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Integrated High Speed Silicon Microring Electro-Optical Modulators

Ming Gong

Supervised by Hui Wu

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https://rochester.zoom.us/j/99354674008

Optical interconnects present a viable solution to meet the ever increasing demands for bandwidth in tele- and data communications. High speed and energy efficient photonic devices and transceivers, therefore, are at the center and front of this technological advance in semiconductors and integrated circuits. Silicon-on-insulator (SOI) is an excellent technology platform for building integrated optical interconnect devices because it is enabled by the well-established complementary metal-oxide semiconductor (CMOS) technologies, resulting in low fabrication cost and more importantly, large scale integration. CMOS compatibility also allows building photonic and electronic devices on a single chip, leading to the developments of electronic-photonic integrated circuits (EPIC). In the last two decades, both passive and active silicon photonic components on SOI platforms have been comprehensively studied thanks to enormous research efforts, and their performance is steadily improving. Recently, heterogeneous and monolithic integrated silicon photonics technologies have enabled the implementation of large-scale EPICs with high device density.

In EPIC designs, the large bandwidth difference between fast photonics and relatively slow electronics presents a fundamental bottleneck. High speed electro- optic (EO) modulators are one of the most critical components in photonic transceivers, located right at this critical E-O interface. A silicon EO modulator usually re- quires a large RF driving voltage to compensate for the relatively low modulation efficiency, especially in high-speed operations. The high RF voltage swing not only increases the power consumption, but makes the supporting high speed CMOS driver circuits challenging to design and implement. Novel designs of EO modulators are needed to overcome these challenges and achieve better performance trad-offs.

To reduce the challenge due to E-O bandwidth mismatch, many multiplexing technologies have been proposed. Optical time-division multiplexing (OTDM) technology is one of them. In this thesis, we propose a new time-interleaving de-multiplexer (DEMUX). Microrings are used as building blocks in this new circuit, because their compact size is well suited for high density integration. Our work focuses on the time-domain characteristics of ring modulators. A differential microring modulator as a sampler is proposed in the OTDM DEMUX to cancel out the power leakage due to its relatively low extinction ratio (ER). Multiple stages are coupled into a signal waveguide in series and controlled by a pair of electrical differential driving signals. In the sampling window, the input signal is sampled by all stages and de-multiplexed into corresponding output channels. The power leakage on output signals is canceled out by comparators, which improves the signal-to-noise ratio (SNR). A 100 Gbps and 200 Gbps prototypes are designed with simulation results presented.

In the second work in this thesis, we propose a new design methodology for nonlinear ring resonators that can efficiently analyze their nonlinear effects. In comparison, existing nonlinear microring designs rely on finite-difference time domain(FDTD) simulation of the whole ring device consumes large computational resources and time. Our new method is based on nonlinear waveguide simulations and microring transfer matrix analysis iterations. Design parameters can be changed without an entirely new FDTD simulation run, and design space studies are now possible. Two-photon-absorption (TPA) effects in silicon ring resonators are estimated using the proposed methodology, which helps determine the input power capability of a ring modulator with various parameters.

In the third work of this thesis, we propose a novel ring-assisted Mach–Zehnder modulator (RAMZM) to further improve the modulator performance compared to our differential microring sampler. By utilizing microring phase modulation, RAMZM can exhibit advantages of both ring and MZM: high ER, better linearity, and low power consumption. We propose to utilize strongly over-coupled rings with lightly-doped low loss pn junctions, to achieve better trade-offs: insertion loss (IL), ER, and modulation amplitude versus operation speed. The operation principle, device design, and optimization are presented. A prototype chip is fabricated on IMEC 50G standard SOI technology. Frequency- and time-domain measurement results show significant performance improvements: a 22 dB static ER and 10 dB ER at 12 Gbps; ring induced IL is 4.6 dB; 3 dB small-signal modulation bandwidth is estimated to be 45 GHz; and the measurements indicate over 35 GHz bandwidth.

In the OTDM de-multiplexer and new RAMZM circuits, we have explored the intensity and phase modulation effects, respectively. However, it is possible to utilize both effects simultaneously to enhance the OMA and ER. In the fourth work of this thesis, we propose to use a waveguide coupler as an interferometer to recombine two outputs of an add-drop ring, which can achieve power redistribution between two ports hence enlarge the output variation. The operation principle and design optimization are analyzed, which shows that the modulator performance is significantly improved in various aspects. The static and transient simulations are used to quantify the improvements on different figures-of-merit. A chip prototype is designed and being fabricated using AIM Photonics SOI technology.