Optoelectronic Oscillators: Progress from Classical Designs to Integrated Systems

: Optoelectronic oscillators (OEOs) have emerged as indispensable tools for generating low-phase-noise microwave and millimeter-wave signals, critical for a variety of high-performance applications. These include radar systems, wireless communications, satellite links, electronic warfare, and advanced instrumentation. The ability of OEOs to produce signals with exceptionally low phase noise makes them ideal for scenarios demanding high signal purity and stability. In radar systems, low-phase-noise signals enhance target detection accuracy and resolution, while in communication networks, such signals enable higher data throughput and improved signal integrity over extended distances. Furthermore, OEOs play a pivotal role in precision instrumentation, where even minor noise can compromise the performance of sensitive equipment. This review examines the progress in OEO technology, transitioning from classical designs relying on long optical fiber delay lines to modern integrated systems that leverage photonic integration for compact, efficient, and tunable solutions. Key advancements, including classical setups, hybrid designs, and integrated configurations, are discussed, with a focus on their performance improvements in phase noise and frequency tunability. The challenges in achieving fully integrated OEOs, particularly concerning stability and phase noise at higher frequencies, are also explored. This paper provides a comprehensive overview of the state-of-the-art in OEO technology, highlighting future directions and potential applications.