Stepped-Frequency Microwave Waveform Generation Based on a Fourier Domain Mode-Locked Optoelectronic Oscillator

We propose and experimentally demonstrate an approach to generating a stepped-frequency microwave waveform (SFMW) with a large time-bandwidth product (TBWP) based on a modified Fourier domain mode-locked optoelectronic oscillator (FDML-OEO). A uniformly stepped-frequency optical carrier is generated by an optical frequency-shifting loop (FSL), which is used as a light source of the FDML-OEO. The joint operation of the stepped-frequency optical carrier generator, a phase modulator (PM), a phase-shifted fiber Bragg grating (PS-FBG), and a photodetector (PD) in the FDML-OEO forms a tunable microwave photonic filter (MPF), with the tuning interval being identical to the step duration of the stepped-frequency optical carrier. When the round-trip time of the OEO is equal to an integer multiple of the tuning interval of the MPF, Fourier domain mode-locking is achieved, and an SFMW is generated. The operation of the proposed SFMW generator is experimentally evaluated. An SFMW with a large TBWP of $3.25\times 10 ^{5}$ is generated.