Photonic generation of a linearly chirped microwave waveform with a large time-bandwidth product based on self-heterodyne technique

We propose and experimentally demonstrate a photonic approach to generating a chirped microwave waveform with a large time-bandwidth product (TBWP) based on self-heterodyne technique. We use the chirp characteristic of a distributed feedback (DFB) laser (LD) to generate a frequency-modulated optical pulse, to directly modulate the LD using an injection current with a parabolic current shape that is controlled by a microwave waveform generated by a low-speed arbitrary waveform generator (AWG). The frequency-modulated optical waveform is then sent to an unbalanced Mach-Zehnder interferometer (MZI) to get two time-delayed replicas of the input waveform. A chirped microwave waveform is then generated by beating the time-delayed waveforms at a high speed photodetector (PD). The proposed technique is theoretically studied and experimentally demonstrated. Using an AWG with sampling rate of 100 MS/s and an MZI with an arm length difference of 144 m, we experimentally generate a linearly chirped microwave waveform with a long time duration of 1 μs and a bandwidth of 4.2 GHz, with a TBWP of 4200.