Microwave photonic approaches for the generation of microwave signals have attracted substantial attention in recent years, thanks to the significant advantages brought by photonics technology, such as high frequency, large bandwidth, and immunity to electromagnetic interference. An optoelectronic oscillator (OEO) is a paradigmatic microwave photonic oscillator that produces microwave signals with ultra-low phase noise, thanks to the high-quality-factor of the OEO cavity that is achieved with the help of optical energy storage elements, such as low-loss optical fiber or a high-quality-factor optical resonator. Different OEO architectures have been proposed to generate spectrally pure single-frequency microwave signals with ultra-low phase noise. Multiple oscillation mode control methods have been proposed in recent years to obtain different kinds of microwave signals. With the rapid development of photonic integration technologies, prototypes of integrated OEOs have been demonstrated with compact size and low power consumption. Moreover, OEOs have also been used for sensing, computing, and signal processing. This Tutorial aims to provide a comprehensive introduction to the developments of OEOs. We first discuss the basic principle and the key phase noise property of OEOs and then focus on its developments in spectrally pure low phase noise signal generation and mode control methods, its chip-scale integration, and its applications in various fields.

The optoelectronic oscillator (OEO) has attracted a great deal of attention in recent years as a high-performance microwave source. Thanks to the high quality-factor (Q-factor) provided by the optical delay line, the OEO can generate spectrally pure microwave signals with ultralow phase noise. Various approaches for realization of OEO architectures for the generation of single-frequency microwave signals have been demonstrated in the past two decades. Seeded by the microwave signal generated by OEO, the generation of complex microwave waveforms has been demonstrated, which can be used in applications such as radar and communication systems. OEO has also been demonstrated to perform other specific functionalities in different applications, including optical pulses and frequency comb generation, optical sensing, low-power microwave signal detection, and optical signal processing. This paper reviews the developments of OEO over the past two decades. An integrated OEO with monolithically integrated photonic parts and radio frequency subsections is reported as a promising example toward the monolithic integration of OEO. Key features of OEO and prospects for future development of the integration of OEO are also described.

We propose and demonstrate a tunable Fourier domain mode locked optoelectronic oscillator (FDML OEO) using a frequency scanning microwave photonics filter (MPF) based on stimulated Brillouin scattering (SBS). Fourier domain mode locking operation is achieved by synchronizing the tuning period of the SBS-based MPF to the cavity round-trip time of OEO. As a result, thousands of longitudinal modes are stimulated simultaneously in the OEO cavity. Linearly chirped microwave waveforms with a chirp rate up to 0.84 GHz/μs and a time bandwidth product as high as 22,446 are generated in the experiment. The proposed approach provides an important solution for chirped microwave waveforms generation which is widely used in modern radar and communication systems, and has the potential to be integrated on a chip.

A silica-based external cavity narrow linewidth laser is fabricated and experimentally demonstrated based on a commercial silica platform. An add-drop microring resonator and a 2×2 directional coupler in the silica chip form a wavelength-dependent and self-injection-locked external cavity. The linewidth-suppressing effect of the external cavity is theoretically calculated based on the chirp reduction theory, which shows that a long external cavity length at the resonant wavelength can be used to suppress the noise of a free-running laser. In the self-injection locked state, the measured intrinsic linewidth of the fabricated external cavity laser is 27 Hz, which is greatly reduced compared to the 49.7-kHz linewidth of the free-running state. The frequency stability is also measured to be improved by an order of magnitude from 4.52 × 10 ⁻⁸ to 9.07 × 10 ⁻¹⁰ at 1 s averaging time. The proposed narrow-linewidth laser shows the potential for massive production and can be used in various applications such as integrated microwave photonics, LIDAR, high-resolution metrology, and satellite communications.

We experimentally demonstrate Tb/s fast random bit generation from random signals generated in a broadband random optoelectronic oscillator. The broadband random optoelectronic oscillator is used as photoelectron entropy source to generate wideband random signal. We sample the random waveform with a resolution of 10 bits and a sampling rate of 128 GS/s. All 10 bits can be preserved and a generation rate of 2.5 Tb/s (128 GS/s $\times $ 10 bits $\times $ 2 data) is achieved by using time-shift bit-order-reverse bitwise exclusive-or operation as a more complicated post-processing method. The randomness of random bit sequences is verified by using NIST Special Publication 800–22 statistical tests.

We propose a microwave photonic frequency measurement scheme based a dual-polarization dual-drive Mach-Zehnder modulator (DP-DDMZM) and frequency -to-time mapping. A compact frequency measurement system is guaranteed thanks to the use of a single modulator, and multi-tone measurement ability is achieved due to the principle of frequency-to-time mapping. In the proposed scheme, the unknown microwave signal is mixed with a linearly chirped microwave signal with the help of the DP-DDMZM, and the mixing product is selected by an electrical bandpass filter to achieve frequency-to-time mapping. Single and multi-tone microwave frequency measurement are successfully demonstrated in the experiment, and the measurement errors are less than ± 3 MHz.