| Optical microring resonator based filters and modulators are the potential building blocks of Photonic VLSI circuits. The ratio of optical power coupled between ring and bus waveguides in microring based devices is crucial to the desired device characteristics, and is the subject of study in the first part of this thesis. High-intensity laser photobleaching is used to demonstrate precise and fast post-fabrication trimming of coupling in (1) high-Q passive polymer microrings vertically coupled to chromophore-doped waveguides, and in (2) laterally coupled electro-optic ring modulators. In the first case, a decrease in coupling causes a 33% decrease in bandwidth, and in the second case an increase in coupling causes a 6-dB increase in extinction ratio. The latter ring is brought closer to critical coupling, as confirmed by phase characterization results, which indicate a significant increase in peak negative group-delay of the undercoupled ring. Photobleaching is an all-optical method of coupling control, via change of the refractive index of chromophore-doped polymer material, in the coupling region. A simple directional-coupler based mathematical model is developed for both cases, which analytically estimates the experimental change in coupling with index decrease.;We also develop a model for predicting the temporal evolution of refractive index change, averaged over the bleached thin-film depth. The concept of saturated absorption is applied to model the effect that high-intensity of light has on photobleaching-induced index change. The values of the photostability figure-of-merit and/or saturation-intensity for modern pi-conjugated non-linear-optical chromophores, such as CLD-1 and AJL8, are obtained here by fitting experimental data.;The second part of this thesis explores applications of microring based devices to digital optical communications---both quantum and classical. A novel device is proposed, which comprises of Cascaded Over- and Under-coupled Resonators (COUR). COUR can be used as a notch filter and phase modulator that overcomes the trade-offs of extinction-ratio--group-delay and sensitivity--bandwidth. Applications in Quantum-Key-Distribution are proposed. Finally, designs of polymer microring based Differential-Phase-Shift-Keyed modulator and demodulator are proposed. The above proposed modulators are based either on novel chromophore-polymer material, corona poled to obtain high electro-optic coefficient, or on thin-film lithium niobate technology, also explored here. |
