Photonic Generation of NLFM Microwave Pulses From DFB-Laser Chirp

This letter reports the photonic generation of apodized-spectrum non-linear frequency-modulated (NLFM) microwave pulses by using an optical self-heterodyne scheme. The distributed feedback (DFB) laser diode was modeled as a linear time-invariant system, relating modulation current inputs with optical frequency outputs through its transfer function, which was determined with a time-resolved spectroscopy technique. The suitable current stimulus on the DFB-laser diode required to generate radio frequency chirped pulses was calculated. Both up- and down-chirp NLFM signals were experimentally generated, covering radar bands ranging from L-band to part of Ku-band. The experimental results agreed well with the analytical calculations. A simulation of the echo detection of a radar system using the experimentally generated NLFM signals showed that the peak-to-sidelobe ratio (PSLR) of the compressed pulses was 10 dB better than that of LFM signals with the same bandwidth.