Performance Analysis of Photonic Ring Resonator for Sensing Application

Abstract

The growing demand for high-speed communication systems, miniaturised optical components and advanced sensing platforms has accelerated the growth of photonic technologies that are emerging as a replacement for traditional electronic systems. As data traffic increases with advances in artificial intelligence and biomedical research, the limitations of conventional electronic systems, such as bandwidth constraints, response speed and high energy consumption, are becoming significant challenges. Therefore, the global research efforts are increasingly focused on photonic integrated circuits (PICs) that are capable of precise signal routing, effective wavelength control and high-performance sensing with low energy consumption. In this context, microring resonators (MRRs) have emerged as a crucial component. Their compact size, high Q factor and wavelength selectivity make them effective in high-speed data transmission and sensing. The work focuses on enhancing the performance of MRR systems used in optical filtering, sensing and switching. Initial investigations on basic MRR architectures, such as all-pass and add-drop configurations, exhibited that these are capable of performing wavelength-selective operations. Their compact design makes them well-suited for integration into photonic circuits. However, they have several limitations, such as limited bandwidth, reduced spectral selectivity, non-uniform passband responses and a trade-off between Q-factor and signal isolation, which limit their use for high-speed optical communication systems. To address these challenges, the present study investigates high-order MRRs designed to achieve flatter spectral responses, a wide free spectral range and improved signal separation for efficient filtering and switching. In addition, highly sensitive MRR based sensors are also presented, which can identify minor changes in the surrounding refractive index, highlighting their potential for biological sensing applications. Thus, the research aims to develop low-loss, reconfigurable and high-performance photonic devices for next-generation integrated optical systems. The work begins by exploring high-order optical filters based on serially coupled MRRs to support efficient on-chip photonic interconnects. Using the transfer matrix method and coupled-mode theory, multiple cascaded arrangements up to the seventh order are designed and analysed through the continued fraction method to achieve flat-top transmission characteristics, steep roll-off, and enhanced group delay. The inter-resonator coupling coefficients are optimised to achieve a maximally flat passband, leading to improved filter characteristics. The results showed that the designed high-order filters provide strong rejection of out-of-band signals and maintain a nearly constant free spectral range (FSR). The proposed design achieved an improved group delay of 7.457 ps at the optimised coupling coefficient, representing excellent spectral performance. The achieved box-shaped spectrum and strong out-of-band suppression prove that the proposed design is well-suited for communication networks and on-chip optical platforms. These characteristics provide a strong foundation for designing compact, low-loss and reconfigurable photonic integrated systems. Additionally, the work also explores applications in optical switching where MRR based structures are employed to implement switches for Optical Network-on-Chip (ONoC) architectures. These switches are designed to improve on-chip routing, reduced latency and allow for the control of optical signals with improved energy efficiency. Three different switch configurations, i.e. 1×2, 2×2 and CRIT-based 2×2 switch have been developed with a compact footprint. The 1×2 configuration provides efficient wavelength-selective routing exhibiting minimal insertion loss with a high extinction ratio, making it suitable for optical signal selection. The 2×2 design supports bar and cross states and offers flexible signal control with moderate performance trade-offs. The CRIT-based 2×2 switch further improves performance by achieving a narrow transmission bandwidth and reduced power consumption through coherent interference of coupled resonators. These results indicate that compact MRR based switches provide scalable, low-power and high-speed performance for future photonic interconnect applications. The work is further extended by applying ring resonators for sensing applications. In this, two sensor designs, i.e. a butterfly-shaped absorber employing elliptical ring resonators and a multi-ring graphene-based refractive index (RI) sensor are presented. The butterfly-shaped absorber uses three layers comprising a metallic ground plane, a dielectric substrate and a top resonant layer patterned in butterfly geometry. The near-perfect absorption and strong field confinement provides enhance sensitivity to refractive index variations in the surrounding medium. The coupling between electric and magnetic fields enhances resonance sharpness and increases the absorption efficiency, enabling precise detection of even minor analyte changes. After this, the graphene-based sensor integrates multiple ring resonators with a monolayer graphene to achieve tunable multi-band absorption. The tunability of graphene through changes in its chemical potential and relaxation time provides dynamic control on resonance frequency, resulting in a reconfigurable sensor suitable for multiple sensing applications. The coupling between multiple ring resonators enhances the sharpness of the resonance with a higher quality factor leading to enhanced sensitivity. The simulation results indicate red shifts in resonance frequencies with changes in the analyte refractive index, thereby highlighting the suitability of proposed design for biomedical sensing. This design enables multi-analyte detection, thus extending the role of MRRs beyond communication to precision sensing in the THz regime. Overall, this thesis integrates the progress in filtering, switching and sensing photonic design approach based on ring resonator architectures. Each design is supported by simulations and parametric performance analysis, with key parameters thoroughly evaluated and optimised. The progression from single-ring to multi-ring architectures demonstrates consistent improvements in bandwidth, tunability, and energy efficiency, while the proposed configurations exhibit high scalability and compatibility for integration into PICs. Beginning from fundamental optical filtering concepts, advancing towards optical switching, and extending into terahertz (THz) metamaterial and graphene-assisted sensing, the results confirm that optimised architectures can overcome the limitations of conventional devices, offering compact and high-performance solutions for all-optical communication and biosensing applications.