Title: Broadband microwave waveform generation with programmable chirp shapes via recirculating phase-modulated optical fiber loop controlled by low-speed electronics

Abstract: Broadband microwave waveforms with programmable chirp shapes are captivating in numerous practical applications. Compared with electronic technology, photonic-assisted solutions exhibit excellent performance in bandwidth and flexibility, but still suffer from complex architecture and requirement of high-speed electronics. Besides, rapid manipulation of chirp shape is still a challenge in the scientific community. In this paper, we propose and demonstrate a novel concept for generating broadband microwave waveforms with programmable chirp shapes. This concept is realized on a simple fiber-optic platform involving a continuous-wave laser source, a recirculating phase-modulated optical fiber loop, and low-speed electronics with a sampling rate at the level of MS/s. Based on this method, chirped microwave waveforms with a bandwidth up to tens of GHz can be generated, where the chirp shape is identical to the low-frequency driving waveform of the recirculating phase-modulated optical fiber loop. In addition, all the parameters of the generated chirped microwave waveforms can be easily reconfigured in real time, including the bandwidth, the central frequency, and the temporal duration. In the experiment, broadband microwave waveforms with customized chirp shapes are generated, where the center frequency and bandwidth tuning ranges exceed 21 GHz, the temporal duration is tuned in the range of 9 ns to 180 ns, and the coherent time of the generated microwave waveform is larger than 100 {\mu}s. This simple fiber-optic platform paves a way to generate broadband microwave waveforms with user-definable chirp shapes, which can find applications in broadband radar systems, electronic warfare and wireless communications.