Oscillators that are capable of producing radio frequency (RF) and microwave signals are widely used in our modern society, such as communication link, radar, medical treatment, remote sensing, radio astronomy, spectroscopy and RF energy. One of the key parameters of an RF/microwave oscillator is its phase noise performance, which reflects the stability of the output signal. Conventional electronic oscillators such as quartz oscillators are commonly used for the generation of low-phase-noise RF/microwave signals at low frequencies. However, the generation of low-phase-noise signals at high frequencies is challenging because the quality factor (Q-factor) of an electronic oscillator, which indicates how well energy is stored within the resonant cavity, is low due to the lack of high-Q energy storage elements at high frequencies.

An optoelectronic oscillator (OEO) is another type of oscillator that designed in optoelectronic domain. The OEO can create high frequency RF/microwave oscillation with an ultra-low phase noise that can rival the best electronic oscillators thanks to the use of a high-Q optical energy storage element, such as a long and low loss fiber delay line. In addition to the excellent phase noise performance, the OEO also has other significant features such as ultrawide bandwidth, simple structure and capable of generating signals in both RF/microwave and optical domain simultaneously. As a result, OEOs have been widely investigated among various applications where an RF/microwave signal is generated, processed or received.

In recent years, new mode control and selection methods, as well as chip scale integration of OEOs have been developed and attracted considerable attention. Mode control based on Fourier domain mode locking (FDML) breaks the limitation of mode building time in OEOs and allows the generation of chirped microwave waveforms directly from the OEO cavity, which is not possible for conventional OEOs. Mode selection based on parity-time (PT) symmetry has also been demonstrated in OEOs to achieve a single mode operation, which avoid the use of ultranarrow band filters as in traditional OEOs. Moreover, integrated OEOs with small size and low power consumption have also been demonstrated in indium phosphide (InP) and silicon platforms, which are key steps towards a new generation of compact and versatile OEOs for demanding applications.

In the review article published in Advanced Photonics by Prof. Ming Li from Institute of Semiconductors, Chinese Academy of Sciences and colleagues (Tengfei Hao, Yanzhong Liu, Jian Tang, et al. Recent advances in optoelectronic oscillators[J]. Advanced Photonics, 2020, 2(4): 044001), a detailed introduction of the progresses in the field of OEOs, especially the recent advances including new mode control and selection methods, as well as chip scale integration of OEOs (see Fig.1) are presented, for the sake of more comprehensive understanding of the background and recent studies of OEOs for a broad range of readers. The developments and operation principle of OEO are first presented. Then recent advances in OEOs, including mode control based on FDML, mode selection based on PT symmetry and integration of OEOs toward achieving the compact chip-scale device are reviewed. Finally, a discussion is given on the future prospects of OEOs, including the generation of different types of RF/microwave signals and fully integrated OEOs.

Fig. 1 Selected emerging advances in OEOs in recently years. (a) A Fourier domain mode-locked (FDML) OEO. Chirped microwave waveforms can be generated directly from the FDML OEO cavity; (b) A Parity-time symmetric OEO. Single-mode operation is achieved without the needs of ultranarrow band filters; (c) An indium phosphide integrated OEO with compact size and low power consumption.