Mode Engineering in Micro Ring Resonators and Their Application

Abstract

Silicon Photonics (SiP) has emerged as the prominent platform for Photonic Integrated Circuits (PICs). CMOS technology compatible fabrication processes, high index contrast of the waveguide core-cladding leading to sharp bends, and low propagation loss are the key advantageous features of SiP circuits in Silicon on Insulator (SOI). Various functional units are already in their mature stage where Micro Ring Resonators (MRRs) have been widely used to realize wavelength selective devices in a PIC. Compact design, high Q-Factor, scalable spectral properties, and the ability to create complex higher-order signal processing architectures are some of its basic advantages. Due to these benefits of MRR, it has found a wide range of applications ranging from sensors, optical communication, and filters. MRRs resonate at particular resonance wavelengths dictated by the interference condition. However, fabrication imperfections and parasitic coupling at various interfaces in MRR excite undesirable degenerate cavity modes that can lead to unpredictable resonance splitting. The extent of splitting and the shape of split resonances are uncontrollable and unpredictable within a reasonable degree of accuracy and are only identified during the device characterization stage. Such split response limits the use of MRR, otherwise a versatile component in PIC. In this work, we attempted to tackle the resonance splitting problem by engineering mode interaction within the cavity. We proposed and demonstrated a unique Self-Coupled MRR (SCMRR) that provides a predictable and controllable resonance split by regulating the excitation of the degenerate cavity mode. We also worked over multiple cavity systems like loaded MRR and quadruple resonance split MRR to gain control over not only the extent of splitting but also the resonance shape. Finally, the proposed devices were exploited for applications in three different domains i.e. sensing, optical communication and RF signal processing using photonics. Optical Communication: we demonstrated four channel multicasting at 48Gbps (4 x12 Gbps) by selectively splitting the MRR resonance into four notches. Multicasting is achieved using Two Photon Absorption (TPA) induced Free Carrier Dispersion (FCD) in Silicon. To the best of our knowledge, we achieved the highest data rate/channel of 12 Gbps using a MRR based device. Sensing: we demonstrated an on-chip self-calibrated sensor interrogator. In this patented technique, we used SCMRR as an interrogator to scan the shift in FBG sensor spectrum that can automatically calibrate the system performance against the natural decay of the SCMRR thermal tuners and fluctuations in the ambient environment. Unlike a single MRR, SCMRR interrogator response certain spectral characteristics that can be processed to identify the change in FBG spectrum as well as the SCMRR resonance split. The SCMRR split is then fed back to the system to calibrate the thermal tuners for SCMRR. RF signal processing using photonics: we proposed a RF Phase Shifter (PS) and generation of on-chip Single SideBand with carrier (SSB+C) for Radio over Fiber (RoF) based applications. In PS, we achieved continuous tuning of RF phase from 00 to 1800 with a record low power penalty of sub-1dB for a wide bandwidth RF (8 GHz-43 GHz). In RoF, we proposed a method of generating SSB+C signal by suppressing one of the sidebands of a Double SideBand with Carrier (DSB+C) signal. We achieved a tunable suppression ratio, high dynamic range, and almost zero dispersion-based power penalty, unlike DSB+C signals, over a transmission length of 43 Km and a frequency range of 1 GHz-20 GHz. The suppression is achieved using DSB+C signal from bulk modulator as well as an on-chip modulator