Optical microwave waveforms generation based on the round-trip phase of the DFB laser

In this paper, we theoretically and experimentally analyze the influence of the round-trip phase of distributed feedback (DFB) lasers on the harmonic-phase of photocurrent, caused by the external injection current. The results indicate that the harmonic-phase of photocurrent introduced by the round-trip phase (HPIRP) reduces with the increase of modulation depth at a fixed frequency. Then, based on the HPIRP of the DFB laser, a simple photonic generation method of optical microwave waveforms is proposed and verified. In this scheme, three lasers with different wavelengths are mainly used. By adjusting the tunable optical delay lines and attenuators, the amplitude and phase of the photocurrent of the three branches converted by the photodetector (PD) can be controlled individually. The photocurrent of the superposition of the three branches can be controlled properly to form the target waveform. Dispersive elements and complex microwave photonic filtering are not required. Through experiments, rectangular and triangular waveforms with a repetition frequency of 3.75 GHz are generated.