Tai Lieu Chat Luong Fiber Optics Fedor Mitschke Fiber Optics Physics and Technology 123 Prof. Fedor Mitschke Universität Rostock Institut für Physik Universitätsplatz 3 18055 Rostock Germany fedor.mitschke@uni-rostock.de ISBN 978-3-642-03702-3 e-ISBN 978-3-642-03703-0 DOI 10.1007/978-3-642-03703-0 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2009938485 c Springer-Verlag Berlin Heidelberg 2009 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer.
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Printed on acid-free paper Springer is part of Springer Science+Business Media (www.com) Absent a Telephone, a Bicyclist Had to Save the World On the height of the Cuban missile crisis in 1962, no direct telecommunication line existed between the White House and the Kremlin. All messages going back and forth had to be sent through intermediaries. The world teetered on the brink of nuclear Armageddon when in the evening of October 23 President John F. Kennedy sent his brother, Robert Kennedy, over to the Soviet Embassy for a last-ditch effort to resolve the crisis peacefully.
Robert presented a proposal how both sides could stand down without losing face. Right after the meeting, Ambassador Anatoly Dobrynin hastened to write a report to Nikita Khrushchev in Moscow. A bicycle courier was called in to take this letter to a Western Union telegraph station, and Dobrynin personally instructed him to go straight to the station because the message was important – which was hardly an exaggeration. That man on the bicycle, in my view, has saved the world.
Most likely, without even knowing. A year later, a direct telegraph line was installed which was popularly called the “red telephone.” (There never was an actual red telephone sitting in the Oval Office.) A lesson had been learned: Communication can be vital when it comes to solving conflicts. Today the situation is vastly different from what it was less than half a cen- tury ago. The world is knit together by a network of connections of economic, political, cultural, and other nature.
That is only possible because virtually instantaneous long-distance communication at affordable cost has become ubiq- uitous. In earlier centuries, important news – like the outcome of a battle, say – often was received only several weeks later. Today we are not the least bit as- tonished when we watch unfolding events in the remotest corner of the planet in real time, living color, and stereophonic sound. The biggest machine on earth is the international telephone network.
It allows you to call this minute, on a lark, your neighbor, your friend in New Zealand, or the Department of Sanitation in Tokyo. And we got used to it! Behind the scenes, of course, there is a substantial investment in technology going into this, and more effort is required to keep up with society’s ever-rising demands. Consider international calls: For some time satellites seemed to be the most efficient and elegant means. Just a decade or two later, they were no more up to the growing task, and a new, earthbound technology took over: optical fiber transmission.
V VI Absent a Telephone, a Bicyclist Had to Save the World Meanwhile, the amount of data handled by fibers exceeds anything that older technology could have handled ever. Today’s Internet traffic would not exist without fiber, and the cost of a long-distance phone call would still be as expensive as it was a quarter century ago. Optical fibers, mostly made of glass but sometimes also other materials, are the subject of this book. The development toward their maturity we enjoy to- day was mostly driven by the challenges of telecommunications applications.
Research has faced quite a number of questions concerning basic physics of guided-wave optics, and many researchers around the world toiled for answers. As a result, fibers can do more than was anticipated: Besides the obvious appli- cation in telecommunications, they have also become useful in data acquisition. This is why engineers and technicians working in either field need to know not only their electrical engineering, but increasingly also some optics. At the same time, it emerges that nonlinear physical processes in fibers will lead to exciting new technology.
This book has its origin in lectures for students of physics and engineering which I gave at the universities in Hannover, Münster, Rostock (all in Germany), and Luleå (Sweden). The book first appeared in the German language. It was well received, but the German-speaking part of the world is not very big, and I heard opinions that an English version would find a larger audience. The book presents the physical foundations in some detail, but in the in- terest of limited mathematical challenges, there is no fully vectorial treatment of the modes.
On the other hand, I found it important to devote some space to nonlinear processes on grounds that over the years, they can only become more relevant than they already are. I proceed in outlining the limitation of the data-carrying capacity of fibers as they will be reached in a couple of years, i., at a time when the student readers of this book will have entered their professional life as engineers or scientists, dealing with these questions. For the English edition, I have expanded certain sections slightly, to keep up to date with current developments. It is my hope that both natural scientists and engineers will find the book helpful.
Maybe physicist will think that some segments are quite “technical,” while engineers may feel that a treatment of nonlinear optics may be not so much for them. My answer to that is that either subject is required to form the full picture. In this context, it is sometimes unfortunate that the structure of our universities emphasizes the distinction between natural scientists and engineers more than is warranted. I envision that, in analogy to electronics engineers, we will see the emergence of photonics engineers.
They would have good practical skills on the technical side and at the same time a deep understanding of the underlying physical mechanisms. Contents I Introduction 1 1 A Quick Survey 3 II Physical Foundations 13 2 Treatment with Ray Optics 15 2.1 Waveguiding by Total Internal Reflection .2 Step Index Fiber .4 Gradient Index Fibers .6 Shortcomings of the Ray-Optical Treatment. 24 3 Treatment with Wave Optics 25 3.3 Linear and Nonlinear Refractive Index .4 Separation of Coordinates .9 Field Amplitude Distribution of the Modes .11 Number of Modes .12 A Remark on Microwave Waveguides .1 Treatment with Derivatives to Wavelength .2 Treatment with Derivatives to Frequency .2 Waveguide and Profile Dispersion .3 Normal, Anomalous, and Zero Dispersion .4 Impact of Dispersion. 55 VII VIII Contents 4.5 Optimized Dispersion: Alternative Refractive Index Profiles .1 Gradient Index Fibers .4 Quadruple-Clad Fibers .5 Dispersion-Shifted or Dispersion-Flattened? .6 Polarization Mode Dispersion .1 Quantifying Polarization Mode Dispersion .2 Avoiding Polarization Mode Dispersion .2 Photonic Crystal Fibers .1 Loss Mechanisms in Glass .4 Ultimate Reach and Possible Alternative Constructions .2 Hollow Core Fibers.
83 III Technical Conditions for Fiber Technology 85 6 Manufacturing and Mechanical Properties 87 6.1 Glass as a Material .3 How Glass Breaks .2 Manufacturing of Fibers .2 Pulling Fibers from the Preform .3 Mechanical Properties of Fibers .2 Reduction of Structural Stability. 99 7 How to Measure Important Fiber Characteristics 101 7.3 Geometry of Fiber Structure .4 Geometry of Amplitude Distribution .1 Near-Field Methods .2 Far-Field Methods .6 Optical Time Domain Reflectometry (OTDR). 114 Contents IX 8 Components for Fiber Technology 117 8.2 Preparation of Fiber Ends .4 Elements for Spectral Manipulation .1 Fabry–Perot Filters .2 Fiber–Bragg Structures .5 Elements for Polarization Manipulation .6 Direction-Dependent Devices .1 Power Splitting/Combining Couplers .2 Wavelength-Dependent Couplers .8 Optical Amplifiers .1 Amplifiers Involving Active Fibers .2 Amplifiers Involving Semiconductor Devices .1 Light from Semiconductors .1 Principle of pn and pin Photodiodes. 149 IV Nonlinear Phenomena in Fibers 151 9 Basics of Nonlinear Processes 153 9.1 Nonlinearity in Fibers vs.3 Nonlinear Wave Equation .1 Envelope Equation Without Dispersion .2 Introducing Dispersion by a Fourier Technique .3 The Canonical Wave Equation: NLSE .4 Discussion of Contributions to the Wave Equation .4 Solutions of the NLSE .2 The Fundamental Soliton .3 How to Excite the Fundamental Soliton .4 Collisions of Solitons .5 Higher-Order Solitons .5 Digression: Solitons in Other Fields of Physics .7 Inelastic Scattering Processes .1 Stimulated Brillouin Scattering .2 Stimulated Raman Scattering.
188 10 A Survey of Nonlinear Processes 193 10.3 Chirped Amplification .4 Optical Wave Breaking .4 The Soliton Laser and Additive Pulse Mode Locking .6 Self-Frequency Shift .7 Long-Haul Data Transmission with Solitons. 207 V Technological Applications of Optical Fibers 209 11 Applications in Telecommunications 211 11.1 Fundamentals of Radio Systems Engineering .5 Multiplexing in Time and Frequency: TDM and WDM .6 On and Off: RZ and NRZ .8 Transmission and Channel Capacity .1 A Single Wavelength Channel .2 Several Wavelength Channels .1 Monitoring of Operations .3 Filtering to Reduce Crosstalk .4 Telecommunication: A Growth Industry .2 The Limits to Growth. 243 12 Fiber-Optic Sensors 247 12.1 Why Sensors? Why Fiber-Optic? .4 The Status Today. 256 VI Appendices 257 A Decibel Units 259 A.4 Beer’s Attenuation and dB Units.
261 B Skin Effect 263 C Bessel Functions 265 C.1 Terminology for the Various Functions .2 Relations Between These Functions .4 Properties of Jm and Km .5 Zeroes of J0 , J1 , and J2 .6 Graphs of the Most Frequently Used Functions. 267 D Optics with Gaussian Beams 269 D.1 Why Gaussian Beams? .2 Formulae for Gaussian Beams .3 Gaussian Beams and Optical Fibers. 271 E Relations for Secans Hyperbolicus 273 F Autocorrelation Measurement 275 F.1 Measurement of Ultrashort Processes .4 A Catalogue of Autocorrelation Shapes. 278 Bibliography 281 Glossary 293 Index 299 Part I Introduction An optical fiber in comparison to a paper clip.
On the far left, part of the fiber’s plastic coating is visible; mostly the fiber is bare, though. Only a small fraction of its diameter of 125 μm near the fiber axis serves the waveguiding directly. Chapter 1 A Quick Survey Visual, and hence optical, communication is older than language. Hand signals, waving of the arms, and fire and smoke signals are basic means of communi- cation, and except under detrimental environmental conditions like pitch-black darkness or fog, they are useful over longer distances than shouting; besides, they are not thwarted by noises like surf at the seashore.
Normally we communicate verbally. Hence, when optical means are em- ployed, there is a necessity to agree on a code that serves to translate the visible signs into a meaningful message. Certain signs of nontrivial meaning are understood universally and even independent of language: consider the handwaving sign for “come here.” On the other hand, the vocabulary of such signs is too limited to convey truly complex messages. Codes that represent smaller units of language – syllables, phonemes, or individual letters – are much more universal.
The best-known example may be the Morse alphabet. Of course, it is mandatory that both sender and receiver of the transmitted message have agreed on the code ahead of time. In today’s computerized environment, codes of various kinds are of tremendous importance. The range (maximum distance) of optical transmission of messages can be increased by concatenation of several shorter spans.
In the Greek tragedy of Agamemnon (part of The Oresteia), Aeschylus (ca.