Introduction to Semiconductor Lasers for Optical Communications: An Applied Approach

Preface
Acknowledgements
Contents
1 Introduction: The Basics of Optical Communications
	1.1 Introduction
	1.2 Introduction to Optical Communications
		1.2.1 The Basics of Optical Communications
		1.2.2 A Remarkable Coincidence
		1.2.3 Optical Amplifiers
		1.2.4 A Complete Technology
	1.3 A Picture of Semiconductor Lasers
	1.4 Organization of the Book
	1.5 Summary and Learning Points
	1.6 Questions and Problems
2 The Basics of Lasers
	2.1 Introduction
	2.2 Introduction to Lasers
		2.2.1 Black Body Radiation
		2.2.2 Statistical Thermodynamics Viewpoint of Black Body Radiation
		2.2.3 Some Probability Distribution Functions
		2.2.4 Density of States
		2.2.5 Spectrum of a Black Body
	2.3 Black Body Radiation: Einstein’s View
	2.4 Implications for Lasing
	2.5 Differences Between Spontaneous Emission, Stimulated Emission, and Lasing
	2.6 Some Example of Laser Systems
		2.6.1 Erbium-Doped Fiber Laser
		2.6.2 HeNe Gas Laser
	2.7 Summary and Learning Points
	2.8 Questions
	2.9 Problems
3 Semiconductors as Laser Materials 1: Fundamentals
	3.1 Introduction
	3.2 Energy Bands and Radiative Recombination
	3.3 Semiconductor Laser Material System
	3.4 Determining the Band Gap
		3.4.1 Vegard’s Law: Ternary Compounds
		3.4.2 Vegard’s Law: Quaternary Compounds
	3.5 Lattice Constant, Strain, and Critical Thickness
		3.5.1 Thin Film Epitaxial Growth
		3.5.2 Strain and Critical Thickness
	3.6 Direct and Indirect Bandgaps
		3.6.1 Dispersion Diagrams
		3.6.2 Features of Dispersion Diagrams
		3.6.3 Direct and Indirect Band Gaps
		3.6.4 Phonons
	3.7 Summary and Learning Points
	3.8 Questions
	3.9 Problems
4 Semiconductors as Laser Materials 2: Density of States, Quantum Wells, and Gain
	4.1 Introduction
	4.2 Density of Electrons and Holes in a Semiconductor
		4.2.1 Modifications to Eq. 4.9: Effective Mass
		4.2.2 Modifications to Eq. 4.9: Including the Band Gap
	4.3 Quantum Wells as Laser Materials
		4.3.1 Energy Levels in an Ideal Quantum Well
		4.3.2 Energy Levels in a Real Quantum Well
	4.4 Density of States in a Quantum Well
	4.5 Number of Carriers
		4.5.1 Quasi-Fermi Levels
		4.5.2 Number of Holes Versus Number of Electrons
	4.6 Condition for Lasing
	4.7 Optical Gain
	4.8 Semiconductor Optical Gain
		4.8.1 Joint Density of States
		4.8.2 Occupancy Factor
		4.8.3 Proportionality Constant
		4.8.4 Linewidth Broadening
	4.9 Summary and Learning Points
	4.10 Learning Points
	4.11 Questions
	4.12 Problems
5 Semiconductor Laser Operation
	5.1 Introduction
	5.2 A Simple Semiconductor Laser
	5.3 A Qualitative Laser Model
	5.4 Absorption Loss
		5.4.1 Band-to-Band and Free Carrier Absorption
		5.4.2 Band-to-Impurity Absorption
	5.5 Rate Equation Models
		5.5.1 Carrier Lifetime
		5.5.2 Consequences in Steady State
		5.5.3 Units of Gain and Photon Lifetime
		5.5.4 Slope Efficiency
	5.6 Facet-Coated Devices
	5.7 A Complete DC Analysis
	5.8 Summary and Learning Points
	5.9 Questions
	5.10 Problems
6 Electrical Characteristics of Semiconductor Lasers
	6.1 Introduction
	6.2 Basics of p–n Junctions
		6.2.1 Carrier Density as a Function of Fermi Level Position
		6.2.2 Band Structure and Charges in p–n Junction
		6.2.3 Currents in an Unbiased p–n Junction
			6.2.3.1 Diffusion Current
			6.2.3.2 Drift Current
		6.2.4 Built-in Voltage
		6.2.5 Width of Space Charge Region
	6.3 Semiconductor p–n Junctions with Applied Bias
		6.3.1 Applied Bias and Quasi-Fermi Levels
		6.3.2 Recombination and Boundary Conditions
		6.3.3 Minority Carrier Quasi-Neutral Region Diffusion Current
	6.4 Semiconductor Laser p–n Junctions
		6.4.1 Diode Ideality Factor
		6.4.2 Clamping of Quasi-Fermi Levels at Threshold
	6.5 Summary of Diode Characteristics
	6.6 Metal Contact to Lasers
		6.6.1 Definition of Energy Levels
		6.6.2 Band Structures
	6.7 Realization of Ohmic Contacts for Lasers
		6.7.1 Current Conduction Through a Metal–Semiconductor Junction: Thermionic Emission
		6.7.2 Current Conduction Through a Metal–Semiconductor Junction: Tunneling Current
		6.7.3 Diode Resistance and Measurement of Contact Resistance
	6.8 Summary and Learning Points
	6.9 Questions
	6.10 Problems
7 The Optical Cavity
	7.1 Introduction
	7.2 Chapter Outline
	7.3 Overview of a Fabry-Perot Optical Cavity
	7.4 Longitudinal Optical Modes Supported by a Laser Cavity
		7.4.1 Optical Modes Supported by an Etalon: The Laser Cavity in 1D
		7.4.2 Free Spectral Range in a Long Etalon
		7.4.3 Free Spectral Range in a Fabry-Perot Laser Cavity
		7.4.4 Optical Output of a Fabry-Perot Laser
		7.4.5 Longitudinal Modes
	7.5 Calculation of Gain from Optical Spectrum
	7.6 Lateral Modes in an Optical Cavity
		7.6.1 Importance of Lateral Modes in Real Lasers
		7.6.2 Total Internal Reflection
		7.6.3 Transverse Electric and Transverse Magnetic Modes
		7.6.4 Quantitative Analysis of the Waveguide Modes
	7.7 Two-Dimensional Waveguide Design
		7.7.1 Confinement in Two Dimensions
		7.7.2 Effective Index Method
		7.7.3 Waveguide Design Targets for Lasers
	7.8 Summary and Learning Points
	7.9 Questions
	7.10 Problems
8 Laser Modulation
	8.1 Introduction: Digital and Analog Optical Transmission
	8.2 Specifications for Digital Transmission
	8.3 Small Signal Laser Modulation
		8.3.1 Measurement of Small Signal Modulation
		8.3.2 Small Signal Modulation of LEDs
		8.3.3 Rate Equations for Lasers, Revisited
		8.3.4 Derivation of Small Signal Homogenous Laser Response
		8.3.5 Small Signal Laser Homogenous Response
	8.4 Laser AC Current Modulation
		8.4.1 Outline of the Derivation
		8.4.2 Laser Modulation Measurement and Equation
		8.4.3 Analysis of Laser Modulation Response
		8.4.4 Demonstration of the Effects of τc
	8.5 Limits to Laser Bandwidth
	8.6 Relative Intensity Noise Measurements
	8.7 Large Signal Modulation
		8.7.1 Modeling the Eye Pattern
		8.7.2 Considerations for Laser Systems
	8.8 Summary and Conclusions
	8.9 Learning Points
	8.10 Questions
	8.11 Problems
9 Distributed Feedback Lasers
	9.1 A Single-Wavelength Laser
	9.2 Need for Single-Wavelength Lasers
		9.2.1 Realization of Single-Wavelength Devices
		9.2.2 Narrow Gain Medium
		9.2.3 High Free Spectral Range and Moderate Gain Bandwidth
		9.2.4 External Bragg Reflectors
	9.3 Distributed Feedback Lasers: Overview
		9.3.1 Distributed Feedback Lasers: Physical Structure
		9.3.2 Bragg Wavelength and Coupling
		9.3.3 Unity Round Trip Gain
		9.3.4 Gain Envelope
		9.3.5 Distributed Feedback Lasers: Design and Fabrication
		9.3.6 Distributed Feedback Lasers: Zero Net Phase
	9.4 Experimental Data from Distributed Feedback Lasers
		9.4.1 Influence of κ on Threshold Current and Slope Efficiency
		9.4.2 Influence of Phase on Threshold Current
		9.4.3 Influence of Phase on Cavity Power Distribution and Slope
		9.4.4 Influence of Phase on Single-Mode Yield
	9.5 Modeling of Distributed Feedback Lasers
	9.6 Coupled Mode Theory
		9.6.1 A Graphical Picture of Diffraction
		9.6.2 Coupled Mode Theory in Distributed Feedback Laser
		9.6.3 Measurement of κ
	9.7 Inherently Single-Mode Lasers
	9.8 Other Types of Gratings
	9.9 Learning Points
	9.10 Questions
	9.11 Problems
10 Assorted Miscellany: Dispersion, Fabrication, and Reliability
	10.1 Introduction
	10.2 Dispersion and Single Mode Devices
	10.3 Temperature Effects on Lasers
		10.3.1 Temperature Effects on Wavelength
		10.3.2 Temperature Effects on DC Properties
	10.4 Laser Fabrication: Wafer Growth, Wafer Fabrication, Chip Fabrication, and Testing
		10.4.1 Substrate Wafer Fabrication
		10.4.2 Laser Design
		10.4.3 Heterostructure Growth
			10.4.3.1 Heterostructure Growth: Molecular Beam Epitaxy (MBE)
			10.4.3.2 Heterostructure Growth: Metallorganic Chemical Vapor Deposition (MOCVD)
	10.5 Grating Fabrication
		10.5.1 Grating Fabrication
		10.5.2 Grating Overgrowth
	10.6 Wafer Fabrication
		10.6.1 Wafer Fabrication: Ridge Waveguide
		10.6.2 Wafer Fabrication: Buried Heterostructure Versus Ridge Waveguide
		10.6.3 Wafer Fabrication: Vertical Cavity Surface-Emitting Lasers (VCSELS)
	10.7 Chip Fabrication
	10.8 Wafer Testing and Yield
	10.9 Reliability
		10.9.1 Individual Device Testing and Failure Modes
		10.9.2 Definition of Failure
		10.9.3 Arrhenius Dependence of Aging Rates
		10.9.4 Analysis of Aging Rates, FITS, and MTBF
		10.9.5 Electrostatic Discharge and Electrical Overstresses
		10.9.6 Optical Overstress and Snap Test
	10.10 Design for …
		10.10.1 Design Tools
		10.10.2 Design for High Speed Directly Modulated Lasers
		10.10.3 Design for High Power
		10.10.4 Design for Low Linewidth
		10.10.5 Design Over Temperature
	10.11 Summary and Learning Points
	10.12 Questions
	10.13 Problems
11 Laser Communication Systems I: Amplitude Modulated Systems
	11.1 Introduction
	11.2 Evolution of Optical Speed
	11.3 Evolutionary Changes
	11.4 Multiplexing
		11.4.1 Wavelength Division Multiplexing
		11.4.2 Wavelength Division Multiplexing and Demultiplexing
		11.4.3 Optical Add Drop Multiplexors
	11.5 Overview of Amplitude-Modulated Communication
		11.5.1 Definitions for Amplitude Modulation Formats
		11.5.2 Bits Versus Symbols
		11.5.3 Pulse Amplitude Modulation
	11.6 External Modulation
		11.6.1 Quantum-Confined Stark Effect
		11.6.2 Absorption Modulation Through the Quantum-Confined Stark Effect
		11.6.3 Mach–Zehnder Modulator from Electooptic Materials
		11.6.4 Phase Shifting with Plasma Effect
	11.7 Laser Linewidth
		11.7.1 Inherent Laser Linewidth
		11.7.2 Linewidth Enhancement Factor
	11.8 Direct Detection Receivers
	11.9 Summary and Learning Points
	11.10 Questions
	11.11 Problems
12 Coherent Communication Systems
	12.1 Introduction
	12.2 Phasor Representation of Light
		12.2.1 Reminder: Phasor Representation of Electrical Signals
		12.2.2 Phasor Representation of Optical Signals
	12.3 Phasor Descriptions of Coherent Optical Transmission
		12.3.1 Binary (and More) Phase Shift Keying
		12.3.2 Differential Phase Shift Keying
		12.3.3 Quadrature Amplitude Modulation
		12.3.4 Polarization Division Multiplexing
		12.3.5 Polarization-Maintaining Fiber
	12.4 Coherent Optical Transmitters
		12.4.1 Binary (or More) Phase Shift Keying Transmitter
		12.4.2 Quadrature Amplitude Modulation
	12.5 Receivers
		12.5.1 Reference Signal
		12.5.2 Balanced Photodiode
		12.5.3 A Full Coherent System
	12.6 Coherent Transmission in Context
		12.6.1 Comparison of Coherent and Incoherent (Amplitude Shift Keying) Systems
		12.6.2 Communication Formats
	12.7 Limits to Transmission Distance in Optical Systems
		12.7.1 Optical Signal-to-Noise Ratio
		12.7.2 Eye Diagram-Based Signal-to-Noise Ratio
		12.7.3 Bit Error Rate Versus Transmission Format and Signal-to-Noise Ratio
	12.8 Noise Sources
		12.8.1 Relative Intensity Noise
		12.8.2 Shot Noise
		12.8.3 Erbium-Doped Fiber Amplifier Noise
		12.8.4 Thermal Johnson Noise
		12.8.5 Combination of Noise Sources
		12.8.6 Other Noise Sources
	12.9 Final Words
	12.10 Summary and Learning Points
	12.11 Questions
	12.12 Problems
Index