Broadband microwave signal generation with programmable chirp shapes via low-speed electronics-controlled phase-modulated optical loop.

Broadband microwave signals with customized chirp shapes are highly captivating in practical applications. Compared with electronic technology, photonic solutions are superior in bandwidth but suffer from flexible and rapid manipulation of chirp shape or frequency. Here, we demonstrate a concept for generating broadband microwave signals with programmable chirp shapes. Our realization is based on a recirculating phase-modulated optical loop to ultrafast manipulate the laser frequency, which breaks the limitation of the buildup time of the laser from spontaneous emission. Through heterodyne beating the frequency-agile lasers with a continuous-wave laser, microwave signals with ultrafast and programmable chirp shapes are generated. Besides, signal parameters, such as bandwidth, center frequency, and temporal duration, can be reconfigured. In the experiment, highly coherent microwave signals with various customized chirp shapes are generated, where the time resolution for programming the chirp shape is 649 ps. This flexible frequency manipulation characteristic holds promise for many applications, including LiDAR, broadband radar systems, and spectroscopy.