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Integrated Silicon-based Optical Modulators: 100 Gb/s and Beyond
Author(s): Kensuke Ogawa
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Book Description

This book discusses the principles and the latest progress of silicon optical modulators as cutting-edge integrated photonic devices on silicon-photonic platforms, which play key roles in modern optical communications with low power consumption, small footprints, and low manufacturing costs. Silicon Mach-Zehnder optical modulators are emphasized as the principal small-footprint optical modulator because of their superior performance in high-speed optical modulation at operational temperatures beyond 100 degrees Celsius without power-consuming thermo-electric cooling in spectral bands over 100 nm.

Book Details

Date Published: 17 May 2019
Pages: 254
ISBN: 9781510625815
Volume: PM302

Table of Contents
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Table of Contents

1 Introduction

2 Background
2.1 High-Capacity Optical Networks
     2.1.1 Overview
     2.1.2 Basic elements
     2.1.3 Transmission capacity
     2.1.4 Energy efficiency
2.2 Optical Modulators in High-Capacity Optical Networks
     2.2.1 Optical modulator in optical transmitter
     2.2.2 Semiconductor optical modulators
     2.2.3 Integrated optical modulators on silicon-photonics platforms

3 Introduction to Integrated Optical Modulators
3.1 Classification of Optical Modulators
     3.1.1 Electro-absorption optical modulators
     3.1.2 Ring-resonator optical modulator using electro-refraction effects
     3.1.3 Mach-Zehnder optical modulator using electro-refraction effects
3.2 High-Speed Broadband Mach-Zehnder Optical Modulators
     3.2.1 Mach-Zehnder interferometer with RF electrodes
     3.2.2 High-contrast intensity modulation
     3.2.3 High-Q phase modulation
3.3 Integrated Silicon-Based Mach–Zehnder Optical Modulators
     3.3.1 Optical-waveguide elements
     3.3.2 Monolithic modulator on chip
     3.3.3 Fabrication processes

4 Optical Circuits and Waveguides in Integrated Mach-Zehnder Optical Modulators
4.1 Optical Circuits
     4.1.1 Single Mach-Zehnder optical modulator
     4.1.2 Quadrature Mach-Zehnder optical modulator
     4.1.3 Polarization-division-multiplexed Mach-Zehnder optical modulator
4.2 Transfer-Matrix Framework
     4.2.1 Representation in transfer matrices
     4.2.2 Transfer matrices of Mach-Zehnder optical modulators
4.3 Optical Waveguide and Optical Mode
     4.3.1 Guided wave in ray trace
     4.3.2 Mode field and wave propagation
4.4 Optical Waveguide Features
     4.4.1 Channel and rib waveguides
     4.4.2 Optical splitter/coupler
     4.4.3 Polarization-division multiplexer
     4.4.4 Other building blocks based on optical waveguides

5 Electronic and Opto-Electronic Properties of High-Speed Phase Shifters
5.1 Physics in Phase Modulation
     5.1.1 Pockels effect
     5.1.2 Intraband free-carrier plasma dispersion and Drude model
     5.1.3 Interband dipole transition processes
     5.1.4 Spectral and thermal characteristics
     5.1.5 Frequency chirping
5.2 Classification of Phase Shifters Using Free-Carrier Plasma Dispersion
     5.2.1 Lateral PN-junction phase shifter
     5.2.2 Vertical PN-junction phase shifter
     5.2.3 Other types of phase shifter
5.3 Design and Modeling of PN-Junction Phase Shifters
     5.3.1 Semi-analytical method
     5.3.2 Computational method
     5.3.3 Equivalent-circuit model
     5.3.4 Remarks on designing traveling-wave electrodes

6 Optical, Electrical, and Electro-Optical Characteristics of Integrated Silicon-based Optical Modulators
6.1 DC Optical Characteristics
     6.1.1 Optical loss
     6.1.2 Phase shift and chromatic dispersion
6.2 DC Electrical Characteristics
     6.2.1 Current-voltage characteristics
     6.2.2 Microscopic imaging of the PN junction
6.3 RF Frequency Characteristics
     6.3.1 S-parameter characteristics
     6.3.2 Effect of parasitics
6.4 Transient Characteristics
     6.4.1 Response limitation by RC time constant
     6.4.2 Intensity modulation characteristics at various modulation speeds
     6.4.3 Intensity modulation characteristics at high temperatures
     6.4.4 Phase modulation characteristics and chirp parameter

7 Transmission Characteristics of Integrated Silicon-based Optical Modulators
7.1 Applications in Optical Network Domains at 100 Gb/s and Beyond
7.2 On-Off Keying and Pulse Amplitude Modulation
     7.2.1 Apparatus and device for OOK transmission
     7.2.2 Characteristics of OOK transmission
     7.2.3 PAMn scheme
7.3 Phase-Shift Keying
     7.3.1 Apparatus and device for PSK transmission
     7.3.2 Characteristics of PSK transmission
7.4 Polarization-Division-Multiplexed Quadrature Phase-Shift Keying
     7.4.1 Apparatus and device for PDM IQ transmission
     7.4.2 Characteristics of PDM IQ transmission
7.5 Discrete Multi-Tone Scheme
     7.5.1 Apparatus for DMT transmission
     7.5.2 Characteristics of DMT transmission
7.6 Note on Transmission Characteristics

8 Photonic–Electronic Integration with Silicon-based Optical Modulators
8.1 Integration with Electronic and Photonic Devices
     8.1.1 Monolithic integration
     8.1.2 Wafer-bonding integration: silicon on silicon
     8.1.3 Die-bonding integration: III-V on silicon
     8.1.4 Hybrid integration
     8.1.5 Optical coupling and packaging
8.2 Integration of Optical Performance Monitoring
     8.2.1 Technical background
     8.2.2 Conventional approach
     8.2.3 Optical layout for integration
     8.2.4 Photonic integrated performance-monitoring circuit
     8.2.5 All-silicon performance monitoring

A.1 Bit Rates and Modulation Formats in High-Capacity Optical Networks
     A.1.1 Bit rates
     A.1.2 Formats in intensity modulation
     A.1.3 Formats in phase modulation
     A.1.4 Format in sub-carrier modulation
A.2 Kramers–Kronig Transformation
     A.2.1 General principle
     A.2.2 Computational method


With the advent of semiconductor diode lasers that emit a coherent light beam under current injection and low-loss silica optical fibers that transmit a light beam over long distances, the technology of optical communications became practically available in the late 1960s to the early 1970s. Since then, optical communications have grown ceaselessly over the decades and have nowadays become indispensable for a variety of networks, such as 5G mobile communication, social networking, video streaming, the Internet of Things, artificial intelligence, virtual/augmented reality, and large-scale simulation. The data transmission capacity (measured in bit rate per wavelength channel, which is allocated in a single core of silica optical fiber) was 10 Mb/s in the 1970s and a few Gb/s in the 1980s. This capacity has reached 100 Gb/s and beyond in modern optical communications—an increase of more than four orders of magnitude, or roughly ten times per decade from the dawn of optical communications.

Such remarkable growth of data traffic in optical communications was enabled by the progress of high-speed photonic devices that are capable of processing and transmitting optical signals at the bit rates cited above. Photonic integration technologies provide design and fabrication platforms that facilitate the high-density integration of various elements of photonic devices with small footprints. These platforms thereby allow the manufacture of small-form-factor photonic assemblies that consist of photonic integrated circuits. Among these platforms, silicon-photonics platforms are most suited to the design and fabrication of photonic integrated circuits with low power consumption and cost because the platforms are based on electronic design tools and fabrication process lines developed for high-volume manufacturing of silicon integrated circuits that consist of ultra-low-voltage complementary metal–oxide–semiconductor devices. Therefore, there is increasing interest in the research and development of photonic integrated circuits on silicon-photonics platforms to accommodate the demand for versatile and affordable small-form-factor optical equipment in high-volume applications such as intra- and inter-datacenter optical communications around the world.

This book focuses on high-speed, integrated silicon-based optical modulators as the cutting-edge, key photonic device at the forefront of modern optical-communication equipment. A high-speed optical modulator allows electrical-to-optical conversion in an optical transmitter for optical signal generation in advanced modulation formats with complex intensityand phase- modulation schemes at bit rates of 100 Gb/s and beyond per wavelength channel. Silicon-photonics platforms allow integration of entire elements on chip to miniaturize the high-speed optical modulator within a footprint less than 1 cm2. The following chapters describe the basic principles to the latest developments of integrated silicon-based optical modulators. It is hoped that this book will inspire readers to further enhancements of modulator performance and sophistication of photonic integration technology, including diode lasers to realize ultra-compact, high-speed integrated optical transmitters in the next generation of optical communications.

Kensuke Ogawa
February 2019

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