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.
This paper focuses on latest progress in experimental and theoretical studies on silicon-based carrier-depletion PNjunction phase shifters in terms of high modulation efficiency for energy-efficient photonic networks of high transmission capacity. Modulation efficiency of rib-waveguide phase shifters having various PN-junction configuration are characterized with respect to DC figure of merit defined for phase shifters using carrier-plasma dispersion as the physical principle of refractive-index modulation. In addition, RF drive voltage required for 10-Gb/s on-off keying is characterized for rib-waveguide phase shifters including lateral and vertical PN-junction configurations.
In this work we reported the efficiency and loss performance of a depletion silicon rib phase shifter, with an overlayer of 220 nm, rib width of 500 nm, and etch depth of 125 nm. We identified a range of doping concentrations that allow the phase shifter to operate at <6 V and <5 dB loss. Junction placement variances are done with doping concentrations in this range. The study suggested that with reduced p dopant concentration (2×10<sup>17</sup> cm<sup>-3</sup>), both loss and phase performance will improve by 32% and 20% respectively when p region > n region, compared to central junction.
Latest computational and experimental studies on high-speed monolithic silicon-based Mach-Zehnder optical modulators
are studied in the light of photonic integrated circuits for digital coherent communication at a bit rate as fast as 128 Gb/s
per wavelength channel. Lateral PN-junction rib-waveguide phase shifters are elaborated with experimental
characteristics of DC phase shifter response in comparison with computational characteristics. High-speed response in
refractive-index dynamics including electron and hole transport in the PN junction is simulated to study speed limit of
the phase shifters. The performance in quadrature phase-shift keying signal generation is characterized in experimental
and computational constellation diagrams. Silicon waveguides for polarization-division multiplexing are designed in
common design rules with the rib-waveguide phase shifters. Long-haul transmission in polarization-multiplexed
quadrature phase-shift keying in 1000-km single-mode fiber link is confirmed with a monolithic silicon Mach-Zehnder
modulator assembled with modulator drivers in a ceramic-based metal package.
Modulation format is a key determinant to the performance of an optical communication network. In this work, we study
a silicon phase shifter arranged around rib waveguide topologies and explore its potential to be applied in quadraturephase-
shift-keying (QPSK) optical modulators. Optical QPSK is implemented with four silicon phase shifters embedded
in a nested Mach-Zehnder waveguide. Modulated QPSK signal is simulated by calculating the interference between light
beams pass through each phase shifter. The constellation diagram of modulated QPSK signal is calculated after coherent
A monolithically integrated silicon-based optical modulator is reviewed with respect to design and high-speed
performance. The integrated silicon-based optical modulator consists of nested in-phase/quadrature Mach-Zehnder
modulator operated in quadrature phase-shift keying formats. Design and performance of high-speed silicon ribwaveguide
phase shifters and high-frequency coplanar-waveguide traveling-wave electrode are presented as key
modulator elements which allow high-speed zero-chirp operation of the integrated optical modulator in the quadrature
phase-shift keying formats. Transmission performance of the integrated optical modulator in differential quadrature
phase-shift keying format is characterized in direct-detection constellation-diagram and bit-error-rate measurements
towards 44.6-Gbit/s optical-fiber transmission. High-speed quadrature phase-shift keying operation is characterized in
coherent-detection constellation-diagram measurements in C and L bands, and QPSK at bit rates up to 64-Gbit/s is
presented. A partial-rib-waveguide polarization rotator, which is essential for 128-Gbit/s small-footprint silicon-based
optical modulator for digital coherent communication, is described and high-extinction ratio low-loss polarization
conversion over C and L bands is evidenced.
Low-loss high-speed traveling-wave silicon Mach-Zehnder modulator with reduced series resistance is studied in
microwave and optical measurements. Microwave impedance and propagation loss under reverse bias are characterized
by S-parameter measurements. Resonant loss due to series inductance-resistance-capacitance coupling limits microwave
performances of the traveling-wave modulator. High-speed optical performances are characterized, based on eyediagram
measurements in on-off keying at 10-32 Gb/s and constellation and eye-diagram measurements in differential
phase-shift keying at 20 Gb/s. Dispersion tolerance in error-free transmission in 10-Gb/s on-off keying and 20-Gb/s
differential phase-shift keying is obtained as +/-950 ps/nm and +/-220 ps/nm, respectively by path-penalty measurements.
Transmission performance in 10-Gbps on-off keying is comparable with lithium niobate Mach-Zehnder modulator.
In this work, we demonstrate two- and three-dimensional (3D) simulations of an active silicon-based photonic crystal
chromatic dispersion compensator utilizing the free carrier dispersion effect. The device has a low power consumption
of 114nW and its intrinsic device modulation speed is predicted to function at 40.5MHz. Due to the device architecture,
simulation must be carried out in 3D so as to fully encapsulate the effects of the photonic crystal contributions in the
active silicon. The novel device allows waveguiding and electrical transport to be individually tailored to a large extent.