Understanding
`Fiber Optics
`
`5th edition
`
`Jeff Hecht
`
`Laser Light Press
`
`MASIMO 2014
` PART 1
`Apple v. Masimo
`IPR2020-01526
`
`

`

`Understanding
`Fiber Optics
`
`Fifth edition, revised
`
`Jeff Hecht
`
`LaserLight Press
`Auburndale, Massachusetts
`
`

`

`Hecht, Jeff
`Understanding fiber optics I [Jeff Hecht].-5th ed.
`p.
`
`cm.
`Includes index.
`1. Fiber optics.
`
`TA1800.H43 2006
`621.36'92-dc22
`
`I. Title.
`
`Copyright ©2015, 2006, 2002, 1999, 1993, 1987 by
` Jeff Hecht
`
`Published by Laser Light Press,
`525 Auburn St., Auburndale, Massachusetts 02466 USA
`Previously published by Pearson Education, Inc.
`
`All rights reserved. Printed in the United States of America. This publication
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`(cid:83)(cid:72)(cid:85)(cid:80)(cid:76)(cid:86)(cid:86)(cid:76)(cid:82)(cid:81)(cid:86)(cid:15)(cid:3)(cid:90)(cid:85)(cid:76)(cid:87)(cid:72)(cid:3)(cid:87)(cid:82)(cid:3)(cid:53)(cid:76)(cid:74)(cid:75)(cid:87)(cid:86)(cid:3)(cid:68)(cid:81)(cid:71)(cid:3)(cid:51)(cid:72)(cid:85)(cid:80)(cid:76)(cid:86)(cid:86)(cid:76)(cid:82)(cid:81)(cid:86)(cid:3)(cid:39)(cid:72)(cid:83)(cid:68)(cid:85)(cid:87)(cid:80)(cid:72)(cid:81)(cid:87)(cid:15)(cid:3)(cid:47)(cid:68)(cid:86)(cid:72)(cid:85)(cid:3)(cid:47)(cid:76)(cid:74)(cid:75)(cid:87)(cid:3)(cid:51)(cid:85)(cid:72)(cid:86)(cid:86)(cid:15)(cid:3)(cid:24)(cid:21)(cid:24)(cid:3)(cid:36)(cid:88)(cid:69)(cid:88)(cid:85)(cid:81)(cid:3)(cid:54)(cid:87)(cid:15)(cid:3)
`(cid:36)(cid:88)(cid:69)(cid:88)(cid:85)(cid:81)(cid:71)(cid:68)(cid:79)(cid:72)(cid:15)(cid:3)(cid:48)(cid:36)(cid:3)(cid:19)(cid:21)(cid:23)(cid:25)(cid:25)(cid:15)(cid:3)(cid:56)(cid:54)(cid:36)
`
`

`

`Preface to the Laser Light Press Edition
`
`About This Edition
`Except for this preface, the front matter, and the errata that follows, this Laser Light
`Press edition reprints the fifth edition of Understanding Fiber Optics published in 2006
`by Pearson Education, Inc. I am planning a sixth edition, but because that will take
`a while to prepare and with the Pearson edition is no longer available, I am
`reprinting the fifth through Laser Light Press. It may not cover the cutting edge of
`fiber optics, but it does cover the fundamentals you need to understand the field.
` This edition also is an experiment. I want to see how reducing the book's price
`will affect sales and make Understanding Fiber Optics more accessible to students. Thus
`Laser Light Press offers a low-cost PDF electronic version and a relatively
`inexpensive print-on-demand paperback. The many diagrams make an e-reader
`version more difficult.
` Whether you are an instructor, a student or a general reader, I would appreciate
`your comments and suggestions. If you are teaching a course based on the book,
`please contact me at jeff@jeffhecht.com for an instructor's manual. You can find
`more information on the book's status and on associated material at through
`http://www.understandingfiberoptics.com or through http://www.jeffhecht.com.
`
`About Fiber Optics
`Fiber optics has come a long way since I wrote the first edition of Understanding Fiber
`Optics in 1987. Optical-fiber communications was a radical new technology then,
`used mostly for high-capacity, long-distance transmission of telephone signals. I
`used a 1200-baud modem to send text messages from my computer through
`proprietary networks. Today a fiber-optic cable to my home provides a broadband
`connection to the Internet. A global network of fiber-optic cables links my phone
`and my computer to every continent except Antarctica, and a new cable is being laid
`through the Arctic Ocean.
` Fiber optics has revolutionized telecommunications in the same way the railroads
`revolutionized land transportation in the years my great-great-grandfather worked
`for one. Like the railroad business, the fiber-optic business has had its spectacular
`booms and busts. The telecommunications bubble brought dreams of riches, but
`the bust that followed left nightmares of ruin and grim jokes about the stocks of
`once high-flying companies. Yet the bubble and its aftermath are reminders that
`fiber optics is a technology that may be too good for its own good. Like the
`railroads and the Internet, fiber optics was something so good that the stock market
`wildly overvalued it; and like the Internet, fiber optics will be part of our future.
` I wrote the first edition of this book mainly for self-study, but it is now used
`widely in classroom settings. My goal is to explain principles rather than to detail
`procedures. When you finish, you should indeed understand fiber optics. You
`should be able to understand what the field is all about, comprehend what you read
`in trade journals such as Lightwave or Laser Focus World, make sense of what people
`in the field are saying, and explain fiber optics to your Aunt Millie or your niece.
`You won't be ready to design a brand new system, but you will be literate in the field.
`
`

`

`Think of it as Fiber Optics 101, a foundation for your understanding of a growing
`technology.
` To explain the fundamentals of fiber optics, I start with ideas that may seem basic
`to some readers; the details will follow. To make concepts accessible, I include
`drawings to show how things work, limit math to simple algebra, and step through
`some simple calculation to show how they work. I compare fiber optics with other
`common technologies and highlight similarities and differences. I have organized
`the book to facilitate cross-referencing and review of concepts, and made a point of
`adding a thorough index to make its contents accessible. I also include some
`information on the business side of the technology, and boxes that talk about key
`issues that the fiber-optics community needs to think about.
` The book introduces basic concepts first, then digs deeper into hardware and
`applications. The chapters are organized as follows:
`
`• The first three chapters are an introduction and overview. Chapter 1 tells
`how fiber optics are used and how the technology developed. Chapter 2 introduces
`optics, light, and the concept of light guiding. Chapter 3 introduces other basic
`concepts of communications and fiber-optic systems. They assume no background
`in optics or telecommunications.
`
`• Chapters 4 through 8 cover optical fibers, their properties, and how they
`are assembled into cables. The material is divided into five chapters to make it
`easier to digest. Chapters 4 through 6 explain the fiber concepts used in the rest of
`the book. Chapter 7 covers special-purpose fibers used in optical amplifiers and
`fiber gratings, photonic-crystal or microstructured fibers, and planar waveguides.
`Chapter 8 is an overview of cabling.
`
`• Chapters 9 to 12 cover laser and LED light sources including diode and fiber
`lasers, optical transmitters, optical detectors, receivers, optical amplifiers, and
`electro-optic regenerators. Chapter 12 compares and contrasts the operation of
`optical amplifiers and electro-optic regenerators.
`
`• Chapters 13 to 16 cover other components. Chapter 13 covers connectors
`and splices that join fibers. Chapter 14 covers optical couplers and other passive
`components in simple fiber systems and describes integrated optics. Chapter 15
`covers optics that send signals at many separate wavelengths through the same
`fibers. Chapter 16 covers optical modulation and switching for optical networking.
`
`• Chapter 17 covers fundamentals of optical and fiber-optic measurements and
`explains the quirks of optical measurements. Chapter 18 describes fiber-optic testing.
`
`• Chapters 19 to 22 cover general principles of fiber communication. Chapter
`19 describes fundamental concepts of fiber-optic systems and optical networking
`and how they work in practice. Chapter 20 describes communication standards.
`Chapter 21 outlines design of point-to-point single-wavelength systems, with
`sample calculations, so you can understand their operation. Chapter 22 describes the
`design of optical networks.
`
`• Chapters 23 to 27 explain how fiber optics fit into networks used for global
`and regional telephone and Internet transmission, cable television, and data
`networks. These chapters focus on different levels and aspects of the global network
`to keep concepts manageable. Chapter 28 covers special systems that don't fit
`elsewhere, such as networks in cars, military systems, and aircraft.
`
`• The final two chapters describe non-communication applications. Chapter 29
`explains the principles and operation of fiber-optic sensors. Chapter 30 covers
`imaging and illumination with fiber optics.
` The glossary at the back of the book gives you quick translations of specialized
`terms and acronyms.
`
`

`

` Appendices tabulate useful information, including values of important physical
`constants, conversion factors, and a few key formulas. They're all in one place to
`make them easier to find. They also include an annotated list of resources, in
`addition to the suggestions for further reading in each chapter. So many resources
`are available on the Internet that I can't hope to compile a thorough list; I
`encourage you to use search engines creatively. I welcome your comments,
`questions, and suggestions at jeff@jeffhecht.com.
`
`
`Acknowledgments
`Over the years many members of the fiber-optics community have given generously
`of their time to patiently answer my questions. I owe special thanks to John Jay,
`Shane Nipple, Craig Kegerise, Jerry Jackson, Eric Udd, Dana McEntire, and Joel
`Orban for feedback on draft chapters of this edition. Thanks to Kevin Able, Bill
`Chang, David Charlton, Marc Duchesne, Erich Dzakler, Robert Gallawa, Jim Hayes,
`Dennis Horwitz, Larry Johnson, Jim Masi, Nick Massa, Mike Pepper, Jim Refi, John
`Schlager, and Wayne Siddal for help on earlier editions and other material. Thanks
`to Jeffrey Rankinen, Pennsylvania College of Technology; Richard Windley, FCPI
`College of Technology; and Dave Whitmore, Champlain College for their helpful
`reviews. Any errors that remain are my own.
` This book draws on a series of articles on optical networking that I wrote for
`Laser Focus World. I think Steve Anderson for commissioning and editing them,
`Carol Settino for ably steering them into print, and the magazine's readers for
`feedback. I thank the Optical Society of America and SPIE - The International
`Society for Optical Engineering for inviting me to reach short courses based on
`Understanding Fiber Optics.
`
`I owe special thanks to the editorial and production staff at Pearson Education
`for their excellent work and their assistance in making this book possible. Thanks
`also to Lisa Cohen for updating me on the changing world of book publishing.
`
`
` Jeff Hecht, Auburndale, MA
` March 2015
`
`
`
`
`
`
`
`
`
`

`

`Errata
`
`What have you learned? item 6 on page 35 should read:
`Refractive index (n) of a material is the speed of light in a vacuum divided by the
`speed of light in the material. It is always greater than 1.0 at optical wavelengths.
`
`Figure 14.12 on page 356 does not correctly show the operators performed on
`light of different polarizations in an optical circulator.
`
`Figure 15.1 on page 365 should have an * on (cid:1)4 on the right side of the drawing to
`
`show that wavelength comes from the local transmitter at the bottom.
`
`Table A.3 on page 764 should give the value of Planck's constant in J-s (joule-
`seconds) or eV-s (electronvolt-seconds), not J/s or eV/s. The numerical values are
`correct, but the units are not.
`
`

`

`Contents
`
`Chapter 1 An Introduction to Fiber
`Optics, 1
`A Personal View: Ups and Downs, 1 * The
`Roots of Fiber Optics, 2 • The Very Basics of
`Communications, 8 • Fiber Terms: Terminology
`and Units, 12
`
`Chapter 2 Fundamentals of Fiber-Optic
`Components, 17
`Basics of Optics, 17 • Light G uiding, 26
`• Fiber Transmission, 28 • Electro-Optics and
`Other Components, 33 • Fiber-Optic
`Applications, 34
`
`Chapter 3 Fundamentals of
`Communications, 39
`Communications Concepts, 39 • Signals
`and Formats, 46 • Connectivity, 50
`• Communications Services, 54 • The Business
`of Telecommunications, 58
`
`Chapter 4 Types of Optical Fibers, 65
`Light Guiding, 65 • Step-lndex Multimode
`Fiber, 68 • Modes and Their Effects, 71
`• Graded-lndex Multimode Fiber, 75
`• Single-Mode Fiber, 77 • Dispersion-Shifted
`Single-Mode Fiber, 80 • Polarization in
`Single-Mode Fiber, 85 • Other Fiber Types, 87
`
`Chapter 5 Properties of Optical
`Fibers, 93
`Fiber Attenuation, 93 • Light Collection and
`Propagation, 99 • Dispersion, 103 • Nonlinear
`Effects, 1 1 5 * Mechanical Properties, 119
`
`Chapter 6 Fiber Materials, Structure, and
`Manufacture, 127
`Requirements for Making Optical Fibers, 1 27
`• Glass Fibers, 128 • Fused-Silica Fibers, 130
`
`• Plastic Fibers, 137 • Exotic Fibers and Light
`Guides, 140
`
`Chapter 7 Specialty Fibers, 151
`W hat Are "Specialty" Fibers?, 151
`• Dispersion-Compensating Fibers, 152
`• Polarization-Maintaining Fibers, 153
`• Bend-Insensitive and Coupling Fibers, 153
`• Reduced-Cladding Fibers, 155 • Doped
`Fibers for Amplifiers and Lasers, 156 • Fiber
`Gratings and Photosensitive Fibers, 159
`• Photonic or "Holey" Fibers, 165 • Special
`Noncommunications Fibers, 166
`
`Chapter 8 Cabling, 173
`Cabling Basics, 173 • Reasons for
`Cabling, 174 • Types of Cable, 178
`• Elements of Cable Structure, 183
`• Cable Installation, 190 • Cable Changes
`and Failure, 191
`
`Chapter 9 Light Sources, 197
`Light Source Considerations, 197 • LED
`Sources, 200 • The Laser Principle, 203
`• Simple Semiconductor Lasers, 207
`’ Laser Wavelength, 213 • Fiber
`Lasers, 219 • Other Solid-State Laser
`Sources, 221
`
`Chapter 10 Transmitters, 227
`Transmitter Terminology, 227 • Operational
`Considerations, 228 • Multiplexing, 232
`• Modulation, 234 • Single-Channel
`Transmitter Design, 238
`• Sample Transmitters, 241
`
`Chapter 11 Receivers, 249
`Defining Receivers, 249 • Performance
`Considerations, 258 • Electronic Functions, 265
`• Sample Receiver Circuits, 267
`
`

`

`Contents
`
`Chapter 12 Amplification, Regeneration,
`and Wavelength Conversion, 275
`Amplification and Regeneration, 275
`• System Requirements, 279 • Repeaters and
`Regenerators, 280 • Optical Amplifiers, 281
`• Erbium-Doped Fiber Amplifiers, 284
`• Other Doped Fiber Amplifiers, 291 • Raman
`Amplification in Fibers, 292 • Semiconductor
`Optical Amplifiers, 295 • Optical Regeneration,
`299 • Wavelength Conversion, 299
`
`Chapter 13 Connectors and Splices, 307
`Applications of Connectors and Splices, 307
`• Fiber-to-Fiber Attenuation, 309 • Internal
`Reflections, 314 • Mechanical Considerations
`in Connectors, 315 • Connector Structures,
`317 • Standard Connector Types, 320
`• Splicing and Its Applications, 326 • Splicing
`Issues and Performance, 327 • Types of
`Splicing, 328
`
`Chapter 14 Couplers and Other Passive
`Components, 339
`Coupler Concepts and Applications, 339
`• Coupler Characteristics, 343 • Coupler Types
`and Technologies, 347 • Attenuators, 353
`• Optical Isolators, 354 • Optical
`Circulators, 355
`
`Chapter 15 Wavelength-Division
`Multiplexing Optics, 363
`W D M Requirements, 363 • W D M
`Systems, 364 • Optical Filters and W DM , 370
`• W D M Technologies, 375 • Building
`Multiplexers and Demultiplexers, 385
`
`Chapter 16 Optical Switches,
`Modulators, and Other Active
`Components, 391
`Defining Active Components, 391 • Modulators
`and Modulation, 392 • Switching in Optical
`
`Networks, 397 • Optical Switching
`Technologies, 403 • Wavelength Switching
`and Conversion, 409 • Integrated Optics, 410
`
`Chapter 17 Fiber-Optic
`Measurements, 417
`Basics of Optical Power Measurement, 417
`• Wavelength and Frequency Measurements,
`425 • Phase and Interference Measurements,
`428 • Polarization Measurements, 430
`• Time and Bandwidth Measurements, 430
`• Signal Q uality Measurements, 434
`• Fiber-Specific Measurements, 436
`
`Chapter 18 Troubleshooting and
`Test Equipment, 447
`Fiber-Optic Troubleshooting, 4 47 • Test
`and Measurement Instruments, 450
`• Troubleshooting Procedures, 462
`
`Chapter 19 System and Optical
`Networking Concepts, 471
`An Evolving Network, 471 • Telecommunication
`Network Structure, 473 • Transmission
`Topologies, 475 • Directing Signals, 481
`• Signal Formats, 483 • Transmission
`Capacity, 487
`
`Chapter 20 Fiber System
`Standards, 499
`W hy Standards Are Needed, 499 • Families of
`Standards, 501 • Layers of Standards, 502
`• Interchange Standards, 507 • Fiber
`Transmission Standards, 509 • Current
`Standards Issues, 513
`
`Chapter 21 Single-Channel System
`Design, 521
`Variables, 521 • Power Budgeting, 523
`• Examples of Loss Budgeting, 528
`• Transmission Capacity Budget, 534
`• Cost/Performance Trade-offs, 541
`
`

`

`Chapter 22 Optical Networking System
`Design, 549
`Optical Networking Concepts, 549 • Optical
`Channel Density, 550 • Operating Ranges of
`W D M Systems, 555 • Factors in W D M Design,
`557 • Optical Amplification and W D M Design,
`562 • Switching and Optical Networking, 563
`• Design Examples, 566
`
`Chapter 23 Global Telecommunications
`Applications, 573
`Defining Telecommunications, 574 • The
`Global Telecommunications Network, 577
`• Internet Transmission, 582 • Submarine
`Cables, 585 • Long-Haul Terrestrial
`Systems, 594 • Types of Long-Distance
`Services, 598
`
`Chapter 24 Regional and Metro
`Telecommunications, 605
`Defining Regional and Metro Telecommunications,
`605 • Regional Distribution, 606 • Regional
`Telecommunications Networks, 610 • Metro
`Networks, 612 • Regional/Metro Services and
`Equipment, 614
`
`Chapter 25 Local Telephone or "Access"
`Networks, 623
`Structure of the Local Phone Network, 623
`• Subscriber and Access Services, 630
`• Emerging Services and Competing
`Technologies, 632 • Fiber to the Home or
`Premises, 636
`
`Internet Access and
`Chapter 26
`Local-Area Networks, 651
`Data and Voice Transmission, 651 • The Internet
`and Its Structure, 654 • Data Transmission
`Technologies, 660 • Fiber Data-Link
`Design, 665 • Fiber in Standard Data
`Networks, 665
`
`Contents
`
`Chapter 27 Video Transmission, 677
`Video Basics, 677 • Transmission Media, 684
`• Cable Television Architecture, 686
`• HDTV and Cable, 691 • Other Video
`Applications, 692
`
`Chapter 28 Mobile Fiber-Optic
`Communications, 699
`M obile Systems, 699 • Remotely Controlled
`Robotic Vehicles, 700 • Fibers in Aircraft, 703
`’ Shipboard Fiber-Optic Networks, 705
`• Automotive Fiber Optics, 706
`
`Chapter 29 Fiber-Optic Sensors, 713
`Fiber-Sensing Concepts, 713 • Fiber-Optic
`Probes, 714 • Fiber-Sensing Mechanisms, 716
`• Some Fiber Sensor Examples, 719
`• Fiber-Optic Gyroscopes, 722 • Smart Skins
`and Structures, 724
`
`Imaging and Illuminating
`Chapter 30
`Fiber Optics, 729
`Basics of Fiber Bundles, 729 • Optics of
`Bundled Fibers, 734 • Imaging Applications,
`737 • Light Piping and Illumination, 740
`
`Appendices, 745
`Appendix A: Important Constants, Units,
`Conversion Factors, and Equations, 745
`• Appendix B: Decibels and Equivalents, 749
`• Appendix C: Standard Time-Division
`Multiplexing Rates, 751 • Appendix D: ITU
`Frequencies and Wavelengths for L- and
`C-bands, 100-GHz Spacing, 100
`Channels, 753 • Appendix E: Laser and Fiber
`Safety, 755 • Appendix F: Fiber-Optic
`Resources, 757
`
`Glossary, 761
`
`Index, 775
`
`

`

`book is dedicated to the memory of Heather Williamson Messenger,
`gifted editor, good friend, and victim of domestic violence.
`
`

`

`An Introduction
`to Fiber Optics
`
`1
`
`About This Chapter
`
`This chapter is a starting point to look around and see where you’re going before you
`dig inro details. The goal is to put fiber optics and communications into context and
`show how they go together. I start with a personal commentary about the turbulent
`times of the past several years, then explain the plan for this book. A brief history of
`fiber optics follows, which introduces some important concepts. Then a brief history
`of communications explains the need for bandwidth and how fiber optics filled that
`need, perhaps too well. Finally, I explain some of the terminology of the field to help
`you in your looking about.
`
`A Personal View: Ups and Downs
`
`Fiber optics has come a long way in the nearly three decades I’ve been watching its
`development. For many years the field grew steadily, with new technology creating
`new applications, and new applications, in turn, supplying money to develop more new
`technology. The growth sped out of control in the late 1990s as the Internet fed a seem-
`ingly limitless thirst for bandwidth that only optical fibers could provide. The boom
`turned into a bubble, and the bubble into a bust as I watched in amazement.
`We knew the bubble was too good to be true, but none of us wanted it to end. We
`told ourselves that the communications industry was in better shape than the dot-coms
`because it had real hardware, not just web sites. Then the industry ran right off a cliff
`and landed with an ugly splat. We traded grim jokes, noting that we would have done
`better to invest in cases of beer and return the empties in a state with a bottle-deposit
`law. Employment dropped nearly as badly. The industry seemed a vast, smoking crater.
`
`Fiber
`revolutionized
`telecommunications
`by supplying
`tremendous
`bandwidth.
`
`o
`
`

`

`Chapter 1
`
`•
`Fiber-optic
`technology
`remains ea y.
`
`Light normally
`goes in straight
`lines, but
`sometimes we
`want it to go
`around corners.
`
`That depressing view is as much of an exaggeration as was the euphoric overenthusiasm
`0f tbe bubble. We’ll never see that manic growth again, and that’s just as well. But fiber-optic
`technology remains healthy, with advances continuing at a more sober rate. Fiber optics has
`become the backbone of the global telecommunications network, giving us instant access
`to Web sites and telephones around the world. That network continues to reach toward
`homes and businesses. Cable television companies, telephone companies, Internet
`providers, and power companies have their own fiber-optic networks. When you use a cell
`phone, your calls usually go wireless only to the tower, where a fiber-optic cable runs to the
`backbone telephone network. The demand for bandwidth continues to rise, although
`there’s a lot of surplus fiber in the ground right now.
`Fiber revolutionized telecommunications in the twentieth century, just as the railroads
`revolutionized transportation in the nineteenth century. Overbuilding of railroads caused
`spectacular busts in the latter half of the nineteenth century, but railroads remained the
`backbone of the national transportation network until the spread of the interstate highway
`system in the 1950s and 1960s. Railroads still carry people and freight today— especially
`in Europe.
`The fiber-optic gold rush is over, and the field has had a roller-coaster ride of dramatic
`ups and downs. We’ve gained some experience and a few gray hairs in the process, but we’ve
`survived. Fiber has carved itself a vital niche in the communications world and will play a
`growing role around the world as other countries expand their own communications
`networks. Fiber is here to stay.
`
`The Roots of Fiber Optics
`
`Fiber optics did not begin as a communications technology. Optical fibers evolved from
`devices developed to guide light for illumination or displays, and were first used to
`look inside the human body. Bundles of optical fibers are still used to examine the
`stomach and the colon because they can reach into otherwise inaccessible areas. It’s
`worth looking at how this idea began— it will teach you the basic ideas of light guiding
`in a fiber.
`Piping Light
`Think of optical fibers as pipes that carry light. Lenses can bend light and mirrors can
`deflect it, but otherwise light travels in straight lines. The working of optical devices,
`from our eyes to giant telescopes and sensitive microscopes, depends on light going in
`straight lines. Yet sometimes it is nice to be able to pipe light around corners and look
`into inaccessible places. The first steps in that direction were taken in the nineteenth
`century.
`In 1880, William Wheeler, a young engineer from Concord, Massachusetts, filed for a
`patent on a way to pipe light through buildings. Thomas Edison had already made the
`first incandescent light bulbs but hadn’t gotten all the bugs out. Wheeler wanted to dis-
`tribute light from an electric arc, a light source that was better developed at the time, but
`was blindingly bright. He planned to put arc lamps in the basements of buildings and
`
`

`

`{I* ItfeL)
`
`W. W H EELER.
`A PPABATV S FOB LIGHTING DW ELLIN GS OB OTHEB 8TBU 0TU BE1.
`Ho. 247,229.
`Patented Sept. 20,1881.
`
`An Introduction to Fiber Optics
`
`FIGURE 1.1
`W heeler’s plan fo r
`piping light into
`rooms (U.S.
`Patent 247,229).
`
`distribute the light to distant rooms through a set of pipes coated with a reflective layer
`inside, as shown in Figure 1.1. Diffusers at the ends of the pipes would spread the light
`out inside each room.
`Wheeler was a solid engineer who became an expert in designing water works. He
`later founded a successful company that made reflectors for street lamps. His design was
`logical at the time since air seemed to be a much clearer medium than any known solid.
`But his light pipes never caught on, and Edison’s incandescent bulbs eventually worked
`much better than arc lamps.
`Total Internal Reflection
`Even before Wheeler’s time, scientists knew how to trap light inside a solid. A phenomenon
`called total internal reflection, described in Chapter 2, can confine light inside glass or other
`transparent materials. This phenomenon involves sending light through the material in
`such a way that it strikes the surface exposed to air at a glancing angle. Then the light is
`reflected back into the solid. You can see the effect in diamond or cut glass, in which one
`surface acts like a mirror to reflect light to your eye.
`Glassblowers may have been the first to realize this effect could guide light along a bent
`glass rod, but it wasn’t widely recognized until 1841 when a Swiss physicist, Daniel Colladon,
`
`Total internal
`reflection can
`guide light along
`a glass rod or
`water jet.
`
`

`

`FIGURE 1.2
`Light guided
`down a water jet.
`
`Light beam becomes more diffuse
`as it passes down the water jet,
`because turbulence breaks up surface.
`
`adapted the trick for a jet of flowing water in his popular science lectures. Figure 1.2 shows
`how he directed a bright light down a horizontal pipe leading out of a tank of water.
`When he opened the spout, water flowed out in a jet and the pull of gravity bent the water
`jet into a parabolic arc. Total internal reflection trapped the light inside the water jet. The
`light beam bounced off the top surface, then off the lower surface, until turbulence in the
`flowing water broke up the beam.
`Others borrowed the idea for their own demonstrations. The Paris Opera used it on
`stage in 1853. The great Victorian exhibitions of the 1880s adapted the idea to make illu-
`minated fountains that fascinated fairgoers who hadn’t seen bright artificial lights. But the
`water jet remained essentially a parlor trick of little practical use.
`Glass Light Guides and Imaging
`Inventors soon adapted the idea of guiding light to more practical purposes. By the early
`1900s, they had patented a scheme for guiding light through a bent glass rod to illuminate the
`inside of the mouth for dentistry. This technique was much better than sticking a gas lamp
`into a patient’s mouth, but it was far from perfect. Illuminated tongue depressors followed,
`A fine glass fiber is actually a very thin, flexible rod, so it can guide light in the same way.
`Assemble glass fibers into a bundle, and they can carry an image from one end to the other,
`
`^
`An optical fiber
`guides light
`like a very thin
`glass rod.
`
`

`

`An Introduction to Fiber Optics
`
`as you will learn in Chapter 30. Clarence W. Hansell, an American electrical engineer and
`prolific inventor, was the first to take this logical step and patented the idea in the late
`1920s. Hansell thought the bundles could be used for inspecting out-of-the-way places, for
`medical applications, or even for a facsimile machine.
`Heinrich Lamm, a German medical student, made the first image-transmitting bundle
`in 1930 and was able to photograph the bright filament of a lamp. He combed the fibers to
`align them, but the bundle didn’t work well because it consisted of bare fibers, in which
`total internal reflection was at the surface exposed to the air. Light can easily leak through
`that surface if anything touches or scratches it, and the fibers inevitably touched and
`scratched each other in Lamm’s bundle. Light even leaked out at places where fingerprint
`oil was smudged on the surface.
`Neither Hansell nor Lamm got very far. The same problems bedeviled other men who
`independently invented fiber bundles for imaging in the early 1950s. These men were a
`Danish inventor, Holger Moller Hansen, two eminent optics professors, Abraham van
`Heel and Harold H. Hopkins, and Hopkins’ student, Narinder Kapany.
`Solving the problem required a fresh look at the requirements for total internal
`reflection. We normally think of it as occurring where light is unable to enter the air, but
`what really matters is a quantity called the refractive index, which you’ll learn about in
`Chapter 2. Total internal reflection can happen when light travelling in one medium tries
`to enter another medium with a lower refractive index. Air has a much lower refractive
`index than glass, but the difference does not have to be large. Oils, beeswax, and many plas-
`tics have refractive indexes that are higher than air but lower than glass. Coat the glass fiber
`with one of those materials, and total internal reflection can still occur, but the surface is
`protected from scratches, fingerprints, and leakage of light into other glass fibers, as shown
`in Figure 1.3.
`Moller Hansen tried coating a fiber with margarine, but the results were impractically
`messy. Brian O ’Brien, a noted American optical physicist, suggested the idea to van Heel,
`who coated his fibers with plastic and beeswax, which were more practical. In December
`1956, Larry Curtiss, an undergraduate student at the University of Michigan, slipped a rod
`of glass with high refractive index into a tube of glass with lower index and made the first
`glass-clad fiber.
`The technology has been refined considerably since then, but glass-clad fiber remains the
`most common type. Fiber bundles were the key to making flexible endoscopes, gastroscopes,
`and colonoscopes to examine the throat, stomach, and colon. Other imaging applications
`soon emerged, as described in Chapter 30. Fiber bundles