On-chip High-Q Micro-ring Resonators and Their Applications in Microwave Photonics （Invited）

On-chip high-Q Micro-Ring Resonators （MRRs） have emerged as essential building blocks for high-performance microwave photonic systems， thanks to their compact footprint， narrow resonance bandwidth， and excellent tunability. They are widely employed in applications such as ultra-narrowband Microwave Photonic Filters （MPFs） for channel selection in radio-over-fiber networks， low-phase-noise Optoelectronic Oscillators （OEOs） for high-frequency microwave signal generation， and broadband optical frequency combs for high-resolution spectroscopy and microwave-to-optical conversion. Recent advances have focused on pushing Q factors into the multi-million range or even higher to further enhance device performance. This paper reviews the fundamental principles， design strategies， and material considerations for on-chip high-Q MRRs， with emphasis on their integration and practical deployment in these representative microwave photonic applications.The Q factor of a MRR characterizes the energy storage capability and loss level within the optical cavity. A higher Q indicates a longer photon lifetime， lower round-trip loss， and a narrower resonance linewidth. In microwave photonic system， this translates into a narrower filtering bandwidth of MPFs， a lower phase noise of OEOs， and enhances nonlinear optical effects. Accordingly， we present a detailed discussion of the fundamental structure， theoretical framework， and key parameters of high-Q MRRs， establishing a solid foundation for their design and optimization. From a coupling perspective， MRR behavior can be understood via two complementary models： power coupling and energy coupling， which correspond to frequency-domain steady-state analysis and time-domain transient analysis， respectively.Subsequently， we elaborate on implementation strategies for high‑Q MRRs from three key aspects： material platform selection （including silicon， silicon nitride， lithium niobate， and lithium tantalate）， fabrication process optimization， and waveguide engineering. A cross‑platform comparison is carried out to evaluate material systems in terms of refractive index contrast， optical transparency window， nonlinear optical properties， and fabrication maturity. This analysis reveals the inherent trade‑offs among platforms—for instance， silicon offers high index contrast and mature CMOS compatibility but suffers from two‑photon absorption in the telecom band， whereas silicon nitride exhibits broad transparency and ultralow loss， albeit requiring more advanced processing to reach ultrahigh Q values. On the waveguide engineering front， techniques such as adiabatic width tapering， Matched bend， Euler bend， and Bezier bend are employed to minimize radiative and bending losses， enabling smooth mode transitions and supporting ultrahigh‑Q operation in compact footprints. Recent progress in high‑Q MRR development across these material platforms is also reviewed.Furthermore， we examine the typical implementations of high‑Q MRRs in microwave photonic systems， with a particular focus on MPFs and OEOs. For MPFs， we begin by highlighting the potential of ultra‑high‑Q MRRs with large Free Spectral Range （FSR） in constructing ultra‑narrowband， widely tunable MPFs. We then review various system architectures developed to achieve ultra‑high out‑of‑band rejection and high reconfigurability. However， for real‑world RF applications， MRR‑based MPFs still require significant improvement in key RF performance metrics such as insertion loss， Noise Figure （NF）， and Spurious-Free Dynamic Range （SFDR）. Regarding OEOs， we discuss developments in on‑chip OEOs based on MRR， as well as emerging PT‑symmetry breaking schemes that eliminate the requirement for additional narrowband optical or electrical filters. In recent years， on‑chip integrated OEOs have made notable progress in frequency tuning range， Side‑Mode Suppression Ratio （SMSR）， and system compactness. However， the limited on‑chip effective delay， combined with additional noise introduced by active components， results in overall phase noise performance that remains considerably higher than that of discrete fiber‑based systems—particularly evident at intermediate and low frequency offsets. We also summarize recent progress in fully integrated on‑chip MPFs and OEOs， which is of great significance for achieving compact， lightweight microwave photonic systems.Prospectively， high‑Q MRR‑based microwave photonic devices are poised to transition into practical use across a range of fields， including radar signal processing， 5 G/6 G wireless communications， and RF signal generation. By leveraging programmable architectures and adaptive control schemes， these devices can simultaneously achieve miniaturization， low power consumption， and high performance. This progress is expected to drive the broader adoption of microwave photonics in cutting‑edge domains such as telecommunications， national defense， aerospace， and intelligent sensing， while gradually narrowing the performance gap between on chip solutions and discrete high performance systems.