Silicon PhotonicsAn Introduction

Graham T. ReedAdvanced Technology Institute,

University of Surrey, Guildford, UK

Andrew P. KnightsMcMaster University, Hamilton, Ontario, Canada

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Silicon Photonics

Silicon PhotonicsAn Introduction

Graham T. ReedAdvanced Technology Institute,

University of Surrey, Guildford, UK

Andrew P. KnightsMcMaster University, Hamilton, Ontario, Canada

Copyright 2004 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester,West Sussex PO19 8SQ, England

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GTR dedicates this book to the following people:

To Alison, Hannah and Matthew for love and inspiration;To my parents Colleen and John for a lifetime of support;To Jackie for sibling rivalry!

APK dedicates this book to Melanie

Contents

About the Authors xiii

Foreword xv

Acknowledgements xix

1 Fundamentals 11.1 What is Phase? 11.2 What is Polarisation? 41.3 What is Interference? 6

2 The Basics of Guided Waves 112.1 The Ray Optics Approach to Describing Planar

Waveguides 112.2 Reflection Coefficients 122.3 Phase of a Propagating Wave and its Wavevector 152.4 Modes of a Planar Waveguide 16

2.4.1 The Symmetrical Planar Waveguide 182.4.2 The Asymmetrical Planar Waveguide 202.4.3 Solving the Eigenvalue Equations for

Symmetrical and Asymmetrical Waveguides 212.4.4 Monomode Conditions 222.4.5 Effective Index of a Mode 24

2.5 A Taste of Electromagnetic Theory 252.6 Simplifying and Solving the Wave Equation 272.7 Another Look at Propagation Constants 32

viii CONTENTS

2.8 Mode Profiles 332.9 Confinement Factor 34

2.10 The Goos–Hänchen Shift 35

3 Characteristics of Optical Fibres for Communications 373.1 The Structure of Optical Fibres 373.2 Modes of an Optical Fibre 39

3.2.1 Modes of a Step-index Fibre 403.2.2 Modes of a Graded-index Fibre 42

3.3 Numerical Aperture and Acceptance Angle 433.4 Dispersion in Optical Fibres 46

3.4.1 Intermodal Dispersion 463.4.2 Intramodal Dispersion 49

3.5 Single-mode Fibres: Mode Profile, Mode-field Diameter,and Spot Size 52

3.6 Normalised Frequency, Normalised PropagationConstant, and Cutoff Wavelength 54References 56

4 Silicon-on-Insulator (SOI) Photonics 574.1 Introduction 574.2 Silicon-on-Insulator Waveguides 58

4.2.1 Modes of Two-dimensional Waveguides 604.3 The Effective Index Method of Analysis 604.4 Large Single-mode Rib Waveguides 644.5 Refractive Index and Loss Coefficient in Optical

Waveguides 694.6 Contributions to Loss in an Optical Waveguide 70

4.6.1 Scattering 704.6.2 Absorption 734.6.3 Radiation 74

4.7 Coupling to the Optical Circuit 764.7.1 Grating Couplers 784.7.2 Butt Coupling and End-fire Coupling 814.7.3 Robust Coupling to Waveguides for

Commercial Applications 874.7.4 Measurement of Propagation Loss in Integrated

Optical Waveguides 914.8 Optical Modulation Mechanisms in Silicon 97

4.8.1 Electric Field Effects 98

CONTENTS ix

4.8.2 Carrier Injection or Depletion 1014.8.3 The Thermo-optic Effect 103

4.9 Other Advantages and Disadvantages of SiliconPhotonics 103References 108

5 Fabrication of Silicon Waveguide Devices 1115.1 Silicon-on-Insulator (SOI) 111

5.1.1 Separation by IMplanted OXygen (SIMOX) 1125.1.2 Bond and Etch-back SOI (BESOI) 1145.1.3 Wafer Splitting (SmartCut Process to Produce

Unibond Wafers) 1165.1.4 Silicon Epitaxial Growth 1175.1.5 Deciding on the SOI 118

5.2 Fabrication of Surface Etched Features 1195.2.1 Photolithography 1195.2.2 Silicon Etching 1235.2.3 Critical Dimension Control 125

5.3 Oxidation 1275.4 Formation of Submicron Silicon Waveguides 129

5.4.1 Silicon Dioxide Thickness 1295.4.2 Surface and Interface Roughness 1305.4.3 Sidewall Roughness 131

5.5 Silicon Doping 1325.5.1 Ion Implantation 1335.5.2 The Implantation System 1345.5.3 Implantation Parameters 1365.5.4 Dopant Activation and Drive-in 137

5.6 Metallisation 1385.6.1 Via Formation 1385.6.2 Metal Deposition 1385.6.3 Material Choice 1405.6.4 Sintering and Barrier Materials 140

5.7 Summary 141References 141

6 A Selection of Photonic Devices 1456.1 Optical Phase Modulators and Variable Optical

Attenuators 1456.1.1 The Optical Phase Modulator 1466.1.2 Modelling of Semiconductor Devices 146

x CONTENTS

6.1.3 Basic Device Geometry, and the Aim ofModelling 147

6.1.4 Effect of Parametric Variation on the DCEfficiency of an Optical Modulator 150

6.1.5 Predicted Device Operation as a PhaseModulator 157

6.1.6 Fabrication and Experimental Results 1596.1.7 Influence of the Thermo-optic Effect on

Experimental Devices 1626.1.8 Switching Characteristics of the Optical Phase

Modulator 1636.2 The Mach–Zehnder Interferometer 1656.3 The Waveguide Bend 1676.4 The Waveguide-to-Waveguide Coupler 172

6.4.1 Applications of the Waveguide-to-WaveguideCoupler 174

6.5 The Arrayed Waveguide Grating (AWG) 1766.5.1 Interference of N Coherent Light Sources 1766.5.2 Operation of the AWG 180

6.6 Waveguide Couplers for Small-dimension Waveguides 186References 189

7 Polarisation-dependent Losses: Issues for Consideration 1917.1 The Effect of Waveguide Thickness 1917.2 Surface Scattering Loss for Different Waveguide

Thickness and Polarisation 1977.3 Polarisation-dependent Coupling Loss 2017.4 Birefringence 204

7.4.1 Birefringence in Planar Silicon Waveguides 2057.4.2 Birefringence in Silicon Rib Waveguides 207

7.5 The Effect of Stress 2137.6 Discussion 214

7.6.1 The Effect of Polarisation and MultimodeSections on the AWG 215

7.6.2 The Effect of PDL on Other Devices 2197.7 Conclusion 219

References 220

8 Prospects for Silicon Light-emitting Devices 2238.1 Erbium Doping 223

8.1.1 Erbium Ion Implantation 224

CONTENTS xi

8.1.2 Optical Efficiency of Er-implanted Si 2268.1.3 Optical Intensity Quenching 2288.1.4 Electroluminescent (EL) Devices 229

8.2 Low-dimensional Structures 2328.2.1 Porous Silicon 2328.2.2 Nano-crystals 2358.2.3 Nano-crystals with Erbium 239

8.3 Dislocation-engineered Emitters 2428.4 Raman Excitation 244

8.4.1 Spontaneous Raman Effect 2458.4.2 Stimulated Raman Effect 2468.4.3 Raman Emission from Silicon Waveguides

at 1.54 µm 2468.5 Summary 248

References 248

Index 251

About the Authors

Graham T Reed BSc (Hons), PhD, FIEE, CEng

Silicon Photonics Research Group, Advanced Technology Institute,University of Surrey, UK

Graham Reed is Professor of Optoelectronics at the University of Surreyin the UK. He graduated in 1983 with a First Class Honours degree inElectronic and Electrical Engineering. Subsequently he obtained a PhDin Integrated Optics in 1987. After a brief period as leader of the Electro-Optics Systems Group at ERA Technology Ltd, he joined the Universityof Surrey in 1989, where he established the Silicon Photonics Group.As such this was one of the pioneering groups in silicon photonics,and has made a significant impact on the state of the art. The groupis currently the leading group in the UK in this field, and ProfessorReed is acknowledged as the individual who initiated research on siliconphotonic circuits and devices in the UK. The aim of the silicon work hasbeen to develop a technology that would have a variety of applications,although telecommunications remains the dominant application area.The work has been carried out with collaborators from all around theworld, both from academic and industrial institutions. Professor Reedhas published extensively in the international scientific literature, hascontributed presentations to numerous international conferences bothas a submitting and an invited speaker, and has served on a variety ofinternational committees.