Advances in Broadband Arbitrary Waveform Optical Synthesis Based on Mode-locked Optoelectronic Oscillators （Invited）

The ability to generate dynamically reconfigurable broadband arbitrary waveforms is of paramount importance for advanced radar imaging， next-generation wireless communications， integrated sensing and communication， and electronic warfare systems. These applications demand waveforms with large time-bandwidth products， complex modulation， and low phase noise to ensure high resolution， superior data throughput， and robust anti-jamming resilience. However， traditional electronic approaches are fundamentally constrained by the electronic bottleneck， which severely restricts instantaneous bandwidth， tuning agility and power efficiency， particularly at high frequencies. In this context， microwave photonics has emerged as a transformative paradigm by leveraging the intrinsic advantages of photonic technologies， including ultra-wide bandwidth， low transmission loss， and immunity to electromagnetic interference. In this field， opto-electronic oscillators are well known for generating microwave signals with ultra-high spectral purity， which is enabled by high-quality-factor resonators formed using long optical fiber delay lines. However， the inherently single-frequency nature of conventional opto-electronic oscillators renders them ill-suited for complex and wideband waveform synthesis.The incorporation of mode-locking mechanism into opto-electronic oscillator architectures has given rise to mode-locked opto-electronic oscillators， representing a major breakthrough that enables the coherent synthesis of broadband waveforms while maintaining low phase noise. This review systematically presents three representative mode-locked opto-electronic oscillator architectures： the Fourier-domain mode-locked opto-electronic oscillator， the wideband injection-locked opto-electronic oscillator， and the active mode-locked opto-electronic oscillator. The Fourier-domain mode-locked opto-electronic oscillator employs a high-speed tunable filter that is periodically scanned in synchronization with the cavity round-trip time， thereby creating a dynamic spectral storage mechanism that enables the sequential amplification of distinct frequency components. Initially demonstrated for the generation of wideband linear frequency-modulated waveforms， the Fourier-domain mode-locked opto-electronic oscillator has since evolved to produce dual-chirp signals， phase-coded linear frequency-modulated waveforms， and fully programmable arbitrary waveforms through precise control of the filter scanning trajectory. In contrast to the Fourier-domain mode-locked opto-electronic oscillator， the wideband injection-locked opto-electronic oscillator introduces an external broadband seed signal， typically generated by an arbitrary waveform generator， into a free-running opto-electronic oscillator cavity. The high-quality-factor resonator functions as a spectral purifier， substantially reducing phase noise and enhancing the signal-to-noise ratio while faithfully replicating the modulation profile of the injected seed signal. Alternately， the active mode-locked opto-electronic oscillator employs periodic gain modulation at a frequency matched to the cavity free spectral range or its harmonics， thereby actively phase-locking multiple longitudinal modes to form a coherent microwave frequency comb that manifests as a pulse train in the time domain. The active mode-locked opto-electronic oscillator facilitates precise repetition-rate control and has evolved to support tunable dual-band operation， multidimensional pulse coding （time， phase， and amplitude）， and fully programmable architectures for seamless switching between diverse waveform formats.This review provides a comprehensive analysis of the operating principles and system architectures for each approach， alongside a comparative evaluation of critical metrics such as bandwidth， phase noise， and time-bandwidth product. The wideband arbitrary waveforms generated by mode-locked opto-electronic oscillators hold great potentials for various applications. In high-resolution radar and imaging， these waveforms provide the bandwidth necessary to achieve millimeter-scale spatial resolution while ensuring robust resilience against electronic interference. In integrated sensing and communication systems， the wideband microwave signals leverage a unified hardware platform to support high-throughput communications alongside high-resolution sensing， effectively eliminating the trade-offs between data rate and detection accuracy. Furthermore， in advanced LiDAR systems， such waveforms support high-fidelity 4D imaging， capturing 3D structural details and instantaneous Doppler velocity with millimeter-level accuracy.Future research is expected to advance along several key frontiers multi-material hybrid integration using materials such as InP， Si3N4， and LiNbO3 to realize compact， stable， chip-scale devices； extension of high-frequency and ultra-wideband operation into the millimeter-wave and terahertz regimes； and the realization of software-defined， intelligent reconfiguration enabled by artificial intelligence for cognitive waveform adaptation. In summary， mode-locked opto-electronic oscillators constitute a versatile and powerful photonic platform that effectively overcomes the electronic bottleneck. They establish a robust foundation for the generation of wideband software-defined microwave waveforms， which are essential for advancing next-generation radar， communication， and sensing systems.