Integrated radar-communication refers to waveform fusion based on hardware sharing to simultaneously perform radar and communication functions using one signal. Consequently, the system performance can be optimized, and the frequency-spectrum resources can be saved. In this paper, a systematic overview of the integrated radar-communication waveform is provided. Specifically, the technical connotation and development stage of the radar-communication integration are presented, the application potential of linear frequency modulation (LFM) signal in the integrated system is analyzed. And the research progresses of LFM-integrated waveform design, high-frequency broadband LFM signal, and LFM-integrated waveform optical generation and processing are summarized. Studies have shown that microwave photons play an important role in the future of integrated radar-communication systems, and radar-communication integration is a system integrating optical and electrical technologies and linking analog and digital technologies.