Sources 98

Polarization-Preserving Optical Fibers 99

Fiber Depolarizer 100

Useful Techniques 101

Manually Scribing and Breaking a Fiber 101

Using UV Curing Epoxy 102

Experimental Use of a Lock-In Amplifier 103

Equation Summary 104

Bibliography 110

Index 114

Glossary

a

Fiber core radius

A

Amplitude or some constant value

APD

Avalanche photodiode

B

Bandwidth

B

Magnetic flux vector

c

Speed of propagation of an electromagnetic wave

ce

Effective speed of light

d

Grating period, Bragg grating period, or lateral misalignment

d0, d1

Chirped grating parameters

Di

Distance from a photodetector to the ith element

dij

Distance between optical elements i and j

e

Electric charge or base of natural logarithms

E

Electric-field vector

EH

Hybrid mode with more electric field than magnetic field to the direction of propagation

f

Frequency (in Hertz), or the lens focal length

F

Finesse

F

Power emitted by a blackbody

FBG

Fiber Bragg grating

fD

Disk rotation frequency

FDM

Frequency division multiplexing

fM

Modulation frequency

FTIR

Frustrated total internal reflection

fσ

Material stress-optic coefficient

Ge

Germanium

GRIN

Graded index

h

Height

h

Planck’s constant

HE

Hybrid mode with more magnetic field than electric field to the direction of propagation

He-Ne

Helium-neon

I

Optical power

i0

Reverse “dark current”

IB

Background optical power

Id

Dark current

is

Photocurrent

k

Wave vector

kB

Boltzman’s constant

L

Length of a GRIN lens

LED

Light-emitting diode

m

Diffraction order

n

Index of refraction

N

Number of slots around a disk edge, or number of excited electrons

NA

Optical-fiber numerical aperture

neff

Effective index of refraction

OD

Optical density

OFDR

Optical frequency domain reflectometry

OTDR

Optical time domain reflectometry

p

Pitch of a GRIN lens

P

Pressure or detected optical power

PD

Photodiode

Pe

Photoelastic constant

Pmin

Minimum detectable acoustic pressure

PVF

Polyvinyl fluoride

r

⊥ distance from optical axis

r

Position vector

R

Electrical resistance or responsivity

R

Reflectivity

RE , RT

Fluorescent decay constants

RGB

Red–green–blue

Rp

p-polarized intensity reflectivity

Rs

s-polarized intensity reflectivity

s

Optical fiber longitudinal misalignment

s1

Distance of an object from the center of a lens

s2

Distance of an image from the center of a lens

Sn

Fluorescent signal level

SNR

Signal-to-noise ratio

t

Time

T

Absolute temperature

TA

Fiber axial displacement fractional transmission

TDM

Time division multiplexing

TE

Mode with an electric field to the direction of propagation

TEM

Mode with both magnetic and electric fields to the direction of propagation

Ti

Transducer function

TIR

Total internal reflection

TL

Fiber longitudinal fractional transmission

TM

Mode with magnetic field to the direction of propagation

UV

Ultraviolet

V

V-number, or voltage

VB

Reverse bias voltage

VOD

Velocity of detonation

VR

Voltage from a reference photodetector

W

Watts

WDM

Wavelength division multiplexing

z

FTIR variable

α

Fractional transmission factor, fluorescent decay probability, or thermal expansion coefficient

αm

Angle of the mth diffraction order

β

Mode-propagation constant, or FTIR variable

ΔE

Energy-level difference in fluorescent decay

ε

Dielectric permittivity, or mechanical strain

ζ

Thermo-optic coefficient

η

Quantum efficiency

θ

Angle

θD

Angle of diffraction

λ

Wavelength

λB

Wavelength Bragg grating maximum reflection

Λ

Microbend optimum deformer spacing

μ

Magnetic susceptibility

μW

Microwatts

σ

Linear stress

τ

e–1 fluorescent decay time

ϕ

Phase

ω

Angular frequency in radians/sec

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