  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.
