David J. Klotzkin Introduction to Semiconductor Lasers for Optical Communications An Applied Approach Introduction to Semiconductor Lasers for Optical Communications David J. Klotzkin Introduction to Semiconductor Lasers for Optical Communications An Applied Approach 123 David J. Klotzkin Department of Electrical and Computer Engineering Binghamton University Binghamton, NY USA ISBN 978-1-4614-9340-2 ISBN 978-1-4614-9341-9 (eBook) DOI 10.1007/978-1-4614-9341-9 Springer New York Heidelberg Dordrecht London Library of Congress Control Number: 2013953201 Ó Springer Science+Business Media New York 2014 This work is subject to copyright. 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Violations are liable to prosecution under the respective Copyright Law. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Preface Nobody questions the importance of semiconductor lasers. The information they transmit is the backbone of the World Wide Web, and they are increasingly finding new applications in solid-state lighting and in spectroscopy, and at new wave- lengths ranging all the way from the ultraviolet on gallium nitride to the extremely long wavelengths produced by quantum cascade lasers. Even in optical commu- nications, lasers are used in different ways, from metropolitan links using directly modulated devices to 100 Gb/s transmission systems incorporating advanced detection and modulation schemes. In this book, I introduce semiconductor lasers from an operational perspective to those who have a background in engineering or optics, but no familiarity with lasers. The objective here is to present semiconductor lasers in a way that is both accessible and interesting to advanced undergraduate students and to first-year graduate students. The target audience for this book is someone who is potentially interested in careers in semiconductor lasers, and the decision of what topic to cover is driven both by the importance of the topic and how fundamental it is to the whole field. I hope to make the reader very comfortable with both the scientific and engineering aspects of this discipline. The topics and emphasis were selected based largely on my experience in the semiconductor laser industry. My goal is that after reading the book, the reader appreciates most of the aspects of laser fabrication and performance so that they could then get immediately, actively involved in the engineering of this material. The book starts with talking generally about optical communications and the need for semiconductor lasers. It then discusses the general physics of lasers, and moves on to the relevant specifics of semiconductors. There are chapters on optical cavities, direct modulation, distributed feedback, and electrical properties of semiconductor lasers. Topics like fabrication and reliability are also covered. The book is appropriate as the primary text for a one-semester course on semiconductor lasers at the advanced undergraduate or introductory graduate level, or would also be appropriate as one of the texts in a general course in photonics, optoelectronics, or optical communications. Binghamton, NY, USA David J. Klotzkin v Acknowledgments Let me start by thanking my doctoral research advisor, Prof. Pallab Bhattacharya, for getting me started on this fascinating field. I appreciate the opportunity to work at Lasertron, Lucent (which later became Agere), Ortel (which later became part of Agere, and then part of Emcore), and Binoptics. At all of these places, there were always laser problems to work on! I also had the invaluable opportunity to work with many knowledgeable and helpful people, particularly Malcolm Green, Phil Kiely, Julie Eng, Richard Sahara, and Jia-Sheng Huang. A particular thanks to Binoptics for allowing me to use some data in this book. My laser course and students were always the motivation for this work, and I appreciate their feedback on what was well presented and what could be improved. In particular, I would like to thank Arwa Fraiwan for her careful reading of the chapters and editing. I thank Merry Stuber and Michael Luby at Springer for their work in getting reviews and their patience in keeping this project moving forward. I am happy again to thank Mary Lanzerotti for her enormous help at both the beginning and the end of this project. Without her to suggest the idea, it would probably have not gotten started. She also went through the chapters with great care and diligence, and was the best editor anyone could want. Finally, much thanks to my wife, Shari, and my family, for their support over the time this has taken. I am glad to get the time back that I had been spending on this book to spend with them. vii Contents 1 Introduction: The Basics of Optical Communications.......... 1 1.1 Introduction . 1 1.2 Introduction to Optical Communications . 1 1.2.1 The Basics of Optical Communications . 1 1.2.2 A Remarkable Coincidence . 3 1.2.3 Optical Amplifiers . 5 1.2.4 A Complete Technology . 5 1.3 A Picture of Semiconductor Lasers . 5 1.4 Organization of the Book. 6 1.5 Questions and Problems. 8 2 The Basics of Lasers.................................. 11 2.1 Introduction . 11 2.2 Introduction to Lasers . 11 2.2.1 Black Body Radiation . 12 2.2.2 Statistical Thermodynamics Viewpoint of Black Body Radiation . 13 2.2.3 Some Probability Distribution Functions . 14 2.2.4 Density of States . 15 2.2.5 Spectrum of a Black Body. 19 2.3 Black Body Radiation: Einstein’s View. 19 2.4 Implications for Lasing . 21 2.5 Differences Between Spontaneous Emission, Stimulated Emission, and Lasing . 23 2.6 Some Example Laser Systems . 24 2.6.1 Erbium-Doped Fiber Laser. 25 2.6.2 He–Ne Gas Laser . 25 2.7 Summary and Learning Points . 28 2.8 Questions. 28 2.9 Problems . 29 3 Semiconductors as Laser Material 1: Fundamentals .......... 31 3.1 Introduction . 31 3.2 Energy Bands and Radiative Recombination . 32 ix x Contents 3.3 Semiconductor Laser Materials System . 33 3.4 Determining the Bandgap . 36 3.4.1 Vegard’s Law: Ternary Compounds . 36 3.4.2 Vegard’s Law: Quaternary Compounds . 38 3.5 Lattice Constant, Strain, and Critical Thickness . 39 3.5.1 Thin Film Epitaxial Growth . 40 3.5.2 Strain and Critical Thickness . 41 3.6 Direct and Indirect Bandgaps . 43 3.6.1 Dispersion Diagrams . 43 3.6.2 Features of Dispersion Diagrams . 46 3.6.3 Direct and Indirect Bandgaps . 46 3.6.4 Phonons. 48 3.7 Summary and Learning Points . 49 3.8 Questions. 50 3.9 Problems . 51 4 Semiconductors as Laser Materials 2: Density of States, Quantum Wells, and Gain ............................. 53 4.1 Introduction . 53 4.2 Density of Electrons and Holes in a Semiconductor . 53 4.2.1 Modifications to Equation 4.9: Effective Mass . 55 4.2.2 Modifications to Equation 4.9: Including the Bandgap. 58 4.3 Quantum Wells as Laser Materials . 59 4.3.1 Energy Levels in an Ideal Quantum Well . 60 4.3.2 Energy Levels in a Real Quantum Well . 62 4.4 Density of States in a Quantum Well . 63 4.5 Number of Carriers . 65 4.5.1 Quasi-Fermi Levels. 66 4.5.2 Number of Holes Versus Number of Electrons. 67 4.6 Condition for Lasing . 68 4.7 Optical Gain . 69 4.8 Semiconductor Optical Gain . 70 4.8.1 Joint Density of States. 71 4.8.2 Occupancy Factor . 72 4.8.3 Proportionality Constant . 73 4.8.4 Linewidth Broadening . 74 4.9 Summary and Learning Points . 75 4.10 Learning Points . 75 4.11 Questions. 76 4.12 Problems . 77 5 Semiconductor Laser Operation ......................... 81 5.1 Introduction . 81 5.2 A Simple Semiconductor Laser . 82 Contents xi 5.3 A Qualitative Laser Model. 82 5.4 Absorption Loss . 86 5.4.1 Band to Band and Free Carrier Absorption . 87 5.4.2 Band-to-Impurity Absorption . 88 5.5 Rate Equation Models . 88 5.5.1 Carrier Lifetime . 91 5.5.2 Consequences in Steady State . 92 5.5.3 Units of Gain and Photon Lifetime . 94 5.5.4 Slope Efficiency . 95 5.6 Facet-Coated Devices . 97 5.7 A Complete DC Analysis . 100 5.8 Summary and Learning Points .