Optical Signal Processing using Nested Ring Resonator Based Photonic Integrated Circuits

Abstract

The continuous advancement of integrated photonics demands compact, reconfigurable, and multifunctional building blocks capable of supporting a wide range of optical signal processing functionalities within a single platform. In this context, ring resonators on SOI platforms have emerged as promising candidates due to their inherent wavelength selectivity, compactness, and CMOS compatibility. However, conventional single microring resonators (MRRs) exhibit Lorentzian transmission characteristics with limited tunability and spectral diversity, restricting their performance in applications requiring sharp modulation and enhanced field confinement. To overcome these limitations, this thesis investigates the Nested Ring Resonator (NRR) configuration that introduces an additional feedback coupled waveguide within a standard ring structure. This internal feedback loop facilitates multiple resonant pathways, leading to rich interference dynamics analogous to Electromagnetically induced phenomena and hence emergence of non-Lorentzian lineshapes such as Electromagnetically Induced Transparency (EIT), Electromagnetically Induced Absorption (EIA), and Fano Resonances (FR). Using a signal flow graph-based analytical framework, the spectral transmission function of the NRR is systematically derived, and the influence of coupling and phase parameters on resonance behavior is comprehensively analyzed. The results demonstrate that by modulating the intra-cavity coupling coefficients and detuning parameters, the NRR can transition smoothly between Lorentzian, Fano, and other asymmetric lineshapes. This intrinsic ability to support multiple resonant phenomena without additional external interferometers highlights NRR as a compact and versatile spectral control unit. Despite its multifunctionality, the NRR exhibits relatively low spectral contrast, which limits its effectiveness in high-extinction Optical Signal Processing applications such as sensing, switching or narrowband filtering. To address this limitation, a modified configuration—the Nested Ring Resonator with Straight Waveguide (NRRSW)—is proposed. The inclusion of a straight bus waveguide enhances coupling interaction, introduces an additional interference path, and thereby improves the extinction ratio and resonance sharpness. Analytical modeling and numerical simulations confirm that the NRRSW retains the asymmetric spectral response of the NRR while offering superior contrast and tunability, making it highly suitable for photonic integrated circuits (PICs). The thesis further explores two major application domains of the proposed structures. First, in biosensing, the steep dispersion and high sensitivity associated with the Fano resonance in NRRSW enable precise detection of refractive index variations, leading to enhanced sensing resolution. Second, in optical switching, the abrupt transmission change near the Fano peak–dip region is exploited to achieve a high-contrast bistable response using a PN-depletion-based phase shifter integrated within the resonator. The bistability characteristics demonstrate strong potential for low-power, high-speed reconfigurable photonic switching. The proposed NRR and NRRSW architectures are designed with fabrication feasibility in mind, ensuring full compatibility with standard silicon photonic foundry processes. Their compactness, robustness, and tunability make them promising candidates for next-generation programmable photonic systems. Overall, this work establishes a unified framework for understanding and engineering asymmetric resonances in nested photonic resonators, paving the way toward integrated, multifunctional devices for sensing, modulation, and all-optical signal processing.