KEYNOTE: Reconfigurable Microwave Photonic Systems: Innovations and Outlook

We review recent progress in optoelectronic oscillators (OEOs) and microwave photonic (MWP) signal processing, emphasizing reconfigurable systems that enable tunability, multifunctional capability, and improved performance metrics. First, we introduce OEO architectures that support dynamic reconfiguration between single-tone, dual-tone, and mode-locked operation using programmable MWP filters. Building on this foundation, we demonstrate that incorporating cascaded zero-dispersion recirculating loops effectively suppresses multimode competition, achieving record-high single-mode suppression ratio while ensuring excellent phase noise performance. We then extend this concept to ultra-high-harmonic mode-locking and realize orders up to the 946th harmonic, enabling the generation of broadband microwave combs. Complementary efforts on dispersion-tuned mode-locking provide frequency agility without the need to introduce complex system setups. Second, we describe quantum dash optical frequency comb sources (QD-OFCs) for multifunctional MWP processors capable of performing a broad range of signal processing functions, including integer and fractional Hilbert transforms, differentiation and integration, analog-to-digital conversion, instantaneous frequency measurement, and reconfigurable beamforming. These systems exploit the key features of QD-OFCs, namely uniform (flat) comb spectra, large number of comb lines, and high optical signal-to-noise ratio, and have strong potential for integration. Finally, we review recent advances in programmable integrated photonics, particularly in silicon and lithium niobate on insulator, for multifunctional MWP signal processing, positioning programmable MWP technologies as a cornerstone for future agile, multifunctional RF/optical front ends. Collectively, these developments underscore how reconfigurable MWP systems can deliver high-performance signal processing solutions for RF communications, sensing, and instrumentation.