High-speed Electrical Circuits For Silicon Ach-zehnder Electro-optic Modulators

Silicon photonics has developed rapidly in recent year. Silicon is the traditionalmaterial in silicon microelectronics as it has the unparalleled cost advantage. It can also beused for developing integrated high-speed optical modulators. Optical modulators are thekey devices in optical communications, optical interconnects, and optical signalprocessing. Particularly, silicon optical modulators based on the free-carrier plasmadispersion effect have the key merits of high modulation speed, low cost, and massproduction capabilities.This thesis focuses on the design of high-speed electrical circuits for siliconMach-Zehnder modulators, including on-chip electrical interconnection, single-endpush-pull traveling-wave electrode, and two types of modulator driving circuits. The maincontent of this thesis is divided into the following three parts:(1) Traveling wave electrode of the Mach-Zehnder modulatorFirstly, the equivalent circuit model of the traveling wave electrode is analyzed. Theresistance, inductance, and capacitance are calculated using theoretical formula. Theinfluence of these components on the electrode characteristics is studied. The results showthat the junction capacitance and the equivalent inductance have great impact on thehigh-speed performance of the electrode. Two types of traveling-wave electrodes (CPSand CPW) are presented and optimized. The high-speed on-chip interconnection lines andmicrowave I/O interface are also analyzed. The results show the impedance nearly matches50-ohm. The simulation of microwave scattering coefficients S21and S11indicates the low-loss and low-reflection transmission property of the travelling-waveelectrode. The3dB bandwidth of the electrode is larger than30GHz.(2) Single-end push-pull traveling-wave electrode.In order to design an efficient and compact QPSK modulator, a single-end push-pulltraveling-wave electrode is proposed. In this configuration, the signal and ground lines ofthe electrode are placed outside of the Mach-Zehnder arms. The slab layer in middle of thetwo arms is highly-doped to connect with the external DC bias. Simulation results showthat the impedance of the optimized electrode is near50-ohm with good impedance match.The experimental results show the Mach-Zehnder modulator based on this electrode canhave20Gb/s modulation speed. With this electrode, a QPSK modulator is designed.(3) External driving circuits for modulatorsIn order to make a module for electro-optic modulators, two high-speed drivingcircuit boards are designed. In one design, discrete components like bias tees and GaAsamplifier transistors are used to construct the circuit board. In another design, thecommercial amplifier and bias tee chips are used. The full-wave simulation of PCB boardis performed. The signal integrity is analyzed and the impedance and electromagneticisolation are optimized. Simulation results show that the two circuit boards have goodperformances. They can be used to drive the modulator chips upon wire bonding.