Fiber-Optic Communication Systems

GOVIND P. AGRAWAL The Institute of Optics University of Rochester

Ш A WILEY-INTERSCIENCE PUBLICATION JOHN WILEY & SONS, INC. NEW YORK / CHICHESTER / BRISBANE / TORONTO / SINGAPORE

Contents

1 Introduction 1

1.1 Historical Perspective 1 1.2 Basic Concepts 6

1.2.1 Analog and Digital Signals 6 1.2.2 Channel Multiplexing 10 1.2.3 Modulation Formats 12

1.3 Optical Communication Systems 13 1.4 Lightwave System Components 15

1.4.1 Optical Fibers as a Communication Channel 15 1.4.2 Optical Transmitters 16 1.4.3 Optical Receivers 17

1.5 Channel Capacity 18 Problems 19 References 20

2 Optical Fibers 22

2.1 Geometrical-Optics Description 22 2.1.1 Step-Index Fibers 23 2.1.2 Graded-Index Fibers 26

2.2 Wave Propagation in Step-Index Fibers 28 2.2.1 Maxwell's Equations 28 2.2.2 Fiber Modes 31 2.2.3 Single-Mode Fibers 35

2.2.3.1 Single-Mode Condition 35 2.2.3.2 Mode Index 36 2.2.3.3 Field Distribution 36 2.2.3.4 Birefringence 36 2.2.3.5 Spot Size 37

ix

X CONTENTS

2.3 Dispersion in Single-Mode Fibers 39 2.3.1 Group-Velocity Dispersion 39 2.3.2 Material Dispersion 41 2.3.3 Waveguide Dispersion 42 2.3.4 Higher-Order Dispersion 44 2.3.5 Polarization Dispersion 45

2.4 Dispersion-Induced Pulse Broadening 46 2.4.1 Basic Propagation Equation 46 2.4.2 Gaussian Input Pulses 48 2.4.3 Limitations on the Bit Rate 51 2.4.4 Fiber Bandwidth 55

2.5 Fiber Loss 57 2.5.1 Attenuation Coefficient 57 2.5.2 Material Absorption 58 2.5.3 Rayleigh Scattering 59 2.5.4 Waveguide Imperfections 60

2.6 Nonlinear Optical Effects 61 2.6.1 Stimulated Light Scattering 61 2.6.2 Nonlinear Refraction 63 2.6.3 Four-Wave Mixing 64

2.7 Fiber Manufacturing 65 2.7.1 Design Issues 65 2.7.2 Fabrication Methods 66 2.7.3 Cables and Connectors 69

Problems 70 References 72

3 Optical Sources and Transmitters 75

3.1 Basic Concepts 76 3.1.1 Emission and Absorption Rates 77 3.1.2 The p-n junction 80 3.1.3 Nonradiative Recombination 83 3.1.4 Semiconductor Materials 84

3.2 Light-Emitting Diodes 87 3.2.1 Light-Current Characteristics 87 3.2.2 Spectral Distribution 89 3.2.3 Modulation Response 90 3.2.4 LED Structures 91

3.3 Semiconductor Lasers 93 3.3.1 Optical Gain 94 3.3.2 Feedback and Laser Threshold 96

CONTENTS xi

3.3.3 Laser Structures 98 3.3.3.1 Gain-Guided Semiconductor Lasers 99 3.3.3.2 Index-Guided Semiconductor Lasers 100

3.3.4 Laser Modes 101 3.3.5 Single-Longitudinal-Mode Operation 105

3.3.5.1 DFB Semiconductor Lasers 106 3.3.5.2 Coupled-Cavity Semiconductor Lasers 107

3.3.6 Light-Current Characteristics 110 3.3.7 Modulation Response 114

3.3.7.1 Small-Signal Modulation 115 3.3.7.2 Large-Signal Modulation 116

3.3.8 Laser Noise 118 3.3.8.1 Intensity Noise 119

3.3.8.2 Spectral Linewidth 121 3.4 Optical Transmitters 123

3.4.1 Source-Fiber Coupling 124 3.4.2 Driving Circuitry 126 3.4.3 Reliability and Packaging 128

Problems 129 References 132

4 Optical Detectors and Receivers 136

4.1 General Concepts 136 4.2 Photodiodes 139

4.2.1 p-n Photodiodes 139 4.2.2 p-i-n Photodiodes 142 4.2.3 Avalanche Photodiodes 143 4.2.4 Advanced APD Structures 147

4.3 Receiver Components 149 4.3.1 Front End 150 4.3.2 Linear Channel 151 4.3.3 Data Recovery 153 4.3.4 Integrated Receivers 154

4.4 Receiver Noise 155 4.4.1 Noise Mechanisms 155

4.4.1.1 Shot Noise 155 4.4.1.2 Thermal Noise 157

4.4.2 p-i-n Receivers 158 4.4.3 APD Receivers 159

CONTENTS

4.5 Receiver Sensitivity 162 4.5.1 Bit-Error Rate 163 4.5.2 Minimum Average Received Power 166 4.5.3 Quantum Limit of Photodetection 169

4.6 Sensitivity Degradation 170 4.6.1 Extinction Ratio 170 4.6.2 Intensity Noise 172 4.6.3 Timing Jitter 174

4.7 Receiver Performance 176 Problems 180 References 182

System Design and Performance 185

5.1 System Architectures 185 5.1.1 Point-to-Point Links 186 5.1.2 Broadcast and Distribution Networks 187 5.1.3 Local-Area Networks 189

5.2 Operating Wavelength and System Limitations 191 5.2.1 Loss-Limited Communication Systems 191 5.2.2 Dispersion-Limited Communication Systems 193 5.2.3 Telecommunication Fiber Links 195

5.3 System Design 196 5.3.1 Power Budget 197 5.3.2 Rise-Time Budget 198

5.4 Sources of Power Penalty 201 5.4.1 Modal Noise 201 5.4.2 Dispersion Broadening 203 5.4.3 Mode-Partition Noise 205 5.4.4 Frequency Chirping 209 5.4.5 Reflection Feedback and Noise 214

5.5 Computer-Aided Design 218 Problems 219 References 221

Coherent Lightwave Systems 226

6.1 Basic Concepts 227 6.1.1 Local Oscillator 227 6.1.2 Homodyne Detection 228 6.1.3 Heterodyne Detection 229 6.1.4 Signal-to-Noise Ratio (SNR) 230

CONTENTS xiii

6.2 Modulation Formats 231 6.2.1 ASK Format 232 6.2.2 PSK Format 235 6.2.3 FSK Format 236

6.3 Demodulation Schemes 238 6.3.1 Heterodyne Synchronous Demodulation 238 6.3.2 Heterodyne Asynchronous Demodulation 239

6.4 Bit-Error Rate and Receiver Sensitivity 241 6.4.1 Synchronous ASK Receivers 242 6.4.2 Synchronous PSK Receivers 243 6.4.3 Synchronous FSK Receivers 245 6.4.4 Asynchronous ASK Receivers 245 6.4.5 Asynchronous FSK Receivers 248 6.4.6 Asynchronous DPSK Receivers 249

6.5 Sensitivity-Degradation Mechanisms and System Design 250 6.5.1 Phase Noise 250 6.5.2 Intensity Noise 253 6.5.3 Polarization Mismatch 256 6.5.4 Fiber Dispersion 258 6.5.5 Other Limiting Factors 260

6.6 System Performance 261 6.6.1 Asynchronous Heterodyne Systems 261 6.6.2 Synchronous Heterodyne Systems 262 6.6.3 Homodyne Systems 263 6.6.5 Fie'd Trials 264

Problems 264 References 266

7 Multichannel Communication Systems 272

7.1 Classification Scheme for Multichannel Systems 273 7.2 Frequency-Division Multiplexing 274

7.2.1 High-Capacity Point-to-Point Links 275 7.2.2 Broadcast and Distribution Networks 276 7.2.3 Multiple-Access Local-Area Networks 279

7.3 Multiplexing and Demultiplexing Components 282 7.3.1 Multiplexers and Demultiplexers 282 7.3.2 Passive Star Couplers 286 7.3.3 Tunable Optical Filters 288 7.3.4 Tunable Semiconductor Lasers 291

7.4 Crosstalk and Other Performance Issues 292 7.4.1 Linear Crosstalk 292 7.4.2 Nonlinear Crosstalk 296

7.4.2.1 Stimulated Raman Scattering 7.4.2.2 Stimulated Brillouin Scattering 7.4.2.3 Cross-Phase Modulation 7.4.2.4 Four-Wave Mixing

7.4.3 Other Design and Performance Issues 7.5 Subcarrier Multiplexing

7.5.1 Analog SCM Lightwave Systems 7.5.2 Coherent SCM Lightwave Systems 7.5.3 Multiwavelength SCM Lightwave Systems

7.6 Code-Division Multiplexing Problems References

Optical Amplifiers

8.1 General Concepts

8.1.1 Gain Spectrum and Bandwidth 8.1.2 Gain Saturation 8.1.3 Amplifier Noise 8.1.4 Amplifier Applications

8.2 Semiconductor Laser Amplifiers 8.2.1 Amplifier Design 8.2.2 Amplifier Characteristics 8.2.3 Multichannel Amplification 8.2.4 Pulse Amplification 8.2.5 System Applications

8.3 Fiber Raman Amplifiers

8.3.1 Raman Gain and Bandwidth 8.3.2 Gain Saturation 8.3.3 Amplifier Performance

8.4 Fiber Brillouin Amplifiers 8.4.1 Brillouin Gain and Amplifier Bandwidth 8.4.2 Gain Saturation 8.4.3 Amplifier Performance

8.5 Erbium-Doped Fiber Amplifiers 8.5.1 Pumping Characteristics 8.5.2 Gain Spectrum 8.5.3 Small-Signal Gain and Saturation Characteristics 8.5.4 Amplifier Noise 8.5.5 Multichannel Amplification 8.5.6 Ultrashort Pulse Amplification 8.5.7 System Applications

CONTENTS xv

8.6 Design and Performance Issues 374 8.6.1 Direct-Detection Lightwave Systems 374 8.6.2 Coherent Lightwave Systems 377 8.6.3 Cascaded In-Line Amplifiers 379

Problems 381 References 383

9 Soliton Communication Systems 391 9.1 Fiber Solitons and Their Properties 391

9.1.1 The Nonlinear Schrödinger Equation 392 9.1.2 Fundamental and Higher-Order Solitons 393 9.1.3 Dark Solitons 397

9.2 Fiber Loss and Soliton Amplification 399 9.2.1 Loss-Induced Soliton Broadening 399 9.2.2 Loss Compensation Through Soliton Amplification 401

9.3 Design Issues 404 9.3.1 Soliton Interaction 404 9.3.2 Frequency Chirp 408 9.3.3 Amplifier Spacing 410 9.3.4 Amplifier Noise 414 9.3.5 Soliton Self-Frequency Shift 415

9.4 Experimental Progress 417 9.5 Multichannel Soliton Communication Systems 420 9.6 Concluding Remarks 423 Problems 423 References 425

Appendix A International System of Units 429

Appendix В Decibel Units 430

Appendix С List of Acronyms 432

Index 435