Dissipative solitons relying on the double balance between nonlinear and linear effects as well as cavity loss and gain have attracted increasing attention in recent years, since they give rise to novel operating states of various dissipative nonlinear systems. An optoelectronic oscillator (OEO) is a dissipative nonlinear microwave photonic system with a high quality factor that has been widely investigated for generating ultra-low noise single-frequency microwave signals. Here, we report a novel operating state of an OEO related to dissipative solitons, i.e., spontaneous frequency hopping related to the formation of dissipative microwave photonic solitons. In this operating state, dissipative microwave photonic solitons occur due to the double balance between nonlinear gain saturation and linear filtering as well as cavity loss and gain in the OEO cavity, creating spontaneous frequency-hopping microwave signals. The generation of wideband tunable frequency-hopping microwave signals with a fast frequency-hopping speed up to tens of nanoseconds is observed in the experiment, together with the corresponding soliton sequences. This work reveals a novel mechanism between the interaction of nonlinear and linear effects in an OEO cavity, extends the suitability and potential applications of solitons, and paves the way for a new class of soliton microwave photonic systems for the generation, processing, and control of microwave and RF signals.

微波光子学是一门融合了微波技术和光子技术的交叉学科，是研究光波和微波在媒质中的相互作用以及在光频域实现微波信号的产生、处理、传输及接收的微波光波融合系统。由于现有的微波光子系统大多由分立器件组成，在体积、功耗、稳定性、成本等方面仍有待提升，因此集成化是微波光子技术发展的必然趋势。文中探讨了微波光子集成技术面临的主要科学与技术问题，总结了该技术的发展现状和前沿研究进展，并对其未来发展前景进行了展望。

微波光子学可借助光电子器件实现微波信号的产生、处理、接收和分配等功能，具有宽带、低传输损耗、轻重量、快速可重构及抗电磁干扰等优势。随着微波光子学的理论方法和技术应用的不断发展，微波光子与多学科交叉融合成为其核心发展方向。文中对微波光子与部分学科交叉融合的现状进行了总结，并对微波光子与激光技术、集成光电子学、量子技术和人工智能等前沿学科的交叉融合进行了展望。

An optoelectronic oscillator (OEO) is a microwave photonic system that produces microwave signals with ultralow phase noise using a high-quality-factor optical energy storage element. This type of oscillator is desired in various practical applications, such as communication links, signal processing, radar, metrology, radio astronomy, and reference clock distribution. Recently, new mode control and selection methods based on Fourier domain mode-locking and parity-time symmetry have been proposed and experimentally demonstrated in OEOs, which overcomes the long-existing mode building time and mode selection problems in a traditional OEO. Due to these mode control and selection methods, continuously chirped microwave waveforms can be generated directly from the OEO cavity and single-mode operation can be achieved without the need of ultranarrowband filters, which are not possible in a traditional OEO. Integrated OEOs with a compact size and low power consumption have also been demonstrated, which are key steps toward a new generation of compact and versatile OEOs for demanding applications. We review recent progress in the field of OEOs, with particular attention to new mode control and selection methods, as well as chip-scale integration of OEOs.