Significance Signal sources are key elements of modern electronic information systems such as radar, communication, measurement, and electronic warfare systems. High-performance microwave signal sources with high center frequency and ultra-low phase noise are essential to meet the rapid development trends of large bandwidth and high sensitivity in modern electronic information systems. However, achieving high center frequency and ultra-low phase noise simultaneously is very challenging for conventional electrical signal sources. Ultra-low phase noise is generally achievable at low center frequencies for conventional electrical signal sources. Although high center frequency microwave signals can be obtained by multiplying a low-frequency signal, the phase noise will also be deteriorated by a factor of 20log10N in the frequency multiplication process. An optoelectronic oscillator (OEO) is a microwave photonic signal source with a close optoelectronic feedback loop. High center frequency and ultra-low phase noise can be achieved simultaneously, which breaks the frequency and phase noise bottlenecks of conventional electrical signal sources. The high center frequency is obtained with the help of broadband optoelectronic devices in the OEO loop, whose bandwidth is as large as tens of GHz. The ultra-low phase noise is enabled by using low-loss or high-Q-factor energy storage elements, such as low-loss optical fiber. Moreover, the phase noise of the OEO can be independent of the center frequency according to the Yao-Maleki model, thus ultra-low phase noise can be maintained for high center frequencies and the OEO can find a wide range of applications in modern electronic information systems. A series of novel OEOs have been proposed and demonstrated in recent years, including parity-time symmetric OEO that achieves single-mode oscillation without filters, frequency scanning Fourier domain mode-locked OEO that overcomes mode building time limitations, optoelectronic parametric oscillator (OEPO) that enables phase-locked stable oscillation, broadband random OEO with random output frequency, soliton OEO with spontaneous frequency-hopping, actively and passively mode-locked OEO with pulsed output, as well as integrated OEO with compact size, greatly expanding the signal generation capability and application of OEOs. Progress Our study reviews the novel OEOs proposed and demonstrated in recent years. We first discuss the singlemode parity-time symmetric OEO, then review the multi-mode OEOs, including the frequency scanning Fourier domain mode-locked OEO, phase-locked OEPO, broadband random OEO, frequency-hopping soliton OEO and pulsed actively/passively mode-locked OEO. Integrated OEOs with compact size and the applications of these novel OEOs are subsequently discussed. Finally, we present an outlook on future development trends of OEOs. Conclusions and Prospects By properly controlling the oscillation modes or integrating key elements of the OEO, a series of novel OEOs, such as the Fourier domain mode-locked OEO, OEPO, broadband random OEO, frequency-hopping soliton OEO, actively/ passively mode-locked OEO, and integrated OEO, have been proposed and demonstrated in recent years, significantly improving the signal generation capability of the OEO and expanding its applications. For future developments of OEOs, further investigation into oscillation mode control methods remains interesting. More novel OEO schemes capable of producing various complex microwave waveforms can be expected in the near future by introducing new mode control methods. Integrated OEOs with higher integration levels and improved overall performance are also anticipated, holding the potential to meet the requirements of a wide range of applications for compact and highperformance microwave signal sources. Furthermore, new principles and research directions may emerge through crossdisciplinary research between OEO and other fields, such as quantum technology and artificial intelligence, marking another important development trend for OEOs.
