Photonic generation of flexible ultra-wide linearly-chirped microwave waveforms

Thanks to the large time bandwidth product (TBWP), linearly chirped microwave waveforms (LCMWs) are widely used in modern radar systems to achieve high-resolution detection and imaging. To overcome the challenge of small unmanned aerial vehicle detection and tracking, radar systems are required to have a higher resolution and multi-function operation, in which an ultra-wideband LCMW is highly preferred with a flexible tuning in the center frequency, instantaneous bandwidth, and multi-band operation. In this paper, we propose and experimentally demonstrate an approach to generating flexible ultra-wide LCMWs based on a Fourier-domain mode-locked optoelectronic oscillator (FDML-OEO) incorporating a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator. In the DP-QPSK modulator, two dual-parallel Mach-Zehnder modulators (DP-MZMs) are integrated. With the use of the upper DP-MZM, an FDML-OEO is produced to generate a wideband LCMW with a tuning in the center frequency and instantaneous bandwidth. With the injection of the generated LCMW into the lower DP-MZM, an ultra-wideband LCMW is generated via microwave frequency multiplication, and multi-band waveform generation is enabled by controlling the bias condition of the lower DP-MZM. An experiment is performed and an LCMW with a maximum bandwidth as broad as 10.8 GHz is generated. By adjusting the driving signal applied to the FDML-OEO, the generated LCMW can be tuned in the center frequency from 16.2 to 23.2 GHz and the bandwidth from 3.6 to 10.8 GHz. By controlling the bias point of the lower DP-MZM, a dual-band LCMW is also experimentally demonstrated. Thanks to the ultra-wide bandwidth and strong flexibility of the generated LCMWs in terms of tunable center frequency, instantaneous bandwidth and multiband operation, the proposed approach offers a promising LCMW generator in the next-generation high-resolution radar systems.