Towards Direct Sampling of Ultra-Broadband Microwave Signals: A Review of Integrated Photonic Analog-to-Digital Converter Technologies

Photonic analog-to-digital converters (PADCs) have been investigated for nearly five decades as a promising solution for next-generation high-performance signal acquisition, thanks to their inherent advantages such as low jitter, high bandwidth, and unique multiplexing techniques. This paper reviews the fundamental principles and recent advancements in four well-known categories of PADCs based on the role of photonic technology in the signal digitization process: photonic assisted PADCs, electronic sampling and photonic quantizing PADCs, photonic sampling and electronic quantizing PADCs, and photonic sampling and quantizing PADCs. We then focus on photonic sampling and electronic quantizing PADCs, which have emerged as the most widely used architecture, owing to their rapid integration progress and practical applications in direct RF sampling. This architecture overcomes the limitations of electronic systems by achieving bandwidths exceeding 100 GHz while maintaining a resolution of 8-14 bits. The modular design, which integrates optical components like modulators, optical demultiplexers, and photodetectors with chiplet-based photonic-electronic co-packaging, holds great potential for hybrid integration and monolithic implementation. Demonstrated in direct RF sampling applications, such as radar and communication systems with >8 GHz instantaneous bandwidth, these PADCs offer a transformative solution for next-generation signal acquisition and processing. Furthermore, this paper reviews the latest integration results of these PADC architectures and their applications in radar and communication systems. In the end, acknowledging the existing challenges, we highlight potential strategies and provide insights into the future development of PADCs for integration and direct RF sampling applications.