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Electronic Tuning of Semiconductor Lasers with Multiple Quantum Well ELECTRONIC TUNING OF SEMICONDUCTOR LASERS WITH MULTIPLE QUANTUM WELL DEVICES Submitted to University of London in Fulfillment of the Requirements for the Degree of Philosophiae Doctor Thesis Author: BO CAI Thesis Supervisor: ALWYN SEEDS Department of Electronic and Electrical Engineering University College London Torrington Place, London WC1E 7JE October, 1991 1 ProQuest Number: 10797639 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10797639 Published by ProQuest LLC(2018). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 Electronic Tuning of Semiconductor Lasers With Multiple Quantum Well Devices Abstract This thesis has been devoted to theoretical and experimental work on tunable semiconductor lasers, devices which play an important role in optical communication. The use of multiple quantum well (MQW) materials as tuning elements has been studied. In theoretical work the static and dynamic characteristics of single and multi-cavity tunable laser systems have been investigated in considerable detail. The various tuning structures were discussed. The refractive index change in MQW materials has been modelled and comparison between MQW and bulk materials has been made. The design details of integrated MQW-Bragg phase modulators are given. The possibility of integrated tunable semiconductor laser utilizing refractive index change in MQW materials was investigated. In experimental work, the electric field induced refractive index change in a PIN MQW device has been used in an electronically tuned external cavity GaAs/AlGaAs laser system. Discontinuous and continuous tuning experiments have been carried out using this structure. The experimental o results show a discontinuous tuning range of over 600GHz (14A in wavelength) for a 6 V change in device bias with less than 0.6 dB variation in laser output power and a continuous tuning range of over 2 GHz without significant output power change. The results also confirm the predicted refractive index change from absorption measurement. The optical FM Response was also measured using a high resolution Fabry-Perot interferometer and birefringent fibre filter system. A flat modulation response from very low frequencies up to 200 MHz was observed and no thermal tuning effect was found. 2 Acknowledgement I am grateful to Dr. A J. Seeds for his supervision, for his support of this thesis and for his dedication and patience in reading and correcting the manuscript. His helpful comments and constructive suggestions for improving the manuscript are greatly appreciated. I would also like to thank the many staff and students in this department who have helped during the course of this work. In particular, I would like to thank Professor G. Parry and his students for their help in the theoretical and experimental aspects of MQW modulators. I have benefitted tremendously from their pioneer work in this area. Thanks are also due to Mr. A. Rivers, Mr. C. Watson and Mr. F. Stride for their help and supervision on device processing. Dr. J. Roberts in SERC III-V semiconductor Centre of the University of Sheffield has given generous help in the aspects of MQW material growth. It is a pleasure to acknowledge my close association with my fellow students, N. Gomes, I. Benchflower, R. Ramos, C. Zaglanikis, and most recently S. Hoskyns. They have been a fertile source of ideas and comments. I would also like to acknowledge the generous support and encouragement of my former colleagues at the Institute of Optics and Electronics of the Chinese Academy of Sciences. The Chinese Academy of Sciences and the U.K University Vice Chancellor’s Committee have financially supported my study in the U.K. through an Overseas Postgraduate Scholarship and an Overseas Research Student Award. Finally I would like to thank my wife for her understanding and support during the course of this work. 3 TABLE OF CONTENTS Contents Page No. Cover Page 1 Abstract 2 Acknowledgement 3 Table of Contents 4 List of Figures 7 List of Tables 10 List of Symbols 11 Chapter One: Introduction 20 1.1 Needs for Tunable Semiconductor Lasers 20 1.2 Historical Review 21 1.3 Structure of this Thesis 28 References 33 Chapter Two: Static Theory of Tunable Semiconductor Lasers 39 2.1 Introduction 39 2.2 Single Cavity Structures 40 2.3 Multiple Cavity Structures 44 2.4 Conclusion 54 References 56 Chapter Three: Dynamic Theory of Tunable Semiconductor Lasers 58 3.1 The Limitation of Tuning Speed 58 3.2 Operating Speed of Tuning Elements 59 3.3 Dynamic Characteristics of Mode Selection 64 3.3.1 The Fabry-Perot Resonator with Gain Medium 64 3.3.2 The Round Trip Effect in a Fabry-Perot Filter 66 3.3.3 Mode Switching Time 68 3.4 Dynamic Characteristics of Continuous Tuning 69 3.4.1 Use of Parametric Resonators for Frequency Tuning 69 4 Table of Contents Contents Page No. 3.4.2 Oscillation in a Parametric Resonator 70 3.4.3 FM Response for a Sinusoidally Modulated Resonator 73 3.4.4 Three Special Cases 74 3.5 Conclusion 75 References 77 Chapter Four: Refractive Index Change in Quantum Well Materials 79 4.1 Introduction 79 4.2 Effective Mass Approximation Model 81 4.3 Exciton Absorption 83 4.3.1 Solution Under Zero Field 84 4.3.2 Solution Under Applied Field 86 4.3.3 Light Hole Absorption Peak Red Shift 87 4.3.4 Oscillation Strength 88 4.4 Continuum Band Absorption 90 4.5 Binding Energy 92 4.6 Absorption Spectrum Broadening 94 4.7 Refractive Index Change 100 4.8 Summary 101 References 104 Chapter Five: Comparison of Various Tuning Mechanisms 107 5.1 Introduction 107 5.2 Application Requirements and Performance Targets 108 5.3 Quantum Confined Stark Effect 111 5.3.1 Phase Shift and Insertion Loss 111 5.3.2 The Upper Cutoff Frequency 112 5.4 Franz-Keldysh Effect 117 5.4.1 Theoretical Model of FKE Induced Band Gap Shift 117 5.4.2 Comparison Between FKE and QCSE 119 5.5 Carrier Injection Effect 121 5.5.1 Plasma Effect 122 5.5.2 Band Gap Shift With Injection Carrier Density 123 5.5.3 Carrier Density Modulation 125 5.5.4 Thermal Effect 128 5 Table of Contents Contents Page No. 5.5.5 Comparison Between CIE and QCSE 129 5.6 Conclusion 131 References 133 Chapter Six: MQW Tuned External Cavity Laser Experiments 136 6.1 Introduction 136 6.2 Discontinuous Tuning Experiments 136 6.2.1 Experimental Arrangement 136 6.2.2 Experimental Results 143 6.2.3 Discussion 145 6.3 Continuous Tuning Experiments 149 6.3.1 Experimental Arrangement 149 6.3.2 Experimental Results 150 ‘ 6.3.3 Discussion 153 6.4 Measurements of the Dynamic Tuning Characteristics 155 6.4.1 Fast Tuning Elements 155 6.4.2 Fabry-Perot Interferometer Method 156 6.4.3 Birefringent Filter Method 164 6.5 Conclusion 167 References 169 Chapter Seven: Conclusion 170 7.1 Tunable Laser Structures 170 7.2 Tuning Mechanisms 171 7.3 MQW Tuned External Cavity Laser Experiments 171 7.4 Proposed Work 173 7.5 Summary 173 Appendices A: Antireflection Coating of Semiconductor Laser Facet 176 B: Data for HLP-1400 Semiconductor Lasers 183 C: Design of Semiconductor Bragg Reflectors 185 D: Measurements of Semiconductor Bragg Reflectors 193 E: Device Processing 197 6 LIST OF FIGURES Figures Page No. Fig. 1.2.1 Grating external cavity semiconductor laser 25 Fig. 1.2.2 Continuously tunable grating external cavity laser 25 Fig. 1.2.3 External cavity laser with an additional reflection surface 26 Fig. 1.2.4 Continuously tunable twin segment Fabry-Perot laser 26 Fig. 1.2.5 Discontinuous tuning twin segment Fabry-Perot laser 30 Fig. 1.2.6 Multiple segment DFB laser 30 Fig. 1.2.7 Twin guide DFB laser 31 Fig. 1.2.8 Three segment DBR laser 31 Fig. 1.2.9 Example of various tunable structures 32 Fig.2.2.1 Single Fabry-Perot resonator 41 Fig.2.3.1 The multiple cavity structure 45 Fig.2.3.2 Block A: a multilayer stack 45 Fig.2.3.3 External cavity laser 49 Fig.2.3.4 Modelling of mode selection 51 Fig.2.3.5 Tuning characteristics of external cavity lasers 53 Fig.3.2.1 The equivalent circuit of a MQW device 60 Fig.3.2.2 Cutoff frequency as a function of device size 62 Fig.3.2.3 A structure for fast MQW modulators 63 Fig.3.3.1 Fabry-Perot resonators as filters 67 Fig.3.4.1 General model for tunable lasers 72 Fig.3.4.2 Model for tunable external cavity lasers 72 Fig.3.4.3 Relative peak frequency deviation as a function of modulation frequency 76 Fig.4.3.1 An n region system 85 Fig.4.3.2 Eigen energies of electron, light and heavy holes under applied field 89 7 List of Figures Figures Page No. Fig.4.3.3 The square of the electron-hole overlap 89 Fig.4.4.1 The shape of exciton in QW 91 Fig.4.4.2 Dimensional coefficient Tl1 and Tih 93 dim dim Fig.4.5.1 Binding energy (+, * from reference 12) 93 Fig.4.6.1 Well thickness floating due to interface roughness 96 Fig.4.6.2 Applied field induced Gaussian broadening 96 o Fig.4.6.3 Absorption spectra of 47A well 99 o Fig.4.7.1.
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