M.Sc Thesis

M.Sc StudentKodanev Anna
SubjectSi-PI Charge-Discharge Microring Modulator for Analog and
Digital Coding
DepartmentDepartment of Electrical and Computer Engineering
Supervisors PROFESSOR EMERITUS Moshe Nazarathy
PROF. Meir Orenstein


Substantial efforts are applied today to develop Compact Integrated Silicon Photonic electro-optical modulators, which could support modern transmission rates of 100Gb/s. Due to low EO effects in Si, the typical MZI-based modulator implementation is characterized by length in the order of millimeters, which makes multichannel modulation on a single chip difficult to implement and requires complex design of travelling-wave RF-lines for high speed modulation. Microring modulators with typical device footprint of tenth micrometers achieve high modulation depth with modest power consumptions. Small-signal analysis has shown that coupling strength modulation in microrings is preferable upon the intracavity one. It allows modulation rate beyond the resonator line-width. Cavity quality factor (Q) was determined as the main limitation of the modulation bandwidth (BW) and extinction ratio (ER).

In this work we provide large-signal analysis of coupling modulation dynamic in microring resonators. We have shown that the main problem, which limits modulation BW and ER in the coupling control based MRs, is the decay of the circulating optical energy inside the ring during the modulation. The effect is accumulative in time.

In order to solve this problem, we proposed novel doubly coupled charge-discharge configuration of microring resonator. We derived analytical relation between two tunable couplers, which allows keeping constant cavity optical field. This configuration has instantaneous response invariant in time. Therefore the BW and ER are not limited by the nature of microring modulator. We have built dynamic model of the charge-discharge modulator and simulated separately digital and analog modulations up to 100 Gb/s and 100 GHz respectively. The results show no distortions due to cavity energy degradation and predict unprecedentedly high ER. For Q>104, the relative level of the second order harmony is less than -55 dB and the third order is less than -100 dB. Relative level of the third order inter-modulations is less than -100 dB. These characteristics are indicators of high-quality analog modulation.

The fabrication of such high-Q microrings is a great challenge by itself, where the main restricting factor is the insertion loss of the tunable couplers. In previously proposed microring resonators, based on coupling modulation, the tunable coupler was implemented by relatively short (~100-200μm) MZIs. In order to reduce the loss, we propose the usage of tunable asynchronous directional couplers instead. Using FDTD tools, we modeled both implementations of the coupler and compared their performance. The comparison shows that the directional coupler configuration is preferable upon the MZI.